3,412 research outputs found
Studying the first galaxies with ALMA
We discuss observations of the first galaxies, within cosmic reionization, at
centimeter and millimeter wavelengths. We present a summary of current
observations of the host galaxies of the most distant QSOs (). These
observations reveal the gas, dust, and star formation in the host galaxies on
kpc-scales. These data imply an enriched ISM in the QSO host galaxies within 1
Gyr of the big bang, and are consistent with models of coeval supermassive
black hole and spheroidal galaxy formation in major mergers at high redshift.
Current instruments are limited to studying truly pathologic objects at these
redshifts, meaning hyper-luminous infrared galaxies (
L). ALMA will provide the one to two orders of magnitude improvement in
millimeter astronomy required to study normal star forming galaxies (ie.
Ly- emitters) at . ALMA will reveal, at sub-kpc spatial
resolution, the thermal gas and dust -- the fundamental fuel for star formation
-- in galaxies into cosmic reionization.Comment: to appear in Science with ALMA: a new era for Astrophysics}, ed. R.
Bachiller (Springer: Berlin); 5 pages, 7 figure
Icequake Source Mechanisms for Studying Glacial Sliding
Improving our understanding of glacial sliding is crucial for constraining basal drag in ice dynamics models. We use icequakes, sudden releases of seismic energy as the ice slides over the bed, to provide geophysical observations that can be used to aid understanding of the physics of glacial sliding and constrain ice dynamics models. These icequakes are located at the bed of an alpine glacier in Switzerland and the Rutford Ice Stream, West Antarctica, two extremes of glacial settings and spatial scales. We investigate a number of possible icequake source mechanisms by performing full waveform inversions to constrain the fundamental physics and stress release during an icequake stick-slip event. Results show that double-couple mechanisms best describe the source for the events from both glacial settings and the icequakes originate at or very near the ice-bed interface. We also present an exploratory method for attempting to measure the till shear modulus, if indirect reflected icequake radiation is observed. The results of this study increase our understanding of how icequakes are associated with basal drag while also providing the foundation for a method of remotely measuring bed shear strengt
Ionization near-zones associated with quasars at z ~ 6
We analyze the size evolution of HII regions around 27 quasars between z=5.7
to 6.4 ('quasar near-zones' or NZ). We include more sources than previous
studies, and we use more accurate redshifts for the host galaxies, with 8 CO
molecular line redshifts and 9 MgII redshifts. We confirm the trend for an
increase in NZ size with decreasing redshift, with the luminosity normalized
proper size evolving as: R_{NZ,corrected} = (7.4 \pm 0.3) - (8.0 \pm 1.1)
\times (z-6) Mpc. While derivation of the absolute neutral fraction remains
difficult with this technique, the evolution of the NZ sizes suggests a
decrease in the neutral fraction of intergalactic hydrogen by a factor ~ 9.4
from z=6.4 to 5.7, in its simplest interpretation. Alternatively, recent
numerical simulations suggest that this rapid increase in near-zone size from
z=6.4 to 5.7 is due to the rapid increase in the background photo-ionization
rate at the end of the percolation or overlap phase, when the average mean free
path of ionizing photons increases dramatically. In either case, the results
are consistent with the idea that z ~ 6 to 7 corresponds to the tail end of
cosmic reionization. The scatter in the normalized NZ sizes is larger than
expected simply from measurement errors, and likely reflects intrinsic
differences in the quasars or their environments. We find that the near-zone
sizes increase with quasar UV luminosity, as expected for photo-ionization
dominated by quasar radiation.Comment: 16 pages, aas format, 4 figures, to appear in the ApJ letter
Spatial and Temporal Variation of Offshore Wind Power and its Values Along the Central California Coast
The analysis of the spatiotemporal variability of wind power remains limited during the planning stage of an offshore wind farm. This study provides a framework to investigate how offshore wind power varies along the Central California Coast over diurnal and seasonal time scales, which is critical for reliability and functionality of the grid system. We find that offshore wind power in this region peaks during evening hours across all seasons and maximizes in spring and summer. The timing of peak offshore wind power production better aligns with that of peak demand across California than solar and land-based wind power production, highlighting its potential to fill the supply gap when demand is high and power production from other renewable energy sources is low. We further assess the value of offshore wind power using demand-based and wholesale market metrics. Both metrics indicate high potential value of offshore wind power over most areas in this region. Finally, we show that the estimate of power production is significantly biased when using mean wind speeds that do not account for temporal variability, leading to potentially inaccurate predictions about locations that are expected to produce the most power. These results reiterate the importance in considering spatiotemporal variability in wind power for accurately calculating the value of offshore wind development
PENELLOPE V. The magnetospheric structure and the accretion variability of the classical T Tauri star HM Lup
HM Lup is a young M-type star that accretes material from a circumstellar
disk through a magnetosphere. Our aim is to study the inner disk structure of
HM Lup and to characterize its variability. We used spectroscopic data from
HST/STIS, X-Shooter, and ESPRESSO taken in the framework of the ULLYSES and
PENELLOPE programs, together with photometric data from TESS and AAVSO. The
2021 TESS light curve shows variability typical for young stellar objects of
the "accretion burster" type. The spectra cover the temporal evolution of the
main burst in the 2021 TESS light curve. We compared the strength and
morphology of emission lines from different species and ionization stages. We
determined the mass accretion rate from selected emission lines and from the UV
continuum excess emission at different epochs, and we examined its relation to
the photometric light curves. The emission lines in the optical spectrum of HM
Lup delineate a temperature stratification along the accretion flow. While the
wings of the H I and He I lines originate near the star, the lines of species
such as Na I, Mg I, Ca I, Ca II, Fe I, and Fe II are formed in an outer and
colder region. The shape and periodicity of the 2019 and 2021 TESS light
curves, when qualitatively compared to predictions from magnetohydrodynamic
models, suggest that HM Lup was in a regime of unstable ordered accretion
during the 2021 TESS observation due to an increase in the accretion rate.
Although HM Lup is not an extreme accretor, it shows enhanced emission in the
metallic species during this high accretion state that is produced by a density
enhancement in the outer part of the accretion flow.Comment: 15 pages, 14 figures. Accepted for publication in A&
INTEGRAL discovery of a bright highly obscured galactic X-ray binary source IGR J16318-4848
INTEGRAL regularly scans the Galactic plane to search for new objects and in
particular for absorbed sources with the bulk of their emission above 10-20
keV. The first new INTEGRAL source was discovered on 2003 January 29, 0.5
degree from the Galactic plane and was further observed in the X-rays with
XMM-Newton. This source, IGR J16318-4848, is intrinsically strongly absorbed by
cold matter and displays exceptionally strong fluorescence emission lines. The
likely infrared/optical counterpart indicates that IGR J16318-4848 is probably
a High Mass X-Ray Binary neutron star or black hole enshrouded in a Compton
thick environment. Strongly absorbed sources, not detected in previous surveys,
could contribute significantly to the Galactic hard X-ray background between 10
and 200 keV.Comment: 6 pages, 4 figures (fig 1 quality lowered), accepted for publication
in A&A letters (INTEGRAL special issue
An assessment of Li abundances in weak-lined and classical T Tauri stars of the Taurus-Auriga association
Accurate measurements of lithium abundances in young low-mass stars provide
an independent and reliable age diagnostics. Previous studies of nearby star
forming regions have identified significant numbers of Li-depleted stars,often
at levels inconsistent with the ages indicated by their luminosity. We aim at a
new and accurate analysis of Li abundances in a sample of ~100 pre-main
sequence stars in Taurus-Auriga using a homogeneous and updated set of stellar
parameters and model atmospheres appropriate for the spectral types of the
sample stars.We compute Li abundances using published values of the equivalent
widths of the Li 6708 A doublet obtained from medium/high resolution spectra.
We find that the number of significantly Li-depleted stars in Taurus-Auriga is
greatly reduced with respect to earlier results. Only 13 stars have abundances
lower than the interstellar value by a factor of 5 or greater. All of them are
weak-lined T Tauri stars drawn from X-ray surveys; with the exception of four
stars located near the L1551 and L1489 dark clouds, all the Li-depleted stars
belong to the class of dispersed low-mass stars, distributed around the main
sites of current star formation. If located at the distance of Taurus-Auriga,
the stellar ages implied by the derived Li abundances are in the range 3-30
Myr, greater than the bulk of the Li-rich population with implication on the
star formation history of the region. In order to derive firm conclusions about
the fraction of Li-depleted stars of Taurus-Auriga, Li measurements of the
remaining members of the association should be obtained, in particular of the
group of stars that fall in the Li-burning region of the HR diagram.Comment: Accepted for publication in Astronomy & Astrophysics. 20 pages, 5
figure
Weak 13CO in the Cloverleaf Quasar: evidence for a young, early generation starburst
Observations of 12CO at high redshift indicate rapid metal enrichment in the
nuclear regions of at least some galaxies in the early universe. However, the
enrichment may be limited to nuclei that are synthesized by short-lived massive
stars, excluding classical secondary nuclei like 13CO. Testing this idea, we
tentatively detect the 13CO J=3-2 line at a level of 0.3 Jy km/s toward the
Cloverleaf Quasar at redshift 2.5. This is the first observational evidence for
13CO at high redshift. The 12CO/13CO J=3-2 luminosity ratio is with at least 40
much higher than ratios observed in molecular clouds of the Milky Way and in
the ultraluminous galaxy Arp 220, but may be similar to that observed toward
NGC 6240. Large Velocity Gradient (LVG) models simulating seven 12CO
transitions and the 13CO line yield 12CO/13CO abundance ratios in excess of 100
for the Cloverleaf. It is possible that the measured ratio is affected by a
strong submillimeter radiation field, which reduces the contrast between the
13CO line and the background. It is more likely, however, that the ratio is
caused by a real deficiency of 13CO. A potential conflict with optical data,
indicating high abundances also for secondary nuclei in quasars of high
redshift, may be settled if the bulk of the CO emission is originating
sufficiently far from the active galactic nucleus.Comment: 7 pages, 5 figures, accepted for publication in A&A (Main Journal
Major Merging: The Way to Make a Massive, Passive Galaxy
We analyze the projected axial ratio distribution, p(b/a), of galaxies that
were spectroscopically selected from the Sloan Digital Sky Survey (DR6) to have
low star-formation rates. For these quiescent galaxies we find a rather abrupt
change in p(b/a) at a stellar mass of ~10^{11} M_sol: at higher masses there
are hardly any galaxies with b/a<0.6, implying that essentially none of them
have disk-like intrinsic shapes and must be spheroidal. This transition mass is
~3-4 times higher than the threshold mass above which quiescent galaxies
dominate in number over star-forming galaxies, which suggests these mass scales
are unrelated. At masses lower than ~10^{11} M_sol, quiescent galaxies show a
large range in axial ratios, implying a mix of bulge- and disk-dominated
galaxies. Our result strongly suggests that major merging is the most
important, and perhaps only relevant, evolutionary channel to produce massive
(>10^{11} M_sol), quiescent galaxies, as it inevitably results in spheroids.Comment: Minor changes to match published version in ApJ Letter
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