599 research outputs found
A New Measurement of the Stellar Mass Density at z~5: Implications for the Sources of Cosmic Reionization
We present a new measurement of the integrated stellar mass per comoving
volume at redshift 5 determined via spectral energy fitting drawn from a sample
of 214 photometrically-selected galaxies with z'<26.5 in the southern GOODS
field. Following procedures introduced by Eyles et al. (2005), we estimate
stellar masses for various sub-samples for which reliable and unconfused
Spitzer IRAC detections are available. A spectroscopic sample of 14 of the most
luminous sources with =4.92 provides a firm lower limit to the stellar mass
density of 1e6 Msun/Mpc^3. Several galaxies in this sub-sample have masses of
order 10^11 Msun implying significant earlier activity occurred in massive
systems. We then consider a larger sample whose photometric redshifts in the
publicly-available GOODS-MUSIC catalog lie in the range 4.4 <z 5.6. Before
adopting the GOODS-MUSIC photometric redshifts, we check the accuracy of their
photometry and explore the possibility of contamination by low-z galaxies and
low-mass stars. After excising probable stellar contaminants and using the z'-J
color to exclude any remaining foreground red galaxies, we conclude that 196
sources are likely to be at z~5. The implied mass density from the unconfused
IRAC fraction of this sample, scaled to the total available, is 6e6 Msun/Mpc^3.
We discuss the uncertainties as well as the likelihood that we have
underestimated the true mass density. Including fainter and quiescent sources
the total integrated density could be as high as 1e7 Msun/Mpc^3. Using the
currently available (but highly uncertain) rate of decline in the star
formationhistory over 5 <z< 10, a better fit is obtained for the assembled mass
at z~5 if we admit significant dust extinction at early times or extend the
luminosity function to very faint limits. [abridged]Comment: Accepted for Publication in ApJ, 39 page
Atom-diatom scattering dynamics of spinning molecules
We present full quantum mechanical scattering calculations using spinning
molecules as target states for nuclear spin selective atom-diatom scattering
of reactive D+H2 and F+H2 collisions. Molecules can be forced to rotate uni-
directionally by chiral trains of short, non-resonant laser pulses, with
different nuclear spin isomers rotating in opposite directions. The
calculations we present are based on rotational wavepackets that can be
created in this manner. As our simulations show, target molecules with
opposite sense of rotation are predominantly scattered in opposite directions,
opening routes for spatially and quantum state selective scattering of close
chemical species. Moreover, two-dimensional state resolved differential cross
sections reveal detailed information about the scattering mechanisms, which
can be explained to a large degree by a classical vector model for scattering
with spinning molecules
Searching for massive galaxies at z>=3.5 in GOODS-North
We constrain the space density and properties of massive galaxy candidates at
redshifts of z>=3.5 in the GOODS-N field. By selecting sources in the
Spitzer+IRAC bands, a highly stellar mass-complete sample is
assembled,including massive galaxies which are very faint in the
optical/near-IR bands that would be missed by samples selected at shorter
wavelengths. The z>=3.5 sample was selected down to 23 mag at 4.5 micron using
photometric redshifts that have been obtained by fitting the galaxies SEDs at
optical, near-IR and IRAC bands. We also require that the brightest band in
which candidates are detected is the IRAC 8 micron band in order to ensure that
the near-IR 1.6 micron (rest-frame) peak is falling in or beyond this band. We
found 53 z>=3.5 candidates, with masses in the range of M~10^10-10^11M_sun. At
least ~81% of these galaxies are missed by traditional Lyman Break selection
methods based on UV light. Spitzer+MIPS emission is detected for 60% of the
sample of z>=3.5 galaxy candidates. Although in some cases this might suggest a
residual contamination from lower redshift star-forming galaxies or AGN, 37% of
these objects are also detected in the sub-mm/mm bands in recent SCUBA,AzTEC
and MAMBO surveys, and have properties fully consistent with vigorous starburst
galaxies at z>=3.5. The comoving number density of galaxies with stellar masses
>= 5x10^10M_sun(a reasonable stellar mass completeness limit for our sample) is
2.6x10^-5Mpc^-3 (using the volume within 3.5<z<5), and the corresponding
stellar mass density is ~2.9x10^6M_sunMpc^-3, or~3% of the local density above
the same stellar mass limit.For the sub-sample of MIPS-undetected galaxies,we
find a number density of ~0.97x10^-5Mpc^-3 and a stellar mass density of
~1.15x10^6M_sun Mpc^-3.[abridged]Comment: Accepted by A&A; 35 pages, 15 figures, references update
Vitamin D in the general population of young adults with autism in the Faroe Islands
Vitamin D deficiency has been proposed as a possible risk factor for developing autism spectrum disorder (ASD). 25-Hydroxyvitamin D3 (25(OH)D3) levels were examined in a cross-sectional population-based study in the Faroe Islands. The case group consisting of a total population cohort of 40 individuals with ASD (aged 15–24 years) had significantly lower 25(OH)D3 than their 62 typically-developing siblings and their 77 parents, and also significantly lower than 40 healthy age and gender matched comparisons. There was a trend for males having lower 25(OH)D3 than females. Effects of age, month/season of birth, IQ, various subcategories of ASD and Autism Diagnostic Observation Schedule score were also investigated, however, no association was found. The very low 25(OH)D3 in the ASD group suggests some underlying pathogenic mechanism
The period and amplitude changes of Polaris (alpha UMi) from 2003 to 2007 measured with SMEI
We present an analysis of 4.5 years of high precision (0.1%) space-based
photometric measurements of the Cepheid variable Polaris, obtained by the broad
band Solar Mass Ejection Imager (SMEI) instrument on board the Coriolis
satellite. The data span from April 2003 to October 2007, with a cadence of 101
minutes and a fill factor of 70%. We have measured the mean peak to peak
amplitude across the whole set of observations to be 25 mmag. There is,
however, a clear trend that the size of the oscillations has been increasing
during the observations, with peak to peak variations less than 22 mmag in
early 2003, increasing to around 28 mmag by October 2007, suggesting that the
peak to peak amplitude is increasing at a rate of 1.39 \pm 0.12 mmag yr^{-1}.
Additionally, we have combined our new measurements with archival measurements
to measure a rate of period change of 4.90 \pm 0.26 s yr^{-1} over the last 50
years. However, there is some suggestion that the period of Polaris has
undergone a recent decline, and combined with the increased amplitude, this
could imply evolution away from an overtone pulsation mode into the fundamental
or a double pulsation mode depending on the precise mass of Polaris.Comment: 6 pages, 7 figures, 2 tables, accepted in MNRA
Early star-forming galaxies and the reionization of the Universe
Star forming galaxies represent a valuable tracer of cosmic history. Recent
observational progress with Hubble Space Telescope has led to the discovery and
study of the earliest-known galaxies corresponding to a period when the
Universe was only ~800 million years old. Intense ultraviolet radiation from
these early galaxies probably induced a major event in cosmic history: the
reionization of intergalactic hydrogen. New techniques are being developed to
understand the properties of these most distant galaxies and determine their
influence on the evolution of the universe.Comment: Review article appearing in Nature. This posting reflects a submitted
version of the review formatted by the authors, in accordance with Nature
publication policies. For the official, published version of the review,
please see http://www.nature.com/nature/archive/index.htm
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