419 research outputs found
Dwarf galaxies in voids: Suppressing star formation with photo-heating
We study structure formation in cosmological void regions using
high-resolution hydrodynamical simulations. Despite being significantly
underdense, voids are populated abundantly with small dark matter halos which
should appear as dwarf galaxies if their star formation is not suppressed
significantly. We here investigate to which extent the cosmological
UV-background photo-evaporates baryons out of halos of dwarf galaxies, and
thereby limits their cooling and star formation rates. Assuming a Haardt &
Madau UV-background with reionisation at redshift z=6, our samples of simulated
galaxies show that halos with masses below a characteristic mass of M_c(z=0) =
6.5 x 10^9 h^{-1} M_sun are baryon-poor, but in general not completely empty,
because baryons that are in the condensed cold phase or are already locked up
in stars resist evaporation. In halos with mass M < M_c, we find that
photo-heating suppresses further cooling of gas. The redshift and UV-background
dependent characteristic mass M_c(z) can be understood from the equilibrium
temperature between heating and cooling at a characteristic overdensity of
\delta ~ 1000. If a halo is massive enough to compress gas to this density
despite the presence of the UV background, gas is free to `enter' the condensed
phase and cooling continues in the halo, otherwise it stalls. By analysing the
mass accretion histories of dwarf galaxies in voids, we show that they can
build up a significant amount of condensed mass at early times before the epoch
of reionisation. Later on, the amount of mass in this phase remains roughly
constant, but the masses of the dark matter halos continue to increase.
(abridged)Comment: revised version as accepted by MNRAS, 15 pages, 15 figures, new
simulation results and a significantly extended discussion have been include
The consequences of a nearby supernova on the early Solar System
If the Sun was born in a relatively compact open cluster, it is quite likely
that a massive (10MSun) star was nearby when it exploded in a supernova. The
repercussions of a supernova can be rather profound, and the current Solar
System may still bear the memory of this traumatic event. The truncation of the
Kuiper belt and the tilt of the ecliptic plane with respect to the Sun's
rotation axis could be such signatures. We simulated the effect of a nearby
supernova on the young Solar System using the Astronomical Multipurpose
Software Environment. Our calculations are realized in two subsequent steps in
which we study the effect of the supernova irradiation on the circumstellar
disk and the effect of the impact of the nuclear blast-wave which arrives a few
decades later. We find that the blastwave of our adopted supernova exploding at
a distance of --\,pc and at an angle of -- with
respect to the angular-momentum axis of the circumsolar disk would induce a
misalignment between the Sun's equator and its disk to ,
consistent with the current value. The blast of a supernova truncates the disk
at a radius between and \,au, which is consistent with the current
edge of the Kuiper belt. For the most favored parameters, the irradiation by
the supernova as well as the blast wave heat the majority of the disk to \,K, which is sufficiently hot to melt chondrules in the circumstellar
disk. The majority of planetary system may have been affected by a nearby
supernova, some of its repercussions, such as truncation and tilting of the
disk, may still be visible in their current planetary system's topology. The
amount of material from the supernova blast wave that is accreted by the
circumstellar disk is too small by several orders of magnitude to explain the
current abundance of the short live radionuclide Al.Comment: Accepted for publication in A&
A high-temperature heat pump for compressed heat energy storage applications: Design, modeling, and performance
The current paper presents the design and performance of a high-temperature heat pump (HTHP) integrated in an innovative, sensible, and latent heat storage system. The HTHP has been designed to work between a heat source from 40 to 100 °C and a heat sink above 130 °C. An initial refrigerant analysis has revealed that R-1233zd(E) is the best candidate to meet the required performance and environmental considerations. The first part of this paper deals with the sizing and selection of the main components while discussing the challenges and working limits. A numerical model is also presented and validated. The second part of the paper is dedicated to develop parametric studies and performance maps under different operating conditions. The results show that the current HTHP, at a source temperature of 80 °C, consumes from 3.23 to 9.88 kW by varying the compressor’s speed from 500 to 1500 rpm. Heat production is achieved in the form of latent heat (7.40 to 21.59 kW) and sensible heat (from 6.35 to 17.94 kW). The heating coefficient of performance (COPHTHP) is around 4.This work has been partially funded by grant agreement No. 764042 (CHESTER project) of the European Union’s Horizon 2020 research and innovation program. The authors would like to express their deep gratitude to Prof. Dr. Jose Miguel Corberán Salvador for his perseverance, encouragement, and invaluable guidance during this work
Chaotic cold accretion onto black holes
Using 3D AMR simulations, linking the 50 kpc to the sub-pc scales over the
course of 40 Myr, we systematically relax the classic Bondi assumptions in a
typical galaxy hosting a SMBH. In the realistic scenario, where the hot gas is
cooling, while heated and stirred on large scales, the accretion rate is
boosted up to two orders of magnitude compared with the Bondi prediction. The
cause is the nonlinear growth of thermal instabilities, leading to the
condensation of cold clouds and filaments when t_cool/t_ff < 10. Subsonic
turbulence of just over 100 km/s (M > 0.2) induces the formation of thermal
instabilities, even in the absence of heating, while in the transonic regime
turbulent dissipation inhibits their growth (t_turb/t_cool < 1). When heating
restores global thermodynamic balance, the formation of the multiphase medium
is violent, and the mode of accretion is fully cold and chaotic. The recurrent
collisions and tidal forces between clouds, filaments and the central clumpy
torus promote angular momentum cancellation, hence boosting accretion. On
sub-pc scales the clouds are channelled to the very centre via a funnel. A good
approximation to the accretion rate is the cooling rate, which can be used as
subgrid model, physically reproducing the boost factor of 100 required by
cosmological simulations, while accounting for fluctuations. Chaotic cold
accretion may be common in many systems, such as hot galactic halos, groups,
and clusters, generating high-velocity clouds and strong variations of the AGN
luminosity and jet orientation. In this mode, the black hole can quickly react
to the state of the entire host galaxy, leading to efficient self-regulated AGN
feedback and the symbiotic Magorrian relation. During phases of overheating,
the hot mode becomes the single channel of accretion (with a different cuspy
temperature profile), though strongly suppressed by turbulence.Comment: Accepted by MNRAS: added comments and references. Your feedback is
welcom
Orbiting Circum-galactic Gas as a Signature of Cosmological Accretion
We use cosmological SPH simulations to study the kinematic signatures of cool
gas accretion onto a pair of well-resolved galaxy halos. Cold-flow streams and
gas-rich mergers produce a circum-galactic component of cool gas that generally
orbits with high angular momentum about the galaxy halo before falling in to
build the disk. This signature of cosmological accretion should be observable
using background-object absorption line studies as features that are offset
from the galaxy's systemic velocity by ~100 km/s. Accreted gas typically
co-rotates with the central disk in the form of a warped, extended cold flow
disk, such that the observed velocity offset is in the same direction as galaxy
rotation, appearing in sight lines that avoid the galactic poles. This
prediction provides a means to observationally distinguish accreted gas from
outflow gas: the accreted gas will show large one-sided velocity offsets in
absorption line studies while radial/bi-conical outflows will not (except
possibly in special polar projections). This rotation signature has already
been seen in studies of intermediate redshift galaxy-absorber pairs; we suggest
that these observations may be among the first to provide indirect
observational evidence for cold accretion onto galactic halos. Cold mode halo
gas typically has ~3-5 times more specific angular momentum than the dark
matter. The associated cold mode disk configurations are likely related to
extended HI/XUV disks seen around galaxies in the local universe. The fraction
of galaxies with extended cold flow disks and associated offset absorption-line
gas should decrease around bright galaxies at low redshift, as cold mode
accretion dies out.Comment: 15 pages, 9 figures, edited to match published version. Includes
expanded discussion, with primary results unchange
Modeling the Dust Properties of z ~ 6 Quasars with ART^2 -- All-wavelength Radiative Transfer with Adaptive Refinement Tree
The detection of large quantities of dust in z ~ 6 quasars by infrared and
radio surveys presents puzzles for the formation and evolution of dust in these
early systems. Previously (Li et al. 2007), we showed that luminous quasars at
z > 6 can form through hierarchical mergers of gas-rich galaxies. Here, we
calculate the dust properties of simulated quasars and their progenitors using
a three-dimensional Monte Carlo radiative transfer code, ART^2 --
All-wavelength Radiative Transfer with Adaptive Refinement Tree. ART^2
incorporates a radiative equilibrium algorithm for dust emission, an adaptive
grid for inhomogeneous density, a multiphase model for the ISM, and a
supernova-origin dust model. We reproduce the SED and dust properties of SDSS
J1148+5251, and find that the infrared emission are closely associated with the
formation and evolution of the quasar host. The system evolves from a cold to a
warm ULIRG owing to heating and feedback from stars and AGN. Furthermore, the
AGN has significant implications for the interpretation of observation of the
hosts. Our results suggest that vigorous star formation in merging progenitors
is necessary to reproduce the observed dust properties of z~6 quasars,
supporting a merger-driven origin for luminous quasars at high redshifts and
the starburst-to-quasar evolutionary hypothesis. (Abridged)Comment: 26 pages, 22 figures, accepted by ApJ. Version with full resolution
images is available at
http://www.cfa.harvard.edu/~yxli/ARTDUST/astroph0706.3706.pd
Protostellar birth with ambipolar and ohmic diffusion
The transport of angular momentum is capital during the formation of low-mass
stars; too little removal and rotation ensures stellar densities are never
reached, too much and the absence of rotation means no protoplanetary disks can
form. Magnetic diffusion is seen as a pathway to resolving this long-standing
problem. We investigate the impact of including resistive MHD in simulations of
the gravitational collapse of a 1 solar mass gas sphere, from molecular cloud
densities to the formation of the protostellar seed; the second Larson core. We
used the AMR code RAMSES to perform two 3D simulations of collapsing magnetised
gas spheres, including self-gravity, radiative transfer, and a non-ideal gas
equation of state to describe H2 dissociation which leads to the second
collapse. The first run was carried out under the ideal MHD approximation,
while ambipolar and ohmic diffusion was incorporated in the second calculation.
In the ideal MHD simulation, the magnetic field dominates the energy budget
everywhere inside and around the first core, fueling interchange instabilities
and driving a low-velocity outflow. High magnetic braking removes essentially
all angular momentum from the second core. On the other hand, ambipolar and
ohmic diffusion create a barrier which prevents amplification of the magnetic
field beyond 0.1 G in the first Larson core which is now fully thermally
supported. A significant amount of rotation is preserved and a small
Keplerian-like disk forms around the second core. When studying the radiative
efficiency of the first and second core accretion shocks, we found that it can
vary by several orders of magnitude over the 3D surface of the cores. Magnetic
diffusion is a pre-requisite to star-formation; it enables the formation of
protoplanetary disks in which planets will eventually form, and also plays a
determinant role in the formation of the protostar itself.Comment: 18 pages, 11 figures, accepted for publication in Astronomy &
Astrophysic
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