63 research outputs found
Establishing the relationship between galaxies and dark matter
We use two methods to establish the relationship between galaxies and dark matter halos. One is based the conditional luminosity function model, which links galaxies and dark matter halos by matching the number density and clustering properties of galaxies with those of dark matter halos in the current CDM model. The second is based on galaxy systems identified from large redshift surveys of galaxies. The galaxy - dark halo relationships established by these two methods match well, and can provide important constraints on how galaxies form and evolve in the univers
Detecting metal-rich intermediate-age globular clusters in NGC4570 using K-band photometry
âThe original publication is available at www.springerlink.comâ. Copyright Springer. DOI: 10.1007/s10509-009-0093-8Globular cluster systems (GCSs) of most early-type galaxies feature two peaks in their optical colour distributions. Blue-peak globular clusters (GCs) are believed to be old and metal-poor, whereas the ages, metallicities, and the origin of the red-peak GCs are still being debated. We obtained deep K-band photometry and combined it with Hubble Space Telescope observations in g and z to yield a full spectral energy distribution from the optical to the near-infrared. This now allows us to break the ageâmetallicity degeneracy. We used our evolutionary synthesis models galev for star clusters to compute a large grid of models with different metallicities and a wide range of ages. Comparing these models to our observations revealed a large population of intermediate-age (1â3 Gyr) and metal-rich (âsolar-metallicity) GCs, that will give us further insights into the formation history of this galaxy.Peer reviewe
Cosmological constraints from the cluster contribution to the power spectrum of the soft X-ray background. New evidence for a low sigma_8
We use the X-ray power spectrum of the ROSAT all-sky survey in the R6 band
(approximately 0.9-1.3 keV) to set an upper limit on the galaxy cluster power
spectrum. The cluster power spectrum is modelled with a minimum number of
robust assumptions regarding the structure of the clusters. The power spectrum
of ROSAT sets an upper limit on the Omega_m-sigma_8 plane which excludes all
the models with sigma_8 above sigma_8 = 0.5/(Omega_m^0.38) in a flat LCDM
universe. We discuss the possible sources of systematic errors in our
conclusions, mainly dominated by the assumed L_x-T relation. Alternatively,
this relation could be constrained by using the X-ray power spectrum, if the
cosmological model is known. Our conclusions suggest that only models with a
low value of sigma_8 (sigma_8 < 0.8 for Omega_m = 0.3) may be compatible with
our upper limit. We also find that models predicting lower luminosities in
galaxy clusters are favoured. Reconciling our cosmological constraints with
these arising by other methods might require either a high entropy floor or
wide-spread presence of cooling flows in the low-redshift clusters.Comment: 14 pages, 19 plots (2 as gif files). MNRAS submitte
Particle Dark Matter Constraints from the Draco Dwarf Galaxy
It is widely thought that neutralinos, the lightest supersymmetric particles,
could comprise most of the dark matter. If so, then dark halos will emit radio
and gamma ray signals initiated by neutralino annihilation. A particularly
promising place to look for these indicators is at the center of the local
group dwarf spheroidal galaxy Draco, and recent measurements of the motion of
its stars have revealed it to be an even better target for dark matter
detection than previously thought. We compute limits on WIMP properties for
various models of Draco's dark matter halo. We find that if the halo is nearly
isothermal, as the new measurements indicate, then current gamma ray flux
limits prohibit much of the neutralino parameter space. If Draco has a moderate
magnetic field, then current radio limits can rule out more of it. These
results are appreciably stronger than other current constraints, and so
acquiring more detailed data on Draco's density profile becomes one of the most
promising avenues for identifying dark matter.Comment: 13 pages, 6 figure
Alignment of galaxy spins in the vicinity of voids
We provide limits on the alignment of galaxy orientations with the direction
to the void center for galaxies lying near the edges of voids. We locate
spherical voids in volume limited samples of galaxies from the Sloan Digital
Sky Survey using the HB inspired void finder and investigate the orientation of
(color selected) spiral galaxies that are nearly edge-on or face-on. In
contrast with previous literature, we find no statistical evidence for
departure from random orientations. Expressed in terms of the parameter c,
introduced by Lee & Pen to describe the strength of such an alignment, we find
that c<0.11(0.13) at 95% (99.7%) confidence limit within a context of a toy
model that assumes a perfectly spherical voids with sharp boundaries.Comment: 8 pages, 4 figures; v2 discussion expanded, references fixed, matches
version accepted by JCA
Spiral Galaxies Rotation Curves with a Logarithmic Corrected Newtonian Gravitational Potential
We analyze the rotation curves of 10 spiral galaxies with a newtonian
potential corrected with an extra logarithmic term, using a disc modelization
for the spiral galaxies. There is a new constant associated with the extra term
in the potential. The rotation curve of the chosen sample of spiral galaxies is
well reproduced for a given range of the new constant. It is argued that this
correction can have its origin from string configurations. The compatibility of
this correction with local physics is discussed.Comment: Latex file, 6 pages, 20 figure
Probing the Environment with Galaxy Dynamics
I present various projects to study the halo dynamics of elliptical galaxies.
This allows one to study the outer mass and orbital distributions of
ellipticals in different environments, and the inner distributions of groups
and clusters themselves.Comment: 5 pages, 2 figs, to appear in Proc. ESO Workshop, Groups of Galaxies
in the Nearby Universe (5-9 Dec 2005), eds. I. Saviane, V. Ivanov & J.
Borissova (Springer-Verlag
Merger rates of dark matter haloes: a comparison between EPS and N-body results
We calculate merger rates of dark matter haloes using the Extended
Press-Schechter approximation (EPS) for the Spherical Collapse (SC) and the
Ellipsoidal Collapse (EC) models.
Merger rates have been calculated for masses in the range
to and for
redshifts in the range 0 to 3 and they have been compared with merger rates
that have been proposed by other authors as fits to the results of N-body
simulations. The detailed comparison presented here shows that the agreement
between the analytical models and N-body simulations depends crucially on the
mass of the descendant halo. For some range of masses and redshifts either SC
or EC models approximate satisfactory the results of N-body simulations but for
other cases both models are less satisfactory or even bad approximations. We
showed, by studying the parameters of the problem that a disagreement --if it
appears-- does not depend on the values of the parameters but on the kind of
the particular solution used for the distribution of progenitors or on the
nature of EPS methods.
Further studies could help to improve our understanding about the physical
processes during the formation of dark matter haloes.Comment: 29 pages, 9 figure
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
The Intriguing Distribution of Dark Matter in Galaxies
We review the most recent evidence for the amazing properties of the density
distribution of dark matter around spiral galaxies. Their rotation curves,
coadded according to the galaxy luminosity, conform to an universal profile
which can be represented as the sum of an exponential thin disk plus a
spherical halo with a flat density core. From dwarfs to giants, these halos
feature an inner constant density region. The fine structure of dark matter
halos is obtained from the kinematics of a number of suitable low-luminosity
disk galaxies. The halo circular velocity increases linearly with radius out to
the edge of the stellar disk, implying a constant dark halo density over the
entire disk region. The properties of halos around normal spirals provide
substantial evidence of a discrepancy between the mass distributions predicted
in the Cold Dark Matter scenario and those actually detected around galaxies.Comment: 12 pages, 7 figures. Invited lecture to the 8th Adriatic Meeting:
Particle Physics in the New Millennium, Dubrovnik 4-14 Sep. 2001. To be
published in the series Lecture Notes in Physics, by Springer Verla
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