54 research outputs found
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
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