120 research outputs found
Entropy of gas and dark matter in galaxy clusters
On the basis of a large scale 'adiabatic', namely non-radiative and
non-dissipative, cosmological smooth particle hydrodynamic simulation we
compare the entropy profiles of the gas and the dark matter (DM) in galaxy
clusters. The quantity K_g = T_g \rho_g^{-2/3} provides a measure for the
entropy of the intra-cluster gas. By analogy with the thermodynamic variables
of the gas the velocity dispersion of the DM is associated with a formal
temperature and thereby K_DM = \sigma_DM^2 \rho_DM^{-2/3} is defined. This DM
entropy is related to the DM phase space density by K_DM \propto Q_DM^{-2/3}.
In accord with other studies the DM phase space density follows a power law
behaviour, Q_DM \propto r^{-1.82}, which corresponds to K_DM \propto r^{1.21}.
The simulated intra-cluster gas has a flat entropy core within (0.8 \pm 0.4)
R_s, where R_s is the NFW scale radius. The outer profile follows the DM
behaviour, K_g \propto r^{1.21}, in close agreement with X-ray observations.
Upon scaling the DM and gas densities by their mean cosmological values we find
that outside the entropy core a constant ratio of K_g / K_{DM} = 0.71 \pm 0.18
prevails. By extending the definition of the gas temperature to include also
the bulk kinetic energy the ratio of the DM and gas extended entropy is found
to be unity for r > 0.8 R_s. The constant ratio of the gas thermal entropy to
that of the DM implies that observations of the intra-cluster gas can provide
an almost direct probe of the DM.Comment: 7 pages, 8 figures, accepted for publication in MNRAS, web page of
the The Marenostrum Numerical Cosmology Project :
http://astro.ft.uam.es/~marenostrum
Detection of the large scale alignment of massive galaxies at z~0.6
We report on the detection of the alignment between galaxies and large-scale
structure at z~0.6 based on the CMASS galaxy sample from the Baryon Oscillation
Spectroscopy Survey data release 9. We use two statistics to quantify the
alignment signal: 1) the alignment two-point correlation function which probes
the dependence of galaxy clustering at a given separation in redshift space on
the projected angle (theta_p) between the orientation of galaxies and the line
connecting to other galaxies, and 2) the cos(2theta)-statistic which estimates
the average of cos(2theta_p) for all correlated pairs at given separation. We
find significant alignment signal out to about 70 Mpc/h in both statistics.
Applications of the same statistics to dark matter halos of mass above 10^12
M_sun/h in a large cosmological simulation show similar scale-dependent
alignment signals to the observation, but with higher amplitudes at all scales
probed. We show that this discrepancy may be partially explained by a
misalignment angle between central galaxies and their host halos, though
detailed modeling is needed in order to better understand the link between the
orientations of galaxies and host halos. In addition, we find systematic trends
of the alignment statistics with the stellar mass of the CMASS galaxies, in the
sense that more massive galaxies are more strongly aligned with the large-scale
structure.Comment: 6 pages, 3 figures, accepted for publication in ApJ Letter
The impact of environment on the dynamical structure of satellite systems
We examine the effects of environment on the dynamical structure of satellite
systems based on the Millennium--II Simulation. Satellite halos are defined as
sub--halos within the virial radius of a host halo. The satellite sample is
restricted to those sub--halos which showed a maximum circular velocity above
30 km/s at the time of accretion. Host halo masses range from 10^11 to 10^14
Msol/h. We compute the satellites' average accretion redshift, z_acc, velocity
dispersion, sigma, and velocity anisotropy parameter, beta, utilising stacked
satellite samples of equal mass hosts at similar background densities. The main
results are: (1) On average satellites within hosts in high density
environments are accreted earlier (Delta z~ 0.1$) compared to their
counterparts at low densities. For host masses above 5 times10^13 Msol/h this
trend weakens and may reverse for higher host masses; (2) The velocity
dispersion of satellites in low density environments follows that of the host,
i.e. no velocity bias is observed for host halos at low densities independent
of host mass. However, for low mass hosts in high density environments the
velocity dispersion of the satellites can be up to ~30% larger than that of the
host halo, i.e. the satellites are dynamically hotter than their host halos.
(3) The anisotropy parameter depends on host mass and environment. Satellites
of massive hosts show more radially biased velocity distributions. Moreover in
low density environments satellites have more radially biased velocities (Delta
beta > 0.1) compared to their counterparts in high density environments. We
believe that our approach allows to predict a similar behaviour for observed
satellite galaxy systems.Comment: 7 pages, 4 figures, accepted for publication in MNRA
The velocity--shape alignment of clusters and the kinetic Sunyaev--Zeldovich effect
We use the Millennium simulation to probe the correlation between cluster
velocities and their shapes and the consequences for measurements of the
kinetic Sunyaev-Zeldovich (kSZ) effect. Halos are generally prolate ellipsoids
with orientations that are correlated with those of nearby halos. We measure
the mean streaming velocities of halos along the lines that separate them,
demonstrating that the peculiar velocities and the long axes of halos tend to
be somewhat aligned, especially for the most massive halos. Since the kSZ
effect is proportional to the line-of-sight velocity and the optical depth of
the cluster, the alignment results in a strong enhancement of the kSZ signature
in clusters moving along the line of sight. This effect has not been taken into
account in many analyses of kSZ signatures.Comment: 5 pages, 5 figures, 1 table; accepted for publication in MNRA
Baryonically Closed Galaxy Groups
Elliptical galaxies and their groups having the largest L_x/L_B lie close to
the locus in the L_x,L_B diagram expected for closed systems with baryon
fractions equal to the cosmic mean value, f_b = 0.16. The estimated baryon
fractions for several of these galaxies/groups are also close to 0.16 when the
gas density is extrapolated to the virial radius. Evidently they are the least
massive baryonically closed systems. Gas retention in these groups implies that
non-gravitational heating cannot exceed about 1 keV per particle, consistent
with the heating required to produce the deviation of groups from the L_x - T
correlation for more massive clusters. Isolated galaxies/groups with X-ray
luminosities significantly lower than baryonically closed groups may have
undermassive dark halos, overactive central AGNs, or higher star formation
efficiencies. The virial mass and hot gas temperatures of nearly or completely
closed groups correlate with the group X-ray luminosities and the optical
luminosities of the group-centered elliptical galaxy, an expected consequence
of their merging history. The ratio of halo mass to the mass of the central
galaxy for X-ray luminous galaxy/groups is about 80.Comment: 7 pages; Accepted by ApJ Letter
Clustering of HI galaxies in HIPASS and ALFALFA
We investigate the clustering of HI-selected galaxies in the ALFALFA survey
and compare results with those obtained for HIPASS. Measurements of the angular
correlation function and the inferred 3D-clustering are compared with results
from direct spatial-correlation measurements. We are able to measure clustering
on smaller angular scales and for galaxies with lower HI masses than was
previously possible. We calculate the expected clustering of dark matter using
the redshift distributions of HIPASS and ALFALFA and show that the ALFALFA
sample is somewhat more anti-biased with respect to dark matter than the HIPASS
sample.Comment: 5 Pages 4 Figures accepted by MNRA
The Shape of Dark Matter Halos: Dependence on Mass, Redshift, Radius, and Formation
Using six high resolution dissipationless simulations with a varying box size
in a flat LCDM universe, we study the mass and redshift dependence of dark
matter halo shapes for M_vir = 9.0e11 - 2.0e14, over the redshift range z=0-3,
and for two values of sigma_8=0.75 and 0.9. Remarkably, we find that the
redshift, mass, and sigma_8 dependence of the mean smallest-to-largest axis
ratio of halos is well described by the simple power-law relation = (0.54
+- 0.02)(M_vir/M_*)^(-0.050 +- 0.003), where s is measured at 0.3 R_vir and the
z and sigma_8 dependences are governed by the characteristic nonlinear mass,
M_*=M_*(z,sigma_8). We find that the scatter about the mean s is well described
by a Gaussian with sigma ~ 0.1, for all masses and redshifts. We compare our
results to a variety of previous works on halo shapes and find that reported
differences between studies are primarily explained by differences in their
methodologies. We address the evolutionary aspects of individual halo shapes by
following the shapes of the halos through ~100 snapshots in time. We determine
the formation scalefactor a_c as defined by Wechsler et al. (2002) and find
that it can be related to the halo shape at z = 0 and its evolution over time.Comment: 18 pages, 21 figures, submitted to MNRA
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