11 research outputs found
The Spatial Distribution of Satellite Galaxies Selected from Redshift Space
We investigate the spatial distribution of satellite galaxies using a mock
redshift survey of the first Millennium Run simulation. The satellites were
identified using common redshift space criteria and the sample therefore
includes a large percentage of interlopers. The satellite locations are
well-fitted by a combination of a Navarro, Frenk & White(NFW) density profile
and a power law. At fixed stellar mass, the NFW scale parameter, r_s, for the
satellite distribution of red hosts exceeds r_s for the satellite distribution
of blue hosts. In both cases the dependence of r_s on host stellar mass is
well-fitted by a power law. For the satellites of red hosts, r_s^{red} \propto
(M_\ast / M_\sun)^{0.71 \pm 0.05} while for the satellites of blue hosts,
r_s^{blue} \propto (M_\ast / M_\sun)^{0.48 \pm 0.07}$. For hosts with stellar
masses greater than 4.0E+10 M_sun, the satellite distribution around blue hosts
is more concentrated than is the satellite distribution around red hosts. The
spatial distribution of the satellites of red hosts traces that of the hosts'
halos; however, the spatial distribution of the satellites of blue hosts is
more concentrated than that of the hosts' halos by a factor of ~2. Our
methodology is general and applies to any analysis of satellites in a mock
redshift survey. However, our conclusions necessarily depend upon the
semi-analytic galaxy formation model that was adopted, and different galaxy
formation models may yield different results.Comment: 25 pages, 5 figures, accepted for publication in The Astrophysical
Journa
Locations of Satellite Galaxies in the Two-Degree Field Galaxy Redshift Survey
We compute the locations of satellite galaxies in the Two-Degree Field Galaxy
Redshift Survey using two sets of selection criteria and three sources of
photometric data. Using the SuperCOSMOS r_F photometry, we find that the
satellites are located preferentially near the major axes of their hosts, and
the anisotropy is detected at a highly-significant level (confidence levels of
99.6% to 99.9%). The locations of satellites that have high velocities relative
to their hosts are statistically indistinguishable from the locations of
satellites that have low velocities relative to their hosts. Additionally,
satellites with passive star formation are distributed anisotropically about
their hosts (99% confidence level), while the locations of star-forming
satellites are consistent with an isotropic distribution. These two
distributions are, however, statistically indistinguishable. Therefore it is
not correct to interpret this as evidence that the locations of the
star-forming satellites are intrinsically different from those of the passive
satellites.Comment: 21 pages, 3 figure
The Orientation of Satellite Galaxies: Evidence of Elongation in the Direction of the Host
We use the fourth data release of the Sloan Digital Sky Survey to investigate
the orientations of 4289 satellite galaxies with respect to their hosts. The
orientation of the satellites is inconsistent with a random distribution at the
99.94% confidence level, and the satellites show a preference for elongation in
the direction of their hosts. Further, on scales < 50 kpc the major axes of the
host galaxies and their satellites are preferentially aligned. Phrased in the
terminology of weak lensing, the images of the satellites have a mean shear of
gamma_T = -0.045 +/- 0.010, averaged over scales 10 kpc < r < 50 kpc. In a
galaxy-galaxy lensing study where lenses and sources are separated solely on
the basis of apparent magnitude, we estimate that on scales < 250 kpc satellite
galaxies acount for between 10% and 15% of the objects that are identified as
sources. In such studies, elongation of the satellites will cause a reduction
of the galaxy-galaxy lensing shear by of order 25% to 40%. Hence, the
elongation of satellite galaxies in the direction of their hosts is a
potentially important effect for precision studies of galaxy-galaxy lensing and
argues strongly in favor favor of the use of accurate photometric redshifts in
order to identify lenses and sources in future studies.Comment: ApJ Letters, in press; title change, revised text includes
preliminarly analysis of 2dFGRS satellites and additional null test
The Distribution of Satellite Galaxies in a Lambda-CDM Universe
We compute the locations of satellite galaxies with respect to their hosts
using the Lambda-CDM GIF simulation. If the major axes of the hosts' images are
perfectly aligned with the major axes of their projected mass, the satellites
are located preferentially close to the hosts' major axes. In this case, the
degree of anisotropy in the satellite locations is a good tracer of the
flattening of the hosts' halos. If all hosts have luminous circular disks, the
symmetry axes of the projected mass and light are not perfectly aligned, and
the locations of the satellites depend upon how the hosts' disks are placed
within their halos. If the disk angular momentum vectors are aligned with the
major axes of the halos, the satellites show a pronounced "Holmberg effect". If
the disk angular momentum vectors are aligned with the intermediate axes of the
local large scale structure, the distribution of satellite locations is
essentially isotropic. If the disk angular momentum vectors are aligned with
either the minor axes or with the net angular momentum vectors of the halos,
the satellites are distributed anisotropically about their hosts, with a
preference for being found nearby the hosts' major axes. This agrees well with
the observation that satellite galaxies in the Sloan Digital Sky Survey tend to
be found nearby the major axes of their hosts, and suggests that the mass and
light of SDSS host galaxies must be fairly well aligned in projection on the
sky.Comment: ApJ, in press; substantial revision of text but main results are
unchanged; revised paper includes the locations of satellites when the host
angular momentum vector is aligned with either the halo major axis or the
halo net angular momentu
Anisotropic Locations of Satellite Galaxies: Clues to the Orientations of Galaxies within their Dark Matter Halos
We investigate the locations of the satellites of relatively isolated host
galaxies in the Sloan Digital Sky Survey and the Millennium Run simulation.
Provided we use two distinct prescriptions to embed luminous galaxies within
the simulated dark matter halos (ellipticals share the shapes of their halos,
while disks have angular momenta that are aligned with the net angular momenta
of their halos), we find a fair agreement between observation and theory.
Averaged over scales r_p \le 500 kpc, the satellites of red, high-mass hosts
with low star formation rates are found preferentially near the major axes of
their hosts. In contrast, the satellites of blue, low-mass hosts with low star
formation rates show little to no anisotropy when averaged over the same scale.
The difference between the locations of the satellites of red and blue hosts
cannot be explained by the effects of interlopers in the data. Instead, it is
caused primarily by marked differences in the dependence of the mean satellite
location, , on the projected distance at which the satellites are found.
We also find that the locations of red, high-mass satellites with low star
formation rates show considerably more anisotropy than do the locations of
blue, low--mass satellites with high star formation rates. There are two
contributors to this result. First, the blue satellites have only recently
arrived within their hosts' halos, while the red satellites arrived in the far
distant past. Second, the sample of blue satellites is heavily contaminated by
interlopers, which suppresses the measured anisotropy compared to the intrinsic
anisotropy.Comment: 43 pages, 13 figures, ApJ in pres