1,120 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
CHIRON TOOLS: Integrated Target Submission, Scheduling and Observing Systems for a High Resolution Fiber Fed Spectrograph
The CHIRON spectrometer is a new high-resolution, fiber-fed instrument on the
1.5 meter telescope at Cerro Tololo Inter-America Observatory (CTIO). To
optimize use of the instrument and limited human resources, we have designed an
integrated set of web applications allowing target submission, observing script
planning, nightly script execution and logging, and access to reduced data by
multiple users. The unified and easy to use interface has dramatically reduced
the time needed to submit and schedule observations and improved the efficiency
and accuracy of nightly operations. We present our experience to help
astronomers and project managers who need to plan for the scope of effort
required to commission a queue-scheduled facility instrument.Comment: 14 pages, 3 figures, accepted to PAS
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
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
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.
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
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.Comment: 20 pages, including 11 figures. MNRAS, in pres
Planet Detectability in the Alpha Centauri System
We use more than a decade of radial velocity measurements for Cen A,
B, and Proxima Centauri from HARPS, CHIRON, and UVES to identify the
and orbital periods of planets that could have been detected if they existed.
At each point in a mass-period grid, we sample a simulated, Keplerian signal
with the precision and cadence of existing data and assess the probability that
the signal could have been produced by noise alone. Existing data places
detection thresholds in the classically defined habitable zones at about of 53 M for Cen A, 8.4 M for
Cen B, and 0.47 M for Proxima Centauri. Additionally, we examine the
impact of systematic errors, or "red noise" in the data. A comparison of white-
and red-noise simulations highlights quasi-periodic variability in the radial
velocities that may be caused by systematic errors, photospheric velocity
signals, or planetary signals. For example, the red-noise simulations show a
peak above white-noise simulations at the period of Proxima Centauri b. We also
carry out a spectroscopic analysis of the chemical composition of the
Centauri stars. The stars have super-solar metallicity with ratios of C/O and
Mg/Si that are similar to the Sun, suggesting that any small planets in the
Cen system may be compositionally similar to our terrestrial planets.
Although the small projected separation of Cen A and B currently
hampers extreme-precision radial velocity measurements, the angular separation
is now increasing. By 2019, Cen A and B will be ideal targets for
renewed Doppler planet surveys.Comment: 16 pages, 7 figures, data provided in appendix. Accepted for
publication in Ap
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