4,213 research outputs found
Evidence for a constant IMF in early-type galaxies based on their X-ray binary populations
A number of recent studies have proposed that the stellar initial mass
function (IMF) of early type galaxies varies systematically as a function of
galaxy mass, with higher mass galaxies having bottom heavy IMFs. These bottom
heavy IMFs have more low-mass stars relative to the number of high mass stars,
and therefore naturally result in proportionally fewer neutron stars and black
holes. In this paper, we specifically predict the variation in the number of
black holes and neutron stars based on the power-law IMF variation required to
reproduce the observed mass-to-light ratio trends with galaxy mass. We then
test whether such variations are observed by studying the field low-mass X-ray
binary populations (LMXBs) of nearby early-type galaxies. In these binaries, a
neutron star or black hole accretes matter from a low-mass donor star. Their
number is therefore expected to scale with the number of black holes and
neutron stars present in a galaxy. We find that the number of LMXBs per K-band
light is similar among the galaxies in our sample. These data therefore
demonstrate the uniformity of the slope of the IMF from massive stars down to
those now dominating the K-band light, and are consistent with an invariant
IMF. Our results are inconsistent with an IMF which varies from a
Kroupa/Chabrier like IMF for low mass galaxies to a steep power-law IMF (with
slope =2.8) for high mass galaxies. We discuss how these observations
constrain the possible forms of the IMF variations and how future Chandra
observations can enable sharper tests of the IMF.Comment: 12 pages, 5 figures, 2 tables, submitted to Ap
Is there an integrative center in the vertebrate brain-stem? A robotic evaluation of a model of the reticular formation viewed as an action selection device
Neurobehavioral data from intact, decerebrate, and neonatal rats, suggests that the reticular formation provides
a brainstem substrate for action selection in the vertebrate central nervous system. In this article, Kilmer,
McCulloch and Blum’s (1969, 1997) landmark reticular formation model is described and re-evaluated, both in
simulation and, for the first time, as a mobile robot controller. Particular model configurations are found to
provide effective action selection mechanisms in a robot survival task using either simulated or physical robots.
The model’s competence is dependent on the organization of afferents from model sensory systems, and a genetic
algorithm search identified a class of afferent configurations which have long survival times. The results support
our proposal that the reticular formation evolved to provide effective arbitration between innate behaviors
and, with the forebrain basal ganglia, may constitute the integrative, ’centrencephalic’ core of vertebrate brain
architecture. Additionally, the results demonstrate that the Kilmer et al. model provides an alternative form of
robot controller to those usually considered in the adaptive behavior literature
Ceftolozane-tazobactam activity against phylogenetically diverse Clostridium difficile strains
Ceftolozane-tazobactam (C/T) is approved for the treatment of complicated intra-abdominal and urinary tract infections and has varied activity against anaerobic bacteria. Here, we evaluate the activity of C/T against a phylogenetically diverse collection of Clostridium difficile isolates and report uniformly high MICs (≥256 μg/ml) to C/T
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Reductions in the dietary niche of southern sea otters (Enhydra lutris nereis) from the Holocene to the Anthropocene.
The sea otter (Enhydra lutris) is a marine mammal hunted to near extinction during the 1800s. Despite their well-known modern importance as a keystone species, we know little about historical sea otter ecology. Here, we characterize the ecological niche of ancient southern sea otters (E. lutris nereis) using δ13C analysis and δ15N analysis of bones recovered from archaeological sites spanning ~7,000 to 350 years before present (N = 112 individuals) at five regions along the coast of California. These data are compared with previously published data on modern animals (N = 165) and potential modern prey items. In addition, we analyze the δ15N of individual amino acids for 23 individuals to test for differences in sea otter trophic ecology through time. After correcting for tissue-specific and temporal isotopic effects, we employ nonparametric statistics and Bayesian niche models to quantify differences among ancient and modern animals. We find ancient otters occupied a larger isotopic niche than nearly all modern localities; likely reflecting broader habitat and prey use in prefur trade populations. In addition, ancient sea otters at the most southerly sites occupied an isotopic niche that was more than twice as large as ancient otters from northerly regions. This likely reflects greater invertebrate prey diversity in southern California relative to northern California. Thus, we suggest the potential dietary niche of sea otters in southern California could be larger than in central and northern California. At two sites, Año Nuevo and Monterey Bay, ancient otters had significantly higher δ15N values than modern populations. Amino acid δ15N data indicated this resulted from shifting baseline isotope values, rather than a change in sea otter trophic ecology. Our results help in better understanding the contemporary ecological role of sea otters and exemplify the strength of combing zooarchaeological and biological information to provide baseline data for conservation efforts
Inspiral, merger and ringdown of unequal mass black hole binaries: a multipolar analysis
We study the inspiral, merger and ringdown of unequal mass black hole
binaries by analyzing a catalogue of numerical simulations for seven different
values of the mass ratio (from q=M2/M1=1 to q=4). We compare numerical and
Post-Newtonian results by projecting the waveforms onto spin-weighted spherical
harmonics, characterized by angular indices (l,m). We find that the
Post-Newtonian equations predict remarkably well the relation between the wave
amplitude and the orbital frequency for each (l,m), and that the convergence of
the Post-Newtonian series to the numerical results is non-monotonic. To leading
order the total energy emitted in the merger phase scales like eta^2 and the
spin of the final black hole scales like eta, where eta=q/(1+q)^2 is the
symmetric mass ratio. We study the multipolar distribution of the radiation,
finding that odd-l multipoles are suppressed in the equal mass limit. Higher
multipoles carry a larger fraction of the total energy as q increases. We
introduce and compare three different definitions for the ringdown starting
time. Applying linear estimation methods (the so-called Prony methods) to the
ringdown phase, we find resolution-dependent time variations in the fitted
parameters of the final black hole. By cross-correlating information from
different multipoles we show that ringdown fits can be used to obtain precise
estimates of the mass and spin of the final black hole, which are in remarkable
agreement with energy and angular momentum balance calculations.Comment: 51 pages, 28 figures, 16 tables. Many improvements throughout the
text in response to the referee report. The calculation of multipolar
components in Appendix A now uses slightly different conventions. Matches
version in press in PR
The origin of the Acheulean: the 1.7 million-year-old site of FLK West, Olduvai Gorge (Tanzania)
The appearance of the Acheulean is one of the hallmarks of human evolution. It represents the
emergence of a complex behavior, expressed in the recurrent manufacture of large-sized tools, with
standardized forms, implying more advance forethought and planning by hominins than those required
by the precedent Oldowan technology. The earliest known evidence of this technology dates back to
c. 1.7 Ma. and is limited to two sites (Kokiselei [Kenya] and Konso [Ethiopia]), both of which lack fauna.
The functionality of these earliest Acheulean assemblages remains unknown. Here we present the
discovery of another early Acheulean site also dating to c. 1.7 Ma from Olduvai Gorge. This site provides
evidence of the earliest steps in developing the Acheulean technology and is the oldest Acheulean site in
which stone tools occur spatially and functionally associated with the exploitation of fauna. Simple and
elaborate large-cutting tools (LCT) and handaxes co-exist at FLK West, showing that complex cognition
was present from the earliest stages of the Acheulean. Here we provide a detailed technological study
and evidence of the use of these tools on the butchery and consumption of fauna, probably by early
Homo erectus sensu lato
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