681 research outputs found
The Positive Feedback of Pop III Objects on Galaxy Formation
We study the formation of molecular hydrogen in cooling gas behind shocks
produced during the blow-away process thought to occur in the first collapsed,
luminous (Pop III) objects in the early universe. We find that for a wide range
of physical parameters the fraction is . The
mass produced in such explosions can exceed the amount of relic
destroyed inside the photodissociation region surrounding a given Pop III. We
conclude that these first objects, differently from the suggestion of Haiman et
al 1997, might have a net positive feedback on subsequent galactic formation.
We discuss the effects of radiation and the implications of our results for the
soft-UV background.Comment: 16 pages, aasms4.sty, LaTeX, 2 figures. submitted to ApJ Letter
The O VI Absorbers Toward PG0953+415: High Metallicity, Cosmic-Web Gas Far From Luminous Galaxies
The spectrum of the low-redshift QSO PG0953+415 shows two strong, intervening
O VI absorption systems. To study the nature of these absorbers, we have used
the Gemini Multiobject Spectrograph to conduct a deep spectroscopic galaxy
redshift survey in the 5' x 5' field centered on the QSO. This survey is fully
complete for r' < 19.7 and is 73% complete for r' < 21.0. We find three
galaxies at the redshift of the higher-z O VI system (z = 0.14232) including a
galaxy at projected distance rho = 155 kpc. We find no galaxies in the Gemini
field at the redshift of the lower-z O VI absorber (z = 0.06807), which
indicates that the nearest galaxy is more than 195 kpc away or has L < 0.04 L*.
Previous shallower surveys covering a larger field have shown that the z =
0.06807 O VI absorber is affiliated with a group/filament of galaxies, but the
nearest known galaxy has rho = 736 kpc. The z = 0.06807 absorber is notable for
several reasons. The absorption profiles reveal simple kinematics indicative of
quiescent material. The H I line widths and good alignment of the H I and metal
lines favor photoionization and, moreover, the column density ratios imply a
high metallicity: [M/H] = -0.3 +/- 0.12. The z = 0.14232 O VI system is more
complex and less constrained but also indicates a relatively high metallicity.
Using galaxy redshifts from SDSS, we show that both of the PG0953+415 O VI
absorbers are located in large-scale filaments of the cosmic web. Evidently,
some regions of the web filaments are highly metal enriched. We discuss the
origin of the high-metallicity gas and suggest that the enrichment might have
occurred long ago (at high z).Comment: Submitted for publication in the Astrophysical Journal Letters. Figs.
1 and 2 compressed for astro-ph. High-resolution version available at
http://www.astro.umass.edu/~tripp/astro/qualitypreps/pg0953tripp.pd
Surprisingly Little O VI Emission Arises in the Local Bubble
This paper reports the first study of the O VI resonance line emission (1032,
1038 Angstroms) originating in the Local Bubble (or Local Hot Bubble)
surrounding the solar neighborhood. In spite of the fact that O VI absorption
within the Local Bubble has been observed, no resonance line emission was
detected during our 230 ksec Far Ultraviolet Spectroscopic Explorer observation
toward a ``shadowing'' filament in the southern Galactic hemisphere. As a
result, tight 2 sigma upper limits are set on the intensities in the 1032 and
1038 Angstrom emission lines: 500 and 530 photons cm^{-2} s^{-1} sr^{-1},
respectively. These values place strict constraints on models and simulations.
They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside
in distinct regions of the Local Bubble and are not mixed in a single plasma,
whether in equilibrium with T ~ 10^6 K or highly overionized with T ~ 4 to 6 x
10^4 K. If the line of sight intersects multiple cool clouds within the Local
Bubble, then the results also suggest that hot/cool transition zones differ
from those in current simulations. With these intensity upper limits, we
establish limits on the electron density, thermal pressure, pathlength, and
cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore,
the intensity of O VI resonance line doublet photons originating in the
Galactic thick disk and halo is determined (3500 to 4300 photons cm^{-2} s^{-1}
sr^{-1}), and the electron density, thermal pressure, pathlength, and cooling
timescale of its O VI-bearing plasma are calculated. The pressure in the
Galactic halo's O VI-bearing plasma (3100 to 3800 K cm^{-3}) agrees with model
predictions for the total pressure in the thick disk/lower halo. We also report
the results of searches for other emission lines.Comment: accepted by ApJ, scheduled for May 2003, replacement astro-ph
submission corrects typos and grammatical errors in original versio
Low-temperature photocarrier dynamics in monolayer MoS2
The band structure of MoS strongly depends on the number of layers, and a
transition from indirect to direct-gap semiconductor has been observed recently
for a single layer of MoS. Single-layer MoS therefore becomes an
efficient emitter of photoluminescence even at room temperature. Here, we
report on scanning Raman and on temperature-dependent, as well as time-resolved
photoluminescence measurements on single-layer MoS flakes prepared by
exfoliation. We observe the emergence of two distinct photoluminescence peaks
at low temperatures. The photocarrier recombination at low temperatures occurs
on the few-picosecond timescale, but with increasing temperatures, a
biexponential photoluminescence decay with a longer-lived component is
observed.Comment: 3 pages, 4 figure
Infrared Spectroscopy of Molecular Supernova Remnants
We present Infrared Space Observatory spectroscopy of sites in the supernova
remnants W28, W44, and 3C391, where blast waves are impacting molecular clouds.
Atomic fine-structure lines were detected from C, N, O, Si, P, and Fe. The S(3)
and S(9) lines of H2 were detected for all three remnants. The observations
require both shocks into gas with moderate (~ 100 /cm3) and high (~10,000 /cm3)
pre-shock densities, with the moderate density shocks producing the ionic lines
and the high density shock producing the molecular lines. No single shock model
can account for all of the observed lines, even at the order of magnitude
level. We find that the principal coolants of radiative supernova shocks in
moderate-density gas are the far-infrared continuum from dust grains surviving
the shock, followed by collisionally-excited [O I] 63.2 and [Si II] 34.8 micron
lines. The principal coolant of the high-density shocks is
collisionally-excited H2 rotational and ro-vibrational line emission. We
systematically examine the ground-state fine structure of all cosmically
abundant elements, to explain the presence or lack of all atomic fine lines in
our spectra in terms of the atomic structure, interstellar abundances, and a
moderate-density, partially-ionized plasma. The [P II] line at 60.6 microns is
the first known astronomical detection. There is one bright unidentified line
in our spectra, at 74.26 microns. The presence of bright [Si II] and [Fe II]
lines requires partial destruction of the dust. The required gas-phase
abundance of Fe suggests 15-30% of the Fe-bearing grains were destroyed. The
infrared continuum brightness requires ~1 Msun of dust survives the shock,
suggesting about 1/3 of the dust mass was destroyed, in agreement with the
depletion estimate and with theoretical models for dust destruction.Comment: 40 pages; 10 figures; accepted by ApJ July 11, 200
Energy Distribution of Micro-events in the Quiet Solar Corona
Recent imaging observations of EUV line emissions have shown evidence for
frequent flare-like events in a majority of the pixels in quiet regions of the
solar corona. The changes in coronal emission measure indicate impulsive
heating of new material to coronal temperatures. These heating or evaporation
events are candidate signatures of "nanoflares" or "microflares" proposed to
interpret the high temperature and the very existence of the corona. The energy
distribution of these micro-events reported in the literature differ widely,
and so do the estimates of their total energy input into the corona. Here we
analyze the assumptions of the different methods, compare them by using the
same data set and discuss their results.
We also estimate the different forms of energy input and output, keeping in
mind that the observed brightenings are most likely secondary phenomena. A
rough estimate of the energy input observed by EIT on the SoHO satellite is of
the order of 10% of the total radiative output in the same region. It is
considerably smaller for the two reported TRACE observations. The discrepancy
can be explained partially by different thresholds for flare detection. There
is agreement on the slope and the absolute value of the distribution if the
same method were used and a numerical error corrected. The extrapolation of the
power law to unobserved energies that are many orders of magnitude smaller
remains questionable. Nevertheless, these micro-events and unresolved smaller
events are currently the best source of information on the heating process of
the corona
The Heavy Element Enrichment of Lyman alpha Clouds in the Virgo Supercluster
Using high S/N STIS echelle spectra (FWHM=7 km/s) of 3C 273, we constrain the
metallicities of two Lya clouds in the vicinity of the Virgo cluster. We detect
C II, Si II, and Si III absorption lines in the Lya absorber at z = 0.00530.
Previous observations with FUSE have revealed Ly beta - Ly theta lines at this
redshift, thereby accurately constraining N(H I). We model the ionization of
the gas and derive [C/H] = -1.2^{+0.3}_{-0.2}, [Si/C] = 0.2+/-0.1, and log
n_{H} = -2.8+/-0.3. The model implies a small absorber thickness, ~70 pc, and
thermal pressure p/k ~ 40 cm^{-3} K. It is most likely that the absorber is
pressure confined by an external medium because gravitational confinement would
require a very high ratio of dark matter to baryonic matter. Based on Milky Way
sight lines in which carbon and silicon abundances have been reliably measured
in the same interstellar cloud (including new measurements presented herein),
we argue that the overabundance of Si relative to C is not due to dust
depletion. Instead, this probably indicates that the gas has been predominately
enriched by Type II supernovae. Such enrichment is most plausibly provided by
an unbound galactic wind, given the absence of galaxies within a projected
distance of 100 kpc and the presence of galaxies capable of driving a wind at
larger distances. We also constrain the metallicity and physical conditions of
the Virgo absorber at z = 0.00337 based on detections of O VI and H I and an
upper limit on C IV. If this absorber is collisionally ionized, the O VI/C IV
limit requires T > 10^{5.3} K. For either collisional ionization or
photoionization, we find that [O/H] > -2.0 at z = 0.00337.Comment: Final Ap.J. versio
Intermediate- and High-Velocity Ionized Gas toward zeta Orionis
We combine UV spectra obtained with the HST/GHRS echelle, IMAPS, and
Copernicus to study the abundances and physical conditions in the predominantly
ionized gas seen at high (-105 to -65 km/s) and intermediate velocities (-60 to
-10 km/s) toward zeta Ori. We have high resolution (FWHM ~ 3.3-4.5 km/s) and/or
high S/N spectra for at least two significant ions of C, N, Al, Si, S, and Fe
-- enabling accurate estimates for both the total N(H II) and the elemental
depletions. C, N, and S have essentially solar relative abundances; Al, Si, and
Fe appear to be depleted by about 0.8, 0.3-0.4, and 0.95 dex, respectively.
While various ion ratios would be consistent with collisional ionization
equilibrium (CIE) for T ~ 25,000-80,000 K, the widths of individual
high-velocity absorption components indicate that T ~ 9000 K -- so the gas is
not in CIE. Analysis of the C II fine-structure excitation equilibrium yields
estimated densities (n_e ~ n_H ~ 0.1-0.2 cm^{-3}), thermal pressures (2 n_H T ~
2000-4000 cm^{-3}K), and thicknesses (0.5-2.7 pc) for the individual clouds. We
compare the abundances and physical properties derived for these clouds with
those found for gas at similar velocities toward 23 Ori and tau CMa, and also
with several models for shocked gas. While the shock models can reproduce some
features of the observed line profiles and some of the observed ion ratios,
there are also significant differences. The measured depletions suggest that
\~10% of the Al, Si, and Fe originally locked in dust in the pre-shock medium
may have been returned to the gas phase, consistent with predictions for the
destruction of silicate dust in a 100 km/s shock. The near-solar gas phase
abundance of carbon, however, seems inconsistent with the predicted longer time
scales for the destruction of graphite grains.Comment: 50 pages, 9 figures; aastex; accepted by Ap
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