Ji-Zhou He

Further Studies of the Local Environmental Dependence of Galaxy Properties

Using the three-dimensional galaxy local density in a comoving sphere with a radius of the distance to the fifth nearest galaxy for each galaxy, we construct two samples at both extremes of density from the volume-limited Main galaxy sample of the Sixth Data Release of the Sloan Digital Sky Survey (SDSS DR6). It is found that the sample at low density has a higher proportion of faint, blue, and late-type galaxies and a lower proportion of luminous, red, and early-type galaxies than the sample at high density. We also measure the projected local density , which is computed from the distance to the fifth nearest neighbor within a redshift slice ± 1000 km s-1 of each galaxy, and come to the same conclusions. In addition, we note that there is no significant difference between the mean luminosity and the mean colors of two samples (< 1 σ), which is consistent with the conclusion obtained by Deng and colleagues in 2007.

Comparisons of the environmental dependence of galaxy properties between galaxies above and below M

From the main galaxy sample of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6), we construct two volume-limited samples above and below the value of M* to explore the difference of the environmental dependence of galaxy properties between galaxies above and below the value of M*. We measure the local three-dimensional galaxy density in a comoving sphere with a radius of the distance to the fifth nearest galaxy for each galaxy, and compare basic properties of galaxies in the lowest density regime with those of galaxies in the densest regime. It is found that the galaxy luminosity strongly depend on local environments only for galaxies above M*, but this dependence is very weak for galaxies below M*. It is worth noting that g−r colour, concentration index ci and galaxy morphologies strongly depend on local environments for all galaxies with different luminosities. This shows that M* is a characteristic parameter only for the environmental dependence of galaxy luminosity.

Effects of both extremes of environments on galaxy properties

Aims. We investigate the dependence of galaxy properties on the local environment. Methods. Member galaxies of compact groups (CGs) and isolated galaxies are located at both extremes of density, which are identified from the volume-limited Main galaxy sample of the Sixth Data Release of the Sloan Digital Sky Survey(SDSS DR6), while the volume-limited Main galaxy sample represents the mean density. We perform comparative studies of distributions of various physical properties among member galaxies of CGs, isolated galaxies, and the volume-limited Main galaxy sample. Results. We find that physical properties of galaxies, such as luminosity, size, colors, concentration index, and morphology strongly depend on the local environment: luminous, red, and early-type galaxies exist preferentially in the densest regions of the universe (e.g., in groups), while faint, blue, and late-type galaxies are located preferentially in low-density regions.

The Dependence of Galaxy Luminosity on Environment in a Volume-limited Sample of the Main Galaxy Sample of SDSS DR6

From the volume-limited main galaxy sample of SDSS DR6, we extracted isolated galaxies and identified groups by different algorithms, in order to investigate the dependence of galaxy luminosity on environment. We compared the luminosity distributions of member galaxies of groups with that of isolated galaxies and the volume-limited main galaxy sample, and found that the luminosity distributions of member galaxies of groups are almost the same as that of the volume-limited sample, but are different from that of isolated galaxies: isolated galaxies have a higher proportion of faint galaxies ( -->Mr ? ? 20.61) and a lower proportion of luminous galaxies ( -->Mr ? ? 21.28) than member galaxies of groups. The level of significance is at least 3 ?. This shows that luminous galaxies exist preferentially in the densest regions of the universe (e.g., in groups), but faint galaxies are located preferentially in low-density regions. We also note that the dependence of galaxy luminosity on dense environment (e.g., in groups) is much weaker than that on underdense environment (e.g., isolated galaxies).

The main galaxy groups from the SDSS data release 5

Aims. We investigate the correlations between galaxy properties and environment. Methods. From the main galaxy sample of the SDSS Data Release 5, we have extracted 11 163 groups at the linking length R0 = 0.2 × ¯ −1/3 = 1.97 Mpc by the three-dimensional friends-of-friends (FOF) algorithm, where ¯ n is the mean galaxy density, and compare basic properties of member galaxies of main galaxy groups with those of the main galaxy sample and isolated galaxies in each redshift bin. Results. The luminosities and sizes of galaxies are almost independent of environment, but morphologies and colors are correlated with environment-especially galaxy morphologies.

THE DEPENDENCE OF LUMINOSITY AND g – r COLOR ON THE ENVIRONMENT FOR THE SAME MORPHOLOGICAL TYPES

We divide the volume-limited main galaxy sample of the Sloan Digital Sky Survey Data Release 6 (SDSS DR6) into two samples—the early-type sample and the late-type sample—to investigate the dependence of luminosity and g – r color on the environment for the same morphological types. For each sample, we construct two subsamples at both extremes of local density which is measured within the distance to the fifth nearest neighbor. It turns out that, for the early-type sample and the late-type sample, significant correlation between g – r color and the environment still exists, but the correlation in the late-type sample is much stronger than that in the early-type sample. Our studies also suggest that the dependence of luminosity on the local environment is mainly due to the dependence of galaxy morphologies on the local environment and the correlation between morphologies and luminosity.

THE STATISTICAL PROPERTIES OF GALAXY MORPHOLOGICAL TYPES IN COMPACT GROUPS OF MAIN GALAXIES FROM THE SDSS DATA RELEASE 4

In order to explore the statistical properties of galaxy morphological types in compact groups (CGs), we construct a random group sample which has the same distributions of redshift and number of member galaxies as those of the CG sample. It turns out that the proportion of early-type galaxies in different redshift bins for the CG sample is statistically higher than that for random group sample, and with growing redshift z this kind of difference becomes more significant. This may be due to the existence of interactions and mergers within a significant fraction of SDSS CGs. We also compare statistical results of CGs with those of more compact groups and pairs, but do not observe as large statistical difference as Hicksons [Astrophys. J.255 (1982) 382] results.

ENVIRONMENTAL DEPENDENCE OF OTHER GALAXY PROPERTIES FOR THE SAME STAR FORMATION ACTIVITIES

Using two volume-limited Main galaxy samples of the Sloan Digital Sky Survey Data Release 6 above and below the value of M*, we have investigated the environmental dependence of other galaxy properties for the same star formation activities. Only in the luminous passive class, a strong environmental dependence of the g ? r color is observed, but the environmental dependence of other properties in this class is very weak. In other classes, we can conclude that the local density dependence of luminosity, g ? r color, concentration index ci, and morphologies for star-forming galaxies and passive ones is much weaker than that obtained in the volume-limited Main galaxy samples. This suggests that star formation activity is a galaxy property very predictive of the local environment. In addition, we also note that passive galaxies are more luminous, redder, highly concentrated, and preferentially early type.

The Properties of Isolated Main Galaxies in Sloan Digital Sky Survey Data Release 3 (SDSS DR3)

We have constructed a Main galaxy subsample of 67777 galaxies with redshifts in the range 0.08 ≤ z ≤ 0.12 from the Sloan Digital Sky Survey Data Release 3. Using cluster analysis, two isolated Main galaxy samples were extracted from this subsample. The two isolated Main galaxy samples identified at different radii have the same properties. Additionally, we find fewer early-type galaxies in isolated Main galaxy samples than in a close double galaxy sample.

Super-Large-Scale Structures in the Sloan Digital Sky Survey

We study the super-large-scale structures in the Sloan Digital Sky Survey by cluster analysis, and examine the geometry and the properties of the member galaxies. Two subsamples are selected from the SDSS, Subsample 1 at the celestial equator and Subsample 2 further north. In Subsample 1 we discover two compact super-large-scale structures: the Sloan Great Wall and the CfA Great Wall. The Sloan Great Wall, located at a median redshift of z = 0.07804, has a total length of about 433 Mpc and a mean galaxy density of about six times that of the whole sample. Most of its member galaxies are of medium size and brightness. The CfA Great Wall, located at a median redshift of z = 0.03058, has a total length of about 251 Mpc and includes large percentages of faint and small galaxies and relatively fewer early-type galaxies.