858 research outputs found
The effects of metallicity, UV radiation and non-equilibrium chemistry in high-resolution simulations of galaxies
We present a series of hydrodynamic simulations of isolated galaxies with
stellar mass of . The models use a resolution of per particle and include a treatment for the full
non-equilibrium chemical evolution of ions and molecules (157 species in
total), along with gas cooling rates computed self-consistently using the
non-equilibrium abundances. We compare these to simulations evolved using
cooling rates calculated assuming chemical (including ionisation) equilibrium,
and we consider a wide range of metallicities and UV radiation fields,
including a local prescription for self-shielding by gas and dust. We find
higher star formation rates and stronger outflows at higher metallicity and for
weaker radiation fields, as gas can more easily cool to a cold (few hundred
Kelvin) star forming phase under such conditions. Contrary to variations in the
metallicity and the radiation field, non-equilibrium chemistry generally has no
strong effect on the total star formation rates or outflow properties. However,
it is important for modelling molecular outflows. For example, the mass of
H outflowing with velocities is enhanced
by a factor in non-equilibrium. We also compute the observable line
emission from CII and CO. Both are stronger at higher metallicity, while CII
and CO emission are higher for stronger and weaker radiation fields
respectively. We find that CII is generally unaffected by non-equilibrium
chemistry. However, emission from CO varies by a factor of . This
has implications for the mean conversion factor between CO
emission and H column density, which we find is lowered by up to a factor
in non-equilibrium, and for the fraction of CO-dark molecular gas.Comment: 26 pages, 16 figures, accepted for publication in MNRAS. Minor
changes relative to v
On the importance of local sources of radiation for quasar absorption line systems
A generic assumption of ionization models of quasar absorption systems is
that radiation from local sources is negligible compared with the cosmological
background. We test this assumption and find that it is unlikely to hold for
absorbers as rare as H I Lyman limit systems. Assuming that the absorption
systems are gas clouds centered on sources of radiation, we derive analytic
estimates for the cross-section weighted moments of the flux seen by the
absorbers, of the impact parameter, and of the luminosity of the central
source. In addition, we compute the corresponding medians numerically. For the
one class of absorbers for which the flux has been measured: damped Ly-alpha
systems at z~3, our prediction is in excellent agreement with the observations
if we assume that the absorption arises in clouds centered on Lyman-break
galaxies. Finally, we show that if Lyman-break galaxies dominate the UV
background at redshift 3, then consistency between observations of the UV
background, the UV luminosity density from galaxies, and the number density of
Lyman limit systems requires escape fractions of order 10 percent.Comment: Accepted for publication in the Astrophysical Journal, 11 pages, 1
figure. Version 2: Added alternative method. Decreased fiducial escape
fraction to guarantee consistency between observed luminosity density, mean
free path, and UV background. This increased the column density above which
local radiation is importan
Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime
An accurate treatment of the multiphase interstellar medium (ISM) in
hydrodynamic galaxy simulations requires that we follow not only the thermal
evolution of the gas, but also the evolution of its chemical state, including
its molecular chemistry, without assuming chemical (including ionisation)
equilibrium. We present a reaction network that can be used to solve for this
thermo-chemical evolution. Our model follows the evolution of all ionisation
states of the 11 elements that dominate the cooling rate, along with important
molecules such as H2 and CO, and the intermediate molecular species that are
involved in their formation (20 molecules in total). We include chemical
reactions on dust grains, thermal processes involving dust, cosmic ray
ionisation and heating and photochemical reactions. We focus on conditions
typical for the diffuse ISM, with densities of 10^-2 cm^-3 < nH < 10^4 cm^-3
and temperatures of 10^2 K < T < 10^4 K, and we consider a range of radiation
fields, including no UV radiation. In this paper we consider only gas that is
optically thin, while paper II considers gas that becomes shielded from the
radiation field. We verify the accuracy of our model by comparing chemical
abundances and cooling functions in chemical equilibrium with the
photoionisation code Cloudy. We identify the major coolants in diffuse
interstellar gas to be CII, SiII and FeII, along with OI and H2 at densities nH
> 10^2 cm^-3. Finally, we investigate the impact of non-equilibrium chemistry
on the cooling functions of isochorically or isobarically cooling gas. We find
that, at T < 10^4 K, recombination lags increase the electron abundance above
its equilibrium value at a given temperature, which can enhance the cooling
rate by up to two orders of magnitude. The cooling gas also shows lower H2
abundances than in equilibrium, by up to an order of magnitude.Comment: 26 pages, 13 figures, accepted for publication in MNRAS. Corrected an
error in figure 2. Supplementary material can be found at
http://noneqism.strw.leidenuniv.n
Mapping the Cosmic Web with Ly-alpha Emission
We use a high-resolution cosmological simulation to predict the distribution
of HI Ly-alpha emission from the low-redshift (z<0.5) intergalactic medium
(IGM). Our simulation can be used to reliably compute the emission from
optically thin regions of the IGM but not that of self-shielded gas. We
therefore consider several models that bracket the expected emission from
self-shielded regions. Most galaxies are surrounded by extended (>10^2 kpc)
``coronae'' of optically thin gas with Ly-alpha surface brightness close to the
expected background. Most of these regions contain smaller cores of dense, cool
gas. Unless self-shielded gas is able to cool to T<10^4.1 K, these cores are
much brighter than the background. The Ly-alpha coronae represent ``cooling
flows'' of IGM gas accreting onto galaxies. We also estimate the number of
Ly-alpha photons produced through the reprocessing of stellar ionizing
radiation in the interstellar medium of galaxies; while this mechanism is
responsible for the brightest Ly-alpha emission, it occurs on small physical
scales and can be separated using high-resolution observations. In all cases,
we find that Ly-alpha emitters are numerous (with a space density ~0.1 h^3
Mpc^-3) and closely trace the filamentary structure of the IGM, providing a new
way to map gas inside the cosmic web.Comment: 4 pages, 3 figures, accepted by ApJ
The scatter and evolution of the global hot gas properties of simulated galaxy cluster populations
We use the cosmo-OWLS suite of cosmological hydrodynamical simulations to
investigate the scatter and evolution of the global hot gas properties of large
simulated populations of galaxy groups and clusters. Our aim is to compare the
predictions of different physical models and to explore the extent to which
commonly-adopted assumptions in observational analyses (e.g. self-similar
evolution) are violated. We examine the relations between (true) halo mass and
the X-ray temperature, X-ray luminosity, gas mass, Sunyaev-Zel'dovich (SZ)
flux, the X-ray analogue of the SZ flux () and the hydrostatic mass. For
the most realistic models, which include AGN feedback, the slopes of the
various mass-observable relations deviate substantially from the self-similar
ones, particularly at late times and for low-mass clusters. The amplitude of
the mass-temperature relation shows negative evolution with respect to the
self-similar prediction (i.e. slower than the prediction) for all models,
driven by an increase in non-thermal pressure support at higher redshifts. The
AGN models predict strong positive evolution of the gas mass fractions at low
halo masses. The SZ flux and show positive evolution with respect to
self-similarity at low mass but negative evolution at high mass. The scatter
about the relations is well approximated by log-normal distributions, with
widths that depend mildly on halo mass. The scatter decreases significantly
with increasing redshift. The exception is the hydrostatic mass-halo mass
relation, for which the scatter increases with redshift. Finally, we discuss
the relative merits of various hot gas-based mass proxies.Comment: 31 pages (21 before appendices), 19 figures, 12 tables, accepted by
MNRAS after minor revisio
Calibrated, cosmological hydrodynamical simulations with variable IMFs I: Method and effect on global galaxy scaling relations
The recently inferred variations in the stellar initial mass function (IMF) among local highmass early-type galaxies may require a reinterpretation of observations of galaxy populations and may have important consequences for the predictions of models of galaxy formation and evolution. We present a new pair of cosmological, hydrodynamical simulations based on the EAGLE model that self-consistently adopts an IMF that, respectively, becomes bottomor top-heavy in high-pressure environments for individual star-forming gas particles. In such models, the excess stellarmass-to-light (M/L) ratio with respect to a reference IMF is increased due to an overabundance of low-mass dwarf stars or stellar remnants, respectively. Crucially, both pressure-dependent IMFs have been calibrated to reproduce the observed trends of increasing excessM/L with central stellar velocity dispersion (σe) in early-type galaxies, while maintaining agreement with the observables used to calibrate the EAGLE model, namely the galaxy luminosity function, half-light radii of late-type galaxies, and black holemasses.We find that while theM/L excess is a good measure of the IMF for low-mass slope variations, it depends strongly on the age of the stellar population for high-mass slope variations. The normalization of the [Mg/Fe]-σerelation is decreased (increased) for bottom- (top-)heavy IMF variations, while the slope is not strongly affected. Bottom-heavy variations have little impact on galaxy metallicities, half-light radii of early-type galaxies, or star formation rates, while top-heavy variations significantly increase these quantities for high-mass galaxies, leading to tension with observations. © 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society
Constraining reionization using the thermal history of the baryons
The thermal evolution of the intergalactic medium (IGM) depends on the
reionization history of the universe. Numerical simulations indicate that the
low density IGM, which is responsible for the low column density Ly-alpha
forest, follows a well defined temperature-density relation. This results in a
cut-off in the distribution of line widths as a function of column density. We
use hydrodynamic simulations to calibrate the relation between the cut-off and
the temperature-density relation and apply this relation to Keck spectra
spanning a redshift range z=2-4.5. We find that the temperature peaks at z~3
and interpret this as evidence for reheating due to the reionization of helium.Comment: 4 pages, 2 figures, to appear in "Cosmic evolution and galaxy
formation: Structure, interactions, and feedback", eds. J. Franco et a
Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas
We extend the non-equilibrium model for the chemical and thermal evolution of diffuse interstellar gas presented in Richings et al. to account for shielding from the UV radiation field. We attenuate the photochemical rates by dust and by gas, including absorption by HI, H2, HeI, HeII and CO where appropriate. We then use this model to investigate the dominant cooling and heating processes in interstellar gas as it becomes shielded from the UV radiation. We consider a one-dimensional plane-parallel slab of gas irradiated by the interstellar radiation field, either at constant density and temperature or in thermal and pressure equilibrium. The dominant thermal processes tend to form three distinct regions in the clouds. At low column densities, cooling is dominated by ionized metals such as Si II, FeII, FeIII and C II, which are balanced by photoheating, primarily from HI. Once the hydrogen-ionizing radiation becomes attenuated by neutral hydrogen, photoelectric dust heating dominates, while C II becomes dominant for cooling. Finally, dust shielding triggers the formation of CO and suppresses photoelectric heating. The dominant coolants in this fully shielded region are H2 and CO. The column density of the HI-H2 transition predicted by our model is lower at higher density (or at higher pressure for gas clouds in pressure equilibrium) and at higher metallicity, in agreement with previous photodissociation region models. We also compare the HI-H2 transition in our model to two prescriptions for molecular hydrogen formation that have been implemented in hydrodynamic simulations
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