331 research outputs found
The Large, Oxygen-Rich Halos of Star-Forming Galaxies Are A Major Reservoir of Galactic Metals
The circumgalactic medium (CGM) is fed by galaxy outflows and accretion of
intergalactic gas, but its mass, heavy element enrichment, and relation to
galaxy properties are poorly constrained by observations. In a survey of the
outskirts of 42 galaxies with the Cosmic Origins Spectrograph onboard the
Hubble Space Telescope, we detected ubiquitous, large (150 kiloparsec) halos of
ionized oxygen surrounding star-forming galaxies, but we find much less ionized
oxygen around galaxies with little or no star formation. This ionized CGM
contains a substantial mass of heavy elements and gas, perhaps far exceeding
the reservoirs of gas in the galaxies themselves. It is a basic component of
nearly all star-forming galaxies that is removed or transformed during the
quenching of star formation and the transition to passive evolution.Comment: This paper is part of a set of three papers on circumgalactic gas
observed with the Cosmic Origins Spectrograph on HST, to be published in
Science, together with related papers by Tripp et al. and Lehner & Howk, in
the November 18, 2011 edition. This version has not undergone final
copyediting. Please see Science online for the final printed versio
The COS Absorption Survey of Baryon Harbors (CASBaH): Warm-hot Circumgalactic Gas Reservoirs Traced by Ne VIII Absorption
We survey the highly ionized circumgalactic media (CGM) of 29 blindly
selected galaxies at 0.49 < z_(gal) < 1.44 based on high-S/N ultraviolet
spectra of z > 1 QSOs and the galaxy database from the COS Absorption Survey of
Baryon Harbors (CASBaH). We detect the Ne VIII doublet in nine of the galaxies,
and for gas with N(Ne VIII) > 10^13.3 cm^-2 (> 10^13.5 cm^-2), we derive a Ne
VIII covering fraction f_c = 75 +15/-25% (44 +22/-20%) within impact parameter
(rho) < 200 kpc of M_* = 10^(9.5-11.5) Msol galaxies and f_c = 70 +16/-22% (f_c
= 42 +20/-17%) within rho < 1.5 virial radii. We estimate the mass in Ne
VIII-traced gas to be M_gas(Ne VIII) > 10^9.5 Msol (Z/Zsol)^-1, or 6-20% of the
expected baryonic mass if the Ne VIII absorbers have solar metallicity.
Ionizing Ne VII to Ne VIII requires 207 eV, and photons with this energy are
scarce in the CGM. However, for the median halo mass and redshift of our
sample, the virial temperature is close to the peak temperature for the Ne VIII
ion, and the Ne VIII-bearing gas is plausibly collisionally ionized near this
temperature. Moreover, we find that photoionized Ne VIII requires cool and
low-density clouds that would be highly underpressured (by approximately two
orders of magnitude) relative to the putative, ambient virialized medium,
complicating scenarios where such clouds could survive. Thus, more complex
(e.g., non-equilibrium) models may be required; this first statistical sample
of Ne VIII absorber/galaxy systems will provide stringent constraints for
future CGM studies.Comment: Published in ApJL, Volume 877, Issue 2, Article L2
The COS-Dwarfs Survey: The Carbon Reservoir Around sub-L* Galaxies
We report new observations of circumgalactic gas from the COS-Dwarfs survey,
a systematic investigation of the gaseous halos around 43 low-mass z 0.1
galaxies using background QSOs observed with the Cosmic Origins Spectrograph.
From the projected 1D and 2D distribution of C IV absorption, we find that C IV
absorption is detected out to ~ 0.5 R of the host galaxies. The C IV
absorption strength falls off radially as a power law and beyond 0.5 R,
no C IV absorption is detected above our sensitivity limit of ~ 50-100 m.
We find a tentative correlation between detected C IV absorption strength and
star formation, paralleling the strong correlation seen in highly ionized
oxygen for L~L* galaxies by the COS-Halos survey. The data imply a large carbon
reservoir in the CGM of these galaxies, corresponding to a minimum carbon mass
of 1.2 out to ~ 110 kpc. This mass is
comparable to the carbon mass in the ISM and more than the carbon mass
currently in stars of these galaxies. The C IV absorption seen around these
sub-L* galaxies can account for almost two-thirds of all > 100 m C IV
absorption detected at low z. Comparing the C IV covering fraction with
hydrodynamical simulations, we find that an energy-driven wind model is
consistent with the observations whereas a wind model of constant velocity
fails to reproduce the CGM or the galaxy properties.Comment: 18 Pages, 11 Figures, ApJ 796 13
The intergalactic medium thermal history at redshift z=1.7--3.2 from the Lyman alpha forest: a comparison of measurements using wavelets and the flux distribution
We investigate the thermal history of the intergalactic medium (IGM) in the
redshift interval z=1.7--3.2 by studying the small-scale fluctuations in the
Lyman alpha forest transmitted flux. We apply a wavelet filtering technique to
eighteen high resolution quasar spectra obtained with the Ultraviolet and
Visual Echelle Spectrograph (UVES), and compare these data to synthetic spectra
drawn from a suite of hydrodynamical simulations in which the IGM thermal state
and cosmological parameters are varied. From the wavelet analysis we obtain
estimates of the IGM thermal state that are in good agreement with other
recent, independent wavelet-based measurements. We also perform a reanalysis of
the same data set using the Lyman alpha forest flux probability distribution
function (PDF), which has previously been used to measure the IGM
temperature-density relation. This provides an important consistency test for
measurements of the IGM thermal state, as it enables a direct comparison of the
constraints obtained using these two different methodologies. We find the
constraints obtained from wavelets and the flux PDF are formally consistent
with each other, although in agreement with previous studies, the flux PDF
constraints favour an isothermal or inverted IGM temperature-density relation.
We also perform a joint analysis by combining our wavelet and flux PDF
measurements, constraining the IGM thermal state at z=2.1 to have a temperature
at mean density of T0/[10^3 K]=17.3 +/- 1.9 and a power-law temperature-density
relation exponent gamma=1.1 +/- 0.1 (1 sigma). Our results are consistent with
previous observations that indicate there may be additional sources of heating
in the IGM at z<4.Comment: 15 pages, 14 figures, matches version accepted for publication on
MNRA
Modeling resilience and sustainability in ancient agricultural systems
The reasons why people adopt unsustainable agricultural practices, and the ultimate environmental implications of those practices, remain incompletely understood in the present world. Archaeology, however, offers unique datasets on coincident cultural and ecological change, and their social and environmental effects. This article applies concepts derived from ecological resilience thinking to assess the sustainability of agricultural practices as a result of long-term interactions between political, economic, and environmental systems. Using the urban center of Gordion, in central Turkey, as a case study, it is possible to identify mismatched social and ecological processes on temporal, spatial, and organizational scales, which help to resolve thresholds of resilience. Results of this analysis implicate temporal and spatial mismatches as a cause for local environmental degradation, and increasing extralocal economic pressures as an ultimate cause for the adoption of unsustainable land-use practices. This analysis suggests that a research approach that integrates environmental archaeology with a resilience perspective has considerable potential for explicating regional patterns of agricultural change and environmental degradation in the past
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