3,966 research outputs found
Nonrefoulement Under the Convention Against Torture: How U.S. Allowances for Diplomatic Assurances Contravene Treaty Obligations and Federal Law
Nonrefoulement Under the Convention Against Torture: How U.S. Allowances for Diplomatic Assurances Contravene Treaty Obligations and Federal Law
A Survey of Weak MgII Absorbers at 0.4 < z < 2.4
We present results from a survey of weak MgII absorbers in the VLT/UVES
spectra of 81 QSOs obtained from the ESO archive. In this survey, we identified
112 weak MgII systems within the redshift interval 0.4 < z < 2.4 with 86%
completeness down to a rest-frame equivalent width of W_r(2796) = 0.02A,
covering a cumulative redshift path length of deltaZ=77.3. From this sample, we
estimate that the number of weak absorbers per unit redshift dN/dz increases
from 1.06 +/- 0.04 at =1.9 to 1.76 +/- 0.08 at =1.2 and thereafter
decreases to 1.51 +/- 0.09 at =0.9 and 1.06 +/- 0.10 at =0.6. Thus we
find evidence for an evolution in the population of weak MgII absorbers, with
their number density peaking at z=1.2. We also determine the equivalent width
distribution of weak systems at =0.9 and =1.9. At 0.4 < z < 1.4, there is
evidence for a turnover from a powerlaw of the form n(W_r) \propto W_r^{-1.04}
at W_r(2796) < 0.1A. This turnover is more extreme at 1.4 < z < 2.4, where the
equivalent width distribution is close to an extrapolation of the exponential
distribution function found for strong MgII absorbers. Based on these results,
we discuss the possibility that some fraction of weak MgII absorbers,
particularly single cloud systems, are related to satellite clouds surrounding
strong MgII systems. These structures could also be analogs to Milky Way high
velocity clouds. In this context, the paucity of high redshift weak MgII
absorbers is caused by a lack of isolated accreting clouds on to galaxies
during that epoch.Comment: 14 pages, 11 figures, ApJ accepte
The Chemical and Ionization Conditions in Weak Mg II Absorbers
We present an analysis of the chemical and ionization conditions in a sample
of 100 weak Mg II absorbers identified in the VLT/UVES archive of quasar
spectra. Using a host of low ionization lines associated with each absorber in
this sample, and on the basis of ionization models, we infer that the
metallicity in a significant fraction of weak Mg II clouds is constrained to
values of solar or higher, if they are sub-Lyman limit systems. Based on the
observed constraints, we present a physical picture in which weak Mg II
absorbers are predominantly tracing two different astrophysical
processes/structures. A significant population of weak Mg II clouds, those in
which N(Fe II) is much less than N(Mg II), identified at both low (z ~ 1) and
high (z ~ 2) redshift, are potentially tracing gas in the extended halos of
galaxies, analogous to the Galactic high velocity clouds. These absorbers might
correspond to alpha-enhanced interstellar gas expelled from star-forming
galaxies, in correlated supernova events. On the other hand, N(FeII)
approximately equal to N(Mg II) clouds, which are prevalent only at lower
redshifts (z < 1.5), must be tracing Type Ia enriched gas in small, high
metallicity pockets in dwarf galaxies, tidal debris, or other intergalactic
structures.Comment: 35 pages (including tables & figures). Accepted for publication in
ApJ. A high resolution version of the paper is available at
"http://www.astro.wisc.edu/~anand/weakMgII.pdf
The Kinematic Evolution of Strong MgII Absorbers
We consider the evolution of strong (W_r(2796) > 0.3A) MgII absorbers, most
of which are closely related to luminous galaxies. Using 20 high resolution
quasar spectra from the VLT/UVES public archive, we examine 33 strong MgII
absorbers in the redshift range 0.3 < z < 2.5. We compare and supplement this
sample with 23 strong MgII absorbers at 0.4 < z < 1.4 observed previously with
HIRES/Keck. We find that neither equivalent width nor kinematic spread (the
optical depth weighted second moment of velocity) of MgII2796 evolve. However,
the kinematic spread is sensitive to the highest velocity component, and
therefore not as sensitive to additional weak components at intermediate
velocities relative to the profile center. The fraction of absorbing pixels
within the full velocity range of the system does show a trend of decreasing
with decreasing redshift. Most high redshift systems (14/20) exhibit absorption
over the entire system velocity range, which differs from the result for low
redshift systems (18/36) at the 95% level. This leads to a smaller number of
separate subsystems for high redshift systems because weak absorping components
tend to connect the stronger regions of absorption. We hypothesize that low
redshift MgII profiles are more likely to represent well formed galaxies, many
of which have kinematics consistent with a disk/halo structure. High redshift
MgII profiles are more likely to show evidence of complex protogalactic
structures, with multiple accretion or outflow events. Although these results
are derived from measurements of gas kinematics, they are consistent with
hierarchical galaxy formation evidenced by deep galaxy surveys.Comment: Accepted to the Astrophysical Journa
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Author Correction: Clustering and climate associations of Kawasaki Disease in San Diego County suggest environmental triggers.
A correction to this article has been published and is linked from the HTML and PDF versions of this paper. The error has not been fixed in the paper
Quantum entanglement and Bell violation of two coupled cavity fields in dissipative environment
We study the quantum entanglement between two coupled cavities, in which one
is initially prepared in a mesoscopic superposition state and the other is in
the vacuum in dissipative environment and show how the entanglement between two
cavities can arise in the dissipative environment. The dynamic behavior of the
nonlocality for the system is also investigated.Comment: 12 pages, 5 figure
Racial/ethnic bullying subtypes and alcohol, tobacco, and marijuana use among U.S. adolescents
A survey of weak MgII absorbers at redshift <z>=1.78
The exact nature of weak MgII absorbers (those with W_r(2796) < 0.3 A) is a
matter of debate, but most are likely related to areas of local star formation
or supernovae activity outside of giant galaxies. Using 18 QSO spectra obtained
with the Ultra-Violet Echelle Spectrograph (UVES) on the Very Large Telescope
(VLT), we have conducted a survey for weak MgII absorbers at 1.4 < z < 2.4. We
searched a redshift path length of 8.51, eliminating regions badly contaminated
by atmospheric absorption so that the survey is close to 100% complete to
W_r(2796) = 0.02 A. We found a total of 9 weak absorbers, yielding a number
density of absorbers of dN/dz = 1.06 +/- 0.12 for 0.02 <= W_r(2796) < 0.3 A.
Narayanan et al. (2005) found dN/dz = 1.00 +/- 0.20 at 0 < z < 0.3 and
Churchill et al. (1999) found dN/dz = 1.74 +/- 0.10 at 0.4 < z < 1.4.
Therefore, the population of weak MgII absorbers appears to peak at z~1. We
explore the expected evolution of the absorber population subject to a changing
extragalactic background radiation (EBR) from z = 0.9 to z = 1.78 (the median
redshift of our survey), and find that the result is higher than the observed
value. We point out that the peak epoch for weak MgII absorption at z~1 may
coincide with the peak epoch of global star formation in the dwarf galaxy
environment.Comment: 25 pages, 14 figures, to be published in ApJ 01 March 2006, v639,
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