12,832 research outputs found
PC1643+4631A,B: The Lyman-Alpha Forest at the Edge of Coherence
This is the first measurement and detection of coherence in the intergalactic
medium (IGM) at substantially high redshift (z~3.8) and on large physical
scales (~2.5 h^-1 Mpc). We perform the measurement by presenting new
observations from Keck LRIS of the high redshift quasar pair PC 1643+4631A, B
and their Ly-alpha absorber coincidences. This experiment extends multiple
sightline quasar absorber studies to higher redshift, higher opacity, larger
transverse separation, and into a regime where coherence across the IGM becomes
weak and difficult to detect. We fit 222 discrete Ly-alpha absorbers to
sightline A and 211 to sightline B. Relative to a Monte Carlo pairing test
(using symmetric, nearest neighbor matching) the data exhibit a 4sigma excess
of pairs at low velocity splitting (<150 km/s), thus detecting coherence on
transverse scales of ~2.5 h^-1 Mpc. We use spectra extracted from an SPH
simulation to analyze symmetric pair matching, transmission distributions as a
function of redshift and compute zero-lag cross-correlations to compare with
the quasar pair data. The simulations agree with the data with the same
strength (~4sigma) at similarly low velocity splitting above random chance
pairings. In cross-correlation tests, the simulations agree when the mean flux
(as a function of redshift) is assumed to follow the prescription given by
Kirkman et al. (2005). While the detection of flux correlation (measured
through coincident absorbers and cross-correlation amplitude) is only
marginally significant, the agreement between data and simulations is
encouraging for future work in which even better quality data will provide the
best insight into the overarching structure of the IGM and its understanding as
shown by SPH simulations.Comment: 15 pages, 11 figures; accepted for publication in Astronomical
Journa
Near-Infrared MOSFIRE Spectra of Dusty Star-Forming Galaxies at 0.2<z<4
We present near-infrared and optical spectroscopic observations of a sample
of 450m and 850m-selected dusty star-forming galaxies (DSFGs)
identified in a 400 arcmin area in the COSMOS field. Thirty-one sources of
the 102 targets were spectroscopically confirmed at , identified
primarily in the near-infrared with Keck MOSFIRE and some in the optical with
Keck LRIS and DEIMOS. The low rate of confirmation is attributable both to high
rest-frame optical obscuration in our targets and limited sensitivity to
certain redshift ranges. The high-quality photometric redshifts available in
the COSMOS field allow us to test the robustness of photometric redshifts for
DSFGs. We find a subset (11/31%) of DSFGs with inaccurate () or non-existent photometric redshifts; these have very distinct
spectral energy distributions from the remaining DSFGs, suggesting a decoupling
of highly obscured and unobscured components. We present a composite rest-frame
4300--7300\AA\ spectrum for DSFGs, and find evidence of 20030 km s
gas outflows. Nebular line emission for a sub-sample of our detections indicate
that hard ionizing radiation fields are ubiquitous in high-z DSFGs, even more
so than typical mass or UV-selected high-z galaxies. We also confirm the
extreme level of dust obscuration in DSFGs, measuring very high Balmer
decrements, and very high ratios of IR to UV and IR to H luminosities.
This work demonstrates the need to broaden the use of wide bandwidth technology
in the millimeter to the spectroscopic confirmations of large samples of high-z
DSFGs, as the difficulty in confirming such sources at optical/near-infrared
wavelengths is exceedingly challenging given their obscuration.Comment: 14 pages, 13 figures, ApJ accepted. Composite DSFG Halpha spectrum
available at www.as.utexas.edu/~cmcasey/downloads.htm
Archaeobotanical evidence for pearl millet (Pennisetum glaucum) in sub-Saharan West Africa
The remains of pearl millet (Pennisetum glaucum) dating to 3460±200 and 2960±370 BP have been recovered at the archaeological site of Birimi, northern Ghana, associated with the Kintampo cultural complex. This finding represents the earliest known occurrence of pearl millet in sub-Saharan Africa. Results indicate that Kintampo peoples developed effective subsistence adaptations to savannas as well as tropical forest habitat
A Review of the Relationship of the ENC and DNC Hydrographic Vector Data Products
This document discusses the level of harmonization between the S-57 and DIGEST standards and the relationship between the ENC and DNC products. It is shown that the military and regulated commercial navigation have different needs, and that it is natural to have two different standards. However, in those areas where the needs overlap the standards should be equivalent. Commercial navigation requires "official" data whereas the military require a broad range of the "best available data". These needs are complementary. At the content level ENC data can be a pure subset of DNC data, however additional harmonization is required to achieve this
The Stellar Populations and Evolution of Lyman Break Galaxies
Using deep near-IR and optical observations of the HDF-N from the HST NICMOS
and WFPC2 and from the ground, we examine the spectral energy distributions
(SEDs) of Lyman break galaxies (LBGs) at 2.0 < z < 3.5. The UV-to-optical
rest-frame SEDs of the galaxies are much bluer than those of present-day spiral
and elliptical galaxies, and are generally similar to those of local starburst
galaxies with modest amounts of reddening. We use stellar population synthesis
models to study the properties of the stars that dominate the light from LBGs.
Under the assumption that the star-formation rate is continuous or decreasing
with time, the best-fitting models provide a lower bound on the LBG mass
estimates. LBGs with ``L*'' UV luminosities are estimated to have minimum
stellar masses ~ 10^10 solar masses, or roughly 1/10th that of a present-day L*
galaxy. By considering the effects of a second component of maximally-old
stars, we set an upper bound on the stellar masses that is ~ 3-8 times the
minimum estimate. We find only loose constraints on the individual galaxy ages,
extinction, metallicities, initial mass functions, and prior star-formation
histories. We find no galaxies whose SEDs are consistent with young (< 10^8
yr), dust-free objects, which suggests that LBGs are not dominated by ``first
generation'' stars, and that such objects are rare at these redshifts. We also
find that the typical ages for the observed star-formation events are
significantly younger than the time interval covered by this redshift range (~
1.5 Gyr). From this, and from the relative absence of candidates for quiescent,
non-star-forming galaxies at these redshifts in the NICMOS data, we suggest
that star formation in LBGs may be recurrent, with short duty cycles and a
timescale between star-formation events of < 1 Gyr. [Abridged]Comment: LaTeX, 37 pages, 21 figures. Accepted for publication in the
Astrophysical Journa
Deep observations of the Super-CLASS super-cluster at 325 MHz with the GMRT: the low-frequency source catalogue
We present the results of 325 MHz GMRT observations of a super-cluster field,
known to contain five Abell clusters at redshift . We achieve a
nominal sensitivity of Jy beam toward the phase centre. We
compile a catalogue of 3257 sources with flux densities in the range
within the entire square degree
field of view. Subsequently, we use available survey data at other frequencies
to derive the spectral index distribution for a sub-sample of these sources,
recovering two distinct populations -- a dominant population which exhibit
spectral index trends typical of steep-spectrum synchrotron emission, and a
smaller population of sources with typically flat or rising spectra. We
identify a number of sources with ultra-steep spectra or rising spectra for
further analysis, finding two candidate high-redshift radio galaxies and three
gigahertz-peaked-spectrum radio sources. Finally, we derive the
Euclidean-normalised differential source counts using the catalogue compiled in
this work, for sources with flux densities in excess of Jy. Our
differential source counts are consistent with both previous observations at
this frequency and models of the low-frequency source population. These
represent the deepest source counts yet derived at 325 MHz. Our source counts
exhibit the well-known flattening at mJy flux densities, consistent with an
emerging population of star-forming galaxies; we also find marginal evidence of
a downturn at flux densities below Jy, a feature so far only seen
at 1.4 GHz.Comment: 25 pages, 18 figures, 10 tables. Accepted for publication in MNRA
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