Claude André Faucher-Giguère

Galaxies on FIRE (Feedback In Realistic Environments): Stellar feedback explains cosmologically inefficient star formation

We present a series of high-resolution cosmological simulations of galaxy formation to z = 0, spanning halo masses ∼10^8–10^(13) M⊙, and stellar masses ∼10^4–10^(11) M⊙. Our simulations include fully explicit treatment of the multiphase interstellar medium and stellar feedback. The stellar feedback inputs (energy, momentum, mass, and metal fluxes) are taken directly from stellar population models. These sources of feedback, with zero adjusted parameters, reproduce the observed relation between stellar and halo mass up to M_(halo) ∼ 10^(12) M⊙. We predict weak redshift evolution in the M*–M_(halo) relation, consistent with current constraints to z > 6. We find that the M*–M_(halo) relation is insensitive to numerical details, but is sensitive to feedback physics. Simulations with only supernova feedback fail to reproduce observed stellar masses, particularly in dwarf and high-redshift galaxies: radiative feedback (photoheating and radiation pressure) is necessary to destroy giant molecular clouds and enable efficient coupling of later supernovae to the gas. Star formation rates (SFRs) agree well with the observed Kennicutt relation at all redshifts. The galaxy-averaged Kennicutt relation is very different from the numerically imposed law for converting gas into stars, and is determined by self-regulation via stellar feedback. Feedback reduces SFRs and produces reservoirs of gas that lead to rising late-time star formation histories, significantly different from halo accretion histories. Feedback also produces large short-time-scale variability in galactic SFRs, especially in dwarfs. These properties are not captured by common ‘sub-grid’ wind models.

A new calculation of the ionizing background spectrum and the effects of He II reionization

The ionizing background determines the ionization balance and the thermodynamics of the cosmic gas. It is therefore a fundamental ingredient to theoretical and empirical studies of both the intergalactic medium (IGM) and galaxy formation. We present here a new calculation of its spectrum that satisfies the empirical constraints we recently obtained by combining state-of-the-art luminosity functions and intergalactic opacity measurements. In our preferred model, star-forming galaxies and quasars each contribute substantially to the H I ionizing field at z < 3, with galaxies rapidly overtaking quasars at higher redshifts as quasars become rarer. In addition to our fiducial model, we explore the physical dependences of the calculated background and clarify how recombination emission contributes to the ionization rates. We find that recombinations do not simply boost the ionization rates by the number of re-emitted ionizing photons as many of these rapidly redshift below the ionization edges and have a distribution of energies. A simple analytic model that captures the main effects seen in our numerical radiative transfer calculations is given. Finally, we discuss the effects of He II reionization by quasars on both the spectrum of the ionizing background and on the thermal history of the IGM. In regions that have yet to be reionized, the spectrum is expected to be almost completely suppressed immediately above 54.4 eV, while a background of higher energy (0.5 keV) photons permeates the entire universe owing to the frequency dependence of the photoionization cross section. We provide an analytical model of the heat input during He II reionization and its effects on the temperature-density relation.

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 ~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.

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 ultraluminous 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 vin ≳ 10 000 km s−1, as observed in ultraviolet 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 vin ∼ 1000 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 ∼vin/2vs by work done by the hot post-shock gas, where vs is the velocity of the swept-up material. Energy-conserving outflows driven by fast AGN winds (vin ∼ 0.1c) may therefore explain the momentum fluxes of galaxy-scale outflows recently measured in luminous quasars and ULIRGs. Shocked wind bubbles expanding normal to galactic discs may also explain the large-scale bipolar structures observed in some systems, including around the Galactic Centre, and can produce significant radio, X-ray and γ-ray emission. The analytic solutions presented here will inform implementations of AGN feedback in numerical simulations, which typically do not include all the important physics.

Evolution of the Intergalactic Opacity: Implications for the Ionizing Background, Cosmic Star Formation, and Quasar Activity

We investigate the implications of the intergalactic opacity for the evolution of the cosmic UV luminosity density and its sources. Our main constraint is our measurement of the Ly? forest opacity at redshifts -->2 ? z ? 4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that H I must be reionized by -->z = 6 and He II by -->z ~ 3 and consider estimates of the hardness of the ionizing background from H I-to-He II column density ratios. The derived hydrogen photoionization rate is remarkably flat over the Ly? forest redshift range covered. Because the quasar luminosity function is strongly peaked near -->z ~ 2, the lack of redshift evolution indicates that star-forming galaxies likely dominate the photoionization rate at -->z 3. Combined with direct measurements of the galaxy UV luminosity function, this requires only a small fraction -->fesc ~ 0.5% of galactic hydrogen-ionizing photons to escape their source for galaxies to solely account for the entire ionizing background. Under the assumption that the galactic UV emissivity traces the star formation rate, current state-of-the-art observational estimates of the star formation rate density appear to underestimate the total photoionization rate at -->z ~ 4 by a factor of ~4, are in tension with recent determinations of the UV luminosity function, and fail to reionize the universe by -->z ~ 6 if extrapolated to arbitrarily high redshift. A theoretical star formation history peaking earlier fits the Ly? forest photoionization rate well, reionizes the universe in time, and is in better agreement with the rate of -->z ~ 4 gamma-ray bursts observed by Swift. Quasars suffice to doubly ionize helium by -->z ~ 3 and likely contribute a nonnegligible and perhaps dominant fraction of the hydrogen-ionizing background at their -->z ~ 2 peak.

He II reionization and its effect on the intergalactic medium

Observations of the intergalactic medium (IGM) suggest that quasars reionize He?II in the IGM at z 3. We have run a set of 190 and 430 comoving Mpc simulations of He?II being reionized by quasars to develop an understanding of the nature of He?II reionization and its potential impact on observables. We find that He?II reionization heats regions in the IGM by as much as 25, 000 K above the temperature that is expected otherwise, with the volume-averaged temperature increasing by ~12, 000 K and with large temperature fluctuations on ~50 Mpc scales. Much of this heating occurs far from quasars by photons with long mean free path. We find a temperature-density equation of state of ? ? 1 0.3 during He?II reionization, but with a wide dispersion in this relation having ? T ~ 104 K. He?II reionization by the observed population of quasars cannot produce an inverted relation (? ? 1 < 0). Our simulations are consistent with the observed evolution in the mean transmission of the He?II Ly? forest. We argue that the heat input from He?II reionization is unable to cause the observed depression at z 3.2 in the H?I Ly? forest opacity as has been suggested. We investigate how uncertainties in the properties of QSOs and of He?II Lyman limit systems influence our predictions.

Gusty, gaseous flows of FIRE: galactic winds in cosmological simulations with explicit stellar feedback

We present an analysis of the galaxy-scale gaseous outflows from the Feedback in Realistic Environments (FIRE) simulations. This suite of hydrodynamic cosmological zoom simulations resolves formation of star-forming giant molecular clouds to z = 0, and features an explicit stellar feedback model on small scales. Our simulations reveal that high-redshift galaxies undergo bursts of star formation followed by powerful gusts of galactic outflows that eject much of the interstellar medium and temporarily suppress star formation. At low redshift, however, sufficiently massive galaxies corresponding to L* progenitors develop stable discs and switch into a continuous and quiescent mode of star formation that does not drive outflows far into the halo. Mass-loading factors for winds in L* progenitors are η ≈ 10 at high redshift, but decrease to η ≪ 1 at low redshift. Although lower values of η are expected as haloes grow in mass over time, we show that the strong suppression of outflows with decreasing redshift cannot be explained by mass evolution alone. Circumgalactic outflow velocities are variable and broadly distributed, but typically range between one and three times the circular velocity of the halo. Much of the ejected material builds a reservoir of enriched gas within the circumgalactic medium, some of which could be later recycled to fuel further star formation. However, a fraction of the gas that leaves the virial radius through galactic winds is never regained, causing most haloes with mass M_h ≤ 10^(12) M_⊙ to be deficient in baryons compared to the cosmic mean by z = 0.

A Direct Precision Measurement of the Intergalactic Lyα Opacity at 2 ≤ z ≤ 4.2* **

We measure the evolution of the intergalactic Lyα effective optical depth, -->τeff, over the redshift range -->2 ≤ z≤ 4.2 from a sample of 86 high-resolution, high-S/N 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 Lyα forest from high-resolution spectra in this redshift interval of a factor of 2 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 Lyα 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 --> Δ C/Ctrue is z = 2, 4% at -->z = 3, and 12% at -->z = 4. Previous measurements of -->τeff at -->z 3 based on directly fitting the quasar continua in the Lyα 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 Lyα 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 -->Δ z = 0.2 around -->z = 3, indicates significant departures from the best-fit power-law redshift evolution [ -->τeff = 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 SDSS spectra by Bernardi and coworkers using an independent approach.

Forged in fire: cusps, cores and baryons in low-mass dwarf galaxies

We present multiple ultrahigh resolution cosmological hydrodynamic simulations of M_★ ≃ 10^(4–6.3) M_⊙ dwarf galaxies that form within two M_(vir) = 10^(9.5–10) M_⊙ dark matter halo initial conditions. Our simulations rely on the Feedback in Realistic Environments (FIRE) implementation of star formation feedback and were run with high enough force and mass resolution to directly resolve structure on the ∼200 pc scales. The resultant galaxies sit on the M_★ versus M_(vir) relation required to match the Local Group stellar mass function via abundance matching. They have bursty star formation histories and also form with half-light radii and metallicities that broadly match those observed for local dwarfs at the same stellar mass. We demonstrate that it is possible to create a large (∼1 kpc) constant-density dark matter core in a cosmological simulation of an M_★ ≃ 10^(6.3) M_⊙ dwarf galaxy within a typical M_(vir) = 10^(10) M_⊙ halo – precisely the scale of interest for resolving the ‘too big to fail’ problem. However, these large cores are not ubiquitous and appear to correlate closely with the star formation histories of the dwarfs: dark matter cores are largest in systems that form their stars late (z ≲ 2), after the early epoch of cusp building mergers has ended. Our M_★ ≃ 10^4 M_⊙ dwarf retains a cuspy dark matter halo density profile that matches that of a dark-matter-only run of the same system. Though ancient, most of the stars in our ultrafaint form after reionization; the ultraviolet field acts mainly to suppress fresh gas accretion, not to boil away gas that is already present in the protodwarf.

Arecibo pulsar survey using alfa. I. Survey strategy and first discoveries

We report results from the initial stage of a long-term pulsar survey of the Galactic plane using the Arecibo L-band Feed Array (ALFA), a seven-beam receiver operating at 1.4 GHz with 0.3 GHz bandwidth, and fast-dump digital spectrometers. The search targets low Galactic latitudes, |b| 5°, in the accessible longitude ranges 32° l 77° and 168° l 214°. The instrumentation, data processing, initial survey observations, sensitivity, and database management are described. Data discussed here were collected over a 100 MHz passband centered on 1.42 GHz using a spectrometer that recorded 256 channels every 64 μs. Analysis of the data with their full time and frequency resolutions is ongoing. Here we report the results of a preliminary, low-resolution analysis for which the data were decimated to speed up the processing. We have detected 29 previously known pulsars and discovered 11 new ones. One of these, PSR J1928+1746, with a period of 69 ms and a relatively low characteristic age of 82 kyr, is a plausible candidate for association with the unidentified EGRET source 3EG J1928+1733. Another, PSR J1906+07, is a nonrecycled pulsar in a relativistic binary with an orbital period of 3.98 hr. In parallel with the periodicity analysis, we also search the data for isolated dispersed pulses. This technique has resulted in the discovery of PSR J0628+09, an extremely sporadic radio emitter with a spin period of 1.2 s. Simulations we have carried out indicate that ~1000 new pulsars will be found in our ALFA survey. In addition to providing a large sample for use in population analyses and for probing the magnetoionic interstellar medium, the survey maximizes the chances of finding rapidly spinning millisecond pulsars and pulsars in compact binary systems. Our search algorithms exploit the multiple data streams from ALFA to discriminate between radio frequency interference and celestial signals, including pulsars and possibly new classes of transient radio sources.