4,807 research outputs found
Physical Properties and Baryonic Content of Low-Redshift Intergalactic Ly-alpha and O VI Absorption Systems: The PG1116+215 Sight Line
We present HST and FUSE observations of the intergalactic absorption toward
PG1116+215 in the 900-3000 A spectral region. We detect 25 Ly-alpha absorbers
at rest-frame equivalent widths W_r > 30 mA, yielding (dN/dz)_Ly-alpha =
154+/-18 over an unblocked redshift path of 0.162. Two additional weak Ly-alpha
absorbers with W_r ~ 15-20 mA are also present. Eight of the Ly-alpha absorbers
have large line widths (b > 40 km/sec). The detection of narrow OVI in the
broad Ly-alpha absorber at z=0.06244 supports the idea that the Ly-alpha
profile is thermally broadened in gas with T > 10^5 K. We find dN/dz ~ 50 for
broad Ly-alpha absorbers with W_r > 30 mA and b > 40 km/sec. If the broad
Ly-alpha lines are dominated by thermal broadening in hot gas, the amount of
baryonic material in these absorbers is enormous, perhaps as much as half the
baryonic mass in the low-redshift universe. We detect OVI absorption in several
of the Ly-alpha clouds along the sight line. Two detections at z=0.13847 and
z=0.16548 are confirmed by the presence of other ions at these redshifts, while
the detections at z=0.04125, 0.05895, 0.05928, and 0.06244 are based upon the
Ly-alpha and OVI detections alone. The information available for 13
low-redshift OVI absorbers with W_r > 50 mA along 5 sight lines yields
(dN/dz)_OVI ~ 14 and Omega_b(OVI) > 0.0027/h_75, assuming a metallicity of 0.1
solar and an OVI ionization fraction < 0.2. The properties and prevalence of
low-redshift OVI absorbers suggest that they too may be a substantial baryon
repository, perhaps containing as much mass as stars and gas inside galaxies.
The redshifts of the OVI absorbers are highly correlated with the redshifts of
galaxies along the sight line, though few of the absorbers lie closer than
600/h_75 kpc to any single galaxy. [abbreviated]Comment: 99 pages, 30 figures, aastex format, ApJS in pres
Molecular Hydrogen in High-Velocity Clouds
We present Far Ultraviolet Spectroscopic Explorer (FUSE) observations of
interstellar molecular hydrogen (H_2) in two Galactic high-velocity clouds
(HVCs). Molecular hydrogen absorption is detected in the Magellanic Stream
(abundance ~0.3 solar) toward the Seyfert galaxy Fairall 9 in the lowest three
rotational states (J=0-2) at v(LSR)=+190 km/s, yielding a total H_2 column
density of log N(H_2)=16.40(+0.26)(-0.53). In contrast, no H_2 absorption is
seen in the high-velocity cloud Complex C (abundance ~0.1 solar) toward the
quasar PG 1259+593 (log N(H_2)<13.96 at v(LSR)=-130 km/s) although both HVCs
have similar HI column densities on the order of log N(HI)~20. Weak H_2
absorption is detected in the Intermediate-Velocity Arch (IV Arch; abundance
\~1.0 solar) toward PG 1259+593 (log N(H_2)=14.10(+0.21)(-0.44) at v(LSR)=-55
km/s and log N(HI)=19.5). It thus appears that metal- and dust-poor halo clouds
like Complex C are not able to form and maintain widely distributed H_2,
whereas metal and dust-rich halo clouds like the IV Arch can maintain H_2 even
at low HI column densities.Comment: 12 pages, 3 figures; Accepted for publication in ApJ Letter
GHRS and ORFEUS-II Observations of the Highly Ionized Interstellar Medium Toward ESO141-055
We present Goddard High Resolution Spectrograph and ORFEUS-II measurements of
Si IV, CIV, N V, and O VI absorption in the interstellar medium of the Galactic
disk and halo toward the nucleus of the Seyfert galaxy ESO141-055. The high
ionization absorption is strong, with line strengths consistent with the
spectral signature expected for hot (log T = 5-6) collisionally ionized gas in
either a ``Galactic fountain'' or an inhomogeneous medium containing a mixture
of conductive interfaces and turbulent mixing layers. The total O VI column
density of log N ~ 15 suggests that the scale height of O VI is large (>3 kpc)
in this direction. Comparison of the high ion column densities with
measurements for other sight lines indicates that the highly ionized gas
distribution is patchy. The amount of O VI perpendicular to the Galactic plane
varies by at least a factor of ~4 among the complete halo sight lines thus far
studied. In addition to the high ion absorption, lines of low ionization
species are also present in the spectra. With the possible exception of Ar I,
which may have a lower than expected abundance resulting from partial
photoionization of gas along the sight line, the absorption strengths are
typical of those expected for the warm, neutral interstellar medium. The sight
line intercepts a cold molecular cloud with log N(H2) ~ 19. The cloud has an
identifiable counterpart in IRAS 100-micron emission maps of this region of the
sky. We detect a Ly-alpha absorber associated with ESO141-055 at z = 0.03492.
This study presents an enticing glimpse into the interstellar and intergalactic
absorption patterns that will be observed at high spectral resolution by the
Far Ultraviolet Spectroscopic Explorer.Comment: 24 pages + 8 figures, uses aaspp4.sty. Accepted for publication in
Ap
FUSE and STIS Observations of the Warm-Hot Intergalactic Medium towards PG1259+593
We use FUSE and STIS spectra to study intergalactic absorption towards the
quasar PG1259+593 (z=0.478). We identify 135 intergalactic absorption lines
with equivalent widths >10mA, tracing 78 absorption components in 72 Ly
alpha/beta absorption-line systems. We concentrate on the distribution and
physical properties of the WHIM as sampled by OVI and intrinsically broad Ly
alpha lines. The number of intervening OVI absorbers for equivalent widths W>24
mA is 3-6 over an unobscured redshift path of dz=0.368. This implies a number
density of OVI systems, dN/dz, of ~8-16 along this sight line. This range is
consistent with estimates from other sight lines, supporting the idea that
intervening intergalactic OVI absorbers contain an substantial fraction of the
baryonic mass in the low-redshift Universe. We identify a number of broad Ly
alpha absorbers with large Doppler parameters (b~40-200 km/s) and low column
densities (N(HI)<10^14 cm^-2). For pure thermal broadening, these widths
correspond to temperatures of ~1x10^5 to 3x10^6 K. While these broad absorbers
could be caused by blends of multiple, unresolved lines, continuum undulations,
or by kinematic flows and Hubble broadening, we consider the possibility that
some of these features are single-component, thermally broadened Ly alpha
lines. These systems could represent WHIM absorbers that are too weak, too
metal-poor, and/or too hot to be detected in OVI. If so, their widths and their
frequency in the PG1259+593 spectrum imply that these absorbers trace an even
larger fraction of the baryons in the low-redshift Universe than the OVI
absorbing systems (abridged version).Comment: 71 pages, 25 figures; accepted for publication in ApJ
Identification and Characterization of Genes Required for Compensatory Growth in Drosophila
To maintain tissue homeostasis, some organs are able to replace dying cells with additional proliferation of surviving cells. Such proliferation can be localized (e.g., a regeneration blastema) or diffuse (compensatory growth). The relationship between such growth and the growth that occurs during development has not been characterized in detail. Drosophila melanogaster larval imaginal discs can recover from extensive damage, producing normally sized adult organs. Here we describe a system using genetic mosaics to screen for recessive mutations that impair compensatory growth. By generating clones of cells that carry a temperature-sensitive cell-lethal mutation, we conditionally ablate patches of tissue in the imaginal disc and assess the ability of the surviving sister clones to replace the lost tissue. We have used this system together with a modified whole-genome resequencing (WGS) strategy to identify several mutations that selectively compromise compensatory growth. We find specific alleles of bunched (bun) and Ribonucleoside diphosphate reductase large subunit (RnrL) reduce compensatory growth in the imaginal disc. Other genes identified in the screen, including two alleles of Topoisomerase 3-alpha (Top3α), while also required for developmental growth, appear to have an enhanced requirement during compensatory growth. Compensatory growth occurs at a higher rate than normal growth and may therefore have features in common with some types of overgrowth. Indeed, the RnrL allele identified compromises both these types of altered growth and mammalian ribonucleotide reductase and topoisomerases are targets of anticancer drugs. Finally, the approach we describe is applicable to the study of compensatory growth in diverse tissues in Drosophila
Set-Rationalizable Choice and Self-Stability
A common assumption in modern microeconomic theory is that choice should be
rationalizable via a binary preference relation, which \citeauthor{Sen71a}
showed to be equivalent to two consistency conditions, namely
(contraction) and (expansion). Within the context of \emph{social}
choice, however, rationalizability and similar notions of consistency have
proved to be highly problematic, as witnessed by a range of impossibility
results, among which Arrow's is the most prominent. Since choice functions
select \emph{sets} of alternatives rather than single alternatives, we propose
to rationalize choice functions by preference relations over sets
(set-rationalizability). We also introduce two consistency conditions,
and , which are defined in analogy to and
, and find that a choice function is set-rationalizable if and only if
it satisfies . Moreover, a choice function satisfies
and if and only if it is \emph{self-stable}, a new concept based
on earlier work by \citeauthor{Dutt88a}. The class of self-stable social choice
functions contains a number of appealing Condorcet extensions such as the
minimal covering set and the essential set.Comment: 20 pages, 2 figure, changed conten
Exploring the Origin and Fate of the Magellanic Stream with Ultraviolet and Optical Absorption
(Abridged) We present an analysis of ionization and metal enrichment in the
Magellanic Stream (MS), the nearest gaseous tidal stream, using HST/STIS and
FUSE ultraviolet spectroscopy of two background AGN, NGC 7469 and Mrk 335. For
NGC 7469, we include optical spectroscopy from VLT/UVES. In both sightlines the
MS is detected in low-ion and high-ion absorption. Toward NGC 7469, we measure
a MS oxygen abundance [O/H]_MS=[OI/HI]=-1.00+/-0.05(stat)+/-0.08(syst),
supporting the view that the Stream originates in the SMC rather than the LMC.
We use CLOUDY to model the low-ion phase of the Stream as a photoionized plasma
using the observed Si III/Si II and C III/C II ratios. Toward Mrk 335 this
yields an ionization parameter log U between -3.45 and -3.15 and a gas density
log (n_H/cm^-3) between -2.51 and -2.21. Toward NGC 7469 we derive sub-solar
abundance ratios for [Si/O], [Fe/O], and [Al/O], indicating the presence of
dust in the MS. The high-ion column densities are too large to be explained by
photoionization, but also cannot be explained by a single-temperature
collisional-ionization model (equilibrium or non-equilibrium). This suggests
the high-ion plasma is multi-phase. Summing over the low-ion and high-ion
phases, we derive conservative lower limits on the ratio N(total H II)/N(H I)
of >19 toward NGC 7469 and >330 toward Mrk 335, showing that along these two
directions the vast majority of the Stream has been ionized. The presence of
warm-hot plasma together with the small-scale structure observed at 21 cm
provides evidence for an evaporative interaction with the hot Galactic corona.
This scenario, predicted by hydrodynamical simulations, suggests that the fate
of the MS will be to replenish the Galactic corona with new plasma, rather than
to bring neutral fuel to the disk.Comment: Accepted for publication in ApJ. 18 pages, 7 figures, all in colo
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