482 research outputs found
The Observable Thermal and Kinetic Sunyaev-Zel'dovich Effect in Merging Galaxy Clusters
The advent of high-resolution imaging of galaxy clusters using the
Sunyaev-Zel'dovich Effect (SZE) provides a unique probe of the astrophysics of
the intracluster medium (ICM) out to high redshifts. To investigate the effects
of cluster mergers on resolved SZE images, we present a high-resolution
cosmological simulation of a 1.5E15 M_sun adiabatic cluster using the TreeSPH
code ChaNGa. This massive cluster undergoes a 10:3:1 ratio triple merger
accompanied by a dramatic rise in its integrated Compton-Y, peaking at z =
0.05. By modeling the thermal SZE (tSZ) and kinetic SZE (kSZ) spectral
distortions of the Cosmic Microwave Background (CMB) at this redshift with
relativistic corrections, we produce various mock images of the cluster at
frequencies and resolutions achievable with current high-resolution SZE
instruments. The two gravitationally-bound merging subclusters account for 10%
and 1% of the main cluster's integrated Compton-Y, and have extended merger
shock features in the background ICM visible in our mock images. We show that
along certain projections and at specific frequencies, the kSZ CMB intensity
distortion can dominate over the tSZ due to the large line of sight velocities
of the subcluster gas and the unique frequency-dependence of these effects. We
estimate that a one-velocity assumption in estimation of line of sight
velocities of the merging subclusters from the kSZ induces a bias of ~10%. This
velocity bias is small relative to other sources of uncertainty in
observations, partially due to helpful bulk motions in the background ICM
induced by the merger. Our results show that high-resolution SZE observations,
which have recently detected strong kSZ signals in subclusters of merging
systems, can robustly probe the dynamical as well as the thermal state of the
ICM.Comment: MNRAS, accepted. 13 pages, 9 figure
Pre-Heated Isentropic Gas in Groups of Galaxies
We confirm that the standard assumption of isothermal, shock-heated gas in
cluster potentials is unable to reproduce the observed X-ray luminosity-
temperature relation of groups of galaxies. As an alternative, we construct a
physically motivated model for the adiabatic collapse of pre-heated gas into an
isothermal potential that improves upon the original work of Kaiser (1991). The
luminosity and temperature of the gas is calculated, assuming an appropriate
distribution of halo formation times and radiation due to both bremsstrahlung
and recombination processes. This model successfully reproduces the slope and
dispersion of the luminosity-temperature relation of galaxy groups. We also
present calculations of the temperature and luminosity functions for galaxy
groups under the prescription of this model. This model makes two strong
predictions for haloes with total masses M<10^13 M_sun, which are not yet
testable with current data: (1) the gas mass fraction will increase in direct
proportion to the halo mass; (2) the gas temperature will be larger than the
virial temperature of the mass. The second effect is strong enough that group
masses determined from gas temperatures will be overestimated by about an order
of magnitude if it is assumed that the gas temperature is the virial
temperature. The entropy required to match observations can be obtained by
heating the gas at the turnaround time, for example, to about 3 X 10^6 K at
z=1, which is too high to be generated by a normal rate of supernova
explosions. This model breaks down on the scale of low mass clusters, but this
is an acceptable limitation, as we expect accretion shocks to contribute
significantly to the entropy of the gas in such objects.Comment: Final, refereed version, accepted by MNRAS. One new figure and
several clarifying statements have been added. Uses mn.a4.sty (hacked
mn.sty). Also available from
http://astrowww.phys.uvic.ca/~balogh/entropy.ps.g
The phase-space density distribution of dark matter halos
High resolution N-body simulations have all but converged on a common
empirical form for the shape of the density profiles of halos, but the full
understanding of the underlying physics of halo formation has eluded them so
far. We investigate the formation and structure of dark matter halos using
analytical and semi-analytical techniques. Our halos are formed via an extended
secondary infall model (ESIM); they contain secondary perturbations and hence
random tangential and radial motions which affect the halo's evolution at it
undergoes shell-crossing and virialization. Even though the density profiles of
NFW and ESIM halos are different their phase-space density distributions are
the same: \rho/\sigma^3 ~ r^{-\alpha}, with \alpha=1.875 over ~3 decades in
radius. We use two approaches to try to explain this ``universal'' slope: (1)
The Jeans equation analysis yields many insights, however, does not answer why
\alpha=1.875. (2) The secondary infall model of the 1960's and 1970's,
augmented by ``thermal motions'' of particles does predict that halos should
have \alpha=1.875. However, this relies on assumptions of spherical symmetry
and slow accretion. While for ESIM halos these assumptions are justified, they
most certainly break down for simulated halos which forms hierarchically. We
speculate that our argument may apply to an ``on-average'' formation scenario
of halos within merger-driven numerical simulations, and thereby explain why
\alpha=1.875 for NFW halos. Thus, \rho/\sigma^3 ~ r^{-1.875} may be a generic
feature of violent relaxation.Comment: 4 pages, 1 fig. Proceedings of Science (SISSA), "Baryons in Dark
Matter Haloes", Novigrad, Croatia, 5-9 October 2004; editors: R.-J. Dettmar,
U. Klein, P. Salucci. The full paper is astro-ph/0506571 (with minus sign in
eq.(2.2) corrected
On the Intracluster Medium in Cooling Flow & Non-Cooling Flow Clusters
Recent X-ray observations have highlighted clusters that lack entropy cores.
At first glance, these results appear to invalidate the preheated ICM models.
We show that a self-consistent preheating model, which factors in the effects
of radiative cooling, is in excellent agreement with the observations.
Moreover, the model naturally explains the intrinsic scatter in the L-T
relation, with ``cooling flow'' and ``non-cooling flow'' systems corresponding
to mildly and strongly preheated systems, respectively. We discuss why
preheating ought to be favoured over merging as a mechanism for the origin of
``non-cooling flow'' clusters.Comment: 4 pages, to appear in the proceedings of the "Multiwavelength
Cosmology" Conference held in Mykonos, Greece, June 2003, ed. M. Plionis
(Kluwer
Semi-analytical dark matter halos and the Jeans equation
Although N-body studies of dark matter halos show that the density profiles,
rho(r), are not simple power-laws, the quantity rho/sigma^3, where sigma(r) is
the velocity dispersion, is in fact a featureless power-law over ~3 decades in
radius. In the first part of the paper we demonstrate, using the semi-analytic
Extended Secondary Infall Model (ESIM), that the nearly scale-free nature of
rho/sigma^3 is a robust feature of virialized halos in equilibrium. By
examining the processes in common between numerical N-body and semi-analytic
approaches, we argue that the scale-free nature of rho/sigma^3 cannot be the
result of hierarchical merging, rather it must be an outcome of violent
relaxation. The empirical results of the first part of the paper motivate the
analytical work of the second part of the paper, where we use rho/sigma^3
proportional to r^{-alpha} as an additional constraint in the isotropic Jeans
equation of hydrostatic equilibrium. Our analysis shows that the constrained
Jeans equation has different types of solutions, and in particular, it admits a
unique ``periodic'' solution with alpha=1.9444. We derive the analytic
expression for this density profile, which asymptotes to inner and outer
profiles of rho ~ r^{-0.78}, and rho ~ r^{-3.44}, respectively.Comment: 37 pg, 14 fig. Accepted to ApJ: added two figures and extended
discussion. Note that an earlier related paper (conference proceedings)
astro-ph/0412442 has a mistake in eq.(2.2); the correct version is eq.(5) of
the present submissio
Merging of globular clusters within inner galactic regions. II. The Nuclear Star Cluster formation
In this paper we present the results of two detailed N-body simulations of
the interaction of a sample of four massive globular clusters in the inner
region of a triaxial galaxy. A full merging of the clusters takes place,
leading to a slowly evolving cluster which is quite similar to observed Nuclear
Clusters. Actually, both the density and the velocity dispersion profiles match
qualitatively, and quantitatively after scaling, with observed features of many
nucleated galaxies. In the case of dense initial clusters, the merger remnant
shows a density profile more concentrated than that of the progenitors, with a
central density higher than the sum of the central progenitors central
densities. These findings support the idea that a massive Nuclear Cluster may
have formed in early phases of the mother galaxy evolution and lead to the
formation of a nucleus, which, in many galaxies, has indeed a luminosity
profile similar to that of an extended King model. A correlation with galactic
nuclear activity is suggested.Comment: 18 pages, 10 figures, 3 tables. Submitted to ApJ, main journa
Evolution of Interstellar Clouds in Local Group Dwarf Spheroidal Galaxies in the Context of Their Star Formation Histories
We consider evolution of interstellar clouds in Local Group dwarf spheroidal
galaxies (dSphs) in the context of their observed star formation histories. The
Local Group dSphs generally experienced initial bursts of star formations in
their formation epochs ( Gyr ago), when hot gas originating from the
supernovae can make the cold interstellar clouds evaporate. We find that the
maximum size of evaporating cloud is 10 pc. Thus, clouds larger than 10 pc can
survive during the initial star formation. These surviving clouds can
contribute to the second star formation to produce ``intermediate-age (
3--10 Gyr ago) stellar populations.'' Assuming that collisions between clouds
induce star formation and that the timescale of the second star formation is a
few Gyr, we estimate the total mass of the clouds. The total mass is about
, which is 1--3 orders of magnitude smaller than the typical
stellar mass of a present dSph. This implies that the initial star formation is
dominant over the second star formation, which is broadly consistent with the
observed star formation histories. However, the variety of the dSphs in their
star formation histories suggests that the effects of environments on the dSphs
may be important.Comment: 14 pages LaTeX, no figures, to appear in Ap
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