arXiv subjects

C. -A. Faucher-Giguere

Publications and source records attributed to C. -A. Faucher-Giguere.

At least 19 recordsLinked to original sources

arXiv2020

A Cosmic UV/X-ray Background Model Update

We present an updated model of the cosmic ionizing background from the UV to the X-rays. Relative to our previous model (Faucher-Giguere et al. 2009), the new model provides a better match to a large number of up-to-date empirical constraints, including: 1) new galaxy and AGN luminosity functions; 2) stellar spectra including binary stars; 3) obscured and unobscured AGN; 4) a measurement of the non-ionizing UV background; 5) measurements of the intergalactic HI and HeII photoionization rates at z~0-6; 6) the local X-ray background; and 7) improved measurements of the intergalactic opacity. In this model, AGN dominate the HI ionizing background at z<~3 and star-forming galaxies dominate it at higher redshifts. Combined with the steeply declining AGN luminosity function beyond z~2, the slow evolution of the HI ionization rate inferred from the high-redshift HI Lya forest requires an escape fraction from star-forming galaxies that increases with redshift (a population-averaged escape fraction of ~1% suffices to ionize the intergalactic medium at z=3 when including the contribution from AGN). We provide effective photoionization and photoheating rates calibrated to match the Planck 2018 reionization optical depth and recent constraints from the HeII Lya forest in hydrodynamic simulations.

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arXiv2019

The Fates of the Circumgalactic Medium in the FIRE Simulations

We analyze the different fates of the circumgalactic medium (CGM) in FIRE-2 cosmological simulations, focusing on the redshifts z=0.25 and z=2 representative of recent surveys. Our analysis includes 21 zoom-in simulations covering the halo mass range Mh(z=0) ~ 10^10 - 10^12 Msun. We analyze both where the gas ends up after first leaving the CGM (its "proximate" fate), as well as its location at z=0 (its "ultimate" fate). Of the CGM at z=2, about half is found in the ISM or stars of the central galaxy by z=0 in Mh(z=2) ~ 5e11 Msun halos, but most of the CGM in lower-mass halos ends up in the IGM. This is so even though most of the CGM in M_h(z=2) ~ 5e10 Msun halos first accretes onto the central galaxy before being ejected into the IGM. On the other hand, most of the CGM mass at z=0.25 remains in the CGM by z=0 at all halo masses analyzed. Of the CGM gas that subsequently accretes onto the central galaxy in the progenitors of Mh(z=0) ~10^12 Msun halos, most of it is cool (T~10^4 K) at z=2 but hot (~Tvir) at z=0.25, consistent with the expected transition from cold mode to hot mode accretion. Despite the transition in accretion mode, at both z=0.25 and z=2 >~80% of the cool gas in Mh >~ 10^11 Msun halos will accrete onto a galaxy. We find that the metallicity of CGM gas is typically a poor predictor of both its proximate and ultimate fates. This is because there is in general little correlation between the origin of CGM gas and its fate owing to substantial mixing while in the CGM.

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arXiv2019

Cosmic ray feedback in the FIRE simulations: constraining cosmic ray propagation with GeV gamma ray emission

We present the implementation and the first results of cosmic ray (CR) feedback in the Feedback In Realistic Environments (FIRE) simulations. We investigate CR feedback in non-cosmological simulations of dwarf, sub-$L\star$ starburst, and $L\star$ galaxies with different propagation models, including advection, isotropic and anisotropic diffusion, and streaming along field lines with different transport coefficients. We simulate CR diffusion and streaming simultaneously in galaxies with high resolution, using a two moment method. We forward-model and compare to observations of $γ$-ray emission from nearby and starburst galaxies. We reproduce the $γ$-ray observations of dwarf and $L\star$ galaxies with constant isotropic diffusion coefficient $κ\sim 3\times 10^{29}\,{\rm cm^{2}\,s^{-1}}$. Advection-only and streaming-only models produce order-of-magnitude too large $γ$-ray luminosities in dwarf and $L\star$ galaxies. We show that in models that match the $γ$-ray observations, most CRs escape low-gas-density galaxies (e.g.\ dwarfs) before significant collisional losses, while starburst galaxies are CR proton calorimeters. While adiabatic losses can be significant, they occur only after CRs escape galaxies, so they are only of secondary importance for $γ$-ray emissivities. Models where CRs are ``trapped'' in the star-forming disk have lower star formation efficiency, but these models are ruled out by $γ$-ray observations. For models with constant $κ$ that match the $γ$-ray observations, CRs form extended halos with scale heights of several kpc to several tens of kpc.

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arXiv2019

The Origins of the Circumgalactic Medium in the FIRE Simulations

We use a particle tracking analysis to study the origins of the circumgalactic medium (CGM), separating it into (1) accretion from the intergalactic medium (IGM), (2) wind from the central galaxy, and (3) gas ejected from other galaxies. Our sample consists of 21 FIRE-2 simulations, spanning the halo mass range log(Mh/Msun) ~ 10-12 , and we focus on z=0.25 and z=2. Owing to strong stellar feedback, only ~L* halos retain a baryon mass >~50% of their cosmic budget. Metals are more efficiently retained by halos, with a retention fraction >~50%. Across all masses and redshifts analyzed >~60% of the CGM mass originates as IGM accretion (some of which is associated with infalling halos). Overall, the second most important contribution is wind from the central galaxy, though gas ejected or stripped from satellites can contribute a comparable mass in ~L* halos. Gas can persist in the CGM for billions of years, resulting in well-mixed halo gas. Sight lines through the CGM are therefore likely to intersect gas of multiple origins. For low-redshift ~L* halos, cool gas (T<10^4.7 K) is distributed on average preferentially along the galaxy plane, however with strong halo-to-halo variability. The metallicity of IGM accretion is systematically lower than the metallicity of winds (typically by >~1 dex), although CGM and IGM metallicities depend significantly on the treatment of subgrid metal diffusion. Our results highlight the multiple physical mechanisms that contribute to the CGM and will inform observational efforts to develop a cohesive picture.

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arXiv2018

Recent progress in simulating galaxy formation from the largest to the smallest scales

Galaxy formation simulations are an essential part of the modern toolkit of astrophysicists and cosmologists alike. Astrophysicists use the simulations to study the emergence of galaxy populations from the Big Bang, as well as problems including the formation of stars and supermassive black holes. For cosmologists, galaxy formation simulations are needed to understand how baryonic processes affect measurements of dark matter and dark energy. Owing to the extreme dynamic range of galaxy formation, advances are driven by novel approaches using simulations with different tradeoffs between volume and resolution. Large-volume but low-resolution simulations provide the best statistics, while higher resolution simulations of smaller cosmic volumes can be evolved with more self-consistent physics and reveal important emergent phenomena. I summarize recent progress in galaxy formation simulations, including major developments in the past five years, and highlight some key areas likely to drive further advances over the next decade.

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arXiv2017

A Model for the Origin of Bursty Star Formation in Galaxies

We propose a simple analytic model to understand when star formation is time-steady versus bursty in galaxies. Recent models explain the observed Kennicutt-Schmidt relation between star formation rate and gas surface densities in galaxies as resulting from a balance between stellar feedback and gravity. We argue that bursty star formation occurs when such an equilibrium cannot be stably sustained, and identify two regimes in which galaxy-scale star formation should be bursty: i) at high redshift (z>~1) for galaxies of all masses, and ii) at low masses (depending on gas fraction) for galaxies at any redshift. At high redshift, characteristic galactic dynamical timescales become too short for supernova feedback to effectively respond to gravitational collapse in galactic discs (an effect recently identified for galactic nuclei), whereas in dwarf galaxies star formation occurs in too few bright star-forming regions to effectively average out. Burstiness is also enhanced at high redshift owing to elevated gas fractions in the early Universe. Our model can thus explain the bursty star formation rates predicted in these regimes by recent high-resolution galaxy formation simulations, as well as the bursty star formation histories observationally-inferred in both local dwarf and high-redshift galaxies. In our model, bursty star formation is associated with particularly strong spatio-temporal clustering of supernovae. Such clustering can promote the formation of galactic winds and our model may thus also explain the much higher wind mass loading factors inferred in high-redshift massive galaxies relative to their z~0 counterparts.

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arXiv2017

Low-Redshift Lyman Limit Systems as Diagnostics of Cosmological Inflows and Outflows

We use cosmological hydrodynamic simulations with stellar feedback from the FIRE project to study the physical nature of Lyman limit systems (LLSs) at z<1. At these low redshifts, LLSs are closely associated with dense gas structures surrounding galaxies, such as galactic winds, dwarf satellites, and cool inflows from the intergalactic medium. Our analysis is based on 14 zoom-in simulations covering the halo mass range M_h~10^9-10^13 Msun at z=0, which we convolve with the dark matter halo mass function to produce cosmological statistics. We find that the majority of cosmologically-selected LLSs are associated with halos in the mass range 10^10 < M_h < 10^12 Msun. The incidence and HI column density distribution of simulated absorbers with columns 10^16.2 < N_HI < 2x10^20 cm^-2 are consistent with observations. High-velocity outflows (with radial velocity exceeding the halo circular velocity by a factor >~2) tend to have higher metallicities ([X/H] ~ -0.5) while very low metallicity ([X/H] < -2) LLSs are typically associated with gas infalling from the intergalactic medium. However, most LLSs occupy an intermediate region in metallicity-radial velocity space, for which there is no clear trend between metallicity and radial kinematics. Metal-enriched inflows arise in the FIRE simulations as a result of galactic winds that fall back onto galaxies at low redshift. The overall simulated LLS metallicity distribution has a mean (standard deviation) [X/H] = -0.9 (0.4) and does not show significant evidence for bimodality, in contrast to recent observational studies but consistent with LLSs arising from halos with a broad range of masses and metallicities.

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arXiv2017

Observational Diagnostics of Gas Flows: Insights from Cosmological Simulations

Galactic accretion interacts in complex ways with gaseous halos, including galactic winds. As a result, observational diagnostics typically probe a range of intertwined physical phenomena. Because of this complexity, cosmological hydrodynamic simulations have played a key role in developing observational diagnostics of galactic accretion. In this chapter, we review the status of different observational diagnostics of circumgalactic gas flows, in both absorption (galaxy pair and down-the-barrel observations in neutral hydrogen and metals; kinematic and azimuthal angle diagnostics; the cosmological column density distribution; and metallicity) and emission (Lya; UV metal lines; and diffuse X-rays). We conclude that there is no simple and robust way to identify galactic accretion in individual measurements. Rather, progress in testing galactic accretion models is likely to come from systematic, statistical comparisons of simulation predictions with observations. We discuss specific areas where progress is likely to be particularly fruitful over the next few years.

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arXiv2016

Strongly Time-Variable Ultra-Violet Metal Line Emission from the Circum-Galactic Medium of High-Redshift Galaxies

We use cosmological simulations from the Feedback In Realistic Environments (FIRE) project, which implement a comprehensive set of stellar feedback processes, to study ultra-violet (UV) metal line emission from the circum-galactic medium of high-redshift (z=2-4) galaxies. Our simulations cover the halo mass range Mh ~ 2x10^11 - 8.5x10^12 Msun at z=2, representative of Lyman break galaxies. Of the transitions we analyze, the low-ionization C III (977 A) and Si III (1207 A) emission lines are the most luminous, with C IV (1548 A) and Si IV (1394 A) also showing interesting spatially-extended structures. The more massive halos are on average more UV-luminous. The UV metal line emission from galactic halos in our simulations arises primarily from collisionally ionized gas and is strongly time variable, with peak-to-trough variations of up to ~2 dex. The peaks of UV metal line luminosity correspond closely to massive and energetic mass outflow events, which follow bursts of star formation and inject sufficient energy into galactic halos to power the metal line emission. The strong time variability implies that even some relatively low-mass halos may be detectable. Conversely, flux-limited samples will be biased toward halos whose central galaxy has recently experienced a strong burst of star formation. Spatially-extended UV metal line emission around high-redshift galaxies should be detectable by current and upcoming integral field spectrographs such as the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope and Keck Cosmic Web Imager (KCWI).

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arXiv2016

A Stellar Feedback Origin for Neutral Hydrogen in High-Redshift Quasar-Mass Halos

Observations reveal that quasar host halos at z~2 have large covering fractions of cool dense gas (>~60% for Lyman limit systems within a projected virial radius). Most simulations have so far have failed to explain these large observed covering fractions. We analyze a new set of 15 simulated massive halos with explicit stellar feedback from the FIRE project, covering the halo mass range M_h~2x10^12-10^13 Msun at z=2. This extends our previous analysis of the circum-galactic medium of high-redshift galaxies to more massive halos. AGN feedback is not included in these simulations. We find Lyman limit system covering fractions consistent with those observed around quasars. The large HI covering fractions arise from star formation-driven galactic winds, including winds from low-mass satellite galaxies that interact with cosmological filaments. We show that it is necessary to resolve these satellite galaxies and their winds to reproduce the large Lyman limit system covering fractions observed in quasar-mass halos. Our simulations predict that galaxies occupying dark matter halos of mass similar to quasars but without a luminous AGN should have Lyman limit system covering fractions comparable to quasars.

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arXiv2015

Neutral hydrogen in galaxy halos at the peak of the cosmic star formation history

We use high-resolution cosmological zoom-in simulations from the FIRE project to make predictions for the covering fractions of neutral hydrogen around galaxies at z=2-4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms. Our simulation sample consists of 16 main halos covering the mass range M_h~10^9-6x10^12 Msun at z=2, including 12 halos in the mass range M_h~10^11-10^12 Msun corresponding to Lyman break galaxies (LBGs). We process our simulations with a ray tracing method to compute the ionization state of the gas. Galactic winds increase the HI covering fractions in galaxy halos by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the intergalactic medium. Our simulations predict HI covering fractions for Lyman limit systems (LLSs) consistent with measurements around z~2-2.5 LBGs; these covering fractions are a factor ~2 higher than our previous calculations without galactic winds. The fractions of HI absorbers arising in inflows and in outflows are on average ~50% but exhibit significant time variability, ranging from ~10% to ~90%. For our most massive halos, we find a factor ~3 deficit in the LLS covering fraction relative to what is measured around quasars at z~2, suggesting that the presence of a quasar may affect the properties of halo gas on ~100 kpc scales. The predicted covering fractions, which decrease with time, peak at M_h~10^11-10^12 Msun, near the peak of the star formation efficiency in dark matter halos. In our simulations, star formation and galactic outflows are highly time dependent; HI covering fractions are also time variable but less so because they represent averages over large areas.

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arXiv2013

Feedback-regulated star formation in molecular clouds and galactic discs

We present a two-zone theory for feedback-regulated star formation in galactic discs, consistently connecting the galaxy-averaged star formation law with star formation proceeding in giant molecular clouds (GMCs). Our focus is on galaxies with gas surface density Sigma_g>~100 Msun pc^-2. In our theory, the galactic disc consists of Toomre-mass GMCs embedded in a volume-filling ISM. Radiation pressure on dust disperses GMCs and most supernovae explode in the volume-filling medium. A galaxy-averaged star formation law is derived by balancing the momentum input from supernova feedback with the gravitational weight of the disc gas. This star formation law is in good agreement with observations for a CO conversion factor depending continuously on Sigma_g. We argue that the galaxy-averaged star formation efficiency per free fall time, epsilon_ff^gal, is only a weak function of the efficiency with which GMCs convert their gas into stars. This is possible because the rate limiting step for star formation is the rate at which GMCs form: for large efficiency of star formation in GMCs, the Toomre Q parameter obtains a value slightly above unity so that the GMC formation rate is consistent with the galaxy-averaged star formation law. We contrast our results with other theories of turbulence-regulated star formation and discuss predictions of our model. Using a compilation of data from the literature, we show that the galaxy-averaged star formation efficiency per free fall time is non-universal and increases with increasing gas fraction, as predicted by our model. We also predict that the fraction of the disc gas mass in bound GMCs decreases for increasing values of the GMC star formation efficiency. This is qualitatively consistent with the smooth molecular gas distribution inferred in local ultra-luminous infrared galaxies and the small mass fraction in giant clumps in high-redshift galaxies.

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arXiv2012

The physics of galactic winds driven by active galactic nuclei

Active galactic nuclei (AGN) drive fast winds in the interstellar medium of their host galaxies. It is commonly assumed that the high ambient densities and intense radiation fields in galactic nuclei imply short cooling times, thus making the outflows momentum-conserving. We show that cooling of high-velocity, shocked winds in AGN is in fact inefficient in a wide range of circumstances, including conditions relevant to ultra-luminous infrared galaxies (ULIRGs), resulting in energy-conserving outflows. We further show that fast energy-conserving outflows can tolerate a large amount of mixing with cooler gas before radiative losses become important. For winds with initial velocity v_in>~10,000 km s^-1, as observed in ultra-violet and X-ray absorption, the shocked wind develops a two-temperature structure. While most of the thermal pressure support is provided by the protons, the cooling processes operate directly only on the electrons. This significantly slows down inverse Compton cooling, while free free cooling is negligible. Slower winds with v_in~1,000 km s^-1, such as may be driven by radiation pressure on dust, can also experience energy-conserving phases but under more restrictive conditions. During the energy-conserving phase, the momentum flux of an outflow is boosted by a factor ~v_in/2v_s by work done by the hot post-shock gas, where v_s is the velocity of the swept-up material. Energy-conserving outflows driven by fast AGN winds (v_in~0.1c) may therefore explain the momentum fluxes Pdot>>L_AGN/c of galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked wind bubbles expanding normal to galactic disks may also explain the large-scale bipolar structures observed in some systems, including around the Galactic Center, and can produce significant radio, X-ray, and gamma-ray emission. [Abridged]

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arXiv2012

Concordance models of reionization: implications for faint galaxies and escape fraction evolution

Recent observations have constrained the galaxy UV luminosity function up to z~10. However, these observations alone allow for a wide range of reionization scenarios due to uncertainties in the abundance of faint galaxies and the escape fraction of ionizing photons. We show that requiring continuity with post-reionization (z<6) measurements, where the Lya forest provides a complete probe of the cosmological emissivity of ionizing photons, significantly reduces the permitted parameter space. Models that are simultaneously consistent with the measured UV luminosity function, the Thomson optical depth to the CMB, and the Lya forest data require either: 1) extrapolation of the galaxy luminosity function down to very faint UV magnitudes M_lim ~ -10, corresponding roughly to the UV background suppression scale; 2) an increase of f_esc by a factor > ~10 from z=4 (where the best fit is 4%) to z=9; or 3) more likely, a hybrid solution in which undetected galaxies contribute significantly and f_esc increases more modestly. Models in which star formation is strongly suppressed in low-mass, reionization-epoch haloes of mass up to ~10^10 M_sun (e.g., owing to a metallicity dependence) are only allowed for extreme assumptions for the evolution of f_esc. However, variants of such models in which the suppression mass is reduced (e.g., assuming an earlier or higher metallicity floor) are in better agreement with the data. Concordance scenarios satisfying the available data predict a consistent redshift of 50% ionized fraction z_reion(50%) ~ 10. On the other hand, the duration of reionization is sensitive to the relative contribution of bright versus faint galaxies, with scenarios dominated by faint galaxies predicting a more extended reionization event. Scenarios relying too heavily on high-z dwarfs are disfavored by kinetic Sunyaev-Zeldovich measurements, which prefer a short reionization history.

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arXiv2012

Quasar Absorption Lines from Radiative Shocks: Implications for Multiphase Outflows and Feedback

Photoionization modeling of certain low-ionization broad absorption lines in quasars implies very compact (Delta R~0.01 pc), galaxy-scale (R kpc) absorbers blueshifted by several 1000 km s^-1. While these are likely signatures of quasar outflows, the lifetimes of such compact absorbers are too short for them to be direct ejecta from a nuclear wind. Instead, I argue that the absorbing clouds must be transient and created in situ. Following arguments detailed by Faucher-Giguere, Quataert, & Murray (2011), I show that a model in which the cool absorbers form in radiative shocks arising when a quasar blast wave impacts an interstellar cloud along the line of sight successfully explains the key observed properties. Using this radiative shock model, the outflow kinetic luminosities for three luminous quasars are estimated to be Edot,k~2-5% L_AGN (with corresponding momentum fluxes Pdot~2-15 L_AGN/c), consistent with feedback models of the M-sigma relation. These energetics are similar to those recently inferred of molecular outflows in local ultra-luminous infrared galaxies and in post-starburt winds, suggesting that active galactic nuclei (AGN) are capable of driving such outflows. Radiative shocks probably affect the multiphase structure of outflows in a range of other systems, including narrower and higher-ionization quasar absorption lines, and compact intergalactic absorbers ejected by star formation and AGN activity.

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arXiv2011

A Physical Model of FeLoBALs: Implications for Quasar Feedback

Photoionization modeling of the low-ionization broad absorption lines of certain quasars, known as FeLoBALs, has recently revealed the number density of the wind absorbers and their distance from the central supermassive black hole. From these, the feedback efficiency of the quasars can in principle be derived. The implied properties of the FeLoBALs are, however, surprising, with the thickness of the absorbers relative to their distance from the black hole, Delta R/R, as small as ~10^-5. Such absorbers are unlikely to survive the journey from the supermassive black hole to their inferred location. We show that the observed FeLoBAL properties are readily explained if they are formed in situ in radiative shocks produced when a quasar blast wave impacts a moderately dense interstellar clump along the line of sight. This physical picture differs significantly from the thin shell approximation often assumed, and implies outflow rates, kinetic luminosities and momentum fluxes that differ correspondingly, in some cases at the order of magnitude level. Using the radiative shock model, we estimate the ratio of the outflow kinetic luminosity to bolometric luminosity for three bright FeLoBAL quasars in the literature. We find Edot/Lbol~2-5% (and corresponding momentum fluxes Pdot~2-15 Lbol/c), similar to what is adopted in models reproducing the M-sigma relation. These outflow properties are also comparable to those recently inferred for molecular outflows in local ultra-luminous infrared galaxies, suggesting that active galactic nuclei are capable of driving such outflows.

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arXiv2011

On Lyman-limit Systems and the Evolution of the Intergalactic Ionizing Background

We study the properties of self-shielding intergalactic absorption systems and their implications for the ionizing background. We find that cosmological simulations post-processed with detailed radiative transfer calculations generally are able to reproduce the observed abundance of Lyman-limit systems, and we highlight possible discrepancies between the observations and simulations. This comparison tests cosmological simulations at overdensities of ~100. Furthermore, we show that the properties of Lyman-limit systems in these simulations, in simple semi-analytic arguments, and as suggested by recent observations indicate that a small change in the ionizing emissivity of the sources would have resulted in a much larger change in the amplitude of the intergalactic HI-ionizing background (with this scaling strengthening with increasing redshift). This strong scaling could explain the rapid evolution in the Lyman-alpha forest transmission observed at z = 6. Our calculations agree with the suggestion of simpler models that the comoving ionizing emissivity was constant or even increasing from z = 3 to 6. Our calculations also provide a more rigorous estimate than in previous studies for the clumping factor of intergalactic gas after reionization, which we estimate was 2 - 3 at z = 6.

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arXiv2011

The Baryonic Assembly of Dark Matter Halos

We use a suite of cosmological hydrodynamic simulations to quantify the accretion rates of baryons into dark matter halos and the resulting baryon mass fractions, as a function of halo mass, redshift, and baryon type (including cold and hot gas). We find that the net baryonic accretion rates through the virial radius are sensitive to galactic outflows and explore a range of outflow parameters to illustrate the effects. We show that the cold gas accretion rate is in general not a simple universal factor of the dark matter accretion rate, and that galactic winds can cause star formation rates to deviate significantly from the external gas accretion rates, both via gas ejection and re-accretion. Furthermore, galactic winds can inject enough energy and momentum in the surrounding medium to slow down accretion altogether, especially in low-mass halos and at low redshift. By resolving the accretion rates versus radius from the halo centers, we show how cold streams penetrate the hot atmospheres of massive halos at z>2, but gradually disappear at lower redshift. The total baryon mass fraction is also strongly suppressed by outflows in low-mass halos, but is nearly universal in the absence of feedback in halos above the UV background suppression scale. The transition halo mass, at which the gas mass in halos is equal for the cold and hot components, is roughly constant at ~10^11.5 Msun and does not depend sensitively on the wind prescription. We provide simple fitting formulae for the cold gas accretion rate into halos in the no-wind case. Finally, we show that cold accretion is broadly consistent with driving the bulk of the highly star-forming galaxies observed at z~2, but that the more intense star formers likely sample the high end of the accretion rate distribution, and may be additionally fueled by a combination of gas recycling, gas re-accretion, hot mode cooling, and mergers.

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