41 research outputs found
Kinematic effect in gravitational lensing by clusters of galaxies
Gravitational lensing provides an efficient tool for the investigation of
matter structures, independent of the dynamical or hydrostatic equilibrium
properties of the deflecting system. However, it depends on the kinematic
status. In fact, either a translational motion or a coherent rotation of the
mass distribution can affect the lensing properties. Here, light deflection by
galaxy clusters in motion is considered. Even if gravitational lensing mass
measurements of galaxy clusters are regarded as very reliable estimates, the
kinematic effect should be considered. A typical peculiar motion with respect
to the Hubble flow brings about a systematic error < 0.3%, independent of the
mass of the cluster. On the other hand, the effect of the spin increases with
the total mass. For cluster masses ~ 10^{15}M_{sun}, the effect of the
gravitomagnetic term is < 0.04% on strong lensing estimates and < 0.5% in the
weak lensing analyses. The total kinematic effect on the mass estimate is then
< 1%, which is negligible in current statistical studies. In the weak lensing
regime, the rotation imprints a typical angular modulation in the tangential
shear distortion. This would allow in principle a detection of the
gravitomagnetic field and a direct measurement of the angular velocity of the
cluster but the required background source densities are well beyond current
tecnological capabilities.Comment: 6 pages; accepted for publication in MNRA
Gravitomagnetic corrections to the lensing deflection angle for spiral galaxy models
We investigate the effects of the gravitomagnetic corrections to the usual
gravitational lens quantities for a specific lensing mass distribution modelled
after spiral galaxies. An exponential disk is embedded into two different
spherical halo models where disk and haloes parameters are fixed according to
the observed mass to light ratios, galaxy magnitudes and rotation curves. The
general expressions for the lensing deflection angle are given also taking into
account the orientation of the galaxy disk plane with respect to the lens
plane. It is found that the gravitomagnetic term changes the deflection angle
by a typical amount of the order of ten microarcseconds.Comment: 7 pages, 2 figures, accepted for publication on MNRA
Stirring Up the Pot: Can Cooling Flows In Galaxy Clusters Be Quenched By Gas Sloshing?
X-ray observations of clusters of galaxies reveal the presence of edges in
surface brightness and temperature, known as "cold fronts". In relaxed clusters
with cool cores, these commonly observed edges have been interpreted as
evidence for the "sloshing" of the core gas in the cluster's gravitational
potential. Such sloshing may provide a source of heat to the cluster core by
mixing hot gas from the cluster outskirts with the cool core gas. Using
high-resolution -body/Eulerian hydrodynamics simulations, we model gas
sloshing in galaxy clusters initiated by mergers with subclusters. The
simulations include merger scenarios with gas-filled and gasless subclusters.
The effect of changing the viscosity of the intracluster medium is also
explored. We find that sloshing can facilitate heat inflow to the cluster core,
provided that there is a strong enough disturbance. In adiabatic simulations,
we find that sloshing can raise the entropy floor of the cluster core by nearly
an order of magnitude in the strongest cases. If the ICM is viscous, the mixing
of gases with different entropies is decreased and consequently the heat flux
to the core is diminished. In simulations where radiative cooling is included,
we find that though eventually a cooling flow develops, sloshing can prevent
the significant buildup of cool gas in the core for times on the order of a Gyr
for small disturbances and a few Gyr for large ones. If repeated encounters
with merging subclusters sustain the sloshing of the central core gas as is
observed, this process can provide a relatively steady source of heat to the
core, which can help to prevent a significant cooling flow.Comment: 22 pages, 26 figures, "emulateapj" format. The version accepted by
ApJ, with proof correction
Interpreting the Evolution of the Size - Luminosity Relation for Disk Galaxies from Redshift 1 to the Present
A sample of very high resolution cosmological disk galaxy simulations is used
to investigate the evolution of galaxy disk sizes back to redshift 1 within the
Lambda CDM cosmology. Artificial images in the rest frame B band are generated,
allowing for a measurement of disk scale lengths using surface brightness
profiles as observations would, and avoiding any assumption that light must
follow mass as previous models have assumed. We demonstrate that these
simulated disks are an excellent match to the observed magnitude - size
relation for both local disks, and for disks at z=1 in the magnitude/mass range
of overlap. We disentangle the evolution seen in the population as a whole from
the evolution of individual disk galaxies. In agreement with observations, our
simulated disks undergo roughly 1.5 magnitudes/arcsec^2 of surface brightness
dimming since z=1. We find evidence that evolution in the magnitude - size
plane varies by mass, such that galaxies with M* > 10^9 M_sun undergo more
evolution in size than luminosity, while dwarf galaxies tend to evolve
potentially more in luminosity. The disks grow in such a way as to stay on
roughly the same stellar mass - size relation with time. Finally, due to an
evolving stellar mass - SFR relation, a galaxy at a given stellar mass (or
size) at z=1 will reside in a more massive halo and have a higher SFR, and thus
a higher luminosity, than a counterpart of the same stellar mass at z=0.Comment: Version resubmitted to ApJ, after referee's comment
Are mergers responsible for universal halo properties?
N-body simulations of Cold Dark Matter (CDM) have shown that, in this
hierarchical structure formation model, dark matter halo properties, such as
the density profile, the phase-space density profile, the distribution of axial
ratio, the distribution of spin parameter, and the distribution of internal
specific angular momentum follow `universal' laws or distributions. Here we
study the properties of the first generation of haloes in a Hot Dark Matter
(HDM) dominated universe, as an example of halo formation through monolithic
collapse. We find all these universalities to be present in this case also.
Halo density profiles are very well fit by the Navarro et al (1997) profile
over two orders of magnitude in mass. The concentration parameter depends on
mass as ,reversing the dependence found in a hierarchical
CDM universe. However, the concentration-formation time relation is similar in
the two cases: earlier forming haloes tend to be more concentrated than their
later forming counterparts. Halo formation histories are also characterized by
two phases in the HDM case: an early phase of rapid accretion followed by
slower growth. Furthermore, there is no significant difference between the HDM
and CDM cases concerning the statistics of other halo properties: the
phase-space density profile; the velocity anisotropy profile; the distribution
of shape parameters; the distribution of spin parameter, and the distribution
of internal specific angular momentum are all similar in the two cases. Only
substructure content differs dramatically. These results indicate that mergers
do not play a pivotal role in establishing the universalities, thus
contradicting models which explain them as consequences of mergers.Comment: 10 pages, 13 figures, submitted to MNRA
3D Spectroscopy with VLT/GIRAFFE - IV: Angular Momentum and Dynamical Support of Intermediate Redshift Galaxies
[Abridged] One of the most outstanding problems related to numerical models
of galaxy formation is the so-called ``angular momentum catastrophe''. We study
the evolution of the angular momentum from z~0.6 to z=0 to further our
understanding of the mechanisms responsible for the large angular momenta of
disk galaxies observed today. This study is based on a complete sample of 32,
0.4<z<0.75 galaxies observed with FLAMES/GIRAFFE at the VLT. Their kinematics
had been classified as rotating disks, perturbed rotators, or complex
kinematics .We have computed the specific angular momentum of disks (j_disk)
and the dynamical support of rotating disks through the V/sigma ratio. To study
how angular momentum can be acquired dynamically, we have compared the
properties of distant and local galaxies. We find that distant rotating disks
have essentially the same properties (j_disk and R_d) as local disks, while
distant galaxies with more complex kinematics have a significantly higher
scatter in the j_disk--V_max and R_d--V_max planes. On average, distant
galaxies show lower values of V/sigma than local galaxies. We found
observational evidence for a non-linear random walk evolution of the angular
momentum in galaxies during the last 8 Gyr. The evolution related to galaxies
with complex kinematics can be attributed to mergers. If galaxies observed at
intermediate redshift are related to present-day spirals, then our results fit
quite well with the ``spiral rebuilding'' scenario proposed by Hammer et al.
(2005)Comment: 12 pages, 8 figures. Accepted for publication in A&
A model for the metallicity evolution of damped Lyman-alpha systems
We apply a physically motivated stellar feedback model to analyse the
statistical properties of damped Lyman-alpha systems (DLAs) expected in the
concordance cold dark matter (CDM) model. Our feedback model produces extended
low-metallicity cold gaseous discs around small galaxies. Since the space
density of galaxies with low circular speeds is high, these discs dominate the
cross-section for the identification of DLAs at all redshifts. The combined
effects of star formation, outflows and infall in our models result in mild
evolution of the N_{HI}-weighted metallicity content in DLAs with redshift,
consistent with observations. According to our model, DLAs contribute only a
small fraction of the volume averaged star formation rate at redshifts z \simlt
5. Our model predicts weak evolution in Omega_{HI} over the redshift range
z=0-5. Furthermore, we show that the cosmological evolution of Omega_{HI} and
the cosmic star formation rate are largely disconnected and conclude that the
evolution of Omega_{HI} as a function of redshift is more likely to tell us
about feedback processes and the evolution of the outer gaseous components of
small galaxies than about the cosmic history of star formation.Comment: 18 pages, 12 figures, accepted for publication in MNRAS (minor
changes
On the spin distributions of CDM haloes
We used merger trees realizations, predicted by the extended Press-Schechter
theory, in order to study the growth of angular momentum of dark matter haloes.
Our results showed that: 1) The spin parameter resulting from the
above method, is an increasing function of the present day mass of the halo.
The mean value of varies from 0.0343 to 0.0484 for haloes with
present day masses in the range of to
. 2)The distribution of is close to
a log-normal, but, as it is already found in the results of N-body simulations,
the match is not satisfactory at the tails of the distribution. A new
analytical formula that approximates the results much more satisfactorily is
presented. 3) The distribution of the values of depends only weakly
on the redshift. 4) The spin parameter of an halo depends on the number of
recent major mergers. Specifically the spin parameter is an increasing function
of this number.Comment: 10 pages, 8 figure
Simulated evolution of the dark matter large-scale structure
We analyze evolution of the basic properties of simulated large scale
structure elements formed by dark matter (DM LSS) and confront it with the
observed evolution of the Lyman- forest. In three high resolution
simulations we selected samples of compact DM clouds of moderate overdensity.
Clouds are selected at redshifts with the Minimal Spanning Tree
(MST) technique. The main properties of so selected clouds are analyzed in 3D
space and with the core sampling approach, what allows us to compare estimates
of the DM LSS evolution obtained with two different techniques and to clarify
some important aspects of the LSS evolution. In both cases we find that regular
redshift variations of the mean characteristics of the DM LSS are accompanied
only by small variations of their PDFs, what indicates the self similar
character of the DM LSS evolution. The high degree of relaxation of DM
particles compressed within the LSS is found along the shortest principal axis
of clouds. We see that the internal structure of selected clouds depends upon
the mass resolution and scale of perturbations achieved in simulations. It is
found that the low mass tail of the PDFs of the LSS characteristics depends
upon the procedure of clouds selection.Comment: 14 pages, 5 figure