408 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
Consequences of Mechanical and Radiative Feedback from Black Holes in Disc Galaxy Mergers
We study the effect of AGN mechanical and radiation feedback on the formation
of bulge dominated galaxies via mergers of disc galaxies. The merging galaxies
have mass-ratios of 1:1 to 6:1 and include pre-existing hot gaseous halos to
properly account for the global impact of AGN feedback. Using smoothed particle
hydrodynamics simulation code (GADGET-3) we compare three models with different
AGN feedback models: (1) no black hole and no AGN feedback; (2) thermal AGN
feedback; and (3) mechanical and radiative AGN feedback. The last model is
motivated by observations of broad line quasars which show winds with initial
velocities of 10,000 km/s and also heating associated with the
central AGN X-ray radiation. The primary changes in gas properties due to
mechanical AGN feedback are lower thermal X-ray luminosity from the final
galaxy - in better agreement with observations - and galactic outflows with
higher velocity km/s similar to recent direct observations of
nearby merger remnants. The kinetic energy of the outflowing gas is a factor of
20 higher than in the thermal feedback case. All merger remnants with
momentum-based AGN feedback with km/s and , independent of their progenitor mass-ratios, reproduce the
observed relations between stellar velocity dispersion and black hole mass
() as well as X-ray luminosity () with
erg/s erg/s for
velocity dispersions in the range of 120 km/s 190
km/s. In addition, the mechanical feedback produces a much greater AGN
variability. We also show that gas is more rapidly and impulsively stripped
from the galactic centres driving a moderate increase in galaxy size and
decrease in central density with the mechanical AGN feedback model.Comment: 16 pages, 10 figures, resubmitted to MNRA
A Cosmic Variance Cookbook
Deep pencil beam surveys (<1 deg^2) are of fundamental importance for
studying the high-redshift universe. However, inferences about galaxy
population properties are in practice limited by 'cosmic variance'. This is the
uncertainty in observational estimates of the number density of galaxies
arising from the underlying large-scale density fluctuations. This source of
uncertainty can be significant, especially for surveys which cover only small
areas and for massive high-redshift galaxies. Cosmic variance for a given
galaxy population can be determined using predictions from cold dark matter
theory and the galaxy bias. In this paper we provide tools for experiment
design and interpretation. For a given survey geometry we present the cosmic
variance of dark matter as a function of mean redshift z and redshift bin size
Dz. Using a halo occupation model to predict galaxy clustering, we derive the
galaxy bias as a function of mean redshift for galaxy samples of a given
stellar mass range. In the linear regime, the cosmic variance of these galaxy
samples is the product of the galaxy bias and the dark matter cosmic variance.
We present a simple recipe using a fitting function to compute cosmic variance
as a function of the angular dimensions of the field, z, Dz and stellar mass
m*. We also provide tabulated values and a software tool. We find that for
GOODS at z=2 and with Dz=0.5 the relative cosmic variance of galaxies with
m*>10^11 Msun is ~38%, while it is ~27% for GEMS and ~12% for COSMOS. For
galaxies of m*~10^10 Msun the relative cosmic variance is ~19% for GOODS, ~13%
for GEMS and ~6% for COSMOS. This implies that cosmic variance is a significant
source of uncertainty at z=2 for small fields and massive galaxies, while for
larger fields and intermediate mass galaxies cosmic variance is less serious.Comment: 8 pages, 4 figures, 5 tables, submitted to Ap
Magnetic buoyancy in simulated galactic discs with a realistic circum galactic medium
We present simulations of isolated disc galaxies in a realistic environment
performed with the Tree-SPMHD-Code Gadget-3. Our simulations include a
spherical circum-galactic medium (CGM) surrounding the galactic disc, motivated
by observations and the results of cosmological simulations. We present three
galactic models with different halo masses between 10e10 Msol and 10e12 Msol,
and for each we use two different approaches to seed the magnetic field, as
well as a control simulation without a magnetic field. We find that the
amplification of the magnetic field in the centre of the disc leads to a
biconical magnetic outflow of gas that magnetizes the CGM. This biconical
magnetic outflow reduces the star formation rate (SFR) of the galaxy by roughly
40 percent compared to the simulations without magnetic fields. As the key
aspect of our simulations, we find that small scale turbulent motion of the gas
in the disc leads to the amplification of the magnetic field up to tens of
10e-6 G, as long as the magnetic field strength is low. For stronger magnetic
fields turbulent motion does not lead to significant amplification but is
replaced by an alpha-omega dynamo. The occurance of a small scale turbulent
dynamo becomes apparent through the magnetic power spectrum and analysis of the
field lines' curvature. In accordance with recent observations we find an
anti-correlation between the spiral structure in the gas density and in the
magnetic field due to a diffusion term added to the induction equation.Comment: 22 pages, 16 figures, submitted to MNRA
Star formation in mergers with cosmologically motivated initial conditions
We use semi-analytic models and cosmological merger trees to provide the
initial conditions for multi-merger numerical hydrodynamic simulations, and
exploit these simulations to explore the effect of galaxy interaction and
merging on star formation (SF). We compute numerical realisations of twelve
merger trees from z=1.5 to z=0. We include the effects of the large hot gaseous
halo around all galaxies, following recent obervations and predictions of
galaxy formation models. We find that including the hot gaseous halo has a
number of important effects. Firstly, as expected, the star formation rate on
long timescales is increased due to cooling of the hot halo and refuelling of
the cold gas reservoir. Secondly, we find that interactions do not always
increase the SF in the long term. This is partially due to the orbiting
galaxies transferring gravitational energy to the hot gaseous haloes and
raising their temperature. Finally we find that the relative size of the
starburst, when including the hot halo, is much smaller than previous studies
showed. Our simulations also show that the order and timing of interactions are
important for the evolution of a galaxy. When multiple galaxies interact at the
same time, the SF enhancement is less than when galaxies interact in series.
All these effects show the importance of including hot gas and cosmologically
motivated merger trees in galaxy evolution models.Comment: 19 pages, 15 figures, 6 tables. Accepted for publication in MNRA
From Discs to Bulges: effect of mergers on the morphology of galaxies
We study the effect of mergers on the morphology of galaxies by means of the
simulated merger tree approach first proposed by Moster et al. This method
combines N-body cosmological simulations and semi-analytic techniques to
extract realistic initial conditions for galaxy mergers. These are then evolved
using high resolution hydrodynamical simulations, which include dark matter,
stars, cold gas in the disc and hot gas in the halo. We show that the satellite
mass accretion is not as effective as previously thought, as there is
substantial stellar stripping before the final merger. The fraction of stellar
disc mass transferred to the bulge is quite low, even in the case of a major
merger, mainly due to the dispersion of part of the stellar disc mass into the
halo. We confirm the findings of Hopkins et al., that a gas rich disc is able
to survive major mergers more efficiently. The enhanced star formation
associated with the merger is not localised to the bulge of galaxy, but a
substantial fraction takes place in the disc too. The inclusion of the hot gas
reservoir in the galaxy model contributes to reducing the efficiency of bulge
formation. Overall, our findings suggest that mergers are not as efficient as
previously thought in transforming discs into bulges. This possibly alleviates
some of the tensions between observations of bulgeless galaxies and the
hierarchical scenario for structure formation.Comment: MNRAS Accepted, 17 pages, 11 figures, 3 Table
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