Xiao-Xun Tang

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.

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.

Compact groups of galaxies (CGs) from the SDSS Data Release 6

From the volume-limited MAIN galaxy sample of the SDSS Data Release 6 (SDSS6), we have identified 1298 compact groups of galaxies (CGs) at the neighbourhood radius of R = 1.2 Mpc by three-dimensional cluster analysis. In order to compile the CG catalog, we also estimated velocity dispersion, virial radius, virial mass and crossing time of CGs. In addition, our results show that properties of galaxies may not correlate with merging frequencies.