22,063 research outputs found
Photometric and clustering properties of hydrodynamical galaxies in a cosmological volume: results at z=0
In this work, we present results for the photometric and clustering
properties of galaxies that arise in a LambdaCDM hydrodynamical simulation of
the local universe. The present-day distribution of matter was constructed to
match the observed large scale pattern of the IRAS 1.2-Jy galaxy survey. Our
simulation follows the formation and evolution of galaxies in a cosmological
sphere with a volume of ~130^3 (Mpc/h)^3 including supernova feedback, galactic
winds, photoheating due to an uniform meta-galactic background and chemical
enrichment of the gas and stellar populations. However, we do not consider
AGNs. In the simulation, a total of ~20000 galaxies are formed above the
resolution limit, and around 60 haloes are more massive than ~10^14 M_sun.
Luminosities of the galaxies are calculated based on a stellar population
synthesis model including the attenuation by dust, which is calculated from the
cold gas left within the simulated galaxies. Environmental effects like colour
bi-modality and differential clustering power of the hydrodynamical galaxies
are qualitatively similar to observed trends. Nevertheless, the overcooling
present in the simulations lead to too blue and overluminous brightest cluster
galaxies (BCGs). To overcome this, we mimic the late-time suppression of star
formation in massive halos by ignoring recently formed stars with the aid of a
simple post-processing recipe. In this way we find luminosity functions, both
for field and group/cluster galaxies, in better agreement with observations.
Specifically, the BCGs then follow the observed luminosity-halo mass relation.
However, in such a case, the colour bi-modality is basically lost, pointing
towards a more complex interplay of late suppression of star formation than
what is given by the simple scheme adopted.Comment: Typos corrected. Replaced to match published version. 12 pages, 12
figures. To appear in MNRA
Stellar irradiated discs and implications on migration of embedded planets I: equilibrium discs
The strength and direction of migration of low mass planets depends on the
disc's thermodynamics. In discs where the viscous heating is balanced by
radiative transport, the migration can be directed outwards, a process which
extends the lifetime of growing planetary embryos. We investigate the influence
of opacity and stellar irradiation on the disc thermodynamics. Utilizing the
resulting disc structure, we determine the regions of outward migration. We
perform two-dimensional numerical simulations of equilibrium discs with viscous
heating, radiative cooling and stellar irradiation. We use the hydrodynamical
code NIRVANA that includes a full tensor viscosity and stellar irradiation, as
well as a two temperature solver that includes radiation transport in the
flux-limited diffusion approximation. The migration is studied by using torque
formulae. In the constant opacity case, we reproduce the analytical results of
a black-body disc: the stellar irradiation dominates in the outer regions --
leading to flaring -- while the viscous heating dominates close to the star. We
find that the inner edge of the disc should not be significantly puffed-up by
the stellar irradiation. If the opacity depends on the local density and
temperature, the structure of the disc is different, and several bumps in the
aspect ratio H/r appear, due to transitions between different opacity regimes.
The bumps in the disc can shield the outer disc from stellar irradiation.
Stellar irradiation is an important factor for determining the disc structure
and has dramatic consequences for the migration of embedded planets. Compared
to discs with only viscous heating, a stellar irradiated disc features a much
smaller region of outward migration for a range of planetary masses. This
suggests that the region where the formation of giant planet cores takes place
is smaller, which in turn might lead to a shorter growth phase
Physical origin of the large-scale conformity in the specific star formation rates of galaxies
Two explanations have been put forward to explain the observed conformity
between the colours and specific star formation rates (SFR/) of galaxies
on large scales: 1) the formation times of their surrounding dark matter halos
are correlated (commonly referred to as "assembly bias"), 2) gas is heated over
large scales at early times, leading to coherent modulation of cooling and star
formation between well-separated galaxies (commonly referred to as
"pre-heating") . To distinguish between the pre-heating and assembly bias
scenarios, we search for relics of energetic feedback events in the
neighbourhood of central galaxies with different specific star formation rates.
We find a significant excess of very high mass () galaxies out
to a distance of 2.5 Mpc around low SFR/ central galaxies compared to
control samples of higher SFR/ central galaxies with the same stellar mass
and redshift. We also find that very massive galaxies in the neighbourhood of
low SFR/ galaxies have much higher probability of hosting radio loud
active galactic nuclei. The radio-loud AGN fraction in neighbours with is four times higher around passive, non star-forming centrals at
projected distances of 1 Mpc and two times higher at projected distances of 4
Mpc. Finally, we carry out an investigation of conformity effects in the
recently publicly-released Illustris cosmological hydrodynamical simulation,
which includes energetic input both from quasars and from radio mode accretion
onto black holes. We do not find conformity effects of comparable amplitude on
large scales in the simulations and we propose that gas needs to be pushed out
of dark matter halos more efficiently at high redshifts.Comment: 8 pages, 8 figures, submitted to MNRA
Constraining stellar assembly and AGN feedback at the peak epoch of star formation
We study stellar assembly and feedback from active galactic nuclei (AGN)
around the epoch of peak star formation (1<z<2), by comparing hydrodynamic
simulations to rest-frame UV-optical galaxy colours from the Wide Field Camera
3 (WFC3) Early-Release Science (ERS) Programme. Our Adaptive Mesh Refinement
simulations include metal-dependent radiative cooling, star formation, kinetic
outflows due to supernova explosions, and feedback from supermassive black
holes. Our model assumes that when gas accretes onto black holes, a fraction of
the energy is used to form either thermal winds or sub-relativistic
momentum-imparting collimated jets, depending on the accretion rate. We find
that the predicted rest-frame UV-optical colours of galaxies in the model that
includes AGN feedback is in broad agreement with the observed colours of the
WFC3 ERS sample at 1<z<2. The predicted number of massive galaxies also matches
well with observations in this redshift range. However, the massive galaxies
are predicted to show higher levels of residual star formation activity than
the observational estimates, suggesting the need for further suppression of
star formation without significantly altering the stellar mass function. We
discuss possible improvements, involving faster stellar assembly through
enhanced star formation during galaxy mergers while star formation at the peak
epoch is still modulated by the AGN feedback.Comment: 6 pages, 4 figures, accepted for publication in MNRAS Letter
Radiative Transfer on Perturbations in Protoplanetary Disks
We present a method for calculating the radiative tranfer on a protoplanetary
disk perturbed by a protoplanet. We apply this method to determine the effect
on the temperature structure within the photosphere of a passive circumstellar
disk in the vicinity of a small protoplanet of up to 20 Earth masses. The
gravitational potential of a protoplanet induces a compression of the disk
material near it, resulting in a decrement in the density at the disk's
surface. Thus, an isodensity contour at the height of the photosphere takes on
the shape of a well. When such a well is illuminated by stellar irradiation at
grazing incidence, it results in cooling in a shadowed region and heating in an
exposed region. For typical stellar and disk parameters relevant to the epoch
of planet formation, we find that the temperature variation due to a
protoplanet at 1 AU separation from its parent star is about 4% (5 K) for a
planet of 1 Earth mass, about 14% (19 K) for planet of 10 Earth masses, and
about 18% (25 K) for planet of 20 Earth masses, We conclude that even such
relatively small protoplanets can induce temperature variations in a passive
disk. Therefore, many of the processes involved in planet formation should not
be modeled with a locally isothermal equation of state.Comment: 23 pages, 8 figures (including 3 color figs). Submitted to Ap
Planet Shadows in Protoplanetary Disks. I: Temperature Perturbations
Planets embedded in optically thick passive accretion disks are expected to
produce perturbations in the density and temperature structure of the disk. We
calculate the magnitudes of these perturbations for a range of planet masses
and distances. The model predicts the formation of a shadow at the position of
the planet paired with a brightening just beyond the shadow. We improve on
previous work on the subject by self-consistently calculating the temperature
and density structures under the assumption of hydrostatic equilibrium and
taking the full three-dimensional shape of the disk into account rather than
assuming a plane-parallel disk. While the excursion in temperatures is less
than in previous models, the spatial size of the perturbation is larger. We
demonstrate that a self-consistent calculation of the density and temperature
structure of the disk has a large effect on the disk model. In addition, the
temperature structure in the disk is highly sensitive to the angle of incidence
of stellar irradition at the surface, so accurately calculating the shape of
the disk surface is crucial for modeling the thermal structure of the disk.Comment: 14 pages, 14 figures. To appear in Ap
Dynamics of Circumstellar Disks II: Heating and Cooling
We present a series of 2-d () hydrodynamic simulations of marginally
self gravitating disks around protostars using an SPH code. We implement simple
dynamical heating and we cool each location as a black body, using a
photosphere temperature obtained from the local vertical structure. We
synthesize SEDs from our simulations and compare them to fiducial SEDs derived
from observed systems. These simulations produce less distinct spiral structure
than isothermally evolved systems, especially in the inner third of the disk.
Pattern are similar further from the star but do not collapse into condensed
objects. The photosphere temperature is well fit to a power law in radius with
index , which is very steep. Far from the star, internal heating
( work and shocks) are not responsible for generating a large fraction of
the thermal energy contained in the disk matter. Gravitational torques
responsible for such shocks cannot transport mass and angular momentum
efficiently in the outer disk. Within 5--10 AU of the star, rapid break
up and reformation of spiral structure causes shocks, which provide sufficient
dissipation to power a larger fraction of the near IR energy output. The
spatial and size distribution of grains can have marked consequences on the
observed near IR SED and can lead to increased emission and variability on
year time scales. When grains are vaporized they do not reform
into a size distribution similar to that from which most opacity calculations
are based. With rapid grain reformation into the original size distribution,
the disk does not emit near infrared photons. With a plausible modification to
the opacity, it contributes much more.Comment: Accepted by ApJ, 60pg incl 24 figure
- âŠ