2,103 research outputs found
Discovery of excess O I absorption towards the z = 6.42 QSO SDSS J1148+5251
We present a search for O I in the spectra of nine 4.9 < z_qso < 6.4 QSOs
taken with Keck/HIRES. We detect six systems with N(O I) > 10^13.7 cm^{-2} in
the redshift intervals where O I 1302 falls redward of the Ly-alpha forest.
Four of these lie towards SDSS J1148+5251 (z_qso = 6.42). This imbalance is
unlikely to arise from variations in sensitivity among our data or from a
statistical fluctuation. The excess O I occurs over a redshift interval that
also contains transmission in Ly-alpha and Ly-beta. Therefore, if these O I
systems represent pockets of neutral gas, then they must occur within or near
regions of the IGM that are highly ionized. In contrast, no O I is detected
towards SDSS J1030+0524 (z_qso = 6.30), whose spectrum shows complete
absorption in Ly-alpha and Ly-beta over \Delta z ~ 0.2. Assuming no ionization
corrections, we measure mean abundance ratios = -0.04 +/- 0.06,
= -0.31 +/- 0.09, and = -0.34 +/- 0.07 (2 sigma), which are
consistent with enrichment dominated by Type II supernovae. The O/Si ratio
limits the fraction of silicon in these systems contributed by metal-free very
massive stars to < 30%, a result which is insensitive to ionization
corrections. The ionic comoving mass densities along the z_qso > 6.2
sightlines, including only the detected systems, are \Omega(O I) = (7.0 +/-
0.6) * 10^{-8}, \Omega(Si II) = (9.6 +/- 0.9) * 10^{-9}, and \Omega(C II) =
(1.5 +/- 0.2) * 10^{-8}.Comment: Submitted to ApJ, with changes to reflect referee's comment
Observations of Chemically Enriched QSO Absorbers near z ~ 2.3 Galaxies: Galaxy-Formation Feedback Signatures in the IGM
We present a study of galaxies and intergalactic gas toward the z=2.73 quasar
HS1700+6416, to explore the effects of galaxy formation feedback on the IGM.
Our observations and ionization simulations indicate that the volume within
100-200 h_71^{-1} physical kpc of high-redshift galaxies contains very small,
dense, and metal-rich absorption-line regions. These systems often contain
shock-heated gas seen in OVI, and may exhibit [Si/C] abundance enhancements
suggestive of Type II supernova enrichment. We argue that the absorbers
resemble thin sheets or bubbles, whose physical properties can be explained
with a simple model of radiatively efficient shocks propegating through the
IGM. Their high metallicities suggest that these shocks are being expelled
from--rather than falling into--star forming galaxies. There is a dropoff in
the IGM gas density at galaxy impact parameters beyond ~300 physical kpc that
may trace boundaries of gas structures where the galaxies reside. The local
heavy-element enhancement covers 100-200 kpc; beyond this the observed
abundances blend into the general IGM. Supernova-driven winds or dynamical
stripping of interstellar gas appears to affect the IGM near massive galaxies,
even at R>~100 kpc. However, these feedback systems represent only a few
percent of the Lya forest mass at z~2.5. Their mass could be larger if the more
numerous metal-poor CIV systems at >~200 kpc are tepid remnants of very
powerful winds. Based on present observations it is not clear that this
scenario is to be favored over one involving pre-enrichment by smaller galaxies
at z>~6.Comment: Accepted for publication in the Astrophysical Journal. 26 pages
emulateapj, incl. 5 pages tables, 15 figure
Expansion and Collapse in the Cosmic Web
We study the kinematics of the gaseous cosmic web at high redshift with Lyman
alpha forest absorption in multiple QSO sightlines. Using a simple analytic
model and a cosmological hydrodynamic simulation we constrain the underlying
three-dimensional distribution of velocities from the observed line-of-sight
distribution of velocity shear across the plane of the sky. The distribution is
found to be in good agreement with the intergalactic medium (IGM) undergoing
large scale motions dominated by the Hubble flow. Modeling the Lyman alpha
clouds analytically and with a hydrodynamics simulation, the average expansion
velocity of the gaseous structures causing the Lyman alpha forest in the lower
redshift (z = 2) sample appears about 20 percent lower than the local Hubble
expansion velocity. We interpret this as tentative evidence for some clouds
undergoing gravitational collapse. However, the distribution of velocities is
highly skewed, and the majority of clouds at redshifts from 2 to 3.8 expand
typically about 5 - 20 percent faster than the Hubble flow. This behavior is
explained if most absorbers in the column density range typically detectable
are expanding filaments that stretch and drain into more massive nodes. We find
no evidence for the observed distribution of velocity shear being significantly
influenced by processes other than Hubble expansion and gravitational
instability, like galactic winds. To avoid overly disturbing the IGM, winds may
be old and/or limp by the time we observe them in the Lyman alpha forest, or
they may occupy only an insignificant volume fraction of the IGM. (abridged)Comment: 63 pages, 26 figures, AAS Latex; ApJ, in pres
Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4
We present a study of the spatial coherence of the intergalactic medium
toward two pairs of high-redshift quasars with moderate angular separations
observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and
Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted
flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for
both pairs. This strongly suggests that at least some of the absorbing
structures span the 230-300/h_70 proper kpc transverse separation between
sightlines. We also statistically examine the similarity between paired spectra
as a function of transmitted flux, a measure which may be useful for comparison
with numerical simulations. In investigating the dependence of the correlation
functions on spectral characteristics, we find that photon noise has little
impact for S/N >~ 10 per resolution element. However, the agreement between the
autocorrelation along the line sight and the crosscorrelation between
sightlines, a potential test of cosmological geometry, depends significantly on
instrumental resolution. Finally, we present an inventory of metal lines. These
include a a pair of strong C IV systems at z ~ 3.4 appearing only toward
Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap
A Measurement of the Temperature-Density Relation in the Intergalactic Medium Using a New Lyman-alpha Absorption Line Fitting Method
The evolution of the temperature in the intergalactic medium is related to
the reionization of hydrogen and helium, and has important consequences for our
understanding of the Lya forest and of galaxy formation in gravitational models
of large-scale structure. We measure the temperature-density relation of
intergalactic gas from Lya forest observations of eight quasar spectra with
high resolution and signal-to-noise ratio, using a new line fitting technique
to obtain a lower cutoff of the distribution of line widths from which the
temperature is derived. We carefully test the accuracy of this technique to
recover the gas temperature with a hydrodynamic simulation. The temperature at
redshift z=(3.9, 3.0, 2.4) is best determined at densities slightly above the
mean: T_star=(20200\pm2700, 20200\pm1300, 22600\pm1900)K (statistical error
bars) for gas density (in units of the mean density) Delta_star=(1.42\pm0.08,
1.37\pm0.11, 1.66\pm0.11). The power-law index of the temperature-density
relation, defined by T=T_star(Delta/Delta_star)^{gamma-1}, is gamma-1=
(0.43\pm0.45, 0.29\pm0.30, 0.52\pm0.14) for the same three redshifts. The
temperature at the fixed over-density Delta=1.4 is T_1.4=(20100\pm2800,
20300\pm1400, 20700\pm1900)K. These temperatures are higher than expected for
photoionized gas in ionization equilibrium with a cosmic background, and can be
explained by a gradual additional heating due to on-going HeII reionization.
The measurement of the temperature reduces one source of uncertainty in the
lower limit to the baryon density implied by the observed mean flux decrement.
We find that the temperature cannot be reliably measured for under-dense gas,
because the velocities due to expansion always dominate the widths of the
corresponding weak lines.Comment: submitted to Ap
Cholinergic, but not dopaminergic or noradrenergic, enhancement sharpens visual spatial perception in humans
The neuromodulator acetylcholine modulates spatial integration in visual cortex by altering the balance of inputs that generate neuronal receptive fields. These cholinergic effects may provide a neurobiological mechanism underlying the modulation of visual representations by visual spatial attention. However, the consequences of cholinergic enhancement on visuospatial perception in humans are unknown. We conducted two experiments to test whether enhancing cholinergic signaling selectively alters perceptual measures of visuospatial interactions in human subjects. In Experiment 1, a double-blind placebo-controlled pharmacology study, we measured how flanking distractors influenced detection of a small contrast decrement of a peripheral target, as a function of target-flanker distance. We found that cholinergic enhancement with the cholinesterase inhibitor donepezil improved target detection, and modeling suggested that this was mainly due to a narrowing of the extent of facilitatory perceptual spatial interactions. In Experiment 2, we tested whether these effects were selective to the cholinergic system or would also be observed following enhancements of related neuromodulators dopamine or norepinephrine. Unlike cholinergic enhancement, dopamine (bromocriptine) and norepinephrine (guanfacine) manipulations did not improve performance or systematically alter the spatial profile of perceptual interactions between targets and distractors. These findings reveal mechanisms by which cholinergic signaling influences visual spatial interactions in perception and improves processing of a visual target among distractors, effects that are notably similar to those of spatial selective attention
Transcriptional profiling reveals extraordinary diversity among skeletal muscle tissues
Skeletal muscle comprises a family of diverse tissues with highly specialized functions. Many acquired diseases, including HIV and COPD, affect specific muscles while sparing others. Even monogenic muscular dystrophies selectively affect certain muscle groups. These observations suggest that factors intrinsic to muscle tissues influence their resistance to disease. Nevertheless, most studies have not addressed transcriptional diversity among skeletal muscles. Here we use RNAseq to profile mRNA expression in skeletal, smooth, and cardiac muscle tissues from mice and rats. Our data set, MuscleDB, reveals extensive transcriptional diversity, with greater than 50% of transcripts differentially expressed among skeletal muscle tissues. We detect mRNA expression of hundreds of putative myokines that may underlie the endocrine functions of skeletal muscle. We identify candidate genes that may drive tissue specialization, including Smarca4, Vegfa, and Myostatin. By demonstrating the intrinsic diversity of skeletal muscles, these data provide a resource for studying the mechanisms of tissue specialization
A Unified Account of the Moral Standing to Blame
Recently, philosophers have turned their attention to the question, not when a given agent is blameworthy for what she does, but when a further agent has the moral standing to blame her for what she does. Philosophers have proposed at least four conditions on having âmoral standingâ:
1. Oneâs blame would not be âhypocriticalâ.
2. One is not oneself âinvolved inâ the target agentâs wrongdoing.
3. One must be warranted in believing that the target is indeed blameworthy for the wrongdoing.
4. The targetâs wrongdoing must some of âoneâs businessâ.
These conditions are often proposed as both conditions on one and the same thing, and as marking fundamentally different ways of âlosing standing.â Here I call these claims into question. First, I claim that conditions (3) and (4) are simply conditions on different things than are conditions (1) and (2). Second, I argue that condition (2) reduces to condition (1): when âinvolvementâ removes someoneâs standing to blame, it does so only by indicating something further about that agent, viz., that he or she lacks commitment to the values that condemn the wrongdoerâs action. The result: after we clarify the nature of the non-hypocrisy condition, we will have a unified account of moral standing to blame. Issues also discussed: whether standing can ever be regained, the relationship between standing and our "moral fragility", the difference between mere inconsistency and hypocrisy, and whether a condition of standing might be derived from deeper facts about the "equality of persons"
The stability of the spectator, Dirac, and Salpeter equations for mesons
Mesons are made of quark-antiquark pairs held together by the strong force.
The one channel spectator, Dirac, and Salpeter equations can each be used to
model this pairing. We look at cases where the relativistic kernel of these
equations corresponds to a time-like vector exchange, a scalar exchange, or a
linear combination of the two. Since the model used in this paper describes
mesons which cannot decay physically, the equations must describe stable
states. We find that this requirement is not always satisfied, and give a
complete discussion of the conditions under which the various equations give
unphysical, unstable solutions
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