786 research outputs found
Baryon fractions in clusters of galaxies: evidence against a preheating model for entropy generation
The Millennium Gas project aims to undertake smoothed-particle hydrodynamic
resimulations of the Millennium Simulation, providing many hundred massive
galaxy clusters for comparison with X-ray surveys (170 clusters with kTsl > 3
keV). This paper looks at the hot gas and stellar fractions of clusters in
simulations with different physical heating mechanisms. These fail to reproduce
cool-core systems but are successful in matching the hot gas profiles of
non-cool-core clusters. Although there is immense scatter in the observational
data, the simulated clusters broadly match the integrated gas fractions within
r500 . In line with previous work, however, they fare much less well when
compared to the stellar fractions, having a dependence on cluster mass that is
much weaker than is observed. The evolution with redshift of the hot gas
fraction is much larger in the simulation with early preheating than in one
with continual feedback; observations favour the latter model. The strong
dependence of hot gas fraction on cluster physics limits its use as a probe of
cosmological parameters.Comment: 16 pages, 18 figures, 4 tables. Accepted for publication in MNRA
The Impact of Cooling and Feedback on Disc Galaxies
We present detailed, analytical models for the formation of disc galaxies to
investigate the impact that cooling and feedback have on their structural
properties. In particular, we investigate which observables extracted directly
from the models are best suited as virial mass estimators, and to what extent
they allow the recovery of the model input parameters regarding the feedback
and cooling efficiencies. Contrary to naive expectations, the luminosities and
circular velocities of disc galaxies are extremely poor indicators of total
virial mass. Instead, we show that the product of disc scale length and
rotation velocity squared yields a much more robust estimate. We show that
feedback can cause a narrow correlation between galaxy mass fraction and halo
spin parameter, similar to that found recently by van den Bosch, Burkert and
Swaters from an analysis of dwarf galaxy rotation curves. Finally we
investigate the impact that cooling and feedback have on the colors,
metallicities, star formation histories and Tully-Fisher relation of disc
galaxies.Comment: 20 pages, 12 figures. To be published in MNRA
Gravitational Quenching in Massive Galaxies and Clusters by Clumpy Accretion
We consider a simple gravitational-heating mechanism for the long-term
quenching of cooling flows and star formation in massive dark-matter haloes
hosting ellipticals and clusters. The virial shock heating in haloes >10^12 Mo
triggers quenching in 10^12-13 Mo haloes (Birnboim, Dekel & Neistein 2007). We
show that the long-term quenching in haloes >Mmin~7x10^12 Mo could be due to
the gravitational energy of cosmological accretion delivered to the inner-halo
hot gas by cold flows via ram-pressure drag and local shocks. Mmin is obtained
by comparing the gravitational power of infall into the potential well with the
overall radiative cooling rate. The heating wins if the gas inner density cusp
is not steeper than r^-0.5 and if the masses in the cold and hot phases are
comparable. The effect is stronger at higher redshifts, making the maintenance
easier also at later times. Clumps >10^5 Mo penetrate to the inner halo with
sufficient kinetic energy before they disintegrate, but they have to be <10^8
Mo for the drag to do enough work in a Hubble time. Pressure confined ~10^4K
clumps are stable against their own gravity and remain gaseous once below the
Bonnor-Ebert mass ~10^8 Mo. They are also immune to tidal disruption. Clumps in
the desired mass range could emerge by thermal instability in the outer halo if
the conductivity is not too high. Alternatively, such clumps may be embedded in
dark-matter subhaloes if the ionizing flux is ineffective, but they separate
from their subhaloes by ram pressure before entering the inner halo. Heating by
dynamical friction becomes dominant for massive satellites, which can
contribute up to one third of the total gravitational heating. We conclude that
gravitational heating by cosmological accretion is a viable alternative to AGN
feedback as a long-term quenching mechanism.Comment: 24 pages, 20 figures, some improvements, MNRAS accepted versio
Gravitational Quenching in Massive Galaxies and Clusters by Clumpy Accretion
We consider a simple gravitational-heating mechanism for the long-term
quenching of cooling flows and star formation in massive dark-matter haloes
hosting ellipticals and clusters. The virial shock heating in haloes >10^12 Mo
triggers quenching in 10^12-13 Mo haloes (Birnboim, Dekel & Neistein 2007). We
show that the long-term quenching in haloes >Mmin~7x10^12 Mo could be due to
the gravitational energy of cosmological accretion delivered to the inner-halo
hot gas by cold flows via ram-pressure drag and local shocks. Mmin is obtained
by comparing the gravitational power of infall into the potential well with the
overall radiative cooling rate. The heating wins if the gas inner density cusp
is not steeper than r^-0.5 and if the masses in the cold and hot phases are
comparable. The effect is stronger at higher redshifts, making the maintenance
easier also at later times. Clumps >10^5 Mo penetrate to the inner halo with
sufficient kinetic energy before they disintegrate, but they have to be <10^8
Mo for the drag to do enough work in a Hubble time. Pressure confined ~10^4K
clumps are stable against their own gravity and remain gaseous once below the
Bonnor-Ebert mass ~10^8 Mo. They are also immune to tidal disruption. Clumps in
the desired mass range could emerge by thermal instability in the outer halo if
the conductivity is not too high. Alternatively, such clumps may be embedded in
dark-matter subhaloes if the ionizing flux is ineffective, but they separate
from their subhaloes by ram pressure before entering the inner halo. Heating by
dynamical friction becomes dominant for massive satellites, which can
contribute up to one third of the total gravitational heating. We conclude that
gravitational heating by cosmological accretion is a viable alternative to AGN
feedback as a long-term quenching mechanism.Comment: 24 pages, 20 figures, some improvements, MNRAS accepted versio
On the relation between the Schmidt and Kennicutt-Schmidt star formation laws and its implications for numerical simulations
When averaged over large scales, star formation in galaxies is observed to
follow the empirical Kennicutt-Schmidt (KS) law for surface densities above a
constant threshold. While the observed law involves surface densities,
theoretical models and simulations generally work with volume density laws
(i.e. Schmidt laws). We derive analytic relations between star formation laws
expressed in terms of surface densities, volume densities, and pressures and we
show how these relations depend on parameters such as the effective equation of
state of the multiphase interstellar medium. Our analytic relations enable us
to implement observed surface density laws into simulations. Because the
parameters of our prescription for star formation are observables, we are not
free to tune them to match the observations. We test our theoretical framework
using high-resolution simulations of isolated disc galaxies that assume an
effective equation of state for the multiphase interstellar medium. We are able
to reproduce the star formation threshold and both the slope and the
normalisation of arbitrary input KS laws without tuning any parameters and with
very little scatter, even for unstable galaxies and even if we use poor
numerical resolution. Moreover, we can do so for arbitrary effective equations
of state. Our prescription therefore enables simulations of galaxies to bypass
our current inability to simulate the formation of stars. On the other hand,
the fact that we can reproduce arbitrary input thresholds and KS laws, rather
than just the particular ones picked out by nature, indicates that simulations
that lack the physics and/or resolution to simulate the multiphase interstellar
medium can only provide limited insight into the origin of the observed star
formation laws.Comment: Accepted for publication in MNRAS, 14 pages and 9 figures. Minor
change
Weak Lensing by Galaxies in Groups and Clusters: I.--Theoretical Expectations
Galaxy-galaxy lensing is rapidly becoming one of the most promising means to
accurately measure the average relation between galaxy properties and halo
mass. In order to obtain a signal of sufficient signal-to-noise, one needs to
stack many lens galaxies according to their property of interest, such as
luminosity or stellar mass. Since such a stack consists of both central and
satellite galaxies, which contribute very different lensing signals, the
resulting shear measurements can be difficult to interpret. In the past,
galaxy-galaxy lensing studies have either completely ignored this problem, have
applied rough isolation criteria in an attempt to preferentially select
`central' galaxies, or have tried to model the contribution of satellites
explicitely. However, if one is able to {\it a priori} split the galaxy
population in central and satellite galaxies, one can measure their lensing
signals separately. This not only allows a much cleaner measurement of the
relation between halo mass and their galaxy populations, but also allows a
direct measurement of the sub-halo masses around satellite galaxies. In this
paper, we use a realistic mock galaxy redshift survey to show that galaxy
groups, properly selected from large galaxy surveys, can be used to accurately
split the galaxy population in centrals and satellites. Stacking the resulting
centrals according to their group mass, estimated from the total group
luminosity, allows a remarkably accurate recovery of the masses and density
profiles of their host haloes. In addition, stacking the corresponding
satellite galaxies according to their projected distance from the group center
yields a lensing signal that can be used to accurate measure the masses of both
sub-haloes and host haloes. (Abridged)Comment: 16 pages, 10 figures, Accepted for publication in MNRA
Feedback and the Structure of Simulated Galaxies at redshift z=2
We study the properties of simulated high-redshift galaxies using
cosmological N-body/gasdynamical runs from the OverWhelmingly Large Simulations
(OWLS) project. The runs contrast several feedback implementations of varying
effectiveness: from no-feedback, to supernova-driven winds to powerful
AGN-driven outflows. These different feedback models result in large variations
in the abundance and structural properties of bright galaxies at z=2. We find
that feedback affects the baryonic mass of a galaxy much more severely than its
spin, which is on average roughly half that of its surrounding dark matter halo
in our runs. Feedback induces strong correlations between angular momentum
content and galaxy mass that leave their imprint on galaxy scaling relations
and morphologies. Encouragingly, we find that galaxy disks are common in
moderate-feedback runs, making up typically ~50% of all galaxies at the centers
of haloes with virial mass exceeding 1e11 M_sun. The size, stellar masses, and
circular speeds of simulated galaxies formed in such runs have properties that
straddle those of large star-forming disks and of compact early-type galaxies
at z=2. Once the detailed abundance and structural properties of these rare
objects are well established it may be possible to use them to gauge the
overall efficacy of feedback in the formation of high redshift galaxies.Comment: 16 pages, 12 figures. Accepted for publication in MNRAS. Minor
changes to match published versio
A mass-dependent density profile for dark matter haloes including the influence of galaxy formation
We introduce a mass-dependent density profile to describe the distribution of dark matter within galaxies, which takes into account the stellar-to-halo mass dependence of the response of dark matter to baryonic processes. The study is based on the analysis of hydrodynamically simulated galaxies from dwarf to Milky Way mass, drawn from the Making Galaxies In a Cosmological Context project, which have been shown to match a wide range of disc scaling relationships. We find that the best-fitting parameters of a generic double power-law density profile vary in a systematic manner that depends on the stellar-to-halo mass ratio of each galaxy. Thus, the quantity M⋆/Mhalo constrains the inner (γ) and outer (β) slopes of dark matter density, and the sharpness of transition between the slopes (α), reducing the number of free parameters of the model to two. Due to the tight relation between stellar mass and halo mass, either of these quantities is sufficient to describe the dark matter halo profile including the effects of baryons. The concentration of the haloes in the hydrodynamical simulations is consistent with N-body expectations up to Milky Way-mass galaxies, at which mass the haloes become twice as concentrated as compared with pure dark matter runs. This mass-dependent density profile can be directly applied to rotation curve data of observed galaxies and to semi-analytic galaxy formation models as a significant improvement over the commonly used NFW profile
The dependence of dark matter profiles on the stellar-to-halo mass ratio: a prediction for cusps versus cores
We use a suite of 31 simulated galaxies drawn from the MaGICC project to investigate the effects of baryonic feedback on the density profiles of dark matter haloes. The sample covers a wide mass range: 9.4×109 <Mhalo/M� <7.8×1011, hosting galaxies with stellarmasses in the range 5.0×105 <M∗/M� < 8.3×1010, i.e. from dwarf to L∗. The galaxies are simulated with blastwave supernova feedback and, for some of them, an additional source of energy from massive stars is included. Within this feedback scheme we vary several parameters, such as the initial mass function, the density threshold for star formation, and energy from supernovae and massive stars. The main result is a clear dependence of the inner slope of the dark matter density profile, α in ρ ∝ rα, on the stellar-to-halo mass ratio, M∗/Mhalo. This relation is independent of the particular choice of parameters within our stellar feedback scheme, allowing a prediction for cusp versus core formation. When M∗/Mhalo is low, �0.01 per cent, energy from stellar feedback is insufficient to significantly alter the inner dark matter density, and the galaxy retains a cuspy profile. At higher stellar-to-halo mass ratios, feedback drives the expansion of the dark matter and generates cored profiles. The flattest profiles form where M∗/Mhalo ∼ 0.5 per cent. Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose the supernova-driven expansion process, resulting in cuspier profiles. Combining the dependence of α on M∗/Mhalo with the empirical abundance matching relation between M∗ and Mhalo provides a prediction for how α varies as a function of stellar mass. Further, using the Tully–Fisher relation allows a prediction for the dependence of the dark matter inner slope on the observed rotation velocity of galaxies. The most cored galaxies are expected to have Vrot ∼ 50 km s−1, with α decreasing for more massive disc galaxies: spirals with Vrot ∼ 150 km s−1 have central slopes α ≤−0.8, approaching again the Navarro–Frenk–White profile. This novel prediction for the dependence of α on disc galaxy mass can be tested using observational data sets and can be applied to theoretical modelling of mass profiles and populations of disc galaxies
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