946 research outputs found
The impact of mechanical AGN feedback on the formation of massive early-type galaxies
We employ cosmological hydrodynamical simulations to investigate the effects
of AGN feedback on the formation of massive galaxies with present-day stellar
masses of . Using
smoothed particle hydrodynamics simulations with a pressure-entropy formulation
that allows an improved treatment of contact discontinuities and fluid mixing,
we run three sets of simulations of 20 halos with different AGN feedback
models: (1) no feedback, (2) thermal feedback, and (3) mechanical and radiation
feedback. We assume that seed black holes are present at early cosmic epochs at
the centre of emerging dark matter halos and trace their mass growth via gas
accretion and mergers with other black holes. Both feedback models successfully
recover the observed M_BH - sigma relation and black hole-to-stellar mass ratio
for simulated central early-type galaxies. The baryonic conversion efficiencies
are reduced by a factor of two compared to models without any AGN feedback at
all halo masses. However, massive galaxies simulated with thermal AGN feedback
show a factor of ~10-100 higher X-ray luminosities than observed. The
mechanical/radiation feedback model reproduces the observed correlation between
X-ray luminosities and velocity dispersion, e.g. for galaxies with sigma = 200
km/s, the X-ray luminosity is reduced from erg/s to erg/s.
It also efficiently suppresses late time star formation, reducing the specific
star formation rate from to on
average and resulting in quiescent galaxies since z=2, whereas the thermal
feedback model shows higher late time in-situ star formation rates than
observed.Comment: 13 pages, 11 figures, accepted for the publication in MNRA
An invariance in the kronig-kramers' relation
Invariance in Kronig-Kramers relation to establish some representations used in analysis of relaxation dispersion
SPHGal: Smoothed Particle Hydrodynamics with improved accuracy for Galaxy simulations
We present the smoothed-particle hydrodynamics implementation SPHGal, which
combines some recently proposed improvements in GADGET. This includes a
pressure-entropy formulation with a Wendland kernel, a higher order estimate of
velocity gradients, a modified artificial viscosity switch with a modified
strong limiter, and artificial conduction of thermal energy. With a series of
idealized hydrodynamic tests we show that the pressure-entropy formulation is
ideal for resolving fluid mixing at contact discontinuities but performs
conspicuously worse at strong shocks due to the large entropy discontinuities.
Including artificial conduction at shocks greatly improves the results. In
simulations of Milky Way like disk galaxies a feedback-induced instability
develops if too much artificial viscosity is introduced. Our modified
artificial viscosity scheme prevents this instability and shows efficient shock
capturing capability. We also investigate the star formation rate and the
galactic outflow. The star formation rates vary slightly for different SPH
schemes while the mass loading is sensitive to the SPH scheme and significantly
reduced in our favored implementation. We compare the accretion behavior of the
hot halo gas. The formation of cold blobs, an artifact of simple SPH
implementations, can be eliminated efficiently with proper fluid mixing, either
by conduction and/or by using a pressure-entropy formulation.Comment: Replaced with the version accepted by MNRA
The STACEE-32 Ground Based Gamma-ray Detector
We describe the design and performance of the Solar Tower Atmospheric
Cherenkov Effect Experiment detector in its initial configuration (STACEE-32).
STACEE is a new ground-based gamma ray detector using the atmospheric Cherenkov
technique. In STACEE, the heliostats of a solar energy research array are used
to collect and focus the Cherenkov photons produced in gamma-ray induced air
showers. The large Cherenkov photon collection area of STACEE results in a
gamma-ray energy threshold below that of previous detectors.Comment: 45 pages, 25 figures, Accepted for publication in Nuclear Instruments
and Methods
Shallow Dark Matter Cusps in Galaxy Clusters
We study the evolution of the stellar and dark matter components in a galaxy
cluster of from to the present epoch using
the high-resolution collisionless simulations of Ruszkowski & Springel (2009).
At the dominant progenitor halos were populated with spherical model
galaxies with and without accounting for adiabatic contraction. We apply a
weighting scheme which allows us to change the relative amount of dark and
stellar material assigned to each simulation particle in order to produce
luminous properties which agree better with abundance matching arguments and
observed bulge sizes at . This permits the study of the effect of initial
compactness on the evolution of the mass-size relation. We find that for more
compact initial stellar distributions the size of the final Brightest Cluster
Galaxy grows with mass according to , whereas for more extended
initial distributions, . Our results show that collisionless
mergers in a cosmological context can reduce the strength of inner dark matter
cusps with changes in logarithmic slope of 0.3 to 0.5 at fixed radius. Shallow
cusps such as those found recently in several strong lensing clusters thus do
not necessarily conflict with CDM, but may rather reflect on the initial
structure of the progenitor galaxies, which was shaped at high redshift by
their formation process.Comment: 8 pages, 4 figures, submitted to MNRA
The ATLAS3D project - XXV: Two-dimensional kinematic analysis of simulated galaxies and the cosmological origin of fast and slow rotators
We present a detailed two-dimensional stellar dynamical analysis of as ample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 x 1010Msun âŒâ€ Mstar âŒâ€ 6x 1011Msun. Kinematic maps of the stellar line-of-sight velocity, velocity dispersion, and higher-order Gauss-Hermite moments h3 and h4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the λR-parameter. The velocity, velocity dispersion, h3, and h4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS3D survey. This includes fast (regular), slow, and misaligned rotation, hot spheroids with embedded cold disk components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant in-situ formation of stars, or with additional gas-rich major mergers - resulting in a spin-up - in their formation history, form elongated fast rotators with a clear anti-correlation of h3 and v/Ï. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants. This formation path does not result in anti-correlated h3 and v/Ï. The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in-situ star formation since z âŒÂ 2, rotate slower and have older stellar populations. (shortened)PostprintPeer reviewe
Constraints on the relationship between stellar mass and halo mass at low and high redshift
We use a statistical approach to determine the relationship between the
stellar masses of galaxies and the masses of the dark matter halos in which
they reside. We obtain a parameterized stellar-to-halo mass (SHM) relation by
populating halos and subhalos in an N-body simulation with galaxies and
requiring that the observed stellar mass function be reproduced. We find good
agreement with constraints from galaxy-galaxy lensing and predictions of
semi-analytic models. Using this mapping, and the positions of the halos and
subhalos obtained from the simulation, we find that our model predictions for
the galaxy two-point correlation function (CF) as a function of stellar mass
are in excellent agreement with the observed clustering properties in the SDSS
at z=0. We show that the clustering data do not provide additional strong
constraints on the SHM function and conclude that our model can therefore
predict clustering as a function of stellar mass. We compute the conditional
mass function, which yields the average number of galaxies with stellar masses
in the range [m, m+dm] that reside in a halo of mass M. We study the redshift
dependence of the SHM relation and show that, for low mass halos, the SHM ratio
is lower at higher redshift. The derived SHM relation is used to predict the
stellar mass dependent galaxy CF and bias at high redshift. Our model predicts
that not only are massive galaxies more biased than low mass ones at all
redshifts, but the bias increases more rapidly with increasing redshift for
massive galaxies than for low mass ones. We present convenient fitting
functions for the SHM relation as a function of redshift, the conditional mass
function, and the bias as a function of stellar mass and redshift.Comment: 21 pages, 17 figures, discussion enlarged, one more figure, updated
references, accepted for publication in Ap
Mrk 421, Mrk 501, and 1ES 1426+428 at 100 GeV with the CELESTE Cherenkov Telescope
We have measured the gamma-ray fluxes of the blazars Mrk 421 and Mrk 501 in
the energy range between 50 and 350 GeV (1.2 to 8.3 x 10^25 Hz). The detector,
called CELESTE, used first 40, then 53 heliostats of the former solar facility
"Themis" in the French Pyrenees to collect Cherenkov light generated in
atmospheric particle cascades. The signal from Mrk 421 is often strong. We
compare its flux with previously published multi-wavelength studies and infer
that we are straddling the high energy peak of the spectral energy
distribution. The signal from Mrk 501 in 2000 was weak (3.4 sigma). We obtain
an upper limit on the flux from 1ES 1426+428 of less than half that of the Crab
flux near 100 GeV. The data analysis and understanding of systematic biases
have improved compared to previous work, increasing the detector's sensitivity.Comment: 15 pages, 14 figures, accepted to A&A (July 2006) August 19 --
corrected error in author lis
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