Environmental dependence of galaxy age in the Main galaxy sample of SDSS DR10
aa r X i v : . [ a s t r o - ph . GA ] O c t Bull. Astr. Soc. India (2014) , 1– ?? Environmental dependence of galaxy age in the Main galaxysample of SDSS DR10
Xin-Fa Deng ∗ School of Science, Nanchang University, Jiangxi, China, 330031
Received 2014 May 05; accepted 2014 August 29
Abstract.
Using two volume-limited Main galaxy samples of the Sloan Digital SkySurvey Data Release 10 (SDSS DR10), I investigate the environmental dependence ofgalaxy age, and get the same conclusions in two volume-limited Main galaxy samples:old galaxies exist preferentially in the densest regions of the universe, while younggalaxies are located preferentially in low density regions. Such an age-density relationis likely a combination of a strong age-stellar mass relation and the stellar mass-densityrelation.
Keywords : galaxies: fundamental parameters – galaxies: statistics – galaxies: general
1. Introduction
Environmental dependence of galaxy parameters has been an important subject in the field ofgalaxy studies. Many galaxy parameters, such as galaxy luminosity, colour, morphological type,stellar mass and star formation rate (SFR), exhibit a strong correlation with galaxy environments(e.g., Postman & Geller 1984; Dressler et al. 1997; Hashimoto et al. 1998; Brown et al. 2000;Fasano et al. 2000; Norberg et al. 2001; Zehavi et al. 2002; Blanton et al. 2003, 2005; G ´ o mez etal. 2003; Treu et al. 2003; Hogg et al. 2004; Kau ff mann et al. 2004; Li et al. 2006; Zandivarezet al. 2006; Patel et al. 2009; Deng et al. 2007, 2008a-b, 2009, 2011a, 2012a-b). For example,Blanton et al. (2003) demonstrated that there is a strong correlation between luminosity and localdensity: the most luminous galaxies tend to reside in the densest regions of the Universe. Blantonet al. (2005) argued that galaxy colour is the galaxy property that is most predictive of the localenvironment. Patel et al. (2009) reported that the SFR and the specific star formation rate (SSFR,the star formation rate per unit stellar mass) at z ≃ ≃
0. Some studies focused on the environmental dependence of ∗ Current address: School of Science, Nanchang University, Jiangxi, China, 330031, email: [email protected]
X. F. Deng galaxy age, and concluded that galaxies in low-density environments are generally younger thangalaxies in high-density environments (e.g. Bernardi et al. 1998; Trager et al. 2000; Kuntschneret al. 2002; Terlevich & Forbes 2002; Proctor et al. 2004; Mendes de Oliveira et al. 2005;Thomas et al. 2005; Gallazzi et al. 2006; S´ a nchez-Bl´ a zquez et al. 2006; Si´ l chenko 2006; Reedet al. 2007; Rakos et al. 2007; Wegner & Grogin 2008; Smith et al. 2012). For example, Proctoret al. (2004) and Mendes de Oliveira et al. (2005) reported that the member galaxies of compactgroups are generally older than field galaxies. Thomas et al. (2005) found that massive early-typegalaxies in low-density regions appear on average ≃ r petro = Ω = .
3, cosmological constant Ω Λ = .
7, Hubble’s constant H = − Mpc − withh =
2. Data
The tenth data release (DR10) (Ahn et al. 2014) of the SDSS-III is already available. In thiswork, the data of the Main galaxy sample was downloaded from the Catalog Archive Server ofSDSS Data Release 10 (Ahn et al. 2014) by the SDSS SQL Search (http: // / dr10 / ).In the SDSS, the target flags can be used to select out objects that were targeted for some par-ticular reason. The Main galaxy targets have one of the LEGACY_TARGET1 bits "GALAXY","GALAXY_BIG" and "GALAXY_BRIGHT_CORE" set (bits 6, 7 and 8). This corresponds tothe requirement: LEGACY_TARGET1 & (64 | | >
0. I extract 633172 Main galaxieswith the redshift 0 . ≤ z ≤ .
2. The data set of age and stellar mass measurements is from theStellarMassStarFormingPort table obtained with the star-forming template and the Kroupa IMF(Maraston et al. 2013). I consider the mass lost via stellar evolution and use best-fit age of galaxy[in Gyr] and best-fit stellar mass [in log M sun ].Following Deng (2010), I construct a luminous volume-limited Main galaxy sample whichcontains 129515 galaxies at 0 . ≤ z ≤ .
102 with − . ≤ M r ≤ − .
50 and a faint volume-limited sample which contains 34573 galaxies at 0 . ≤ z ≤ . − . ≤ M r ≤ − . nvironmental dependence of galaxy age in DR10 ff erent redshift and luminosityranges. The absolute magnitude M r is calculated from the r-band apparent Petrosian magnitude,using a polynomial fit formula (Park et al. 2005) for the mean K-correction within 0 < z < K ( z ) = . z − . + . z − . − . lo g (1 + . . Deng (2010) argued that when studying the environmental dependence of galaxy properties, oneneeds to see the di ff erence between the above-mentioned two galaxy samples.
3. Environmental dependence of galaxy age in the Main galaxy sample
The local three-dimensional galaxy density (Galaxies Mpc − ) is defined as the number of galaxies(N =
5) within the three-dimensional distance to the 5th nearest galaxy to the volume of the spherewith the radius of this distance. In previous works (e.g., Deng et al. 2008a, 2009; Deng 2010),such a density estimator was often applied. In this work, it is still used to characterize localgalaxy environment. Like Deng et al. (2008a) did, for each sample, I arrange galaxies in adensity order from the smallest to the largest, select approximately 5% of the galaxies, constructtwo subsamples at both extremes of density according to the density, and compare distribution ofage in the lowest density regime with that in the densest regime.Fig. 1 shows age distribution at both extremes of density for the faint (left panel) and lu-minous (right panel) volume-limited Main galaxy samples. As shown by this figure, in these twovolume-limited Main galaxy samples, ages of galaxies strongly depend on local environments:old galaxies exist preferentially in the densest regions of the Universe, while young galaxies arelocated preferentially in low density regions. I further perform the Kolmogorov-Smirnov (KS)test. The probability of the two distributions in Fig. 1 coming from the same parent distribution isnearly 0, which shows that two independent distributions are significantly di ff erent in this figure.So, this statistical conclusion is robust.There often are tight correlations between galaxy properties (e.g., Bower et al. 1992; Ken-nicutt 1992; Strateva et al. 2001; Blanton et al. 2003; Hopkins et al. 2003; Baldry et al. 2004;Balogh et al. 2004; Christlein et al. 2004; Kelm et al. 2005; Deng et al. 2008c, 2010; Bamford etal. 2009; Grützbauch et al. 2011a-b). For example, Kennicutt (1992) and Bamford et al. (2009)demonstrated that galaxy morphology is strongly correlated with the SFR and stellar mass. Denget al. (2010) reported the correlation between star formation activities and the concentrationindex: passive galaxies are more luminous, redder, highly concentrated and preferentially ”early-type”. Grützbauch et al. (2011a) found that galaxy colour correlates strongly with stellar mass at0.4 < z <
1. In this condition, the strong environmental dependence of a galaxy property is likelydue to the environmental dependence of other galaxy properties and tight correlations betweengalaxy properties. Grützbauch et al. (2011b) argued that stellar mass is the most important factorin determining the colours of galaxies. Blanton et al. (2005) and Deng and Zou (2009) demon-strated that galaxy colour is the galaxy property very predictive of local environments. Thus, thestrong environmental dependence of galaxy age is likely due to the environmental dependence ofstellar mass and tight correlation between stellar mass and age.
X. F. Deng F r a c t i on F r a c t i on Figure 1.
Age distribution at both extremes of density for the faint (left panel) and luminous (right panel)volume-limited Main galaxy samples: red solid line for the subsample at high density, blue dashed line forthe subsample at low density. The error bars of blue lines are 1- σ Poissonian errors. Error-bars of red linesare omitted for clarity.
Some studies showed that there is a strong correlation between stellar mass and environment(e.g., Kau ff mann et al. 2004; Li et al. 2006; Deng et al. 2011a). Fig. 2 shows stellar mass dis-tribution at both extremes of density for the faint (left panel) and luminous (right panel) volume-limited Main galaxy samples. As shown by Fig. 2, in these two volume-limited Main galaxysamples, high mass galaxies exist preferentially in the densest regions of the Universe, while lowmass galaxies are located preferentially in low density regions. The Kolmogorov-Smirnov (KS)test probability in this figure is also 0, which shows that stellar mass of galaxies indeed stronglydepends on environments.I examine average stellar mass as a function of age for the faint (left panel) and luminous(right panel) volume-limited Main galaxy samples. Fig. 3 shows that in these two volume-limitedMain galaxy samples, average stellar mass of galaxies increases substantially with increasing age.This tight age-stellar mass relation and the stellar mass-density relation likely leads to the strongage-density relation.Norberg et al. (2001) and Deng et al. (2009) demonstrated that the correlation between thegalaxy luminosity and environment is fairly di ff erent between galaxies above and below the valueof M ∗ r found for the overall Schechter fit to the galaxy luminosity function. Deng et al. (2009)noted that g-r color, concentration index ci and galaxy morphologies strongly depend on local en-vironments for all galaxies with di ff erent luminosities, and concluded that M ∗ r is a characteristicparameter only for the environmental dependence of galaxy luminosity. Deng et al. (2012c) even nvironmental dependence of galaxy age in DR10 F r a c t i on F r a c t i on Figure 2.
Stellar mass distribution at both extremes of density for the faint (left panel) and luminous (rightpanel) volume-limited Main galaxy samples: red solid line for the subsample at high density, blue dashedline for the subsample at low density. The error bars of blue lines are 1- σ Poissonian errors. Error-bars ofred lines are omitted for clarity. found that M ∗ r is not an important characteristic parameter for the environmental dependence ofthe u-band luminosity. The u-band luminosity of galaxies still strongly depend on local envir-onments in the faint volume-limited sample, like the one in the luminous volume-limited sampledoes. When investigating the environmental dependence of other galaxy properties, some worksalso demonstrated that there is no significant statistical di ff erence between galaxies above andbelow the value of M ∗ r (e.g., Deng 2010; Deng et al. 2011b-c; Deng et al. 2013). In this work, asshown by Figs. 1-3, the same statistical conclusions can be arrived at from two volume-limitedMain galaxy samples above and below the value of M ∗ r .
4. Summary
From the Main galaxy data of SDSS DR10, I construct two volume-limited samples with theluminosity − . ≤ M r ≤ − .
50 and − . ≤ M r ≤ − .
50 respectively, and explore theenvironmental dependence of galaxy age in these two volume-limited Main galaxy samples. Iapply the three-dimensional density estimator within the distance to the 5th nearest neighbor,proceed with the same approach as used by Deng et al. (2008a), and compares distribution of agein the lowest density regime with that in the densest regime. Statistical analyses in two volume-limited Main galaxy samples can reach the same conclusions: old galaxies exist preferentially inthe densest regions of the universe, while young galaxies are located preferentially in low densityregions. Further investigation suggests that such an age-density relation is likely a combinationof a strong age-stellar mass relation and the stellar mass-density relation.
X. F. Deng
Age[inGyr]
Log M * Age[inGyr]
Log M * Figure 3.
Average stellar mass as a function of age for the faint (left panel) and luminous (right panel)volume-limited Main galaxy samples. Error bars represent standard deviation in each redshift bin.
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Acknowledgements
I thank the anonymous referee for providing many useful comments and suggestions. This studywas supported by the National Natural Science Foundation of China (NSFC, Grant 11263005).Funding for SDSS-III has been provided by the Alfred P. Sloan Foundation, the ParticipatingInstitutions, the National Science Foundation, and the U.S. Department of Energy.The SDSS-III web site is http: // / . SDSS-III is managed by the AstrophysicalResearch Consortium for the Participating Institutions of the SDSS-III Collaboration includingthe University of Arizona, the Brazilian Participation Group, Brookhaven National Laboratory,University of Cambridge, University of Florida, the French Participation Group, the German Par-ticipation Group, the Instituto de Astrofisica de Canarias, the Michigan State / Notre Dame //