Orbital Dependence of Galaxy Properties in Satellite Systems of Galaxies

Abstract

We study the dependence of satellite galaxy properties on the distance to the host galaxy and the orbital motion (prograde and retrograde orbits) using the Sloan Digital Sky Survey (SDSS) data. From SDSS Data Release 7, we find 3515 isolated satellite systems of galaxies at z < 0.03 that contain 8904 satellite galaxies. Using this sample, we construct a catalog of 635 satellites associated with 215 host galaxies whose spin directions are determined by our inspection of the SDSS color images and/or by spectroscopic observations in the literature. We divide satellite galaxies into prograde and retrograde orbit subsamples depending on their orbital motion with respect to the spin direction of the host. We find that the number of galaxies in prograde orbit is nearly equal to that of retrograde orbit galaxies: the fraction of satellites in prograde orbit is 50% ± 2%. The velocity distribution of satellites with respect to their hosts is found to be almost symmetric: the median bulk rotation of satellites is −1 ± 8 km s −1 . It is found that the radial distribution of early-type satellites in prograde orbit is strongly concentrated toward the host while that of retrograde ones shows much less concentration. We also find the orbital speed of late-type satellites in prograde orbit increases as the projected distance to the host (R) decreases while the speed decreases for those in retrograde orbit. At R less than 0.1 times the host virial radius (R < 0.1r vir,host ), the orbital speed decreases in both prograde and retrograde orbit cases. Prograde satellites are on average fainter than retrograde satellites for both early and late morphological types. The u − r color becomes redder as R decreases for both prograde and retrograde orbit late-type satellites. The differences between prograde and retrograde orbit satellite galaxies may be attributed to their different origin or the different strength of physical processes that they have experienced through hydrodynamic interactions with their host galaxies.

Detection of the Velocity Shear Effect on the Spatial Distributions of the Galactic Satellites in Isolated Systems

“…We construct a sample of the isolated galactic systems, each of which consists of a single host and at least one satellite, from the SDSS DR7 spectroscopic data sets (Abazajian et al 2009) by following the procedure prescribed in Hwang & Park (2010). Focusing on the local volume in the redshift range of 0 < z < 0.08, we select those galaxies that are at least 1 mag brighter in the r band than the survey limit as the hosts, which amounts to putting a threshold m c = 16.77 as an upper limit on the r-band magnitudes, m r , of the SDSS galaxies.…”

“…As done in Hwang & Park (2010), the host galaxies are identified as the isolated ones if their nearest neighbors satisfy two conditions: r n max{R v , R vn } and Δv n Δv c , where R v and R vn are the projected virial radii of the host galaxy and its nearest neighbor, respectively, where the projected virial radius is defined as the projected radius that encloses the mean mass density as high as 200 times the critical density of the universe.…”

“…These values of Δv c were empirically determined from the work of Park & Choi (2009), who studied the frequency distribution of the differences in the radial velocities between the SDSS galaxy pairs. In the works of Park & Choi (2009) and Hwang & Park (2010), the elliptical and spiral galaxies were classified according to the morphology segregator that had been developed by Park & Choi (2005) based on the gradients of the galaxy colors.…”

“…The true satellites of an isolated host galaxy are identified among the candidates as the ones whose separation distances from the isolated host, r s , in the projected plane of sky are less than some threshold, r sc , given as (Hwang & Park 2010) …”

“…For the detailed description of how to identify the isolated galactic systems, the readers are referred to Park & Choi (2009) and Hwang & Park (2010). A total of 10,646 SDSS galaxies are selected as the isolated hosts having one or more satellites.…”