The nature of the SDSS galaxies in various classes based on morphology, colour and spectral


Mon. Not. R. Astron. Soc. 000, 1–17 (2008)

Printed 1 July 2008

A (MN L TEX style ?le v2.2)

The nature of the SDSS galaxies in various classes based on morphology, colour and spectral features – I. Optical properties

arXiv:0807.0110v1 [astro-ph] 1 Jul 2008

Joon Hyeop Lee1,2?, Myung Gyoon Lee1? , Changbom Park3?, Yun-Young Choi4? 1
Astronomy Program, Department of Physics and Astronomy, Seoul National University, Seoul 151-742, Korea Astronomy and Space Science Institute, Daejeon 305-348, Korea 3 Korea Institute for Advanced Study, Dongdaemun-gu, Seoul 103-722, Korea 4 Astrophysical Research Centre for the Structure and Evolution of the Cosmos, Sejong University, Seoul 143-747, Korea
2 Korea

Accepted 2008 June 28. Received 2008 June 04; in original form 2008 April 14


We present a comprehensive study of the nature of the SDSS galaxies divided into various classes based on their morphology, colour, and spectral features. The SDSS galaxies are classi?ed into early-type and late-type; red and blue; passive, HII, Seyfert, and LINER, returning a total of 16 ?ne classes of galaxies. We examine the luminosity dependence of seven physical parameters of galaxies in each class. We ?nd that more than half of red early-type galaxies (REGs) have star formation or AGN activity, and that these active REGs have smaller axis ratio and bluer outside compared to the passive REGs. Blue early-type galaxies (BEGs) show structural features similar to those of REGs, but their centres are bluer than REGs. HII BEGs are found to have bluer centres than passive BEGs, but HII REGs have bluer outside than passive REGs. Bulge-dominated late-type galaxies have red colours. Passive red late-types are similar to REGs in several aspects. Most blue late-type galaxies (BLGs) have forming stars, but a small fraction of BLGs do not show evidence for current star formation activity. Di?erences of other physical parameters among di?erent classes are inspected, and their implication on galaxy evolution is discussed. Key words: galaxies: general – galaxies: evolution – galaxies: statistics – galaxies: elliptical – galaxies: spiral – galaxies: active



One of the fundamental issues of the observational cosmology is the evolutionary connection between various classes of galaxies. Today, galaxies are classi?ed with various criteria. The most classical classi?cation of galaxies is the Hubble sequence: elliptical galaxies, lenticular galaxies, spiral galaxies, barred spiral galaxies, and irregular galaxies (Hubble 1936; Sandage 1961; De Vaucouleurs 1974). The main criterion of this classi?cation is the morphology of galaxies; that is, the existence, size ratio, and appearance of spiral arms, disc, bulge, and bar. The Hubble sequence was established mainly based on the galaxies without nuclear activity, and such galaxies are often called normal galaxies. On the other hand, more and more galaxies showing active nuclei with broad line emission are being found; they are often called abnormal galaxies. According to the features of


E-mail: jhl@kasi.re.kr (JHL); mglee@astrog.snu.ac.kr (MGL); cbp@kias.re.kr (CBP); yychoi@kias.re.kr (YYC)

the line emission, active galactic nuclei (AGN) host galaxies are classi?ed into several sub-classes: Seyfert 1 galaxies, Seyfert 2 galaxies, broad-line radio galaxies, narrow-line radio galaxies, low ionisation nuclear emission regions (LINERs), and so on. Luminosity is another criterion to classify galaxies. Sandage & Binggeli (1984) de?ned the galaxies with MB > ?18 as dwarf galaxies. In addition to those general classes of galaxies, there are some unusual galaxy classes with interesting properties: E+A galaxies with the spectral features of both very old stellar populations and very young stellar populations (Dressler & Gunn 1992; Yang et al. 2006, 2008); ultra-luminous infrared galaxies (ULIRGs) that are very bright in the mid- and far-infrared bands (Sanders & Mirabel 1996; Hwang et al. 2007); blue compact galaxies that show very compact morphology and high surface brightness with blue colour (Thuan et al. 1997); extremely red objects (EROs) whose optical ? infrared colour is extremely red (Elston et al. 1988); and so on. Recently, to deal with a large amount of survey data, many astronomers have classi?ed galaxies simply into red se-


J. H. Lee et al.
the distribution of photometric, spectroscopic and structural parameters for 350 nearby galaxies using the Millennium Galaxy Catalogue (Liske et al. 2003), arguing that most properties show a clear distinction between early-type galaxies and late-type galaxies. Later, Conselice (2006) carried out principal-component analyses of the properties of 22000 galaxies at z < 0.05 using the Third Reference Catalogue of Bright Galaxies (RC3; De Vaucouleurs et al. 1991), ?nding that the three parameters determining a galaxy’s physical state may be mass, star formation and interactions/mergers. These studies presented how useful to consider various parameters in galaxy classi?cation, although some important components like AGNs were not considered in their analyses. Conselice (2006) showed that multiple independent components, rather than any single component, may determine the various properties of galaxies. Therefore, to understand the nature of galaxies more comprehensively and to give stronger constraints on galaxy evolution models to explain all kinds of galaxies, it is necessary to investigate the nature of galaxies in various and ?nely-divided classes, and to ?nd out the evolutionary connections between the classes. We have been doing a comprehensive study on a set of ?ne galaxy classes in the Sloan Digital Sky Survey (SDSS; York et al. 2000), based on their morphology, colour and spectral features. In this paper, the ?rst in the series, we present the optical properties of galaxies in various ?ne classes. The outline of this paper is as follows. Section 2 shows the data set we used, and §3 describes the methods to classify the SDSS galaxies and to select volume-limited samples. We present the statistics of selected optical parameters and their luminosity dependence in §4. Based on the optical properties, we discuss the nature of galaxies in the ?ne classes in §5. Finally, the conclusions in this paper are given in §6. Throughout this paper, we adopt the cosmological parameters h = 0.7, ?Λ = 0.7, and ?M = 0.3.

quence galaxies and blue sequence galaxies according to their colour-magnitude relation (e.g. Martin et al. 2007). For a long time, the individual properties of galaxies in those various classes have been investigated, and many e?orts have been made to answer fundamental questions about galaxy evolution and connections between galaxy classes. Some examples of such fundamental questions are as follows: Why is there a conspicuous bimodality in the colour distribution of galaxies? Why does the colour bimodality not exactly agree with the morphological segregation? How do the environments a?ect the properties of galaxies? How did active galactic nuclei (AGNs) form? Is there any transition among the di?erent classes of galaxies? Recent studies using large survey data discovered several interesting aspects of galaxy evolution, providing some answers to those fundamental questions. For example, Ball et al. (2006) studied the bivariate luminosity functions with galaxy type classi?cation, and found that there is a clear morphological bimodality supporting the idea that merger and accretion are associated with bulges and discs, respectively. Bernardi et al. (2005) showed that both luminosity and colour of early-type galaxies are correlated with stellar velocity dispersion, and that velocity dispersion may be also closely correlated with the age and metal abundance of early-type galaxies. Choi et al. (2007) found that late-type galaxies show wider dispersion in several physical quantities than early-type galaxies, and that those physical quantities manifest di?erent behaviours across M? ± 1 mag. Park et al. (2007) investigated the environmental e?ects on various physical properties of galaxies, ?nding that a key constraint on galaxy formation models is the morphology-density-luminosity relation. Mateus et al. (2006) analysed the colour, 4000? break, and age in various A spectral classes of galaxies, suggesting that the median lightweighted stellar age of galaxies is directly responsible for the colour bimodality in the galaxy population. Those previous studies, however, are not yet enough to explain the origins of the various galaxy classes and the evolutionary connections between the classes. One of the major limitations is that the galaxy classi?cations in most previous studies were often limited to only one or two properties. For instance, automatically-classi?ed morphology (e.g. Park & Choi 2005; Ball et al. 2006), galaxy colour (e.g. Martin et al. 2007), or spectral line features (e.g. Mateus et al. 2006) are used frequently in recent studies based on large survey data, but a multilateral classi?cation study using all these criteria at the same time has not yet been seen. Such simpli?cations may be often very useful to understand the various and complicated phenomena in galaxy evolution. However, a simple classi?cation can not distinguish detailed aspects of galaxy evolution. For example, early-type galaxies are often regarded as red and passive galaxies in most studies, but that is not always true. Some kinds of galaxies with early-type morphology were found to have blue colours possibly originating from young stellar populations or active nuclei (Abraham et al. 1999; Im et al. 2001; Ferreras et al. 2005; Lee et al. 2006). It is di?cult to understand these kinds of unusual classes in studies with simple classi?cations. Closely related to galaxy classi?cation, a couple of studies searching for principal components of galaxy properties have been conducted recently. Ellis et al. (2005) examined



We use the SDSS Data Release 4 (DR4; Adelman-McCarthy et al. 2006)1 in this study. The DR4 imaging data cover about 6670 deg2 in the ugriz bands, and the DR4 spectroscopic data cover 4783 deg2 . The photometric and spectroscopic observations were conducted with the 2.5-m SDSS telescope at the Apache Point Observatory in New Mexico, USA. The median width of the point-spread-function in the r band photometry is 1.4′′ , and the wavelength coverage in the spectroscopy is 3800 ? 9200?. We use the photometric and structural A parameters from the SDSS pipeline (Stoughton et al. 2002) data, and the spectroscopic parameters from the Max-Planck-Institute for Astronomy catalogue (MPA catalogue; Kau?mann et al. 2003a; Tremonti et al. 2004; Gallazzi et al. 2006). In addition, we use the velocity dispersion estimated using an automated spectroscopic pipeline called idlspec2d version 5 (D. Schlegel et al. 2008, in preparation), and colour gradient, inverse concentration, and axis ratio of the SDSS DR4plus (Choi et al. 2007)


See http://www.sdss.org/dr4/.

Optical properties of SDSS galaxy classes
sample, which is one of the products of the New York University Value-Added Galaxy Catalogue (Blanton et al. 2005). After foreground extinction correction (Schlegel et al. 1998), the magnitude of each galaxy was corrected in two more aspects: the redshift e?ect (k-correction) and galaxy luminosity evolution (evolutionary correction). We used the method of Blanton et al. (2003) to conduct k-correction, and the empirical formula of Tegmark et al. (2004) to conduct evolutionary correction. Using these methods, we corrected the observed magnitude of each galaxy into the magnitude at redshift z = 0.1, where the SDSS galaxies were observed most frequently. Since the corrections are applied optionally in this paper, we denote the magnitude with k-correction as 0.1K m and the magnitude with both k-correction and evolutionary correction as 0.1KE m, if the observed magnitude is m. We use Petrosian magnitudes to represent optical brightness, while we use model magnitudes to calculate galaxy colours. The colour gradient ?0.1K (g ? i) is de?ned as the di?erence in (g ? i) colour between the region at 0.5Rpet < R < Rpet and the region at R < 0.5Rpet (negative ?(g ? i) for blue outside; Choi et al. 2007). The inverse concentration (R50/R90) and axis ratio of galaxies were estimated using ellipsoidal ?tting (Choi et al. 2007).2 The colour gradient, concentration, and axis ratio are corrected for seeing e?ects. The equivalent width of Hα emission and the 4000? break A index, Dn (4000), de?ned in Bruzual (1983), were retrieved from the MPA catalogue.


Figure 1. Segregation between early-type galaxies (dark points) and late-type galaxies (light points) in the colour vs. colourgradient space. The lines represent di?erent segregation guidelines for di?erent magnitude ranges (solid line for 14.5 < rpet < 16.0, short-dashed line for 16.0 < rpet < 16.5, long-dashed line for 16.5 < rpet < 17.0, and dotted line for 17.0 < rpet < 17.5).

3 3.1

ANALYSIS Galaxy classi?cation

In this study, we classi?ed galaxies with three criteria: morphology, colour, and spectral line features. Morphology is one of the most fundamental criteria to classify galaxies. We selected early-type galaxies with the galaxy classi?cation method using the colour versus colourgradient space (Park & Choi 2005) as shown in Fig. 1. In this classi?cation method, colours and colour gradients are the main criteria for classi?cation, and the inverse concentration cut is also applied di?erentially for di?erent magnitude ranges: R50/R90 < 0.43 for 14.5 < rpet < 16.0, R50/R90 < 0.45 for 16.0 < rpet < 16.5, R50/R90 < 0.47 for 16.5 < rpet < 17.0, and R50/R90 < 0.48 for 17.0 < rpet < 17.5. The galaxy colour is another classi?cation criterion that is frequently used in large galaxy surveys. We classi?ed the galaxies into red galaxies and blue galaxies, based on the (g ? r) colour. Since the r band is the standard band in the SDSS photometry and the photometric uncertainty in the u band is relatively large compared to that in the g band, we selected the (g ? r) colour as the index for the galaxy colour segregation. We used the method of Lee et al. (2006) to segregate blue galaxies from red galaxies. First, we divided the redshift range of 0 z 0.4 into eight bins, and derived a 0.1K (g ? r) colour histogram of the early-type galaxies with

Figure 2. The 0.1K (g ? r) colour versus redshift of early-type galaxies. The solid line is the polynomial ?t of Gaussian peaks in the 0.1K (g ? r) distribution of early-type galaxies at 8 redshift bins between z = 0 and z = 0.4. The dashed line is the (peak ?3σ) line, which is used for red/blue galaxy separation.

Rpet is the Petrosian radius, and R50 (R90) is the semimajor axis length of an ellipse containing 50 per cent (90 per cent) of the Petrosian ?ux.


J. H. Lee et al.
Table 1. Abbreviations of the 16 ?ne galaxy classes Early-type Red Blue Passive HII Seyfert LINER pREG hREG sREG lREG pBEG hBEG sBEG lBEG Late-type Red Blue pRLG hRLG sRLG lRLG pBLG hBLG sBLG lBLG

Figure 3. (a) AGN selection in the BPT diagram (Baldwin et al. 1981). We used an empirical criterion (solid line; given by Kau?mann et al. 2003b) to distinguish AGNs from star-forming galaxies. (b) Seyfert-LINER segregation in the [OIII]/Hβ vs. [OI]/Hα diagram. The solid line is the criterion to distinguish between Seyferts and LINERs, given by Kewley et al. (2006).

0.1K Mpet (r) ?20 for each redshift bin. Then, we ?tted the colour distribution with a Gaussian function in each redshift bin, and derived a guideline for colour separation by ?tting a 5th-order polynomial function to the colours corresponding to the peaks in the eight bins, as shown in Fig. 2. We used a polynomial function instead of a linear function for more accurate ?tting, although the resulting peak line seems almost linear at z < 0.3. It is noted that the peak line shows a somewhat complex curve when it extends to higher redshift (z ? 1; Lee et al. 2006). We regard the early-type galaxies that are 3σ bluer than the Gaussian peak colour as blue early-type galaxies, and the other early-type galaxies as red early-type galaxies. The same colour guideline was used to separate the red late-type galaxies and the blue late-type galaxies. In spectroscopic studies, the line features of galaxies are often used to classify galaxies. Based on the spectral line features, we classi?ed the galaxies into passive galaxies, HII galaxies, Seyfert galaxies and LINER galaxies. First, We selected AGN host galaxies using the line ?ux ratio diagram of [OIII]/Hβ versus [NII]/Hα (BPT diagram; Baldwin et al. 1981) as shown in Fig. 3a. We used an empirical criterion to segregate AGN host galaxies from starforming galaxies: [OIII]/Hβ = 0.61/([NII]/Hα ?0.05) + 1.3, given by Kau?mann et al. (2003b). AGN selection was conducted for the sample of galaxies with a signal-to-noise ratio (S/N) of 3 for Hα, Hβ, [OIII] and [NII] lines (Kau?mann et al. 2003b), but we classi?ed some galaxies as AGNs even if their S/N ratios of one or two lines are smaller than 3, in some special cases. For example, a galaxy with log([NII]/Hα) > ?0.2 and its S/N ratios of [NII] and Hα larger than 3 but with its S/N ratios of [OIII] and Hβ

smaller than 3, was classi?ed as an AGN host galaxy. These S/N criteria are too generous if we intend to select a genuine sample of AGNs, but they may be useful to reduce the contamination of HII galaxies due to AGNs. Second, among the selected AGN host galaxies, we distinguished Seyferts from LINERs in the [OIII]/Hβ versus [OI]/Hα diagram. We used an empirical guideline: [OIII]/Hβ = 1.18 [OI]/Hα +1.3, given by Kewley et al. (2006). AGN host galaxies in the LINER domain were classi?ed as LINER galaxies, and the AGN host galaxies that are not LINER galaxies were classi?ed as Seyfert galaxies. In other words, we classi?ed AGN host galaxies without the signal of [OI] emission line as Seyfert galaxies in this paper. Third, we selected as HII galaxies, non-AGN galaxies with Hα emission with S/N 3. This criterion is more generous than those in previous studies, but useful to reduce the contamination of passive galaxies due to HII galaxies. Finally, passive galaxies were selected as galaxies with no or insu?cient (S/N< 3) signal of Hα emission. Based on the three independent classi?cations of galaxies, we classi?ed the galaxies into the ?nal 16 classes: [early-type, late-type] × [red, blue] × [passive, HII, Seyfert, LINER]. Hereafter, we use the following abbreviations for the 16 galaxy classes: REG (red early-type galaxy), BEG (blue early-type galaxy), RLG (red late-type galaxy), BLG (blue late-type galaxy), and p- (passive), h- (HII), s- (Seyfert), l- (LINER), as shown in Table 1. For example, ‘pREGs’ represents ‘passive red early-type galaxies’ and ‘sBLGs’ represents ‘Seyfert blue late-type galaxies’. Fig. 4 presents atlas images and spectra of sample galaxies in the 16 ?ne classes.


Sample selection

The selection of a galaxy sample is very important, because most properties of galaxies are known to be sensitive to their luminosity and redshift. We selected three sample volumes in the luminosity versus redshift space as shown in Fig. 5. Each selected volume is a rectangle because the SDSS spectroscopy has not only a lower-brightness limit but also an upper-brightness limit for completeness. Each volume has a small redshift range and a large luminosity range, which is adequate to investigate the luminosity dependence of galaxy properties. Since our sample covers only the nearby universe (z < 0.1), the e?ect of the redshift dependence is unlikely to be signi?cant. Possible small redshift variations of galaxy properties are checked by comparing the di?erence between the three volumes. The V1 and V2 volumes in Fig. 5 are within the redshift range with reliable spectral information (0.04 < z < 0.1) suggested by Kewley et al. (2006), but the V3 volume is not, implying that the spectroscopic parame-

Optical properties of SDSS galaxy classes


Figure 4. Atlas images and spectra of sample galaxies in the 16 ?ne classes. These images were retrieved from the SDSS.

Table 2. The number of galaxies in each class and in each volume (V1) Passive HII Seyfert LINER Total (V2) Passive HII Seyfert LINER Total (V3) Passive HII Seyfert LINER Figure 5. Three volumes (V1, V2, and V3) selected in this study. The dashed lines are the limits of the complete spectroscopic sample, and the solid lines are the completeness limits corrected for the redshift e?ect (Blanton et al. 2003) and the galaxy luminosity evolution (Tegmark et al. 2004). Total REG 5109 2479 1477 4829 13894 REG 925 705 302 824 2756 REG 116 189 45 72 422 BEG 26 148 98 159 431 BEG 23 141 65 32 261 BEG 40 253 26 14 333 RLG 446 1626 1770 3569 7411 RLG 196 815 547 846 2404 RLG 66 325 78 108 577 BLG 77 9632 1472 1281 12462 BLG 54 6348 396 233 7031 BLG 45 3997 76 41 4159 Total 5658 13885 4817 9838 34198 Total 1198 8009 1310 1935 12452 Total 267 4764 225 235 5491

ters of galaxies in the V3 volume may be less reliable than those in the V1 or V2 volumes.

4 4.1

OPTICAL PROPERTIES Class composition

Table 2 lists the number of galaxies in each class and in each volume. The class composition varies with respect to the volume. The three most dominant classes are hBLGs (28.2 per cent), pREGs (14.9 per cent) and lREGs (14.1 per cent) in V1, while they are hBLGs (51.0 per cent), pREGs

(7.4 per cent) and lRLGs (6.8 per cent) in V2, and hBLGs (72.8 per cent), hRLGs (5.9 per cent) and hBEGs (4.6 per cent) in V3. Since most properties of galaxies are known to depend on their luminosity, it is necessary to study the variation of class properties with respect to their luminosity. Fig. 6 presents the class fraction versus luminosity in each volume, showing that the variation of the class fraction is continuous even between di?erent volumes. Small discontinuities are found in several classes, but those discontinuities are not signi?cant, considering the fractional uncertainty based on the Poisson error. These results indicate that the e?ect of luminosity is more important than that of redshift on the difference in the class composition between di?erent volumes. Choi et al. (2007) showed that the fraction of early-type


J. H. Lee et al.

Figure 6. Class fraction as a function of luminosity. Percentage variation of each class with respect to luminosity was derived for three volumes: V1 (open circle), V2 (open rectangle), and V3 (open triangle). Each errorbar represents the Poisson error.

galaxies decreases as luminosity decreases, which is consistent with our result. In Fig. 6, however, we found that such a trend appears to be better established between the colour classes, than between the morphology classes, among passive galaxies. In other words, passive red galaxies are dominant at the bright end, while passive blue galaxies are dominant at the faint end, on average. The class fraction distribution of HII blue galaxies is similar to that of passive blue galaxies, but the class fraction of HII red galaxies is highest 0.1K at ?20.5 Mpet (r) ?19.5, unlike that of passive red galaxies. It is interesting that the fraction of AGN host BLGs shows decrease at the faint end, unlike that of nonAGN BLGs.

Figure 7. 0.1K (u?r) colour variation with respect to 0.1KE Mpet , for each class. Each open circle shows the median value at give magnitude bin in V1, and open rectangle and open triangle do in V2 and V3, respectively. Each errorbar represents the sample inter-quartile range (SIQR) of 0.1K (u ? r) colour at given magnitude bin. The line across symbols in each panel is the linear least-square ?t.


Optical colour


Luminosity dependence of optical properties

To understand the individual characteristics of each class, we investigate their photometric, structural, and spectroscopic properties, using seven selected physical quantities. Figs. 7 – 13 show the variation of each quantity with respect to luminosity for the 16 classes, respectively. To reduce the biases in those quantities due to internal extinction in latetype galaxies (Choi et al. 2007), late-type galaxies with axis ratio smaller than 0.6 are not used in the analysis of each physical quantity, except for axis ratio itself. To reduce the e?ect of abnormal data points, we use the median statistics.

Fig. 7 shows the luminosity dependence of 0.1K (u ? r) colour for each class. Nine of 16 classes have a clear trend that the fainter galaxies are bluer. Those variations are steady and continuous even in the transition ranges between volumes, implying that there is little e?ect from the redshift di?erence between volumes. pBLGs are the only class that are clearly redder when fainter, but the deviation in such a trend is somewhat large. Table 3 lists the results of linear least-squares ?tting for Fig. 7. We note several interesting features in Fig. 7 and Table 3. First, red galaxies (except for hRLGs) and HII blue galaxies show bluer colour at fainter luminosity. The colour variation in pREGs, called often the colour-magnitude relation (CMR), was explained in many previous studies. That is, the CMR of pREGs may re?ect mainly the di?erence in the metal abundance with respect to the mass of galaxies (Kodama & Arimoto 1997; Kau?mann & Charlot 1998). The colour variation in other classes shows somewhat complicated trends, possibly a?ected by both age and metallicity e?ects. In star-forming galaxies, the fraction of young stellar population may be an important factor making the colour variation, because it is known that galaxies with small mass

Optical properties of SDSS galaxy classes
Table 3. Slopes of the linear ?ts in the 0.1KE M pet plots REG Passive HII Seyfert LINER ?0.087 ± 0.005(2.792) ?0.092 ± 0.004(2.776) ?0.104 ± 0.006(2.771) ?0.104 ± 0.006(2.792) RLG Passive HII Seyfert LINER ?0.092 ± 0.007(2.792) ?0.020 ± 0.014(2.559) ?0.043 ± 0.010(2.617) ?0.046 ± 0.010(2.697)
0.1K (u


? r) versus

BEG ?0.023 ± 0.018(2.426) ?0.218 ± 0.028(2.139) ?0.029 ± 0.023(2.275) ?0.002 ± 0.016(2.407) BLG 0.115 ± 0.067(1.967) ?0.129 ± 0.007(1.810) 0.022 ± 0.016(2.110) 0.020 ± 0.017(2.222)

The slopes of the median 0.1K (u ? r) with respect to 0.1KE Mpet , and 0.1K (u ? r) at 0.1KE Mpet (r) = ?21 within parentheses.

are generally young in the local universe (Bernardi et al. 2005; Treu et al. 2005). Second, we found that pREGs and hREGs have a similar slope in their CMR within 1σ, and that the CMR slope of AGN host REGs is marginally steeper than that of pREGs. The di?erence in the CMR slope between di?erent spectral classes of REGs is relatively small, compared to those of BEGs, RLGs and BLGs, showing that the metallicity e?ect may be dominant in REGs, irrespective of their spectral class. In other words, the e?ects of star formation or AGN activities may be small in REGs. The CMR slope in pREGs is about ?0.09, which is signi?cantly smaller than those in hBLGs (about ?0.13). This indicates that the CMR slope of age-e?ect-dominated galaxies is steeper (larger colour variation) than the CMR slope of metallicity-e?ect-dominated galaxies, con?rming the result of Choi et al. (2007). Third, we found that AGN host galaxies in non-REG classes show very small variation in their colour. Particularly, AGN host blue galaxies do not show signi?cant colour variation with respect to luminosity. Since galaxies with small mass are generally expected to have bluer colours than massive galaxies (Kodama & Arimoto 1997; Bernardi et al. 2005; Choi et al. 2007), such constant colours of AGN host blue galaxies are somewhat unusual. It is inferred that AGN activity may be responsible for the colour constancy, and that such AGN e?ects may be most prominent in blue galaxies. Fourth, hBEGs show the largest variation in colour due to the very blue faint members of the class. The very blue faint hBEGs were also identi?ed by Choi et al. (2007), which may have much younger mean stellar ages or much poorer metal abundance than bright hBEGs. Finally, we found that pBEGs and hRLGs do not show signi?cant variation in their colour. Since both the metallicity-magnitude relation and the age-magnitude relation are known to cause the slope in CMR, the CMR of pBEGs and hRLGs may not be the result of any single mechanism. In other words, the combined e?ects of age and metallicity working di?erentially with respect to luminosity are one possible origin of the CMR of pBEGs and hRLGs. Another possibility is di?erential dust extinction with respect to luminosity.

Figure 8. The same as Fig. 7, but ?(g ? i) as Y-axis.

Table 4. As Table 3 but for ?(g ? i) REG Passive HII Seyfert LINER 0.004 ± 0.002(?0.028) 0.005 ± 0.002(?0.036) 0.015 ± 0.009(?0.056) 0.012 ± 0.008(?0.060) RLG Passive HII Seyfert LINER 0.018 ± 0.008(?0.063) 0.037 ± 0.006(?0.118) 0.033 ± 0.007(?0.152) 0.037 ± 0.005(?0.158) BEG 0.000 ± 0.007(0.011) 0.050 ± 0.005(0.071) 0.020 ± 0.011(0.036) 0.025 ± 0.014(0.026) BLG 0.056 ± 0.021(?0.112) 0.044 ± 0.002(?0.163) 0.057 ± 0.004(?0.180) 0.055 ± 0.006(?0.184)


Colour gradient

In Fig. 8, the luminosity dependence of (g ? i) colour gradient for each class is shown, and the linear ?ts in Fig. 8 are summarised in Table 4. As found by Choi et al. (2007), REGs have negative colour gradients (blue outside) on average, and show little variation of their colour gradients with respect to luminosity. Such a colour gradient in REGs is consistent with the previous studies, and may originate from the internal metallicity gradient (Tamura & Ohta 2003; La Barera et al. 2005). The centres of BEGs are bluer than their outer parts, which results from the shape of the domain in which early-type galaxies were selected (Fig. 1). Choi et al. (2007) reported that the colour gradient of BEGs increases as luminosity decreases, and we additionally found


J. H. Lee et al.
Table 5. As Table 3 but for inverse concentration REG Passive HII Seyfert LINER 0.014 ± 0.003(0.325) 0.012 ± 0.002(0.330) 0.015 ± 0.002(0.335) 0.007 ± 0.002(0.329) RLG Passive HII Seyfert LINER 0.009 ± 0.004(0.372) 0.010 ± 0.002(0.389) 0.000 ± 0.004(0.393) 0.003 ± 0.003(0.378) BEG 0.017 ± 0.003(0.331) 0.018 ± 0.004(0.332) 0.008 ± 0.003(0.334) 0.022 ± 0.002(0.335) BLG ?0.015 ± 0.009(0.472) 0.005 ± 0.002(0.462) ?0.004 ± 0.007(0.435) ?0.005 ± 0.003(0.431)

Figure 9. The same as Fig. 7, but inverse concentration as Yaxis.

that such a trend is most conspicuous in hBEGs. This implies that star formation activity in BEGs may be centrally concentrated, and that the central star formation activity in faint BEGs may be more vigorous than those in bright BEGs. All sub-classes of late-type galaxies have negative colour gradients (i.e. bluer outside than centre) and show signi?cantly increasing colour gradients as luminosity decreases. The increase in BLGs is slightly larger than that in RLGs. Faint late-type galaxies show very small colour di?erence between their centre and outside, which implies that the fraction of Scd- and Im-type galaxies in late-type galaxies may increase as luminosity decreases, as pointed out by Choi et al. (2007). It is noted that AGN host BLGs show variation in their colour gradient larger than hBLGs, which is mainly due to the (negatively) large colour gradients of bright AGN host BLGs. This shows that bright AGN host BLGs may have vigorous star formation activity in their discs, or that the gas cooling in their galactic centre may be suppressed by AGN feedback (Cattaneo et al. 2007). The colours of AGNs cannot be responsible for those trends in colour gradients, because the colours of AGNs are known to be typically blue (Im et al. 2007), which would cause the opposite trend.

ation of inverse concentration are found between di?erent volumes. This may be mainly because we did not conduct morphological k-correction in estimating the inverse concentration. Table 5 lists the linear ?ts in Fig. 9. In earlytype galaxies, there is a clear trend that fainter galaxies are less concentrated, as expected from the early-type selection criteria and consistent with the well-known nonhomology in early-type galaxies (Kormendy & Djorgovski 1989; Michard & Marchal 1994). It is notable that no clear di?erence is found between pREGs, hREGs, pBEGs and hBEGs, which may indicate that the structures of these galaxies are similar. However, this does not necessarily mean that the mass pro?les of REGs and BEGs are similar, because BEGs may have young and bright stellar populations with relatively low mass-to-light ratio in their centre. Choi et al. (2007) showed that the brightest early-type galaxies are less concentrated, which may be due to recent mergers. We found that such a trend is most conspicuous in hBEGs, indicating that the star formation activity in bright hBEGs may be related to recent mergers. It is interesting that faint lBEGs are signi?cantly less concentrated than faint lREGs, which shows a possibility that faint lBEGs may have relatively large disc components. The concentration of late-type galaxies shows larger scatters than that of early-type galaxies, which may be partially due to the uncertainty in estimating the Petrosian radius of late-type galaxies that have typically double components (bulge+disc) in their surface pro?le. On average, RLGs are less concentrated than early-type galaxies, and BLGs are the least concentrated. The fact that RLGs with axis ratio > 0.6 are more concentrated than BLGs with axis ratio > 0.6 shows that the bulge fraction may be an important factor determining the colour of a late-type galaxy with small (i.e. close to face-on) inclination.


Axis ratio


Light concentration

The luminosity dependence of inverse concentration for each class is presented in Fig. 9. Small discontinuities in the vari-

Fig. 10 shows the luminosity dependence of axis ratio for each class. In this ?gure, the axis ratio cut (> 0.6) was not applied to late-type galaxies, unlike the ?gures for the other parameters. Table 6 summarises the linear ?ts in Fig. 10. It is interesting that RLGs have smaller axis ratios than BLGs on average, which may be partially due to the reddening of late-type galaxies with large inclination (Bailin & Harris 2008). In other words, late-type galaxies with large inclination would be classi?ed as RLGs, although they had blue

Optical properties of SDSS galaxy classes


Figure 10. The same as Fig. 7, but axis ratio as Y-axis.

Figure 11. The same as Fig. 7, but the velocity dispersion as Yaxis. The velocity dispersion was NOT corrected for the aperture e?ect.

Table 6. As Table 3 but for axis ratio REG Passive HII Seyfert LINER ?0.007 ± 0.006(0.744) ?0.025 ± 0.005(0.734) 0.003 ± 0.018(0.678) ?0.009 ± 0.013(0.710) RLG Passive HII Seyfert LINER 0.007 ± 0.009(0.711) ?0.031 ± 0.014(0.575) ?0.027 ± 0.010(0.596) ?0.026 ± 0.010(0.628) BEG 0.002 ± 0.021(0.677) ?0.016 ± 0.011(0.734) ?0.008 ± 0.012(0.732) ?0.004 ± 0.017(0.760) BLG ?0.017 ± 0.019(0.688) ?0.047 ± 0.004(0.695) ?0.024 ± 0.013(0.741) ?0.015 ± 0.010(0.742) Table 7. As Table 3 but for velocity dispersion REG Passive HII Seyfert LINER ?44.3 ± 1.7(166) ?43.9 ± 2.2(158) ?42.1 ± 2.6(152) ?41.3 ± 1.7(155) RLG Passive HII Seyfert LINER ?39.4 ± 2.9(152) ?31.1 ± 1.5(126) ?28.8 ± 1.5(125) ?31.2 ± 1.8(132) BEG ?36.9 ± 4.2(125) ?36.0 ± 4.0(118) ?27.9 ± 3.5(117) ?36.6 ± 2.8(121) BLG ?20.6 ± 3.1(87) ?23.1 ± 2.7(86) ?24.5 ± 2.1(101) ?25.1 ± 2.5(102)

colour in the face-on view. We found that the axis ratio of hREGs show signi?cant variation with respect to luminosity, implying that faint hREGs may have larger disc components than bright hREGs, on average. In addition to hREGs, an obvious trend that fainter galaxies have smaller axis ratio is also found in hBLGs. One possibility is that disc components may be more dominant in fainter hBLGs, because the axis ratio of galaxies with large inclination and a small bulge may appear smaller than that of galaxies with large inclination and a large bulge. However, it is also possible that this trend is caused by an inclination e?ect (Choi et al. 2007). In other words, since faint hBLGs are intrinsically bluer than bright BLGs (see Fig. 7), bright hBLGs with large inclination are more easily classi?ed as hRLGs than faint hBLGs with large incli-

nation, which makes the average axis ratio of faint hBLGs smaller than that of bright hBLGs. However, the average axis ratio of faint hRLGs is not particularly larger than that of bright hRLGs. On the contrary, faint hRLGs have marginally smaller axis ratio than bright hRLGs. This may be because hRLGs with large inclination may su?er dimming due to the extinction, causing the small average axis ratio of faint RLGs. This e?ect is well discussed in Choi et al. (2007).


J. H. Lee et al.

Table 8. As Table 3 but for Hα equivalent width REG Passive HII Seyfert LINER 0.01 ± 0.01(0.17) 0.00 ± 0.02(0.49) 0.62 ± 0.14(1.37) 0.32 ± 0.08(1.15) RLG Passive HII Seyfert LINER 0.00 ± 0.01(0.20) 0.07 ± 0.74(3.92) 0.33 ± 0.33(2.67) 1.80 ± 1.04(1.46) BEG 0.01 ± 0.03(0.21) 4.88 ± 1.85(17.78) ?1.06 ± 1.69(9.99) 0.28 ± 0.35(3.67) BLG ?0.40 ± 1.18(1.88) 2.37 ± 0.38(17.38) ?1.65 ± 0.56(10.30) ?0.69 ± 0.45(6.18)


Velocity dispersion

In Fig. 11, the luminosity dependence of velocity dispersion (σv ) for each class is displayed, and Table 7 lists the linear ?ts in Fig. 11. According to Fig. 11 and Table 7, early-type galaxies and red galaxies have large variations in their σv with respect to luminosity, and have large median σv at 0.1KE Mpet (r) = ?21, compared to late-type galaxies and blue galaxies. For example, REGs have very large variation in their σv and very large median σv at 0.1KE Mpet (r) = ?21, whereas BLGs have very small variation in their σv and very small median σv at 0.1KE Mpet (r) = ?21. One main cause for the di?erence in the variation slope may be the di?erence in the dynamical mass pro?le between classes. Since the aperture size of the SDSS spectroscopy is ?xed as 3′′ , the σv of a large galaxy re?ects the small fraction of its centre, while the σv of a small galaxy re?ects a relatively large fraction of that galaxy. Therefore, if the dynamical mass pro?les of galaxies in a class are not concentrated, the σv – 0.1KE Mpet (r) relation slope in that class may be small, because the mass fraction within 3′′ aperture of a large galaxy may be much smaller than that of a small galaxy, in that class. On the other hand, the σv – 0.1KE Mpet (r) relation slope in a class with a highly-concentrated dynamical mass pro?le, may be relatively large, because a large galaxy in that class may have relatively large fraction of its mass within 3′′ aperture. In this sense, the large variation in σv of REGs shows that their dynamical mass pro?les may be highlyconcentrated, and the outer parts of REGs may not signi?cantly a?ect the σv estimation. On the other hand, the small variation in σv of BLGs indicates that their dynamical mass pro?les may not be concentrated and the outer parts of BLGs may a?ect signi?cantly the σv estimation. However, this interpretation is cautiously suggested, because the different luminosity dependence of galaxy mass-to-light ratio between classes may also in?uence the di?erence in slope of the σv – 0.1KE Mpet (r) relation. The rotation of late-type galaxies may also a?ect the σv estimation, but late-type galaxies with small axis ratio (< 0.6) are not used in this analysis. Therefore, the e?ect of late-type galaxy rotation should be relatively small. 4.2.6 Hα equivalent width

Figure 12. The same as Fig. 7, but Hα equivalent width as Yaxis. A positive value of the equivalent width represents an emission line.

The luminosity dependence of Hα equivalent width for each class is presented in Fig. 12, and the linear ?ts in Fig. 12 are

summarised in Table 8. A positive value of Hα equivalent width indicates line emission, and a negative value indicates line absorption. According to the de?nition, passive galaxies do not show any signi?cant Hα emission. We found that red HII galaxies (i.e. hREGs and hRLGs) show small Hα equivalent widths in the almost entire luminosity range, unlike blue HII galaxies. The small Hα equivalent width in most red HII galaxies implies that those galaxies have just a small fraction of current star formation. There are trends of increasing Hα equivalent width as luminosity decreases in blue HII galaxies, which shows that star formation activity may be more vigorous in faint galaxies, within the spectroscopic ?bre aperture. Choi et al. (2007) suggested that those trends in late-type galaxies may be because the ?bre spectra systematically miss the light from the outer discs of bright large galaxies, which is a possible explanation for our hBLGs sample. However, the trend in hBEGs may be intrinsic rather than caused by the ?bre aperture e?ect, because the star formation activity in hBEGs may not be biased to their outer parts, as shown in §4.2.2. The trend of more vigorous star formation activity in fainter galaxies is consistent with previous studies (Bernardi et al. 2005; Treu et al. 2005), supporting the galactic downsizing scenario. The Hα equivalent width of sBLGs shows a somewhat unusual trend: it increases from 0.1KE Mpet = ?23 to 0.1KE Mpet = ?21, but decreases from 0.1KE Mpet = ?21 to 0.1KE Mpet = ?19. In fact, the trend in sBEGs is similar to

Optical properties of SDSS galaxy classes


Figure 14. Sub-sample selection with the same distribution of velocity dispersion in V1. The dashed line is the distribution of velocity dispersion for each class, and the solid line is the selected sub-sample. In each panel, the class name and sub-sample size are denoted at the upper-right corner.

Figure 13. The same as Fig. 7, but the 4000? break index as A Y-axis.

Table 9. As Table 3 but for 4000? break index A REG Passive HII Seyfert LINER ?0.048 ± 0.014(1.949) ?0.048 ± 0.006(1.909) ?0.079 ± 0.009(1.865) ?0.063 ± 0.011(1.885) RLG Passive HII Seyfert LINER ?0.042 ± 0.011(1.944) ?0.018 ± 0.018(1.717) ?0.044 ± 0.009(1.769) ?0.050 ± 0.007(1.814) BEG ?0.038 ± 0.024(1.708) ?0.072 ± 0.017(1.438) 0.032 ± 0.014(1.452) 0.024 ± 0.017(1.546) BLG 0.051 ± 0.042(1.435) ?0.042 ± 0.004(1.363) ?0.001 ± 0.010(1.507) 0.008 ± 0.018(1.568)

ies have larger mean stellar ages than blue or non-passive galaxies. Faint REGs have smaller Dn (4000) than bright REGs regardless of their spectral class, which is consistent with galaxy downsizing (Cowie et al. 1996; Treu et al. 2005), in the sense that bright (and maybe massive) galaxies have more old stellar populations than faint (and maybe lessmassive) galaxies. This trend is also found in hBEGs, pRLGs, sRLGs, lRLGs and hBLGs. It is noted that blue AGN host galaxies show almost constant Dn (4000) with respect to luminosity. This may be because the spectral energy distribution (SED) of those blue AGN host galaxies, within the ?bre aperture, may be dominated by the AGN SED. The Dn (4000) feature in hRLGs seems bimodal with large deviation, and the hRLGs in the V1 volume show a similar Dn (4000) variation slope with the other RLGs. 4.3 Statistics at ?xed σv

that in sBLGs, and is therefore possibly related to the luminosity dependence of AGN activity. However, these trends are not very signi?cant, due to the large error bars.


A 4000?break

A Fig. 13 shows the luminosity dependence of the 4000? break index for each class, and the linear ?ts in Fig. 13 are listed in Table 9. On average, Dn (4000) of red galaxies is larger than that of blue galaxies, and Dn (4000) of passive galaxies is larger than that of non-passive galaxies. Because the Dn (4000) parameter re?ects the fraction of old stellar population in a galaxy, this result shows that red or passive galax-

Since most properties of galaxies depend on each other (Bernardi et al. 2003; Choi et al. 2007), it is necessary to resample each galaxy class with the same distribution of a control parameter, for a direct comparison of galaxy properties between di?erent classes. One of the most frequently used control parameter is luminosity, which represents basically stellar mass. However, the mass-to-light ratio of a galaxy depends on its bulge-to-disc ratio (Yoshino & Ichikawa 2008), so that luminosity is not the best as a control parameter in the studies of diverse galaxy classes. Therefore, we use velocity dispersion instead of luminosity as a control parameter, which represents galaxy dynamical mass. Fig. 14 shows the selection of the sub-sample in each class with the same distribution of velocity dispersion. Since


J. H. Lee et al.
4.3.1 Optical colour

REGs have similar colour regardless of their spectral class, which shows that star formation or AGN activity in REGs is not strong. In a given morphology-colour class, except for REGs, we found that HII galaxies are bluer than passive galaxies, and that LINER galaxies are redder than Seyfert galaxies. The trend of 0.1K (u ? r), ‘REG > RLG > BEG > BLG’ is natural, considering the classi?cation of morphological types in Fig. 1. It is noted that pRLGs have similar colour to that of REGs.


Colour gradient

Figure 15. Median value and sampling error of three parameters in each class, selected with the same distribution of velocity dispersion as shown in Fig. 14. The three parameters are 0.1K (u?r), ?(g ? i) and R50/R90. The values are summarised in Table 10.

the estimation error of σv is very large for σv < 100 km s?1 (Choi et al. 2007), we use galaxies with σv 100 km s?1 only. Since the sample sizes of pBEGs, hBEGs, sBEGs and pBLGs are too small, their sub-samples were not selected with the same σv distribution as the sub-samples of the other classes. Therefore, the results in those classes are less reliable than those in the other classes. In the sub-sample of each class, the median value and the sampling error of six physical quantities were derived as shown in Fig. 15 and Fig. 16. Each sampling error was estimated by calculating the standard deviation of the median values in 200-times-repetitive sampling. To reduce the biases in the results due to internal extinction in late-type galaxies (Choi et al. 2007), late-type galaxies with axis ratio smaller than 0.6 were not used in the analysis of each quantity, except for the axis ratio itself. We note one possible selection bias. The velocity dispersion of each galaxy may not represent the dynamical mass perfectly, because the velocity dispersions are derived within the limited ?bre aperture, and the dynamical mass pro?les of galaxies in each class may not be homogeneous. Therefore, the following results may include more or less biases due to the di?erence in the genuine dynamical mass range of galaxies. Nevertheless, these comparisons are useful, since the velocity dispersion may at least represent the central dynamical mass of each galaxy. A few quantities should be cautiously compared between di?erent classes, considering the selection criteria of the classes. For example, since optical colour is one criterion to classify our sample galaxies (red galaxies versus blue galaxies), it is meaningful to compare the optical colour only in the same colour class. Similar considerations are necessary in analysing the colour gradient and light concentration.

Whereas REGs have negative colour gradients (i.e. red centres), as shown by Choi et al. (2007), BEGs have positive colour gradients (i.e. blue centres), which is consistent with previous studies (Menanteau et al. 2001; Lee et al. 2006). It is interesting that hREGs have more negative colour gradients (i.e. bluer outside) than pREGs, while hBEGs have more positive colour gradients than pBEGs. This di?erence implies that the process of star formation in a galaxy is different between REGs and BEGs. In other words, the star formation in an hREG may be dominant in the outer parts of the galaxy, which is possibly triggered by gas infall. On the other hand, hBEGs have star formation mainly in their centres. We found that AGN host REGs have bluer outsides even than hREGs, which shows a possibility that excessive gas infalling into REGs may trigger the AGN activity. No signi?cant di?erence is found in the colour gradients between most spectral classes of BEGs, due to the large sampling error, except for hBEGs, which have signi?cantly bluer centre than pBEGs, and marginally bluer centre than lBEGs. pRLGs have very small (but still negative) colour gradients, compared to the other RLGs, showing that the disc components in pRLGs may be very small or of red colour. AGN host RLGs have larger negative colour gradients than hRLGs, which is possibly caused by the suppression of gas cooling in the centre of AGN host RLGs by AGNs. Similarly, AGN host BLGs have larger negative colour gradients than non-AGN BLGs.


Light concentration

Early-type galaxies have similar concentrations, and are more concentrated than late-type galaxies, as found by Choi et al. (2007). It is interesting that hBEGs are most concentrated among early-type galaxies, which may be due to the bright young stellar populations in the centre of hBEGs. Meanwhile, hRLGs are signi?cantly more concentrated than hBLGs, which imply that many hRLGs may be bulge-dominated late-type galaxies. It is noted that sBLGs are unusually concentrated, which may be partly due to the existence of a bright AGN in their centre.


Axis ratio

Choi et al. (2007) showed that early-type galaxies have systematically larger axis ratio than late-type galaxies, which is consistent with our results. The median axis ratio of pREGs

Optical properties of SDSS galaxy classes
4.3.6 ? 4000Abreak


? Morphology class hardly a?ects the 4000A break index, while there are clear di?erences in Dn (4000) between different colour and spectral classes. Red galaxies have larger Dn (4000) than blue galaxies, and passive galaxies have larger Dn (4000) than non-passive galaxies. The class with the smallest Dn (4000) is hBLG.



Tables 10 summarises the median values and sampling errors of six physical quantities in the sub-samples selected with the same distribution of velocity dispersion in Fig. 14, for REGs, BEGs, RLGs, and BLGs, respectively. In the following subsections, we discuss the nature of galaxies in each ?ne class, mainly focusing on the results using the σv -?xed sample (§4.3). 5.1
Figure 16. The same as Fig. 15, but for axis ratio, EW(Hα) [?] A and Dn (4000).

Red early-type galaxies (REGs)

is about 0.75, and non-passive REGs have smaller axis ratio than pREGs. Considering that non-passive REGs have bluer outsides than pREGs (see §4.3.2), the small axis ratio of non-passive REGs may be due to the existence of small disc components. On the other hand, hBEGs have a median axis ratio larger than that of pBEGs (but consistent within 1σ sampling error), implying that the relationship between pBEGs and hBEGs may be di?erent from that between pREGs and hREGs. The axis ratios of non-passive RLGs are signi?cantly smaller than those of non-passive REGs, in a given spectral class, which implies that disc components in non-passive RLGs may be larger than those in non-passive REGs. Interestingly, the axis ratios of AGN host RLGs are relatively larger than those of hRLGs, which may be a selection effect that AGNs are less detected in disc galaxies with large inclination. It is noted that the di?erence in the axis ratio between pREGs and pRLGs is marginal, which shows that most pRLGs may be almost face-on or bulge-dominated latetype galaxies. The axis ratios of non-passive BLGs are larger than those of non-passive RLGs, which is because intrinsic BLGs with large inclination may be classi?ed as RLGs due to the strong dust extinction. 4.3.5 Hα equivalent width

pREGs may be typical elliptical galaxies, which are old, red and passively-evolving. Other (non-passive) REGs have similar properties to pREGs, but there are some notable di?erences between pREGs and non-passive REGs. First of all, hREGs have similar colour to pREGs, but their colour gradients are signi?cantly di?erent, in the sense that hREGs show larger negative colour gradients than pREGs. This indicates that hREGs may have blue populations in their outer parts, implying there may have been some gas infall from their outsides. AGN host REGs have larger negative colour gradients even than hREGs, although the optical colour of AGN host REGs is comparable with that of pREGs. We found that the axis ratio of AGN host REGs is signi?cantly smaller than that of pREGs, and the axis ratio of hREGs is marginally smaller than that of pREGs. These structural features imply that the infalling gas may have formed small disc components in the outer parts of non-passive REGs. lREGs are similar to sREGs in their Hα equivalent width and Dn (4000), but lREGs are slightly more concentrated, implying that the disc components may be smaller in lREGs than in sREGs. 5.2 Blue early-type galaxies (BEGs)

According to the de?nition of the spectral classes in §3.1, the Hα equivalent widths of passive galaxies are almost zero. Blue HII galaxies show large median Hα equivalent widths, implying vigorous star formation activity in them. On the other hand, red HII galaxies have small median Hα equivalent widths, which shows that they may have low current star formation. The Hα equivalent width of blue Seyfert galaxies is much larger than that of blue LINER galaxies, while the di?erence in Hα equivalent width between red Seyfert galaxies and red LINER galaxies is not very large.

BEGs are the galaxies in the blue tail of the earlytype galaxies in the colour versus colour gradient diagram (Park & Choi 2005). BEGs have concentration index comparable to that of REGs, except for hBEGs, which are marginally more concentrated than pREGs. The axis ratios of BEGs also agree with those of REGs within very large sampling errors. These features show that BEGs are similar to REGs in their morphological structures. Compared to REGs, however, BEGs have blue colour, positive colour gradient (blue centre), large Hα equivalent width and small Dn (4000), indicating that BEGs have a large number of young stars, compared to REGs. Since BEGs have bluer centre than REGs, it is considered that the young stellar populations in BEGs may be concentrated toward their centre unlike typical disc galaxies, which is consistent with the previous studies about blue spheroidal galaxies


J. H. Lee et al.
Table 10. Summary of the median values for six parameters in each ?ne class pREG ? r) ?(g ? i) R50/R90 Axis ratio EW(Hα) [?] A Dn (4000)
0.1K (u

hREG 2.762±0.008(0.10) ?0.048±0.003(0.04) 0.342±0.001(0.02) 0.720±0.011(0.14) 0.603±0.117(0.34) 1.861±0.008(0.08) hBEG 2.333±0.046(0.15) 0.031±0.013(0.03) 0.320±0.007(0.01) 0.700±0.047(0.09) 12.137±0.540(5.58) 1.475±0.038(0.08) hRLG 2.592±0.011(0.19) ?0.171±0.006(0.07) 0.398±0.003(0.04) 0.540±0.011(0.14) 3.565±0.159(5.16) 1.638±0.013(0.17) hBLG 2.008±0.017(0.17) ?0.171±0.006(0.07) 0.418±0.004(0.05) 0.660±0.014(0.11) 18.494±0.191(9.27) 1.370±0.014(0.09)

sREG 2.763±0.010(0.11) ?0.065±0.004(0.04) 0.347±0.002(0.03) 0.690±0.013(0.16) 1.226±0.148(1.02) 1.848±0.011(0.10) sBEG 2.287±0.040(0.12) 0.010±0.010(0.05) 0.338±0.006(0.02) 0.640±0.036(0.13) 12.778±0.407(6.21) 1.462±0.031(0.07) sRLG 2.621±0.010(0.13) ?0.181±0.006(0.06) 0.392±0.003(0.03) 0.600±0.009(0.14) 2.425±0.166(2.53) 1.759±0.011(0.13) sBLG 2.123±0.022(0.13) ?0.201±0.008(0.09) 0.393±0.005(0.05) 0.690±0.019(0.09) 14.856±0.275(9.38) 1.445±0.017(0.10)

lREG 2.774±0.006(0.10) ?0.078±0.003(0.04) 0.341±0.001(0.02) 0.710±0.010(0.13) 1.242±0.092(0.73) 1.831±0.008(0.09) lBEG 2.392±0.027(0.10) 0.000±0.008(0.04) 0.332±0.004(0.02) 0.740±0.026(0.09) 2.582±0.283(4.34) 1.507±0.023(0.06) lRLG 2.676±0.006(0.13) ?0.190±0.003(0.06) 0.392±0.002(0.04) 0.660±0.007(0.17) 1.687±0.097(1.05) 1.801±0.008(0.10) lBLG 2.248±0.022(0.14) ?0.213±0.009(0.08) 0.411±0.005(0.03) 0.700±0.019(0.12) 7.858±0.233(5.69) 1.528±0.017(0.10)

2.765±0.009(0.08) ?0.034±0.003(0.03) 0.338±0.002(0.02) 0.750±0.013(0.12) 0.169±0.011(0.12) 1.890±0.009(0.07) pBEG

? r) ?(g ? i) R50/R90 Axis ratio EW(Hα) [?] A Dn (4000)

0.1K (u

2.484±0.073(0.06) ?0.014±0.018(0.03) 0.329±0.010(0.01) 0.670±0.061(0.07) 0.205±0.059(0.14) 1.746±0.057(0.10) pRLG

? r) ?(g ? i) R50/R90 Axis ratio EW(Hα) [?] A Dn (4000)

0.1K (u

2.779±0.019(0.09) ?0.101±0.010(0.08) 0.390±0.006(0.03) 0.700±0.024(0.12) 0.160±0.020(0.15) 1.917±0.021(0.06) pBLG

? r) ?(g ? i) R50/R90 Axis ratio EW(Hα) [?] A Dn (4000)

0.1K (u

2.199±0.089(0.35) ?0.176±0.034(0.07) 0.419±0.018(0.09) 0.580±0.065(0.14) 0.087±0.065(0.51) 1.535±0.069(0.17)

Median value ± sampling error of each parameter in the sub-samples, selected in Fig. 14. The value in the parentheses is the SIQR of each parameter in each class.

(Menanteau et al. 2001; Lee et al. 2006; Choi et al. 2007). BEGs are likely to be the early-type galaxies that recently accreted cold gas from a late-type neighbour during a close encounter or merged with less signi?cant gas-rich galaxies (Park et al. 2008). hBEGs have a larger positive colour gradient (bluer centre) than pBEGs, whereas hREGs have a larger negative colour gradient (bluer outside) than pREGs. In other words, the star formation of hBEGs seems to be dominant in their central regions, while the star formation of hREGs seems to be dominant in their outer parts. It is noted that the structural features of hBEGs, pBEGs and pREGs are similar. The major di?erence between these galaxy classes is that pBEGs have bluer centres and a younger stellar population than pREGs, and that hBEGs have even bluer centres than pBEGs. However, the star formation activity in hBEGs will not continue forever, and the young stellar populations in BEGs will become older and redder as time goes on, if they do not su?er any interactions with other objects. This implies that hBEGs will probably evolve into pBEGs after their star formation ends, and that pBEGs may also evolve into pREGs much later. In other words, hBEGs, pBEGs and pREGs may form the evolutionary sequence of hBEGs → pBEGs → pREGs, as suggested by Lee et al. (2006, 2007).

The axis ratio and concentration of sBEGs are in agreement with those of sREGs, implying that Seyfert earlytype galaxies may have outer disc components. However, we need to be cautious when discussing their structural similarity, because the sampling errors in BEGs are very large. lBEGs show noticeable di?erences from sBEGs: lBEGs have marginally larger axis ratios and marginally redder centres than sBEGs. This relationship between lBEGs and sBEGs is similar to that between lREGs and sREGs.


Red late-type galaxies (RLGs)

RLGs are bluer than REGs. Considering that the colour gradients of RLGs are signi?cantly more negative than those of REGs, we note that what makes RLGs bluer than REGs may be the blue outer parts of RLGs (i.e. blue disc components). The existence of blue disc components in RLGs is supported by the facts that RLGs are less concentrated than early-type galaxies (for RLGs with axis ratio > 0.6) and that the axis ratios of RLGs are smaller than those of early-type galaxies. Non-passive RLGs are signi?cantly bluer than pRLGs. The concentrations of RLGs are similar among all spectral classes. The Dn (4000) of pRLGs agrees with that of pREGs,

Optical properties of SDSS galaxy classes
but the Dn (4000) of non-passive RLGs is smaller than that of non-passive REGs, and larger than that of non-passive BEGs. This indicates that the age composition of the stellar populations in RLGs may be intermediate between REGs and BEGs, except for passive galaxies. We remind the reader that these spectral features are based on the central stellar populations in each galaxy, not entire populations. The Dn (4000) features of RLGs, therefore, imply that RLGs may have a larger central bulge with old stellar population, compared to BLGs. The fact that hRLGs are more concentrated than hBLGs also supports this interpretation. The median axis ratio of RLGs (including RLGs with axis ratio < 0.6) is smallest among all of the colourmorphology classes, indicating that there may be many disc galaxies with large inclination within the RLGs. In other words, intrinsic BLGs with large inclination may be classi?ed as RLGs, due to their strong dust extinction. It is noted that hRLGs have axis ratios decreasing as luminosity decreases (see Fig. 10). Intrinsically bright late-type galaxies with large inclination are dimmed due to the extinction in their discs, causing the small median axis ratio at the faint end. pRLGs have signi?cantly larger axis ratio than hRLGs. A good explanation for this di?erence is that RLGs with large inclination may be classi?ed as hRLGs rather than pRLGs, because their disc components may be observed within the spectroscopic ?bre aperture. That is, many pRLGs may be bulge-dominated late-type galaxies with small inclination, so that only their bulge parts were covered in the SDSS spectroscopy. However, pRLGs may also include genuinely passive spiral galaxies (Couch et al. 1998; Yamauchi & Goto 2004; Choi et al. 2007). AGN host RLGs have signi?cantly larger axis ratios than hRLGs (but smaller than pRLGs). This may be a selection e?ect caused by AGN obscuration. Since the AGN emission may be di?cult to observe in disc galaxies with large inclination, the median axis ratio of the observed AGN host galaxies may be overestimated. In other words, AGN host RLGs with small axis ratio (i.e. large inclination) may be classi?ed as hRLGs due to AGN obscuration. 5.4 Blue late-type galaxies (BLGs)


hBLGs. This is possibly due to the di?erence in the brightness of the central AGN between sBLGs and lBLGs. The axis ratio of AGN host BLGs is similar to that of AGN host REGs. However, the bulge-to-disc ratio of AGN host BLGs may be smaller than those of AGN host REGs, because the light pro?le of AGN host BLGs is much less concentrated than those of AGN host REGs. The large median axis ratio of AGN host BLGs may be due to the selection e?ect caused by AGN obscuration, as mentioned in §5.3.



Among all colour-morphology classes, BLGs have the bluest colour, the least concentrated pro?le, the largest Hα equivalent width and the smallest Dn (4000). The colour gradient of BLGs is more negative than that of RLGs, except for HII galaxies. Except for passive galaxies, BLGs have larger axis ratio than RLGs, which may be because intrinsic BLGs with large inclination can be classi?ed as RLGs. hBLGs may be typical late-type galaxies with a large amount of current star formation. Their blue colour (particularly in their outer parts), di?use structure, and large Hα equivalent width are related to the vigorous star formation in the disc of hBLGs. It is di?cult to ?nd any signi?cantly di?erent features between pBLGs and hBLGs, due to the large sampling error of pBLGs, except for the Hα equivalent width and Dn (4000). Those spectral features show that pBLGs may be older than hBLGs in their mean stellar age. Some pBLGs may be genuinely passive spiral galaxies. sBLGs are signi?cantly more concentrated than hBLGs, while the concentration of lBLGs is comparable with that of

We conducted a comprehensive study of the nature of the SDSS galaxies in various classes based on their morphology, colour and spectral features. Using three criteria, we classi?ed the SDSS galaxies into early-type and late-type; red and blue; passive, HII, Seyfert and LINER, resulting in 16 ?ne classes of galaxies in total. We estimated the luminosity dependence of seven physical quantities in each class, and compared the properties among classes, using a sub-sample with the same distribution of the velocity dispersion. From the analysis, we found that each galaxy class has its own distinguishable features. This shows that an analysis based on a simple classi?cation may have a risk of mixing up di?erent kinds of objects with di?erent natures. The red early-type galaxies include well-known typical elliptical galaxies (pREGs), but some REGs show evidence for additional star formation in their outer regions (hREGs). Some other REGs with AGNs (sREGs, lREGs) have structural properties showing the existence of larger disc components than hREGs, indicating the relationship between AGN activity and gas accretion. The blue earlytype galaxies may be in the process of bulge formation. The structural similarity between pREGs, pBEGs and hBEGs, supports an evolutionary sequence of hBEGs → pBEGs → pREGs. Seyfert early-type galaxies have a close relationship with the outer disc components of early-type galaxies, and the disc components in LINER early-type galaxies (lREGs, lBEGs) are smaller than those in Seyfert early-type galaxies, on average. The blue late-type galaxies have properties in agreement with typical spiral galaxies. Most of them are star-forming (hBLGs), but a very small fraction of BLGs do not show any evidence of current star formation (pBLGs). Some of BLGs have an AGN (sBLGs, lBLGs), which are less detected at large inclination, and sBLGs show particularly bright centres on average. The median axis ratio in each class shows that some intrinsic BLGs with large inclination may often be classi?ed as red late-type galaxies, due to strong extinction by dust in their disc. In addition to dust extinction, a large bulge-to-disc ratio may make a late-type galaxy red. Many pRLGs seem to be bulge-dominated latetype galaxies with small inclination, in which line emission is not detected due to the limited size of the spectroscopic ?bre aperture. Like in BLGs, AGN activity is detected in some RLGs (sRLGs, lRLGs), which have small inclination. This paper is the ?rst in the series of comprehensive studies on the nature of the SDSS galaxies in ?nely-divided classes. In the following papers, we will inspect various aspects of galaxies in these classes, focusing on their multiwavelength properties and environmental e?ects.


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ACKNOWLEDGEMENTS This work was supported in part by a grant (R01-2007-00020336-0) from the Basic Research Program of the Korea Science and Engineering Foundation (KOSEF). CBP and YYC acknowledge the support of the KOSEF through the Astrophysical Research Centre for the Structure and Evolution of the Cosmos (ARCSEC). Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS Web site is http://www.sdss.org/. The SDSS is managed by the Astrophysical Research Consortium for the Participating Institutions. The Participating Institutions are the American Museum of Natural History, Astrophysical Institute Potsdam, the University of Basel, the University of Cambridge, Case Western Reserve University, the University of Chicago, Drexel University, Fermilab, the Institute for Advanced Study, the Japan Participation Group, Johns Hopkins University, the Joint Institute for Nuclear Astrophysics, the Kavli Institute for Particle Astrophysics and Cosmology, the Korean Scientist Group, the Chinese Academy of Sciences (LAMOST), Los Alamos National Laboratory, the Max-Planck-Institute for Astronomy (MPIA), the Max Planck Institute for Astrophysics (MPA), New Mexico State University, Ohio State University, the University of Pittsburgh, the University of Portsmouth, Princeton University, the US Naval Observatory, and the University of Washington.

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