242 research outputs found
The [OIII]λ5007 equivalent width distribution at z âŒ2: The redshift evolution of the extreme emission line galaxies
We determine the [OIII] equivalent width (EW) distribution of
rest-frame UV-selected (M) star-forming
galaxies in the GOODS North and South fields. We make use of deep HDUV
broadband photometry catalogues for selection and 3D-HST WFC3/IR grism spectra
for measurement of line properties. The [OIII] EW distribution
allows us to measure the abundance of extreme emission line galaxies (EELGs)
within this population. We model a log-normal distribution to the
[OIII] rest-frame equivalent widths of galaxies in our sample,
with location parameter and variance parameter . This EW distribution has a mean [OIII] EW of
168. The fractions of rest-UV-selected galaxies with
[OIII] EWs greater than and are measured to
be , , and
respectively. The EELG fractions do not vary strongly with UV luminosity in the
range () considered in this paper, consistent with
findings at higher redshifts. We compare our results to and
studies where candidate EELGs have been discovered through
Spitzer/IRAC colours, and we identify rapid evolution with redshift in the
fraction of star-forming galaxies observed in an extreme emission line phase (a
rise by a factor between and ). This
evolution is consistent with an increased incidence of strong bursts in the
galaxy population of the reionisation era. While this population makes a
sub-dominant contribution of the ionising emissivity at , EELGs
are likely to dominate the ionising output in the reionisation era
Using Bars As Signposts of Galaxy Evolution at High and Low Redshifts
An analysis of the NICMOS Deep Field shows that there is no evidence of a
decline in the bar fraction beyond z~0.7, as previously claimed; both
bandshifting and spatial resolution must be taken into account when evaluating
the evolution of the bar fraction. Two main caveats of this study were a lack
of a proper comparison sample at low redshifts and a larger number of galaxies
at high redshifts. We address these caveats using two new studies. For a proper
local sample, we have analyzed 134 spirals in the near-infrared using 2MASS
(main results presented by Menendez-Delmestre in this volume) which serves as
an ideal anchor for the low-redshift Universe. In addition to measuring the
mean bar properties, we find that bar size is correlated with galaxy size and
brightness, but the bar ellipticity is not correlated with these galaxy
properties. The bar length is not correlated with the bar ellipticity. For
larger high redshift samples we analyze the bar fraction from the 2-square
degree COSMOS ACS survey. We find that the bar fraction at z~0.7 is ~50%,
consistent with our earlier finding of no decline in bar fraction at high
redshifts.Comment: In the proceedings of "Penetrating Bars through Masks of Cosmic Dust:
The Hubble Tuning Fork strikes a New Note
From z>6 to z~2: Unearthing Galaxies at the Edge of the Dark Ages
Galaxies undergoing formation and evolution can now be observed over a time
baseline of some 12 Gyr. An inherent difficulty with high-redshift observations
is that the objects are very faint and the best resolution (HST) is only ~0.5
kpc. Such studies thereby combine in a highly synergistic way with the great
detail that can be obtained for nearby galaxies. 3 new developments are
highlighted. First is the derivation of stellar masses for galaxies from SEDs
using HST and now Spitzer data, and dynamical masses from both sub-mm
observations of CO lines and near-IR observations of optical lines like Halpha.
A major step has been taken with evidence that points to the z~2-3 LBGs having
masses that are a few x 10^10 Msolar. Second is the discovery of a population
of evolved red galaxies at z~2-3 which appear to be the progenitors of the more
massive early-type galaxies of today, with dynamical masses around a few x
10^11 Msolar. Third are the remarkable advances that have occurred in
characterizing dropout galaxies to z~6 and beyond, < 1 Gyr from recombination.
The HST ACS has played a key role here, with the dropout technique being
applied to i & z images in several deep ACS fields, yielding large samples of
these objects. This has allowed a detailed determination of their properties
and meaningful comparisons against lower-z samples. The use of cloning
techniques has overcome many of the strong selection biases affecting the study
of these objects. A clear trend of size with redshift has been identified, and
its impact on the luminosity density and SFR estimated. There is a significant
though modest decrease in the SFR from z~2.5 to z~6. The latest data also allow
for the first robust determination of the LF at z~6. Finally, the latest UDF
ACS and NICMOS data has resulted in the detection of some galaxies at z~7-8.Comment: 18 pages, 8 figures. To appear in Penetrating Bars through Masks of
Cosmic Dust: The Hubble Tuning Fork Strikes a New Note, eds. D. Block, K.
Freeman, R. Groess, I. Puerari, & E.K. Block (Dordrecht: Kluwer), in pres
Dynamical tunneling in molecules: Quantum routes to energy flow
Dynamical tunneling, introduced in the molecular context, is more than two
decades old and refers to phenomena that are classically forbidden but allowed
by quantum mechanics. On the other hand the phenomenon of intramolecular
vibrational energy redistribution (IVR) has occupied a central place in the
field of chemical physics for a much longer period of time. Although the two
phenomena seem to be unrelated several studies indicate that dynamical
tunneling, in terms of its mechanism and timescales, can have important
implications for IVR. Examples include the observation of local mode doublets,
clustering of rotational energy levels, and extremely narrow vibrational
features in high resolution molecular spectra. Both the phenomena are strongly
influenced by the nature of the underlying classical phase space. This work
reviews the current state of understanding of dynamical tunneling from the
phase space perspective and the consequences for intramolecular vibrational
energy flow in polyatomic molecules.Comment: 37 pages and 23 figures (low resolution); Int. Rev. Phys. Chem.
(Review to appear in Oct. 2007
Structureâproperties relationships in fibre drawing of bioactive phosphate glasses
New bioactive phosphate glasses suitable for continuous fibre production are investigated in this work. The structure of both bulk and fibres from Na2OâCaOâMgOâP2O5 glasses has been studied by means of Raman and 31P and 23Na nuclear magnetic resonance spectroscopies, and the structural results have been correlated with the mechanical properties of the fibres and the dissolution rate of the bulk glasses. It has been observed that the mechanical properties of the phosphate glass fibres are influenced by the glass network connectivity, while the dissolution rates are governed by the Qi speciation of the PO4 units. As seen in previous studies, molar volume seems to play an important role in the fragility behaviour of phosphate glasses. Here, a lower molar volume resulting from the increase in the oxygen packing density hinders the cooperative flow of the PO4 units throughout the glass network and, therefore, causes a reduction in the kinetic fragility
Rapid Evolution in the Most Luminous Galaxies During the First 900 Million Years
The first 900 million years (Myr) to redshift z~6 (the first seven per cent
of the age of the Universe) remains largely unexplored for the formation of
galaxies. Large samples of galaxies have been found at z~6, but detections at
earlier times are uncertain and unreliable. It is not at all clear how galaxies
built up from the first stars when the Universe was ~300 Myr old (z~12-15) to
z~6, just 600 Myr later. Here we report the results of a search for galaxies at
z~7-8, about 700 Myr after the Big Bang, using the deepest near-infrared and
optical images ever taken. Under conservative selection criteria we find only
one candidate galaxy at z~7-8, where ten would be expected if there were no
evolution in the galaxy population between z~7-8 and z~6. Using less
conservative criteria, there are four candidates, where 17 would be expected
with no evolution. This demonstrates that very luminous galaxies are quite rare
700 Myr after the Big Bang. The simplest explanation is that the Universe is
just too young to have built up many luminous galaxies at z~7-8 by the
hierarchical merging of small galaxies.Comment: Accepted for publication in Nature, 20 pages, 5 figures, 2 tables
(includes Supplementary Information), replaced to match version in pres
Heterochronic faecal transplantation boosts gut germinal centres in aged mice
Ageing is a complex multifactorial process associated with a plethora of disorders, which contribute significantly to morbidity worldwide. One of the organs significantly affected by age is the gut. Age-dependent changes of the gut-associated microbiome have been linked to increased frailty and systemic inflammation. This change in microbial composition with age occurs in parallel with a decline in function of the gut immune system, however it is not clear if there is a causal link between the two. Here we report that the defective germinal centre reaction in Peyerâs patches of aged mice can be rescued by faecal transfers from younger adults into aged mice and by immunisations with cholera toxin, without affecting germinal centre reactions in peripheral lymph nodes. This demonstrates that the poor germinal centre reaction in aged animals is not irreversible, and that it is possible to improve this response in older individuals by providing appropriate stimuli
The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)
We provide an overview of the capabilities and performance of the
Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST)
when used in its multi-object spectroscopy (MOS) mode employing a novel Micro
Shutter Array (MSA) slit device. The MSA consists of four separate 98 arcsec
91 arcsec quadrants each containing individually
addressable shutters whose open areas on the sky measure 0.20 arcsec
0.46 arcsec on a 0.27 arcsec 0.53 arcsec pitch. This is the first time
that a configurable multi-object spectrograph has been available on a space
mission. The levels of multiplexing achievable with NIRSpec MOS mode are
quantified and we show that NIRSpec will be able to observe typically fifty to
two hundred objects simultaneously with the pattern of close to a quarter of a
million shutters provided by the MSA. This pattern is fixed and regular, and we
identify the specific constraints that it yields for NIRSpec observation
planning. We also present the data processing and calibration steps planned for
the NIRSpec MOS data. The significant variation in size of the mostly
diffraction-limited instrument point spread function over the large wavelength
range of 0.6-5.3 m covered by the instrument, combined with the fact that
most targets observed with the MSA cannot be expected to be perfectly centred
within their respective slits, makes the spectrophotometric and wavelength
calibration of the obtained spectra particularly complex. These challenges
notwithstanding, the sensitivity and multiplexing capabilities anticipated of
NIRSpec in MOS mode are unprecedented, and should enable significant progress
to be made in addressing a wide range of outstanding astrophysical problems
Recommended from our members
The Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope II. Multi-object spectroscopy (MOS)
We provide an overview of the capabilities and performance of the
Near-Infrared Spectrograph (NIRSpec) on the James Webb Space Telescope (JWST)
when used in its multi-object spectroscopy (MOS) mode employing a novel Micro
Shutter Array (MSA) slit device. The MSA consists of four separate 98 arcsec
91 arcsec quadrants each containing individually
addressable shutters whose open areas on the sky measure 0.20 arcsec
0.46 arcsec on a 0.27 arcsec 0.53 arcsec pitch. This is the first time
that a configurable multi-object spectrograph has been available on a space
mission. The levels of multiplexing achievable with NIRSpec MOS mode are
quantified and we show that NIRSpec will be able to observe typically fifty to
two hundred objects simultaneously with the pattern of close to a quarter of a
million shutters provided by the MSA. This pattern is fixed and regular, and we
identify the specific constraints that it yields for NIRSpec observation
planning. We also present the data processing and calibration steps planned for
the NIRSpec MOS data. The significant variation in size of the mostly
diffraction-limited instrument point spread function over the large wavelength
range of 0.6-5.3 m covered by the instrument, combined with the fact that
most targets observed with the MSA cannot be expected to be perfectly centred
within their respective slits, makes the spectrophotometric and wavelength
calibration of the obtained spectra particularly complex. These challenges
notwithstanding, the sensitivity and multiplexing capabilities anticipated of
NIRSpec in MOS mode are unprecedented, and should enable significant progress
to be made in addressing a wide range of outstanding astrophysical problems
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