The local environmental dependence of properties of Luminous Red Galaxies (LRGs) from the SDSS data release 6 (SDSS6)

Using the Luminous Red Galaxy (LRG) sample of the SDSS Data Release 6, we investigate the dependence of LRG properties on the local environment. For each LRG, the local three-dimensional density within the distance to the 5th nearest galaxy is calculated. It turns out that there is no strong local environmental dependence of LRG morphologies, and that other galaxy properties do not present significant dependence on the local environment. We further calculate the local three-dimensional galaxy density within the distance to the 10th nearest galaxy, and find that results are similar to those for local density within the distance to the 5th nearest galaxy. Published in Astrofizika, Vol. 51, No. 2, pp. 189–200 (May 2008).     

The influence of environment on galaxy age,stellar velocity dispersion,and stellar mass in the LOWZ sample of the SDSS-III

In this work, I examine the environmental dependence of galaxy age, stellar velocity dispersion and stellar mass in the LOWZ sample of the Sloan Digital Sky Survey Data Release 10 (SDSS DR10). I measure the projected local density Σ₅, divide the LOWZ sample into subsamples with a redshift binning size of Δz = 0.02 and analyze the environmental dependence of galaxy age, stellar velocity dispersion and stellar mass in each redshift bin. It is found that galaxy age, stellar velocity dispersion and stellar mass in the LOWZ galaxy sample are very weakly correlatedwith the local environment, like the one in theCMASS galaxy sample does.     

The environmental dependence of the chemical properties of star-forming galaxies

We use a  0.040 < z < 0.085  sample of 37 866 star-forming galaxies from the Fourth Data Release of the Sloan Digital Sky Survey to investigate the dependence of gas-phase chemical properties on stellar mass and environment. The local density, determined from the projected distances to the fourth and fifth nearest neighbours, is used as an environment indicator. Considering environments ranging from voids, i.e.  log Σ≲−0.8  , to the periphery of galaxy clusters, i.e.  log Σ≈ 0.8  , we find no dependence of the relationship between galaxy stellar mass and gas-phase oxygen abundance, along with its associated scatter, on local galaxy density. However, the star-forming gas in galaxies shows a marginal increase in the chemical enrichment level at a fixed stellar mass in denser environments. Compared with galaxies of similar stellar mass in low-density environments, they are enhanced by a few per cent for massive galaxies to about 20 per cent for galaxies with stellar masses  ≲10^9.5 M_⊙  . These results imply that the evolution of star-forming galaxies is driven primarily by their intrinsic properties and is largely independent of their environment over a large range of local galaxy density.     

Luminosity Function of the Cluster of Galaxies Abell 566

We investigate the Luminosity Function(LF)of the cluster of galaxies Abell 566.The photometric data of 15 intermediate-bands are obtained from the Beijing-Arizona- Taiwan-Connecticut(BATC)photometric sky survey.For each of the 15 wavebands,the LF of cluster galaxies is well modelled by the Schechter function,with characteristic luminosi- ties from-18.0 to-21.9 magnitude,from the a- to the p-band.Morphological dependence of the LF is investigated by separating the cluster members into‘red’and‘blue’subsamples. It is clear that late type galaxies have a steeper shape of LF than the early type galaxies.We also divided the sample galaxies by their local environment.It was found that galaxies in the sparser region have steeper shape of LF than galaxies in the denser region.Combining the results of morphological and environmental dependence of LFs,we show that Abell 566 is a well relaxed cluster with positive evidence of galaxy interaction and merger,and excess number of bright early type galaxies located in its denser region.     

The DEEP2 Galaxy Redshift Survey: the role of galaxy environment in the cosmic star formation history

Using galaxy samples drawn from the Sloan Digital Sky Survey and the DEEP2 Galaxy Redshift Survey, we study the relationship between star formation and environment at   z ∼ 0.1  and 1. We estimate the total star formation rate (SFR) and specific star formation rate (sSFR) for each galaxy according to the measured [O  ii ]λ 3727 Å nebular line luminosity, corrected using empirical calibrations to match more robust SFR indicators. Echoing previous results, we find that in the local Universe star formation depends on environment such that galaxies in regions of higher overdensity, on average, have lower SFRs and longer star formation time-scales than their counterparts in lower density regions. At   z ∼ 1  , we show that the relationship between sSFR and environment mirrors that found locally. However, we discover that the relationship between total SFR and overdensity at   z ∼ 1  is inverted relative to the local relation. This observed evolution in the SFR–density relation is driven, in part, by a population of bright, blue galaxies in dense environments at   z ∼ 1  . This population, which lacks a counterpart at   z ∼ 0  , is thought to evolve into members of the red sequence from   z ∼ 1  to ∼0. Finally, we conclude that environment does not play a dominant role in the cosmic star formation history at   z < 1  : the dependence of the mean galaxy SFR on local galaxy density at constant redshift is small compared to the decline in the global SFR space density over the last 7 Gyr.     

Supernovae in isolated galaxies, in pairs and in groups of galaxies

In order to investigate the influence of environment on supernova (SN) production, we have performed a statistical investigation of the SNe discovered in isolated galaxies, in pairs and in groups of galaxies. 22 SNe in 18 isolated galaxies, 48 SNe in 40 galaxy members of 37 pairs and 211 SNe in 170 galaxy members of 116 groups have been selected and studied.
We found that the radial distributions of core-collapse SNe in galaxies located in different environments are similar, and consistent with those reported by Bartunov, Makarova & Tsvetkov . SNe discovered in pairs do not favour a particular direction with respect to the companion galaxy. Also, the azimuthal distributions inside the host members of galaxy groups are consistent with being isotropics. The fact that SNe are more frequent in the brighter components of the pairs and groups is expected from the dependence of the SN rates on the galaxy luminosity.
There is an indication that the SN rate is higher in galaxy pairs compared with that in groups. This can be related to the enhanced star formation rate in strongly interacting systems.
It is concluded that, with the possible exception of strongly interacting systems, the parent galaxy environment has no direct influence on SN production.     

Correlations between environment and other properties of galaxies from the Sloan Digital Sky Survey at fixed color

From the volume-limited Main galaxy sample of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we construct three samples with g–r color bins , labeled S1–S3, to investigate how other properties of galaxies depend on environment at fixed color. For each sample, we measure the local three-dimensional galaxy density in a comoving sphere with radius equal to the distance to the 5th nearest galaxy for each galaxy, select about 5% galaxies and construct the two subsamples at both extremes of density. Our study suggests that the environmental dependence of luminosity is mainly due to the environmental dependence of galaxy color and the correlation between color and luminosity. In addition, we preferentially conclude that concentration index and morphologies are not strongly correlated with local density at fixed color, and that galaxy color is a galaxy property very predictive of the local environment. Because SDSS spectroscopy is incomplete for bright galaxies at very low redshifts, we also use a volume-limited Main galaxy sample with a lower redshift limit z = 0.05, which contains 94,954 galaxies at 0.05 < z < 0.089 with −22.40 < M_r < −20.16, and reach the same conclusions.Due to the bimodality of the u–r color distribution, we classify galaxies as ‘red’ and ‘blue’, respectively, and further subdivide the samples into star-forming galaxies and passive ones using Hα equivalent width, W₀(Hα). Results show that color and star formation activity of galaxies are galaxy properties very predictive of the local environment.     

The 2dF Galaxy Redshift Survey: luminosity functions by density environment and galaxy type

We use the 2dF Galaxy Redshift Survey to measure the dependence of the b _J-band galaxy luminosity function on large-scale environment, defined by density contrast in spheres of radius  8  h ⁻¹ Mpc  , and on spectral type, determined from principal component analysis. We find that the galaxy populations at both extremes of density differ significantly from that at the mean density. The population in voids is dominated by late types and shows, relative to the mean, a deficit of galaxies that becomes increasingly pronounced at magnitudes brighter than   M _{b J}−5log₁₀ h ≲−18.5  . In contrast, cluster regions have a relative excess of very bright early-type galaxies with   M _{b J}−5log₁₀ h ≲−21  . Differences in the mid- to faint-end population between environments are significant: at   M _{b J}−5log₁₀ h =−18  early- and late-type cluster galaxies show comparable abundances, whereas in voids the late types dominate by almost an order of magnitude. We find that the luminosity functions measured in all density environments, from voids to clusters, can be approximated by Schechter functions with parameters that vary smoothly with local density, but in a fashion that differs strikingly for early- and late-type galaxies. These observed variations, combined with our finding that the faint-end slope of the overall luminosity function depends at most weakly on density environment, may prove to be a significant challenge for models of galaxy formation.     

The DEEP2 galaxy redshift survey: evolution of the colour–density relation at 0.4 < z < 1.35

Using a sample of 19 464 galaxies drawn from the DEEP2 Galaxy Redshift Survey, we study the relationship between galaxy colour and environment at  0.4 < z < 1.35  . We find that the fraction of galaxies on the red sequence depends strongly on local environment out to   z > 1  , being larger in regions of greater galaxy density. At all epochs probed, we also find a small population of red, morphologically early-type galaxies residing in regions of low measured overdensity. The observed correlations between the red fraction and local overdensity are highly significant, with the trend at   z > 1  detected at a greater than 5σ level. Over the entire redshift regime studied, we find that the colour–density relation evolves continuously, with red galaxies more strongly favouring overdense regions at low z relative to their red-sequence counterparts at high redshift. At   z ≳ 1.3  , the red fraction only weakly correlates with overdensity, implying that any colour dependence to the clustering of  ∼ L *  galaxies at that epoch must be small. Our findings add weight to existing evidence that the build-up of galaxies on the red sequence has occurred preferentially in overdense environments (i.e. galaxy groups) at   z ≲ 1.5  . Furthermore, we identify the epoch  ( z ∼ 2)  at which typical  ∼ L *  galaxies began quenching and moved on to the red sequence in significant number. The strength of the observed evolutionary trends at  0 < z < 1.35  suggests that the correlations observed locally, such as the morphology–density and colour–density relations, are the result of environment-driven mechanisms (i.e. 'nurture') and do not appear to have been imprinted (by 'nature') upon the galaxy population during their epoch of formation.     

Environmental dependence of stellar mass,SFR, and SSFR for the Luminous Red Galaxy (LRG) sample of the SDSS DR7

Using the Luminous Red Galaxy (LRG) sample of the Sloan Digital Sky Survey Data Release 7 (SDSS DR7), we investigate the environmental dependence of stellar mass, star formation rate (SFR) and specific star formation rate (SSFR) of LRGs. It is found that stellar mass of LRGs nearly is independent of local environments, and that the environmental dependence of SFR and SSFR in the LRG sample is much weaker than the one in the Main galaxy sample. One possible explanation is that galaxy color and morphology are a pair of galaxy properties most predictive of local environments, while LRGs are a group of galaxies that are likely to be luminous, red and of early types (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Exploring the evolution of spiral galaxies

We have constructed a family of simple models for spiral galaxy evolution to allow us to investigate observational trends in star formation history with galaxy parameters. The models are used to generate broad-band colours from which ages and metallicities are derived in the same way as the data. We generate a grid of model galaxies and select only those that lie in regions of parameter space covered by the sample. The data are consistent with the proposition that the star formation history of a region within a galaxy depends primarily on the local surface density of the gas but that one or two additional ingredients are required to explain the observational data fully. The observed age gradients appear steeper than those produced by the density dependent star formation law, indicating that the star formation law or infall history must vary with galactocentric radius. Furthermore, the metallicity–magnitude and age–magnitude correlations are not reproduced by a local density dependence alone. These correlations require one or both of the following: (i) a combination of mass dependent infall and metal enriched outflow, or (ii) a mass dependent galaxy formation epoch. Distinguishing these possibilities on the basis of current data is extremely difficult.     

Tidal alignments as a contaminant of redshift space distortions

We investigate the effect of orientation-dependent selection effects on galaxy clustering in redshift space. It is found that if galaxies are aligned by large-scale tidal fields, then these selection effects give rise to a dependence of the observed galaxy density on the local tidal field, in addition to the well-known dependences on the matter density and radial velocity gradient. This alters the galaxy power spectrum in a way that is different for Fourier modes parallel to and perpendicular to the line of sight. These tidal galaxy alignments can thus mimic redshift space distortions (RSD), and thus result in a bias in the measurement of the velocity power spectrum. If galaxy orientations are affected only by the local tidal field, then the tidal alignment effect has exactly the same scale and angular dependence as the RSDs in the linear regime, so it cannot be projected out or removed by masking small scales in the analysis. We consider several toy models of tidal alignments and orientation-dependent selection, normalize their free parameter (an amplitude) to recent observations, and find that they could bias the velocity amplitude   f ( z ) G ( z )  by 5–10 per cent in some models, although most models give much smaller contamination. We conclude that tidal alignments may be a significant systematic error in RSD measurements that aim to test general relativity via the growth of large-scale structure. We briefly discuss possible mitigation strategies.     

Global properties of nearby galaxies in various environments

We analyze a sample of the Local Volume that contains 451 galaxies within 10 Mpc. We compare the various global parameters of these galaxies with their tidal index that characterizes the local density of the environment. The closest correlation is observed between the density of the galaxy’s environment and its morphological type. The abundance of neutral hydrogen in the members of close groups was found to be, on average, a factor of 3 lower than that in isolated galaxies. However, much of this difference is attributable to different morphological composition for the group members and field galaxies. The total mass-to-luminosity ratio is virtually independent of the tidal index of the galaxy, which indirectly indicates a low percentage of tidal systems among dwarf galaxies. All of the galaxies with three or more companions in the Local Volume are shown to have masses above the threshold value of 10¹⁰M_⊙.     

Color–Density Relation of Galaxies in the Redshift Region of 0.60 < z < 0.75

In this work, I construct a LRG sample with the redshift of 0.6 ≤ z ≤ 0.75 from the Sloan Digital Sky Survey Data Release 15 (SDSS DR15), which contains 184172 CMASS LRGs and 27158 eBOSS LRGs and examine the environmental dependence of the u–r, u–g, g–r, r–i, and i–z colors in this galaxy sample. I divide this LRG sample into subsamples with a redshift binning size of Δz = 0.01, and analyze the environmental dependence of the u–r, u–g, g–r, r–i, and i–z colors for these subsamples in each redshift bin. Overall, the u–r, u–g, g–r, and r–i colors of galaxies in LRG sample with the redshift of 0.6 ≤ z ≤ 0.75 are very weakly correlated with the local environment, which shows that minimal environmental dependence of galaxy parameters can continue to higher redshifts. It is noteworthy that i–z color of this CMASS + eBOSS LRG sample shows substantial correlation with the local environment in the redshift region 0.70 ≤ z ≤ 0.75.     

The stellar populations of spiral galaxies

We have used a large sample of low-inclination spiral galaxies with radially resolved optical and near-infrared photometry to investigate trends in star formation history with radius as a function of galaxy structural parameters. A maximum-likelihood method was used to match all the available photometry of our sample to the colours predicted by stellar population synthesis models. The use of simplistic star formation histories, uncertainties in the stellar population models and considering the importance of dust all compromise the absolute ages and metallicities derived in this work; however, our conclusions are robust in a relative sense. We find that most spiral galaxies have stellar population gradients, in the sense that their inner regions are older and more metal rich than their outer regions. Our main conclusion is that the surface density of a galaxy drives its star formation history, perhaps through a local density dependence in the star formation law. The mass of a galaxy is a less important parameter; the age of a galaxy is relatively unaffected by its mass; however, the metallicity of galaxies depends on both surface density and mass. This suggests that galaxy‐mass-dependent feedback is an important process in the chemical evolution of galaxies. In addition, there is significant cosmic scatter suggesting that mass and density may not be the only parameters affecting the star formation history of a galaxy.     

The duty cycle of local radio galaxies

We use a volume- and flux-limited sample of local  (0.03 ≤ z ≤ 0.1)  radio galaxies with optical counterparts to address the question of how long a typical galaxy spends in radio active and quiescent states. The length of the active phase has a strong dependence on the stellar mass of the host galaxy. Radio sources in the most massive hosts are also retriggered more frequently. The time spent in the active phase has the same dependence on stellar mass as does the gas cooling rate, suggesting the onset of the quiescent phase is due to fuel depletion. We find radio and emission-line active galactic nuclei (AGN) activity to be independent, consistent with these corresponding to different accretion states.     

A marked correlation function analysis of halo formation times in the Millennium Simulation

We study the environmental dependence of the formation epoch of dark matter haloes in the Millennium Simulation: a ten billion particle N -body simulation of standard Lambda cold dark matter cosmology. A sensitive test of this dependence – the marked correlation function – reveals highly significant evidence that haloes of a given mass form earlier in denser regions. We define a marked cross-correlation function, which helps quantify how this effect depends upon the choice of the halo population used to define the environment. The mean halo formation redshift as a function of the local overdensity in dark matter is also well determined, and we see an especially clear dependence for galaxy-sized haloes. This contradicts one of the basic predictions of the excursion set model of structure formation, even though we see that this theory predicts other features of the distribution of halo formation epochs rather well. It also invalidates an assumption usually employed in the popular halo, or halo occupation distribution, models of galaxy clustering, namely that the distribution of halo properties is a function of halo mass but not of halo environment.