18 research outputs found
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
Dark Matter Angular Momentum Profile from the Jeans Equation
Cosmological simulations of dark matter structures have shown that the
equilibrated dark matter structures have a fairly small angular momentum. It
appears from these N-body simulations that the radial profile of the angular
momentum has an almost universal behavior, even if the different dark matter
structures have experienced very different formation and merger histories. We
suggest a perturbed Jeans equation, which includes a rotational term. This is
done under a reasonable assumed form of the change in the distribution
function. By conjecturing that the (new) subdominant rotation term must be
proportional to the (old) dominant mass term, we find a clear connection, which
is in rather good agreement with the results of recent high resolution
simulations. We also present a new connection between the radial profiles of
the angular momentum and the velocity anisotropy, which is also in fair
agreement with numerical findings. Finally we show how the spin parameter
increases as a function of radius.Comment: 9 pages, 10 figures, accepted for publication in ApJ, Added
reference
The impact of baryon physics on the structure of high-redshift galaxies
We study the detailed structure of galaxies at redshifts z > 2 using
cosmological simulations with improved modeling of the interstellar medium and
star formation. The simulations follow the formation and dissociation of
molecular hydrogen, and include star formation only in cold molecular gas. The
molecular gas is more concentrated towards the center of galaxies than the
atomic gas, and as a consequence, the resulting stellar distribution is very
compact. For halos with total mass above 10^{11} Mo, the median half-mass
radius of the stellar disks is 0.8 kpc at z = 3. The vertical structure of the
molecular disk is much thinner than that of the atomic neutral gas. Relative to
the non-radiative run, the inner regions of the dark matter halo change shape
from prolate to mildly oblate and align with the stellar disk. However, we do
not find evidence for a significant dark disk of dark matter around the stellar
disk. The outer halo regions retain the orientation acquired during accretion
and mergers, and are significantly misaligned with the inner regions. The
radial profile of the dark matter halo contracts in response to baryon
dissipation, establishing an approximately isothermal profile throughout most
of the halo. This effect can be accurately described by a modified model of
halo contraction. The angular momentum of a fixed amount of inner dark matter
is approximately conserved over time, while in the dissipationless case most of
it is transferred outward during mergers. The conservation of the dark matter
angular momentum provides supporting evidence for the validity of the halo
contraction model in a hierarchical galaxy formation process.Comment: 17 pages, 18 figures, submitted to Ap
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
Cosmology: small scale issues
The abundance of dark matter satellites and subhalos, the existence of
density cusps at the centers of dark matter halos, and problems producing
realistic disk galaxies in simulations are issues that have raised concerns
about the viability of the standard cold dark matter (LambdaCDM) scenario for
galaxy formation. This talk reviews these issues, and considers the
implications for cold vs. various varieties of warm dark matter (WDM). The
current evidence appears to be consistent with standard LambdaCDM, although
improving data may point toward a rather tepid version of LambdaWDM - tepid
since the dark matter cannot be very warm without violating observational
constraints.Comment: 7 pages, 1 figure, to appear in the proceedings of the 8th UCLA Dark
Matter Symposium, Marina del Rey, USA, 20-22 February 200