201 research outputs found
Internal alignments of red versus blue discs in dark matter haloes
Large surveys have shown that red galaxies are preferentially aligned with their haloes, while blue galaxies have a more isotropic distribution. Since haloes generally align with their filaments, this introduces a bias in the measurement of the cosmic shear from weak lensing. It is therefore vitally important to understand why this difference arises. We explore the stability of different disc orientations within triaxial haloes. We show that, in the absence of gas, the disc orientation is most stable when its spin is along the minor axis of the halo. Instead when gas cools on to a disc, it is able to form in almost arbitrary orientation, including off the main planes of the halo (but avoiding an orientation perpendicular to the halo's intermediate axis). Substructure helps gasless galaxies reach alignment with the halo faster, but has less effect on galaxies when gas is cooling on to the disc. Our results provide a novel and natural interpretation for why red, gas poor galaxies are preferentially aligned with their halo, while blue, star-forming, galaxies have nearly random orientations, without requiring a connection between galaxies' current star formation rate and their merger histor
Concentration, spin and shape of dark matter haloes: scatter and the dependence on mass and environment
We use a series of cosmological N-body simulations for a flat Λ cold dark matter (ΛCDM) cosmology to investigate the structural properties of dark matter haloes, at redshift zero, in the mass range 3 × 109 h−1 ≲Mvir ≲ 3 × 1013 h−1 M⊙. These properties include the concentration parameter, c, the spin parameter, λ, and the mean axis ratio, . For the concentration-mass relation we find c∝ in agreement with the model proposed by Bullock et al., but inconsistent with the alternative model of Eke et al. The normalization of the concentration-mass relation, however, is 15 per cent lower than suggested by Bullock et al. The results for λ and are in good agreement with previous studies, when extrapolated to the lower halo masses probed here, while c and λ are anticorrelated, in that high-spin haloes have, on average, lower concentrations. In an attempt to remove unrelaxed haloes from the sample, we compute for each halo the offset parameter, xoff, defined as the distance between the most bound particle and the centre of mass, in units of the virial radius. Removing haloes with large xoff increases the mean concentration by ∼10 per cent, lowers the mean spin parameter by ∼15 per cent, and removes the most prolate haloes. In addition, it largely removes the anticorrelation between c and λ, though not entirely. We also investigate the relation between halo properties and their large-scale environment density. For low-mass haloes we find that more concentrated haloes live in denser environments than their less concentrated counterparts of the same mass, consistent with recent correlation function analyses. Note, however, that the trend is weak compared to the scatter. For the halo spin parameters we find no environment dependence, while there is a weak indication that the most spherical haloes reside in slightly denser environments. Finally, using a simple model for disc galaxy formation we show that haloes that host low surface brightness galaxies are expected to be hosted by a biased subset of haloes. Not only do these haloes have spin parameters that are larger than average, they also have concentration parameters that are ∼15 per cent lower than the average at a given halo mass. We discuss the implications of all these findings for the claimed disagreement between halo concentrations inferred from low surface brightness rotation curves, and those expected for a ΛCDM cosmolog
Concentration, Spin and Shape of Dark Matter Haloes: Scatter and the Dependence on Mass and Environment
We use a series of cosmological N-body simulations for a flat LCDM cosmology
to investigate the properties of dark matter haloes in the mass range
3.0e9-3.0e13 Msun. These properties include the concentration parameter (c),
the spin parameter (lambda) and the mean axis ratio (q). For the
concentration-mass relation we find c~M^(-0.11) in agreement with the model
proposed by Bullock et al. even if we find a lower normalization (15%). The
results for lambda and q are in good agreement with previous studies, while c
and lambda are anti-correlated. In an attempt to remove unrelaxed haloes, we
use the offset parameter (xoff), defined as the distance between the most bound
particle and the center of mass. Removing haloes with large xoff increases the
c by ~10%, lowers the lambda by ~15%, and removes the most prolate haloes. In
addition, it largely removes the anti-correlation between c and lambda though
not entirely. We also investigate the effects of the large-scale environment.
We find that more concentrated haloes live in denser environments. Note,
however, that the trend is weak compared to the scatter. For the spin
parameters we find no environment dependence, while there is a weak indication
that the most spherical haloes reside in denser region. Finally, using a simple
model for disk galaxy formation we show that haloes that host low surface
brightness galaxies are expected to be hosted by a biased sub-set of haloes.
Not only do these haloes have spin parameters that are larger than average,
they also have c that are 15% percent lower than the average at a given halo
mass. We discuss the implications of all these findings for the claimed
disagreement between halo concentrations inferred from LSB rotation curves, and
those expected for a LCDM cosmology. (abridged)Comment: 23 pages, 14 figure. Resolution and environment effects discussed in
more details. Conclusions unchanged. References added. Accepted for
publication by MNRAS. A preprint with high-resolution figures is available at
http://www-theorie.physik.unizh.ch/~andrea/ConcParam
Properties of galaxy groups in the Sloan Digital Sky Survey - II. Active galactic nucleus feedback and star formation truncation
Successfully reproducing the galaxy luminosity function (LF) and the bimodality in the galaxy distribution requires a mechanism that can truncate star formation in massive haloes. Current models of galaxy formation consider two such truncation mechanisms: strangulation, which acts on satellite galaxies, and active galactic nucleus (AGN) feedback, which predominantly affects central galaxies. The efficiencies of these processes set the blue fraction of galaxies, fblue(L, M), as a function of galaxy luminosity, L, and halo mass, M. In this paper, we use a galaxy group catalogue extracted from the Sloan Digital Sky Survey (SDSS) to determine fblue(L, M). To demonstrate the potential power of these data as a benchmark for galaxy formation models, we compare the results to the semi-analytical model for galaxy formation of Croton et al. Although this model accurately fits the global statistics of the galaxy population, as well as the shape of the conditional LF, there are significant discrepancies when the blue fraction of galaxies as a function of mass and luminosity is compared between the observations and the model. In particular, the model predicts (i) too many faint satellites in massive haloes, (ii) a blue fraction of satellites that is much too low, and (iii) a blue fraction of centrals that is too high and with an inverted luminosity dependence. In the same order, we argue that these discrepancies owe to (i) the neglect of tidal stripping in the semi-analytical model, (ii) the oversimplified treatment of strangulation, and (iii) improper modelling of dust extinction and/or AGN feedback. The data presented here will prove useful to test and calibrate future models of galaxy formation and, in particular, to discriminate between various models for AGN feedback and other star formation truncation mechanism
Cusps in CDM halos
We resolve the inner region of a massive cluster forming in a cosmological
LCDM simulation with a mass resolution of 2*10^6 Msun and before z=4.4 even
3*10^5 Msun. This is a billion times less than the clusters final virial mass
and a substantial increase over current LCDM simulations. We achieve this
resolution using a new multi-mass refinement procedure and are now able to
probe a dark matter halo density profile down to 0.1 percent of the virial
radius. The inner density profile of this cluster halo is well fitted by a
power-law rho ~ r^-gamma down to the smallest resolved scale. An inner region
with roughly constant logarithmic slope is now resolved, which suggests that
cuspy profiles describe the inner profile better than recently proposed
profiles with a core. The cluster studied here is one out of a sample of six
high resolution cluster simulations of Diemand et al. (2004) and it's inner
slope of gamma = 1.2 lies close to the sample average.Comment: 10 pages,10 figures. Matches version in press. Added Figure 7 and
section 3.3. about upper limits of inner slopes, text (incl, title and
abstract) revised, conclusions unchange
Internal alignments of red versus blue discs in dark matter haloes
Large surveys have shown that red galaxies are preferentially aligned with their haloes, while blue galaxies have a more isotropic distribution. Since haloes generally align with their filaments, this introduces a bias in the measurement of the cosmic shear from weak lensing. It is therefore vitally important to understand why this difference arises. We explore the stability of different disc orientations within triaxial haloes. We show that, in the absence of gas, the disc orientation is most stable when its spin is along the minor axis of the halo. Instead when gas cools on to a disc, it is able to form in almost arbitrary orientation, including off the main planes of the halo (but avoiding an orientation perpendicular to the halo's intermediate axis). Substructure helps gasless galaxies reach alignment with the halo faster, but has less effect on galaxies when gas is cooling on to the disc. Our results provide a novel and natural interpretation for why red, gas poor galaxies are preferentially aligned with their halo, while blue, star-forming, galaxies have nearly random orientations, without requiring a connection between galaxies' current star formation rate and their merger history
The role of spin in the formation and evolution of galaxies
Using the SDSS spectroscopic sample, we estimate the dark matter halo spin
parameter lambda for ~53,000 disk galaxies for which MOPED star formation
histories are available. We investigate the relationship between spin and total
stellar mass, star formation history, and environment. First, we find a clear
anti-correlation between stellar mass and spin, with low mass galaxies
generally having high dark matter spins. Second, galaxies which have formed
more than ~5% of their stars in the last 0.2 Gyr have more broadly distributed
and typically higher spins (including a significant fraction with lambda > 0.1)
than galaxies which formed a large fraction of their stars more than 10 Gyr
ago. Finally, we find little or no correlation between the value of spin of the
dark halo and environment as determined both by proximity to a new cluster
catalog and a marked correlation study. This agrees well with the predictions
from linear hierarchical torquing theory and numerical simulations.Comment: Accepted to MNRAS after moderate revisio
Predicting hospital mortality among frequently readmitted patients: HSMR biased by readmission
<p>Abstract</p> <p>Background</p> <p>Casemix adjusted in-hospital mortality is one of the measures used to improve quality of care. The adjustment currently used does not take into account the effects of readmission, because reliable data on readmission is not readily available through routinely collected databases. We have studied the impact of readmissions by linking admissions of the same patient, and as a result were able to compare hospital mortality among frequently, as opposed to, non-frequently readmitted patients. We also formulated a method to adjust for readmission for the calculation of hospital standardised mortality ratios (HSMRs).</p> <p>Methods</p> <p>We conducted a longitudinal retrospective analysis of routinely collected hospital data of six large non-university teaching hospitals in the Netherlands with casemix adjusted standardised mortality ratios ranging from 65 to 114 and a combined value of 93 over a five-year period. Participants concerned 240662 patients admitted 418566 times in total during the years 2003 - 2007. Predicted deaths by the HSMR model 2008 over a five-year period were compared with observed deaths.</p> <p>Results</p> <p>Numbers of readmissions per patient differ substantially between the six hospitals, up to a factor of 2. A large interaction was found between numbers of admissions per patient and HSMR-predicted risks. Observed deaths for frequently admitted patients were significantly lower than HSMR-predicted deaths, which could be explained by uncorrected factors surrounding readmissions.</p> <p>Conclusions</p> <p>Patients admitted more frequently show lower risks of dying on average per admission. This decline in risk is only partly detected by the current HSMR. Comparing frequently admitted patients to non-frequently admitted patients commits the constant risk fallacy and potentially lowers HSMRs of hospitals treating many frequently admitted patients and increases HSMRs of hospitals treating many non-frequently admitted patients. This misleading effect can only be demonstrated by an analysis over a prolonged period, but occurs, in effect, every day of the year. This finding is relevant for all countries where hospitals use HSMR for monitoring and improving hospital performance. The use of 'admission frequency' as additional adjustment variable may provide a more accurate HSMR.</p
Merger and Ring Galaxy Formation Rates at z<=2
We compare the observed merger rate of galaxies over cosmic time and the
frequency of collisional ring galaxies (CRGs), with analytic models and halo
merger and collision rates from a large cosmological simulation. In the Lambda
cold dark matter (LCDM) model we find that the cosmic {\it merger fraction}
does not evolve strongly between 0.2<z<2, implying that the observed decrease
of the cosmic star formation rate since z~1 might not be tied to a disappearing
population of major mergers. Halos hosting massive galaxies undergo on average
~2 mergers from z~2 up to present day, reflecting the late assembly time for
the massive systems and the related downsizing problem. The cosmic {\it merger
rate} declines with redshift: at the present time it is a factor of 10 lower
than at z~2, in reasonable agreement with the current available data. The rate
of CRG formation derived from the interactions between halo progenitors up to
z=2 is found to be a good tracer of the cosmic merger rate. In the LCDM model
the rate of CRGs as well as the merger rate do not scale as (1+z)^m, as
suggested by previous models. Our predictions of cosmic merger and CRG rates
may be applied to forthcoming surveys such as GOODS and zCOSMOS.Comment: Accepted for a publication on MNRAS. More references added and a
comparison with previous work
Do High-Velocity Clouds trace the Dark Matter subhalo population?
Within the cosmological concordance model, Cold Dark Matter (CDM) subhalos
form the building blocks which merge hierarchically to more massive galaxies.
Since intergalactic gas is accreted by massive galaxies, observable e.g. as
high- velocity clouds (HVCs) around the Milky Way, with extremely low
metallicities, these can be suggested to represent the baryonic content of
primordial Dark Matter (DM) subhalos. Another possibility of their origin is
that they stem from disrupted satellite galaxies, but in this case, these gas
clouds move unaccompanied by a bound DM structure. Since HVCs are observed with
long gas tails and with irregular substructures, numerical models are performed
aiming at exploring their structure and compare them with observations. If HVCs
are engulfed by DM subhalos, their gas must leave the DM gravitational
potential and reflect this in their dynamics. On the other hand, the evolution
and survival of pure gas models must be tested to distinguish between
DM-dominated and DM-free clouds and to allow conclusions on their origin. The
models demonstrate that purely baryonic HVCs with low masses are disrupted by
ram-pressure stripping and Kelvin-Helmholtz instabilities, while more massive
ones survive, losing their initially spherical shape and develop significant
substructures including cometary elongations in the column density distribution
("head-tail structure"). On the contrary, HVCs with DM subhalos survive with
more than 90% of their gas mass still bound and spherically shaped, approaching
the Galactic disk like bullets. In addition, we find that velocity gradients
along the cometary head-tail structures does not necessarily offer a
possibility to distinguish between DM-dominated and purely gaseous HVCs.
Comparison of models with observations let us conclude that HVCs are not
embedded in a DM substructure and do not trace the cosmological subhalo
population.Comment: Accepted for publication in A&
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