42 research outputs found
Constraints on Field Galaxy Halos from Weak Lensing and Satellite Dynamics
Here I summarize constraints on the nature of the dark matter halos of field
galaxies that have been obtained from the most recent investigations of (i)
weak galaxy-galaxy lensing and (ii) the dynamics of satellite galaxies in orbit
about large host galaxies. Both of these techniques are statistical in their in
their nature (i.e., large samples of galaxies are required to obtain a
"signal"), but since they have inherently different selection biases and
systematic errors they are quite complementary to each other. Results of work
over the last several years on weak lensing and satellite dynamics is revealing
a remarkably consistent picture regarding the dark matter halos of bright field
galaxies (L > L*). The halos extend to large physical radii (> 150 kpc/h) and
are flattened in projection on the sky, there is a marked difference in the
depths of the potential wells of early-type galaxies and late-type galaxies,
and the velocity dispersion profiles of the halos decrease at large projected
radii. All of these are expected to hold true in a cold dark matter universe
and, while neither technique can address the the possible small-scale conflicts
between CDM and observed galaxies, on scales > 50 kpc/h both techniques yield
results that are consistent with each other and with the predictions of CDM.Comment: 28 pages, 15 figures, invited review in "The New Cosmology", eds. R.
E. Allen, D. V. Nanopoulos, and C. N. Pope, in pres
Satellite Galaxies in the Illustris-1 Simulation: Poor Tracers of the Mass Distribution
Number density profiles are computed for the satellites of relatively
isolated host galaxies in the Illustris-1 simulation. The mean total mass
density of the hosts is well-fitted by an NFW profile. The number density
profile for the complete satellite sample is inconsistent with NFW and, on
scales < 0.5 r_200, the satellites do not trace the hosts' mass. This differs
substantially from previous results from semi-analytic galaxy formation models.
The shape of the satellite number density profile depends on the luminosities
of the hosts and the satellites, and on the host virial mass. The number
density profile for the faintest satellites is well-fitted by an NFW profile,
but the concentration is much less than the mean host mass density. The number
density profile for the brightest satellites exhibits a steep increase in slope
for host-satellite distances < 0.1 r_200, in qualitative agreement with recent
observational studies that find a steep increase in the satellite number
density at small host-satellite distances. On scales > 0.1 r_200 the satellites
of the faintest hosts trace the host mass reasonably well. On scales > 0.4
r_200, the satellites of the brightest hosts do not trace the host mass and the
satellite number density increases steeply for host-satellite distances < 0.1
r_200. The discrepancy between the satellite number density profile and the
host mass density is most pronounced for the most massive systems, with the
satellite number density falling far below that of the mass density on scales <
0.5 r_200.Comment: 11 pages, 4 figures, accepted for publication in ApJ Letter
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
Satellite galaxies in the Illustris-1 simulation: anisotropic locations around relatively isolated hosts
We investigate the locations of satellite galaxies in the z = 0 redshift
slice of the hydrodynamical Illustris-1 simulation. As expected from previous
work, the satellites are distributed anisotropically in the plane of the sky,
with a preference for being located near the major axes of their hosts. Due to
misalignment of mass and light within the hosts, the degree of anisotropy is
considerably less when satellite locations are measured with respect to the
hosts' stellar surface mass density than when they are measured with respect to
the hosts' dark matter surface mass density. When measured with respect to the
hosts' dark matter surface mass density, the mean satellite location depends
strongly on host stellar mass and luminosity, with the satellites of the
faintest, least massive hosts showing the greatest anisotropy. When measured
with respect to the hosts' stellar surface mass density, the mean satellite
location is essentially independent of host stellar mass and luminosity. In
addition, the satellite locations are largely insensitive to the amount of
stellar mass used to define the hosts' stellar surface mass density, as long as
at least 50% to 70% of the hosts' total stellar mass is used. The satellite
locations are dependent upon the stellar masses of the satellites, with the
most massive satellites having the most anisotropic distributions.Comment: 12 pages, 10 figures, accepted for publication in MNRA
The spatial correlation properties of dark galaxy halos in a CDM universe
We use the Hierarchical Particle Mesh (HPM) N-body code written by J. V. Villumsen (Villumsen, 1989) to investigate the two-point spatial correlation function, xi(r), of dark galaxy halos as a function of halo mass and local environment (i.e. high, low, or average mass density). We assume a standard cold dark matter (CDM) universe (omega = 1, delta = 0, H sub 0 = 50,km/sec/Mpc). Because of the large dynamic ranges in mass and length that can be obtained with the HPM code, it is well-suited to an investigation of this sort
Fast generation of large-scale structure density maps via generative adversarial networks
Generative Adversarial Networks (GANs) are a recent advancement in unsupervised machine learning. They are a cat-and-mouse game between two neural networks: (1) a discriminator network which learns to validate whether a sample is real or fake compared to a training set and (2) a generator network which learns to generate data that appear to belong to the training set. Both networks learn from each other until training is complete and the generator network is able to produce samples that are indistinguishable from the training set. We find that GANs are well-suited for fast generation of novel 3D density maps that are indistinguishable from those obtained from N-body simulations. In a matter of seconds, a fully trained GAN can generate thousands of density maps at different epochs in the history of the universe. These GAN-generated maps can then be used to study the evolution of large-scale structure over time.Accepted manuscrip