1,664 research outputs found
The Micro Slit Gas Detector
We describe the first tests with a new proportional gas detector. Its
geometry consists in slits opened in a copper metallized kapton foil with 30
micron anode strips suspended in these openings. In this way the multiplication
process is similar to a standard MSGC. The fundamental difference is the
absence of an insulating substrate around the anode. Also the material budget
is significantly reduced, and the problems related to charging-up or
polarization are removed. Ageing properties of this detector are under study.Comment: 13 pages tex file, 10 figures ep
The Dearth of z~10 Galaxies in all HST Legacy Fields -- The Rapid Evolution of the Galaxy Population in the First 500 Myr
We present an analysis of all prime HST legacy fields spanning >800 arcmin^2
for the search of z~10 galaxy candidates and the study of their UV luminosity
function (LF). In particular, we present new z~10 candidates selected from the
full Hubble Frontier Field (HFF) dataset. Despite the addition of these new
fields, we find a low abundance of z~10 candidates with only 9 reliable sources
identified in all prime HST datasets that include the HUDF09/12, the HUDF/XDF,
all the CANDELS fields, and now the HFF survey. Based on this comprehensive
search, we find that the UV luminosity function decreases by one order of
magnitude from z~8 to z~10 at all luminosities over a four magnitude range.
This also implies a decrease of the cosmic star-formation rate density by an
order of magnitude within 170 Myr from z~8 to z~10. We show that this
accelerated evolution compared to lower redshift can entirely be explained by
the fast build-up of the dark matter halo mass function at z>8. Consequently,
the predicted UV LFs from several models of galaxy formation are in good
agreement with this observed trend, even though the measured UV LF lies at the
low end of model predictions. In particular, the number of only 9 observed
candidate galaxies is lower, by ~50%, than predicted by galaxy evolution
models. The difference is generally still consistent within the Poisson and
cosmic variance uncertainties. However, essentially all models predict larger
numbers than observed. We discuss the implications of these results in light of
the upcoming James Webb Space Telescope mission, which is poised to find much
larger samples of z~10 galaxies as well as their progenitors at less than 400
Myr after the Big Bang.Comment: 13 pages, 6 figures, minor updates to match accepted versio
Evidence for a cost of immunity when the crustacean Daphnia magna is exposed to the bacterial pathogen Pasteuria ramosa
Pressure-induced hole doping of the Hg-based cuprate superconductors
We investigate the electronic structure and the hole content in the
copper-oxygen planes of Hg based high Tc cuprates for one to four CuO2 layers
and hydrostatic pressures up to 15 GPa. We find that with the pressure-induced
additional number of holes of the order of 0.05e the density of states at the
Fermi level changes approximately by a factor of 2. At the same time the saddle
point is moved to the Fermi level accompanied by an enhanced k_z dispersion.
This finding explains the pressure behavior of Tc and leads to the conclusion
that the applicability of the van Hove scenario is restricted. By comparison
with experiment, we estimate the coupling constant to be of the order of 1,
ruling out the weak coupling limit.Comment: 4 pages, 4 figure
Rest-Frame Optical Emission Lines in z~3.5 Lyman Break selected Galaxies: The Ubiquity of Unusually High [OIII]/Hbeta Ratios at 2 Gyr
We present K-band spectra of rest-frame optical emission lines for 24
star-forming galaxies at z~3.2-3.7 using MOSFIRE on the Keck 1 telescope.
Strong rest-frame optical [O III] and Hbeta emission lines were detected in 18
LBGs. The median flux ratio of [O III]5007 to Hbeta is 5.1+/-0.5, a factor of
5-10x higher than in local galaxies with similar stellar masses. The observed
Hbeta luminosities are in good agreement with expectations from the estimated
star-formation rates, and none of our sources are detected in deep X-ray
stacks, ruling out significant contamination by active galactic nuclei.
Combining our sample with a variety of LBGs from the literature, including 49
galaxies selected in a very similar manner, we find a high median ratio of
[OIII]/Hbeta = 4.8+0.8-1.7. This high ratio seems to be an ubiquitous feature
of z~3-4 LBGs, very different from typical local star-forming galaxies at
similar stellar masses. The only comparable systems at z~0 are those with
similarly high specific star-formation rates, though ~5x lower stellar masses.
High specific star-formation rates either result in a much higher ionization
parameter or other unusual conditions for the interstellar medium, which result
in a much higher [OIII]/Hbeta line ratio. This implies a strong relation
between a global property of a galaxy, the specific star-formation rate, and
the local conditions of ISM in star-forming regions.Comment: 14 pages, 8 figures, 5 color, published in ApJ, updated to reflect
published versio
First Frontier Field Constraints on the Cosmic Star-Formation Rate Density at z~10 - The Impact of Lensing Shear on Completeness of High-Redshift Galaxy Samples
We search the complete Hubble Frontier Field dataset of Abell 2744 and its
parallel field for z~10 sources to further refine the evolution of the cosmic
star-formation rate density (SFRD) at z>8. We independently confirm two images
of the recently discovered triply-imaged z~9.8 source by Zitrin et al. (2014)
and set an upper limit for similar z~10 galaxies with red colors of
J_125-H_160>1.2 in the parallel field of Abell 2744. We utilize extensive
simulations to derive the effective selection volume of Lyman-break galaxies at
z~10, both in the lensed cluster field and in the adjacent parallel field.
Particular care is taken to include position-dependent lensing shear to
accurately account for the expected sizes and morphologies of highly-magnified
sources. We show that both source blending and shear reduce the completeness at
a given observed magnitude in the cluster, particularly near the critical
curves. These effects have a significant, but largely overlooked, impact on the
detectability of high-redshift sources behind clusters, and substantially
reduce the expected number of highly-magnified sources. The detections and
limits from both pointings result in a SFRD which is higher by 0.4+-0.4 dex
than previous estimates at z~10 from blank fields. Nevertheless, the
combination of these new results with all other estimates remain consistent
with a rapidly declining SFRD in the 170 Myr from z~8 to z~10 as predicted by
cosmological simulations and dark-matter halo evolution in LambdaCDM. Once
biases introduced by magnification-dependent completeness are accounted for,
the full six cluster and parallel Frontier Field program will be an extremely
powerful new dataset to probe the evolution of the galaxy population at z>8
before the advent of the JWST.Comment: 10 pages, 7 figures, changed to match accepted version to appear in
Ap
The size-star formation relation of massive galaxies at 1.5<z<2.5
We study the relation between size and star formation activity in a complete
sample of 225 massive (M > 5 x 10^10 Msun) galaxies at 1.5<z<2.5, selected from
the FIREWORKS UV-IR catalog of the CDFS. Based on stellar population synthesis
model fits to the observed restframe UV-NIR SEDs, and independent MIPS 24
micron observations, 65% of galaxies are actively forming stars, while 35% are
quiescent. Using sizes derived from 2D surface brightness profile fits to high
resolution (FWHM_{PSF}~0.45 arcsec) groundbased ISAAC data, we confirm and
improve the significance of the relation between star formation activity and
compactness found in previous studies, using a large, complete mass-limited
sample. At z~2, massive quiescent galaxies are significantly smaller than
massive star forming galaxies, and a median factor of 0.34+/-0.02 smaller than
galaxies of similar mass in the local universe. 13% of the quiescent galaxies
are unresolved in the ISAAC data, corresponding to sizes <1 kpc, more than 5
times smaller than galaxies of similar mass locally. The quiescent galaxies
span a Kormendy relation which, compared to the relation for local early types,
is shifted to smaller sizes and brighter surface brightnesses and is
incompatible with passive evolution. The progenitors of the quiescent galaxies,
were likely dominated by highly concentrated, intense nuclear star bursts at
z~3-4, in contrast to star forming galaxies at z~2 which are extended and
dominated by distributed star formation.Comment: 6 pages, 4 figures, accepted for publication in Ap
A Rest-frame Optical View on z~4 Galaxies I: Color and Age Distributions from Deep IRAC Photometry of the IUDF10 and GOODS Surveys
We present a study of rest-frame UV-to-optical color distributions for z~4
galaxies based on the combination of deep HST/ACS+WFC3/IR data with
Spitzer/IRAC imaging. In particular, we use new, ultra-deep data from the IRAC
Ultradeep Field program (IUDF10). Our sample contains a total of ~2600 galaxies
selected as B-dropout Lyman Break Galaxies (LBGs) in the HUDF and one of its
deep parallel fields, the HUDF09-2, as well as GOODS-North and South. This
sample is used to investigate the UV continuum slopes beta and Balmer break
colors (J_125-[4.5]) as a function of rest-frame optical luminosity. The [4.5]
filter is chosen to avoid potential contamination by strong rest-frame optical
emission lines. We find that galaxies at M_z<-21.5 (roughly corresponding to
L*[z~4]) are significantly redder than their lower luminosity counterparts. The
UV continuum slopes and the J_125-[4.5] colors are well correlated. The most
simple explanation for this correlation is that the dust reddening at these
redshifts is better described by an SMC-like extinction curve, rather than the
typically assumed Calzetti reddening. After correcting for dust, we find that
the galaxy population shows mean stellar population ages in the range 10^8.5 to
10^9 yr, with a dispersion of ~0.5 dex, and only weak trends as a function of
luminosity. In contrast to some results from the literature, we find that only
a small fraction of galaxies shows Balmer break colors which are consistent
with extremely young ages, younger than 100 Myr. Under the assumption of smooth
star-formation histories, this fraction is only 12-19% for galaxies at
M_z<-19.75. Our results are consistent with a gradual build-up of stars and
dust in galaxies at z>4, with only a small fraction of stars being formed in
short, intense bursts of star-formation.Comment: 11 pages, 10 figures; submitted to Ap
Newly Discovered Bright z~9-10 Galaxies and Improved Constraints on Their Prevalence Using the Full CANDELS Area
We report the results of an expanded search for z~9-10 candidates over the
~883 arcmin^2 CANDELS+ERS fields. This study adds 147 arcmin^2 to the search
area we consider over the CANDELS COSMOS, UDS, and EGS fields, while expanding
our selection to include sources with bluer J_{125}-H_{160} colors than our
previous J_{125}-H_{160}>0.5 mag selection. In searching for new z~9-10
candidates, we make full use of all available HST, Spitzer/IRAC, and
ground-based imaging data. As a result of our expanded search and use of
broader color criteria, 3 new candidate z~9-10 galaxies are identified. We also
find again the z=8.683 source previously confirmed by Zitrin+2015. This brings
our sample of probable z~9-11 galaxy candidates over the CANDELS+ERS fields to
19 sources in total, equivalent to 1 candidate per 47 arcmin^2 (1 per 10
WFC3/IR fields). To be comprehensive, we also discuss 28 mostly lower
likelihood z~9-10 candidates, including some sources that seem to be reliably
at z>8 using the HST+IRAC data alone, but which the ground-based data show are
much more likely at z<4. One case example is a bright z~9.4 candidate COS910-8
which seems instead to be at z~2. Based on this expanded sample, we obtain a
more robust LF at z~9 and improved constraints on the volume density of bright
z~9 and z~10 galaxies. Our improved z~9-10 results again reinforce previous
findings for strong evolution in the UV LF at z>8, with a factor of ~10
evolution seen in the luminosity density from z~10 to z~8.Comment: 22 pages, 12 figures, 6 tables, accepted for publication in the
Astrophysical Journa
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