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.Comment: 5 pages, 1 figure. Conference proceedings for "AGN Winds in Charleston," Charleston, SC, Oct 15-18, 2011. To be published by AS

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.Comment: 22 pages, 14 figures, to appear in MNRAS. Substantially improved since original posting (including a new AGN luminosity function and spectral treatment, and an improved IGM opacity model). Electronic data available at http://galaxies.northwestern.edu/uv

40 Years of Pulsars: The Birth and Evolution of Isolated Radio Pulsars

We investigate the birth and evolution of isolated radio pulsars using a population synthesis method, modeling the birth properties of the pulsars, their time evolution, and their detection in the Parkes and Swinburne Multibeam (MB) surveys. Together, the Parkes and Swinburne MB surveys have detected nearly 2/3 of the known pulsars and provide a remarkably homogeneous sample to compare with simulations. New proper motion measurements and an improved model of the distribution of free electrons in the interstellar medium, NE2001, also make revisiting these issues particularly worthwhile. We present a simple population model that reproduces the actual observations well, and consider others that fail. We conclude that: pulsars are born in the spiral arms, with the birthrate of 2.8+/-0.5 pulsars/century peaking at a distance ~3 kpc from the Galactic centre, and with mean initial speed of 380^{+40}_{-60} km/s; the birth spin period distribution extends to several hundred milliseconds, with no evidence of multimodality, implying that characteristic ages overestimate the true ages of the pulsars by a median factor >2 for true ages <30,000 yr; models in which the radio luminosities of the pulsars are random generically fail to reproduce the observed P-Pdot diagram, suggesting a relation between intrinsic radio luminosity and (P, Pdot); radio luminosities L Edot^0.5 provide a good match to the observed P-Pdot diagram; for this favored radio luminosity model, we find no evidence for significant magnetic field decay over the lifetime of the pulsars as radio sources ~100 Myr.Comment: 3 pages, including 2 figures, to appear in the proceedings of "40 Years of Pulsars: Millisecond Pulsars, Magnetars, and More", August 12-17, 2007, McGill University, Montreal, Canada; for full details, see astro-ph/051258

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.Comment: 13 pages, including 2 figures, plus references. Author's version of Perspective article published in Nature Astronom

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.Comment: 23 pages, 10 figures. To appear in MNRA

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 Lya forest opacity at redshifts 2<z<4.2 from 86 high-resolution quasar spectra. In addition, we impose the requirements that HI must be reionized by z=6 and HeII by z~3, and consider estimates of the hardness of the ionizing background from HI to HeII column density ratios. The derived hydrogen photoionization rate is remarkably flat over the Lya 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 f_esc~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 ~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 Lya 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 non-negligible and perhaps dominant fraction of the hydrogen ionizing background at their z~2 peak. [Abridged]Comment: 26 pages, including 14 figures, submitted to ApJ and revised following the referee's repor

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.Comment: 10 pages, 5 figures, submitted to ApJ, added Fig.

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.Comment: replaced with version accepted for publication in MNRA

Numerical Simulations Unravel the Cosmic Web

The universe is permeated by a network of filaments, sheets, and knots collectively forming a "cosmic web.'' The discovery of the cosmic web, especially through its signature of absorption of light from distant sources by neutral hydrogen in the intergalactic medium, exemplifies the interplay between theory and experiment that drives science, and is one of the great examples in which numerical simulations have played a key and decisive role. We recount the milestones in our understanding of cosmic structure, summarize its impact on astronomy, cosmology, and physics, and look ahead by outlining the challenges faced as we prepare to probe the cosmic web at new wavelengths.Comment: 10 pages, 2 figures. Appeared as a solicited Perspective article in the January 4, 2008 special issue of Science on the cosmic we

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.Comment: 9 pages, 2 figures. MNRAS, in pres