1,517 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
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
Sex venue-based network analysis to identify HIV prevention dissemination targets for men who have sex with men
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Cost-based Modeling for Fraud and Intrusion Detection: Results from the JAM Project
We describe the results achieved using the JAM distributed data mining system for the real world problem of fraud detection in financial information systems. For this domain we provide clear evidence that state-of-the-art commercial fraud detection systems can be substantially improved in stopping losses due to fraud by combining multiple models of fraudulent transaction shared among banks. We demonstrate that the traditional statistical metrics used to train and evaluate the performance of learning systems (i.e. statistical accuracy or ROC analysis) are misleading and perhaps inappropriate for this application. Cost-based metrics are more relevant in certain domains, and defining such metrics poses significant and interesting research questions both in evaluating systems and alternative models, and in formalizing the problems to which one may wish to apply data mining technologies. This paper also demonstrates how the techniques developed for fraud detection can be generalized and applied to the important area of intrusion detection in networked information systems. We report the outcome of recent evaluations of our system applied to tcpdump network intrusion data specifically with respect to statistical accuracy. This work involved building additional components of JAM that we have come to call, MADAM ID (Mining Audit Data for Automated Models for Intrusion Detection). However, taking the next step to define cost-based models for intrusion detection poses interesting new research questions. We describe our initial ideas about how to evaluate intrusion detection systems using cost models learned during our work on fraud detection
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
But What About... Cosmic Rays, Magnetic Fields, Conduction, & Viscosity in Galaxy Formation
We present a suite of high-resolution cosmological simulations, using the
FIRE-2 feedback physics together with explicit treatment of magnetic fields,
anisotropic conduction and viscosity, and cosmic rays (CRs) injected by
supernovae (including anisotropic diffusion, streaming, adiabatic, hadronic and
Coulomb losses). We survey systems from ultra-faint dwarf (, ) through Milky Way
masses, systematically vary CR parameters (e.g. the diffusion coefficient
and streaming velocity), and study an ensemble of galaxy properties
(masses, star formation histories, mass profiles, phase structure,
morphologies). We confirm previous conclusions that magnetic fields,
conduction, and viscosity on resolved (pc) scales have small
effects on bulk galaxy properties. CRs have relatively weak effects on all
galaxy properties studied in dwarfs (, ), or at high redshifts (), for
any physically-reasonable parameters. However at higher masses () and , CRs can suppress star
formation by factors , given relatively high effective diffusion
coefficients . At lower
, CRs take too long to escape dense star-forming gas and lose energy to
hadronic collisions, producing negligible effects on galaxies and violating
empirical constraints from -ray emission. But around , CRs escape the galaxy and build up a
CR-pressure-dominated halo which supports dense, cool ( K) gas
that would otherwise rain onto the galaxy. CR heating (from collisional and
streaming losses) is never dominant.Comment: 35 pages, 23 figures. Updated to match published (MNRAS) 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
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