23 research outputs found
Tides or dark matter sub-halos: Which ones are more attractive?
Young tidal dwarf galaxies (TDGs) are observed in the tidal debris of
gas-rich interacting galaxies. In contrast to what is generally assumed to be
the case for isolated dwarf galaxies, TDGs are not embedded in their own dark
matter (DM) sub-halo. Hence, they are more sensitive to stellar feedback and
could be disrupted on a short time-scale. Detailed numerical and observational
studies demonstrate that isolated DM-dominated dwarf galaxies can have
lifetimes of more than 10 Gyr. For TDGs that evolve in a tidal field with
compressing accelerations equal to the gravitational acceleration within a DM
sub-halo typical of an isolated dwarf galaxy, a similar survival time is
expected. The tidal acceleration profile depends on the virial mass of the host
galaxy and the distance between the TDG and its host. We analytically compare
the tidal compression to the gravitational acceleration due to either cuspy or
cored DM sub-halos of various virial masses. For example, the tidal field at a
distance of 100 kpc to a host halo of 10^13 Msol can be as stabilizing as a
10^9 Msol DM sub-halo. By linking the tidal field to the equivalent
gravitational field of a DM sub-halo, we can use existing models of isolated
dwarfs to estimate the survivability of TDGs. We show that part of the
unexpectedly high dynamical masses inferred from observations of some TDGs can
be explained by tidal compression and hence TDGs require to contain less
unobservable matter to understand their rotation curves.Comment: 11 pages, 7 figures, accepted for publication in MNRA
Radiative cooling rates, ion fractions, molecule abundances and line emissivities including self-shielding and both local and metagalactic radiation fields
We use the spectral synthesis code Cloudy to tabulate the properties of gas
for an extensive range in redshift (z=0 to 9), temperature (log T [K] = 1 to
9.5), metallicity (log Z/ = -4 to +0.5, Z=0), and density
(log [] = -8 to +6). This therefore includes
gas with properties characteristic of the interstellar, circumgalactic and
intergalactic media. The gas is exposed to a redshift-dependent UV/X-ray
background, while for the self-shielded lower-temperature gas (i.e. ISM gas) an
interstellar radiation field and cosmic rays are added. The radiation field is
attenuated by a density- and temperature-dependent column of gas and dust.
Motivated by the observed star formation law, this gas column density also
determines the intensity of the interstellar radiation field and the cosmic ray
density. The ionization balance, molecule fractions, cooling rates, line
emissivities, and equilibrium temperatures are calculated self-consistently. We
include dust, cosmic rays, and the interstellar radiation field step-by-step to
study their relative impact. These publicly available tables are ideal for
hydrodynamical simulations. They can be used stand alone or coupled to a
non-equilibrium network for a subset of elements. The release includes a C
routine to read in and interpolate the tables, as well as an easy to use python
graphical user interface to explore the tables.Comment: Accepted for publication in MNRAS; 29 pages, 24 figures; for data
files and more info see: https://www.sylviaploeckinger.com/radcool; minor
changes to match published versio
Tests of subgrid models for star formation using simulations of isolated disk galaxies
We use smoothed-particle hydrodynamics simulations of isolated Milky Way-mass
disk galaxies that include cold, interstellar gas to test subgrid prescriptions
for star formation (SF). Our fiducial model combines a Schmidt law with a
gravitational instability criterion, but we also test density thresholds and
temperature ceilings. While SF histories are insensitive to the prescription
for SF, the Kennicutt-Schmidt (KS) relations between SF rate and gas surface
density can discriminate between models. We show that our fiducial model, with
an SF efficiency per free-fall time of 1 per cent, agrees with
spatially-resolved and azimuthally-averaged observed KS relations for neutral,
atomic and molecular gas. Density thresholds do not perform as well. While
temperature ceilings selecting cold, molecular gas can match the data for
galaxies with solar metallicity, they are unsuitable for very low-metallicity
gas and hence for cosmological simulations. We argue that SF criteria should be
applied at the resolution limit rather than at a fixed physical scale, which
means that we should aim for numerical convergence of observables rather than
of the properties of gas labelled as star-forming. Our fiducial model yields
good convergence when the mass resolution is varied by nearly 4 orders of
magnitude, with the exception of the spatially-resolved molecular KS relation
at low surface densities. For the gravitational instability criterion, we
quantify the impact on the KS relations of gravitational softening, the SF
efficiency, and the strength of supernova feedback, as well as of observable
parameters such as the inclusion of ionized gas, the averaging scale, and the
metallicity.Comment: Submitted to MNRAS, 23 pages, 20 figure
A thermal-kinetic subgrid model for supernova feedback in simulations of galaxy formation
We present a subgrid model for supernova feedback designed for simulations of
galaxy formation. The model uses thermal and kinetic channels of energy
injection, which are built upon the stochastic kinetic and thermal models for
stellar feedback used in the OWLS and EAGLE simulations, respectively. In the
thermal channel, the energy is distributed statistically isotropically and
injected stochastically in large amounts per event, which minimizes spurious
radiative energy losses. In the kinetic channel, we inject the energy in small
portions by kicking gas particles in pairs in opposite directions. The
implementation of kinetic feedback is designed to conserve energy, linear
momentum and angular momentum, and is statistically isotropic. To test and
validate the model, we run simulations of isolated Milky Way-mass and dwarf
galaxies, in which the gas is allowed to cool down to 10 K. Using the thermal
and kinetic channels together, we obtain smooth star formation histories and
powerful galactic winds with realistic mass loading factors. Furthermore, the
model produces spatially resolved star formation rates and velocity dispersions
that are in agreement with observations. We vary the numerical resolution by
several orders of magnitude and find excellent convergence of the global star
formation rates and the mass loading of galactic winds. We show that large
thermal-energy injections generate a hot phase of the interstellar medium (ISM)
and modulate the star formation by ejecting gas from the disc, while the
low-energy kicks increase the turbulent velocity dispersion in the neutral ISM,
which in turn helps suppress star formation.Comment: 22 pages, 17 figures (including appendix); submitted to MNRA
Diagnosing the interstellar medium of galaxies with far-infrared emission lines I. The [C II] 158 microns line at z~0
Atomic fine structure lines have been detected in the local Universe and at
high redshifts over the past decades. The [C II] emission line at 158 m is
an important observable as it provides constraints on the interstellar medium
(ISM) cooling processes. We develop a physically motivated framework to
simulate the production of far-infrared line emission from galaxies in a
cosmological context. This first paper sets out our methodology and describes
its first application, simulating the [C II] 158 m line emission in the
local Universe. We combine the output from EAGLE cosmological hydrodynamical
simulations with a multi-phase model of the ISM. Gas particles are divided into
three phases: dense molecular gas, neutral atomic gas and diffuse ionised gas
(DIG). We estimate the [C II] line emission from the three phases using a set
of Cloudy cooling tables. Our results agree with previous findings regarding
the contribution of these three ISM phases to the [C II] emission. Our model
shows good agreement with the observed -star formation rate
(SFR) relation in the local Universe within 0.4 dex scatter. The fractional
contribution to the [C II] line from different ISM phases depends on the total
SFR and metallicity. The neutral gas phase dominates the [C II] emission in
galaxies with -, but the
ionised phase dominates at lower SFRs. Galaxies above solar metallicity exhibit
lower /SFR ratios for the neutral phase. In comparison, the
/SFR ratio in the DIG is stable when metallicity varies. We
suggest that the reduced size of the neutral clouds, caused by increased SFRs,
is the likely cause for the deficit at high infrared
luminosities, although EAGLE simulations do not reach these luminosities at
.Comment: Accepted for publication in A&A; 16 pages, 11 figures, 4 tables (plus
appendix