7,893 research outputs found
An excess of damped Lyman alpha galaxies near QSOs
We present a sample of 33 damped Lyman alpha systems (DLAs) discovered in the
Sloan Digital Sky Survey (SDSS) whose absorption redshifts (z_abs) are within
6000 km/s of the QSO's systemic redshift (z_sys). Our sample is based on 731
2.5 < z_sys < 4.5 non-broad-absorption-line (non-BAL) QSOs from Data Release 3
(DR3) of the SDSS. We estimate that our search is ~100 % complete for absorbers
with N(HI) >= 2e20 cm^-2. The derived number density of DLAs per unit redshift,
n(z), within v < 6000 km/s is higher (3.5 sigma significance) by almost a
factor of 2 than that of intervening absorbers observed in the SDSS DR3, i.e.
there is evidence for an overdensity of galaxies near the QSOs. This provides a
physical motivation for excluding DLAs at small velocity separations in surveys
of intervening 'field' DLAs. In addition, we find that the overdensity of
proximate DLAs is independent of the radio-loudness of the QSO, consistent with
the environments of radio-loud and radio-quiet QSOs being similar.Comment: Accepted for publication in MNRAS (13 pages, 6 figures
Chemical abundances of damped Lyman alpha systems in the XQ-100 survey
The XQ-100 survey has provided high signal-noise spectra of 100 redshift
3-4.5 quasars with the X-Shooter spectrograph. The metal abundances for 13
elements in the 41 damped Lyman alpha systems (DLAs) identified in the XQ-100
sample are presented, and an investigation into abundances of a variety of DLA
classes is conducted. The XQ-100 DLA sample contains five DLAs within 5000 km/s
of their host quasar (proximate DLAs; PDLAs) as well as three sightlines which
contain two DLAs within 10,000 km/s of each other along the same line-of-sight
(multiple DLAs; MDLAs). Combined with previous observations in the literature,
we demonstrate that PDLAs with logN(HI)<21.0 show lower [S/H] and [Fe/H]
(relative to intervening systems with similar redshift and N(HI)), whilst
higher [S/H] and [Si/H] are seen in PDLAs with logN(HI)>21.0. These abundance
discrepancies are independent of their line-of-sight velocity separation from
the host quasar, and the velocity width of the metal lines (v90). Contrary to
previous studies, MDLAs show no difference in [alpha/Fe] relative to single
DLAs matched in metallicity and redshift. In addition, we present follow-up
UVES data of J0034+1639, a sightline containing three DLAs, including a
metal-poor DLA with [Fe/H]=-2.82 (the third lowest [Fe/H] in DLAs identified to
date) at z=4.25. Lastly we study the dust-corrected [Zn/Fe], emphasizing that
near-IR coverage of X-Shooter provides unprecedented access to MgII, CaII and
TiII lines (at redshifts 3-4) to provide additional evidence for subsolar
[Zn/Fe] ratio in DLAs.Comment: Accepted to MNRAS. 19 pages plus Appendix material (102 pages total
The host galaxy of GRB010222: The strongest damped Lyman-alpha system known
Analysis of the absorption lines in the afterglow spectrum of the gamma-ray
burst GRB010222 indicates that its host galaxy (at a redshift of z=1.476) is
the strongest damped Lyman-alpha (DLA) system known, having a very low
metallicity and modest dust content. This conclusion is based on the detection
of the red wing of Lyman-alpha plus a comparison of the equivalent widths of
ultraviolet Mg I, Mg II, and Fe II lines with those in other DLAs. The column
density of H I, deduced from a fit to the wing of Lyman-alpha, is (5 +/- 2)
10^22 cm^-2. The ratio of the column densities of Zn and Cr lines suggests that
the dust content in our line of sight through the galaxy is low. This could be
due to either dust destruction by the ultraviolet emission of the afterglow or
to an initial dust composition different to that of the diffuse interstellar
material, or a combination of both.Comment: Submitted to MNRAS 12 page
Charge-coupled devices with fast timing for astrophysics and space physics research
A charge coupled device is under development with fast timing capability (15 millisecond full frame readout, 30 microsecond resolution for measuring the time of individual pixel hits). The fast timing CCD will be used in conjunction with a CsI microfiber array or segmented scintillator matrix detector to detect x rays and gamma rays with submillimeter position resolution. The initial application will be in conjunction with a coded aperture hard x ray/gamma ray astronomy instrument. We describe the concept and the readout architecture of the device
Synchronization and Control in Intrinsic and Designed Computation: An Information-Theoretic Analysis of Competing Models of Stochastic Computation
We adapt tools from information theory to analyze how an observer comes to
synchronize with the hidden states of a finitary, stationary stochastic
process. We show that synchronization is determined by both the process's
internal organization and by an observer's model of it. We analyze these
components using the convergence of state-block and block-state entropies,
comparing them to the previously known convergence properties of the Shannon
block entropy. Along the way, we introduce a hierarchy of information
quantifiers as derivatives and integrals of these entropies, which parallels a
similar hierarchy introduced for block entropy. We also draw out the duality
between synchronization properties and a process's controllability. The tools
lead to a new classification of a process's alternative representations in
terms of minimality, synchronizability, and unifilarity.Comment: 25 pages, 13 figures, 1 tabl
HST Observations of SGR 0526-66: New Constraints on Accretion and Magnetar Models
Soft Gamma-ray Repeaters (SGRs) are among the most enigmatic sources known
today. Exhibiting huge X- and Gamma-ray bursts and flares, as well as soft
quiescent X-ray emission, their energy source remains a mystery. Just as
mysterious are the Anomalous X-ray pulsars (AXPs), which share many of the same
characteristics. Thanks to recent Chandra observations, SGR 0526-66, the first
SGR, now appears to be a transition object bridging the two classes, and
therefore observations of it have implications for both SGRs and AXPs. The two
most popular current models for their persistent emission are accretion of a
fossil disk or decay of an enormous (~10^15 G) magnetic field in a magnetar. We
show how deep optical observations of SGR 0526-66, the only SGR with small
enough optical extinction for meaningful observations, show no evidence of an
optical counterpart. These observation place strong new constraints on both
accretion disk and magnetar models, and suggest that the spectral energy
distribution may peak in the hard-UV. Almost all accretion disks are excluded
by the optical data, and a magnetar would require a ~10^15-10^16 G field.Comment: 23 pages, 5 figures. Accepted by Ap
Geographic differences in effects of experimental warming on ant species diversity and community composition
Ecological communities are being reshaped by climatic change. Losses and gains of species will alter community composition and diversity but these effects are likely to vary geographically and may be hard to predict from uncontrolled natural experiments . In this study, we used open-top warming chambers to simulate a range of warming scenarios for ground-nesting ant communities at a northern (Harvard Forest, MA) and southern (Duke Forest, NC) study site in the eastern US. After 2.5 years of experimental warming, we found no significant effects of accumulated growing degree days or soil moisture on ant diversity or community composition at the northern site, but a decrease in asymptotic species richness and changes in community composition at the southern site. However, fewer than 10% of the species at either site responded significantly to the warming treatments. Our results contrast with those of a comparable natural experiment conducted along a nearby elevational gradient, in which species richness and composition responded strongly to changes in temperature and other correlated variables. Together, our findings provide some support for the prediction that warming will have a larger negative effect on ecological communities in warmer locales at lower latitudes and suggest that predicted responses to warming may differ between controlled field experiments and unmanipulated thermal gradients. Key words: Ants; climate change; community; elevational gradient; Formicidae; geographic range; warming experiment
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