526 research outputs found
Breathing FIRE: How Stellar Feedback Drives Radial Migration, Rapid Size Fluctuations, and Population Gradients in Low-Mass Galaxies
We examine the effects of stellar feedback and bursty star formation on
low-mass galaxies ()
using the FIRE (Feedback in Realistic Environments) simulations. While previous
studies emphasized the impact of feedback on dark matter profiles, we
investigate the impact on the stellar component: kinematics, radial migration,
size evolution, and population gradients. Feedback-driven outflows/inflows
drive significant radial stellar migration over both short and long timescales
via two processes: (1) outflowing/infalling gas can remain star-forming,
producing young stars that migrate within their first , and (2) gas outflows/inflows drive strong fluctuations in the
global potential, transferring energy to all stars. These processes produce
several dramatic effects. First, galaxies' effective radii can fluctuate by
factors of over , and these rapid size fluctuations
can account for much of the observed scatter in radius at fixed
Second, the cumulative effects of many outflow/infall episodes steadily heat
stellar orbits, causing old stars to migrate outward most strongly. This
age-dependent radial migration mixes---and even inverts---intrinsic age and
metallicity gradients. Thus, the galactic-archaeology approach of calculating
radial star-formation histories from stellar populations at can be
severely biased. These effects are strongest at , the same regime where feedback most
efficiently cores galaxies. Thus, detailed measurements of stellar kinematics
in low-mass galaxies can strongly constrain feedback models and test baryonic
solutions to small-scale problems in CDM.Comment: Accepted to ApJ (820, 131) with minor revisions from v1. Figure 4 now
includes dark matter. Main results in Figures 7 and 1
A Direct Precision Measurement of the Intergalactic Lyman-alpha Opacity at 2<z<4.2
We directly measure the evolution of the intergalactic Lya effective optical
depth, tau_eff, over the redshift range 2<z<4.2 from a sample of 86
high-resolution, high-signal-to-noise quasar spectra obtained with the ESI and
HIRES spectrographs on Keck, and with the MIKE spectrograph on Magellan. This
represents an improvement over previous analyses of the Lya forest from
high-resolution spectra in this redshift interval of a factor of two in the
size of the data set alone. We pay particular attention to robust error
estimation and extensively test for systematic effects. We find that our
estimates of the quasar continuum levels in the Lya forest obtained by spline
fitting are systematically biased low, with the magnitude of the bias
increasing with redshift, but that this bias can be accounted for using mock
spectra. The mean fractional error is <1% at z=2, 4% at z=3, and 12% at z=4.
Previous measurements of tau_eff at z>~3 based on directly fitting the quasar
continua in the Lya forest, which have generally neglected this effect, are
therefore likely biased low. We provide estimates of the level of absorption
arising from metals in the Lya forest based on both direct and statistical
metal removal results in the literature, finding that this contribution is
~6-9% at z=3 and decreases monotonically with redshift. The high precision of
our measurement, attaining 3% in redshift bins of width Delta z=0.2 around z=3,
indicates significant departures from the best-fit power-law redshift evolution
(tau_eff=0.0018(1+z)^3.92, when metals are left in), particularly near z=3.2.
The observed downward departure is statistically consistent with a similar
feature detected in a precision statistical measurement using Sloan Digital Sky
Survey spectra by Bernardi and coworkers using an independent approach.Comment: 27 pages, including 18 figures, published in Ap
On the deuterium abundance and the importance of stellar mass loss in the interstellar and intergalactic medium
We quantify the gas-phase abundance of deuterium and fractional contribution
of stellar mass loss to the gas in cosmological zoom-in simulations from the
Feedback In Realistic Environments project. At low metallicity, our simulations
confirm that the deuterium abundance is very close to the primordial value. The
chemical evolution of the deuterium abundance that we derive here agrees
quantitatively with analytical chemical evolution models. We furthermore find
that the relation between the deuterium and oxygen abundance exhibits very
little scatter. We compare our simulations to existing high-redshift
observations in order to determine a primordial deuterium fraction of 2.549 +/-
0.033 x 10^-5 and stress that future observations at higher metallicity can
also be used to constrain this value. At fixed metallicity, the deuterium
fraction decreases slightly with decreasing redshift, due to the increased
importance of mass loss from intermediate-mass stars. We find that the
evolution of the average deuterium fraction in a galaxy correlates with its
star formation history. Our simulations are consistent with observations of the
Milky Way's interstellar medium: the deuterium fraction at the solar circle is
85-92 per cent of the primordial deuterium fraction. We use our simulations to
make predictions for future observations. In particular, the deuterium
abundance is lower at smaller galactocentric radii and in higher mass galaxies,
showing that stellar mass loss is more important for fuelling star formation in
these regimes (and can even dominate). Gas accreting onto galaxies has a
deuterium fraction above that of the galaxies' interstellar medium, but below
the primordial fraction, because it is a mix of gas accreting from the
intergalactic medium and gas previously ejected or stripped from galaxies.Comment: Accepted for publication in MNRAS. Revised version: expanded
discussion and added Figure 2 (residual dependence on iron abundance
Properties of the circumgalactic medium in cosmic ray-dominated galaxy haloes
We investigate the impact of cosmic rays (CRs) on the circumgalactic medium (CGM) in FIRE-2 simulations, for ultra-faint dwarf through Milky Way (MW)-mass haloes hosting star-forming (SF) galaxies. Our CR treatment includes injection by supernovae, anisotropic streaming and diffusion along magnetic field lines, and collisional and streaming losses, with constant parallel diffusivity κ∼3×10²⁹ cm² s⁻¹ chosen to match γ-ray observations. With this, CRs become more important at larger halo masses and lower redshifts, and dominate the pressure in the CGM in MW-mass haloes at z ≲ 1–2. The gas in these ‘CR-dominated’ haloes differs significantly from runs without CRs: the gas is primarily cool (a few ∼10⁴), and the cool phase is volume-filling and has a thermal pressure below that needed for virial or local thermal pressure balance. Ionization of the ‘low’ and ‘mid’ ions in this diffuse cool gas is dominated by photoionization, with O VI columns ≳10^(14.5) cm⁻² at distances ≳150kpc. CR and thermal gas pressure are locally anticorrelated, maintaining total pressure balance, and the CGM gas density profile is determined by the balance of CR pressure gradients and gravity. Neglecting CRs, the same haloes are primarily warm/hot (T≳10⁵) with thermal pressure balancing gravity, collisional ionization dominates, O VI columns are lower and Ne VIII higher, and the cool phase is confined to dense filaments in local thermal pressure equilibrium with the hot phase
The origin of ultra diffuse galaxies: stellar feedback and quenching
We test if the cosmological zoom-in simulations of isolated galaxies from the
FIRE project reproduce the properties of ultra diffuse galaxies. We show that
stellar feedback-generated outflows that dynamically heat galactic stars,
together with a passively aging stellar population after imposed quenching
(from e.g. infall into a galaxy cluster), naturally reproduce the observed
population of red UDGs, without the need for high spin halos or dynamical
influence from their host cluster. We reproduce the range of surface
brightness, radius and absolute magnitude of the observed z=0 red UDGs by
quenching simulated galaxies at a range of different times. They represent a
mostly uniform population of dark matter-dominated galaxies with M_star ~1e8
Msun, low metallicity and a broad range of ages. The most massive simulated
UDGs require earliest quenching and are therefore the oldest. Our simulations
provide a good match to the central enclosed masses and the velocity
dispersions of the observed UDGs (20-50 km/s). The enclosed masses of the
simulated UDGs remain largely fixed across a broad range of quenching times
because the central regions of their dark matter halos complete their growth
early. A typical UDG forms in a dwarf halo mass range of Mh~4e10-1e11 Msun. The
most massive red UDG in our sample requires quenching at z~3 when its halo
reached Mh ~ 1e11 Msun. If it, instead, continues growing in the field, by z=0
its halo mass reaches > 5e11 Msun, comparable to the halo of an L* galaxy. If
our simulated dwarfs are not quenched, they evolve into bluer low-surface
brightness galaxies with mass-to-light ratios similar to observed field dwarfs.
While our simulation sample covers a limited range of formation histories and
halo masses, we predict that UDG is a common, and perhaps even dominant, galaxy
type around Ms~1e8 Msun, both in the field and in clusters.Comment: 20 pages, 13 figures; match the MNRAS accepted versio
The failure of stellar feedback, magnetic fields, conduction, and morphological quenching in maintaining red galaxies
The quenching "maintenance'" and related "cooling flow" problems are
important in galaxies from Milky Way mass through clusters. We investigate this
in halos with masses , using
non-cosmological high-resolution hydrodynamic simulations with the FIRE-2
(Feedback In Realistic Environments) stellar feedback model. We specifically
focus on physics present without AGN, and show that various proposed "non-AGN"
solution mechanisms in the literature, including Type Ia supernovae, shocked
AGB winds, other forms of stellar feedback (e.g. cosmic rays), magnetic fields,
Spitzer-Braginskii conduction, or "morphological quenching" do not halt or
substantially reduce cooling flows nor maintain "quenched" galaxies in this
mass range. We show that stellar feedback (including cosmic rays from SNe)
alters the balance of cold/warm gas and the rate at which the cooled gas within
the galaxy turns into stars, but not the net baryonic inflow. If anything,
outflowing metals and dense gas promote additional cooling. Conduction is
important only in the most massive halos, as expected, but even at reduces inflow only by a factor (owing to
saturation effects and anisotropic suppression). Changing the morphology of the
galaxies only slightly alters their Toomre- parameter, and has no effect on
cooling (as expected), so has essentially no effect on cooling flows or
maintaining quenching. This all supports the idea that additional physics,
e.g., AGN feedback, must be important in massive galaxies.Comment: 16 pages, 12 figure
One blind and three targeted searches for (sub)millisecond pulsars
We conducted one blind and three targeted searches for millisecond and
submillisecond pulsars. The blind search was conducted within 3deg of the
Galactic plane and at longitudes between 20 and 110deg. It takes 22073
pointings to cover this region, and 5487 different positions in the sky. The
first targeted search was aimed at Galactic globular clusters, the second one
at 24 bright polarized and pointlike radiosources with steep spectra, and the
third at 65 faint polarized and pointlike radiosources. The observations were
conducted at the large radiotelescope of Nancay Observatory, at a frequency
near 1400 MHz. Two successive backends were used, first a VLBI S2 system,
second a digital acquisition board and a PC with large storage capacity
sampling the signal at 50 Mb/s on one bit, over a 24-MHz band and in one
polarization. The bandwidth of acquisition of the second backend was later
increased to 48 MHz and the sampling rate to 100 Mb/s. The survey used the
three successive setups, with respective sensitivities of 3.5, 2.2, and 1.7
mJy. The targeted-search data were obtained with the third setup and reduced
with a method based on the Hough transform, yielding a sensitivity of 0.9 mJy.
The processing of the data was done in slightly differed time by
soft-correlation in all cases. No new short-period millisecond pulsars were
discovered in the different searches. To better understand the null result of
the blind survey, we estimate the probability of detecting one or more
short-period pulsars among a given Galactic population of synthetic pulsars
with our setup: 25% for the actual incomplete survey and 79% if we had
completed the whole survey with a uniform nominal sensitivity of 1.7 mJy. The
alternative of surveying a smaller, presumably more densely populated, region
with a higher sensitivity would have a low return and would be impractical at a
transit instrument. (abridged)Comment: accepted for publication in Astronomy & Astrophysic
Effects of different cosmic ray transport models on galaxy formation
Cosmic rays (CRs) with ∼GeV energies can contribute significantly to the energy and pressure budget in the interstellar, circumgalactic, and intergalactic medium (ISM, CGM, IGM). Recent cosmological simulations have begun to explore these effects, but almost all studies have been restricted to simplified models with constant CR diffusivity and/or streaming speeds. Physical models of CR propagation/scattering via extrinsic turbulence and self-excited waves predict transport coefficients which are complicated functions of local plasma properties. In a companion paper, we consider a wide range of observational constraints to identify proposed physically motivated cosmic ray propagation scalings which satisfy both detailed Milky Way (MW) and extragalactic γ-ray constraints. Here, we compare the effects of these models relative to simpler ‘diffusion+streaming’ models on galaxy and CGM properties at dwarf through MW mass scales. The physical models predict large local variations in CR diffusivity, with median diffusivity increasing with galactocentric radii and decreasing with galaxy mass and redshift. These effects lead to a more rapid dropoff of CR energy density in the CGM (compared to simpler models), in turn producing weaker effects of CRs on galaxy star formation rates (SFRs), CGM absorption profiles, and galactic outflows. The predictions of the more physical CR models tend to lie ‘in between’ models which ignore CRs entirely and models which treat CRs with constant diffusivity
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