7,382 research outputs found
The Difficulty of Getting High Escape Fractions of Ionizing Photons from High-redshift Galaxies: a View from the FIRE Cosmological Simulations
We present a series of high-resolution (20-2000 Msun, 0.1-4 pc) cosmological
zoom-in simulations at z~6 from the Feedback In Realistic Environment (FIRE)
project. These simulations cover halo masses 10^9-10^11 Msun and rest-frame
ultraviolet magnitude Muv = -9 to -19. These simulations include explicit
models of the multi-phase ISM, star formation, and stellar feedback, which
produce reasonable galaxy properties at z = 0-6. We post-process the snapshots
with a radiative transfer code to evaluate the escape fraction (fesc) of
hydrogen ionizing photons. We find that the instantaneous fesc has large time
variability (0.01%-20%), while the time-averaged fesc over long time-scales
generally remains ~5%, considerably lower than the estimate in many
reionization models. We find no strong dependence of fesc on galaxy mass or
redshift. In our simulations, the intrinsic ionizing photon budgets are
dominated by stellar populations younger than 3 Myr, which tend to be buried in
dense birth clouds. The escaping photons mostly come from populations between
3-10 Myr, whose birth clouds have been largely cleared by stellar feedback.
However, these populations only contribute a small fraction of intrinsic
ionizing photon budgets according to standard stellar population models. We
show that fesc can be boosted to high values, if stellar populations older than
3 Myr produce more ionizing photons than standard stellar population models (as
motivated by, e.g., models including binaries). By contrast, runaway stars with
velocities suggested by observations can enhance fesc by only a small fraction.
We show that "sub-grid" star formation models, which do not explicitly resolve
star formation in dense clouds with n >> 1 cm^-3, will dramatically
over-predict fesc.Comment: 17 pages, 16 figures, MNRAS in pres
Comparing models for IMF variation across cosmological time in Milky Way-like galaxies
One of the key observations regarding the stellar initial mass function (IMF) is its near-universality in the Milky Way (MW), which provides a powerful way to constrain different star formation models that predict the IMF. However, those models are almost universally ‘cloud-scale’ or smaller – they take as input or simulate single molecular clouds (GMCs), clumps or cores, and predict the resulting IMF as a function of the cloud properties. Without a model for the progenitor properties of all clouds that formed the stars at different locations in the MW (including ancient stellar populations formed in high redshift, likely gas-rich dwarf progenitor galaxies that looked little like the Galaxy today), the predictions cannot be fully explored nor safely applied to ‘live’ cosmological calculations of the IMF in different galaxies at different cosmological times. We therefore combine a suite of high-resolution cosmological simulations (from the Feedback In Realistic Environments project), which form MW-like galaxies with reasonable star formation properties and explicitly resolve massive GMCs, with various proposed cloud-scale IMF models. We apply the models independently to every star particle formed in the simulations to synthesize the predicted IMF in the present-day galaxy. We explore models where the IMF depends on Jeans mass, sonic or ‘turbulent Bonnor–Ebert’ mass, fragmentation with a polytropic equation of state, or where it is self-regulated by protostellar feedback. We show that all of these models, except the feedback-regulated ones, predict far more variation (∼0.6–1 dex 1σ scatter in the IMF turnover mass) in the simulations than is observed in the MW
Structure of the clean Ta(100) surface
The clean Ta(100) surface and some aspects of hydrogen adsorption have been studied by LEED and AES. The thorough examination of LEED patterns did not provide any evidence for an atomic reconstruction of the clean surface over the entire temperature range investigated, 150–600 K. The r-factor analysis used for comparison between measured and calculated I–V spectra yields a contraction of the topmost layer spacing of about 11% and an expansion of the second layer spacing of about 1% compared to the bulk value. The hydrogen adsorption does not induce any superstructures, but small hydrogen exposures lass then 1 L influence I–V spectra substantially
The Origin and Evolution of the Galaxy Mass-Metallicity Relation
We use high-resolution cosmological zoom-in simulations from the Feedback in
Realistic Environment (FIRE) project to study the galaxy mass-metallicity
relations (MZR) from z=0-6. These simulations include explicit models of the
multi-phase ISM, star formation, and stellar feedback. The simulations cover
halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0
and have been shown to produce many observed galaxy properties from z=0-6. For
the first time, our simulations agree reasonably well with the observed
mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We
predict the evolution of the MZR from z=0-6 as
log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05
and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04,
for gas-phase and stellar metallicity, respectively. Our simulations suggest
that the evolution of MZR is associated with the evolution of stellar/gas mass
fractions at different redshifts, indicating the existence of a universal
metallicity relation between stellar mass, gas mass, and metallicities. In our
simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction
of their metals inside the halo, because metal-rich winds fail to escape
completely and are recycled into the galaxy. This resolves a long-standing
discrepancy between "sub-grid" wind models (and semi-analytic models) and
observations, where common sub-grid models cannot simultaneously reproduce the
MZR and the stellar mass functions.Comment: 17 pages, 14 figures, re-submitted to MNRAS after revisions on
referee comment
Feedback first: the surprisingly weak effects of magnetic fields, viscosity, conduction, and metal diffusion on galaxy formation
Using high-resolution simulations with explicit treatment of stellar feedback
physics based on the FIRE (Feedback in Realistic Environments) project, we
study how galaxy formation and the interstellar medium (ISM) are affected by
magnetic fields, anisotropic Spitzer-Braginskii conduction and viscosity, and
sub-grid metal diffusion from unresolved turbulence. We consider controlled
simulations of isolated (non-cosmological) galaxies but also a limited set of
cosmological "zoom-in" simulations. Although simulations have shown significant
effects from these physics with weak or absent stellar feedback, the effects
are much weaker than those of stellar feedback when the latter is modeled
explicitly. The additional physics have no systematic effect on galactic star
formation rates (SFRs) . In contrast, removing stellar feedback leads to SFRs
being over-predicted by factors of . Without feedback, neither
galactic winds nor volume filling hot-phase gas exist, and discs tend to
runaway collapse to ultra-thin scale-heights with unphysically dense clumps
congregating at the galactic center. With stellar feedback, a multi-phase,
turbulent medium with galactic fountains and winds is established. At currently
achievable resolutions and for the investigated halo mass range
, the additional physics investigated here (MHD,
conduction, viscosity, metal diffusion) have only weak (-level)
effects on regulating SFR and altering the balance of phases, outflows, or the
energy in ISM turbulence, consistent with simple equipartition arguments. We
conclude that galactic star formation and the ISM are primarily governed by a
combination of turbulence, gravitational instabilities, and feedback. We add
the caveat that AGN feedback is not included in the present work
The Growth of Massive Black Holes in Galaxy Merger Simulations with Feedback by Radiation Pressure
We study the growth of massive black holes (BH) in galaxies using smoothed
particle hydrodynamic simulations of major galaxy mergers with new
implementations of BH accretion and feedback. The effect of BH accretion on gas
in its host galaxy is modeled by depositing momentum at a rate ~ tau L/c into
the ambient gas, where L is the luminosity produced by accretion onto the BH
and tau is the wavelength-averaged optical depth of the galactic nucleus to the
AGN's radiation (a free parameter of our model). The accretion rate onto the BH
is relatively independent of our subgrid accretion model and is instead
determined by the BH's dynamical impact on its host galaxy: BH accretion is
thus self-regulated rather than `supply limited.' We show that the final BH
mass and total stellar mass formed during a merger are more robust predictions
of the simulations than the time dependence of the star formation rate or BH
accretion rate. In particular, the latter depend on the assumed interstellar
medium physics, which determines when and where the gas fragments to form star
clusters; this in turn affects the fuel available for further star formation
and BH growth. Simulations over a factor of ~ 30 in galaxy mass are consistent
with the observed M_BH-sigma relation for a mean optical depth of tau ~ 25.
This requires that most BH growth occur when the galactic nucleus is optically
thick to far-infrared radiation, consistent with the hypothesized connection
between ultra-luminous infrared galaxies and quasars. We find tentative
evidence for a shallower M_BH-sigma relation in the lowest mass galaxies, sigma
< 100 km/s. Our results demonstrate that feedback-regulated BH growth and
consistency with the observed M_BH-sigma relation do not require that BH
feedback terminate star formation in massive galaxies or unbind large
quantities of cold gas.Comment: 21 pages, 17 figures, submitted MNRA
- …