113 research outputs found
Multiwavelength star formation indicators: Observations
We present a compilation of multiwavelength data on different star formation
indicators for a sample of nearby star forming galaxies. Here we discuss the
observations, reductions and measurements of ultraviolet images obtained with
STIS, on board the Hubble Space Telescope, ground-based Halpha, and VLA 8.46
GHz radio images. These observations are complemented with infrared fluxes, as
well as large aperture optical radio and ultraviolet data from the literature.
This database will be used in a forthcoming paper to compare star formation
rates at different wavebands. We also present spectral energy distributions
(SEDs) for those galaxies with at least one far-infrared measurements from ISO,
longward of 100 um. These SEDs are divided in two groups, those which are
dominated by the far-infrared emission, and those where the contribution from
the far-infrared and optical emission is comparable. These SEDs are useful
tools to study the properties of high redshift galaxies.Comment: 39 pages, 17 jpeg figures, 1 eps figure, To appear in ApJS May 200
Properties of Ly-alpha emitters around the radio galaxy MRC 0316-257
Observations of the radio galaxy MRC 0316-257 at z=3.13 and the surrounding
field are presented. Using narrow- and broad-band imaging obtained with the
VLT, 77 candidate Ly-alpha emitters with a rest-frame equivalent width of > 15
A were selected in a ~7'x7' field around the radio galaxy. Spectroscopy of 40
candidate emitters resulted in the discovery of 33 emission line galaxies of
which 31 are Ly-alpha emitters with redshifts similar to that of the radio
galaxy, while the remaining two galaxies turned out to be [OII] emitters. The
Ly-alpha profiles have widths (FWHM) in the range of 120-800 km/s, with a
median of 260 km/s. Where the signal-to-noise was large enough, the Ly-alpha
profiles were found to be asymmetric, with apparent absorption troughs blueward
of the profile peaks, indicative of absorption along the line of sight of an HI
mass of at least 2x10^2 - 5x10^4 M_sun. The properties of the Ly-alpha galaxies
(faint, blue and small) are consistent with young star forming galaxies which
are still nearly dust free. The volume density of Ly-alpha emitting galaxies in
the field around MRC 0316-257 is a factor of 3.3+0.5-0.4 larger compared with
the density of field Ly-alpha emitters at that redshift. The velocity
distribution of the spectroscopically confirmed emitters has a FWHM of 1510
km/s, which is substantially smaller than the width of the narrow-band filter
(FWHM ~ 3500 km/s). The peak of the velocity distribution is located within 200
km/s of the redshift of the radio galaxy. We conclude that the confirmed
Ly-alpha emitters are members of a protocluster of galaxies at z~3.13. The size
of the protocluster is larger than 3.3x3.3 Mpc^2. The mass of this structure is
estimated to be > 3-6x10^14 M_sun and could be the progenitor of a cluster of
galaxies similar to e.g. the Virgo cluster. (Abridged)Comment: 23 Pages, including 20 PostScript figures. Publiced in Astronomy &
Astrophysics. v2: typo fixed and Journal reference adde
Local Star formation triggered by SN shocks in magnetized diffuse neutral clouds
In this work, considering the impact of a SNR with a neutral magnetized cloud
we derived analytically a set of conditions which are favorable for driving
gravitational instability in the cloud and thus star formation. We have built
diagrams of the SNR radius, versus the cloud density, that constrain a domain
in the parameter space where star formation is allowed. The diagrams are also
tested with fully 3-D MHD simulations involving a SNR and a self-gravitating
cloud and we find that the numerical analysis is consistent with the results
predicted by the diagrams. While the inclusion of a homogeneous magnetic field
approximately perpendicular to the impact velocity of the SNR with an intensity
~1 G results only a small shrinking of the star formation triggering zone
in the diagrams, a larger magnetic field (~10 G) causes a significant
shrinking, as expected. Applications of the diagrams to a few regions of our
own galaxy have revealed that star formation in those sites could have been
triggered by shock waves from SNRs. Finally, we have evaluated the effective
star formation efficiency for this sort of interaction and found that it is
smaller than the observed values in our own Galaxy (sfe ~0.01-0.3). This result
is consistent with previous work in the literature and also suggests that the
mechanism presently investigated, though very powerful to drive structure
formation, supersonic turbulence and eventually, local star formation, does not
seem to be sufficient to drive global star formation in normal star forming
galaxies, not even when the magnetic field in the neutral clouds is neglected.
(abridged)Comment: 19 pages, 13 figures, accepted for pubblication in MNRA
Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much of modern
astrophysics. For several decades it has been thought that stellar birth is
primarily controlled by the interplay between gravity and magnetostatic
support, modulated by ambipolar diffusion. Recently, however, both
observational and numerical work has begun to suggest that support by
supersonic turbulence rather than magnetic fields controls star formation. In
this review we outline a new theory of star formation relying on the control by
turbulence. We demonstrate that although supersonic turbulence can provide
global support, it nevertheless produces density enhancements that allow local
collapse. Inefficient, isolated star formation is a hallmark of turbulent
support, while efficient, clustered star formation occurs in its absence. The
consequences of this theory are then explored for both local star formation and
galactic scale star formation. (ABSTRACT ABBREVIATED)Comment: Invited review for "Reviews of Modern Physics", 87 pages including 28
figures, in pres
Angular Momentum and the Formation of Stars and Black Holes
The formation of compact objects like stars and black holes is strongly
constrained by the requirement that nearly all of the initial angular momentum
of the diffuse material from which they form must be removed or redistributed
during the formation process. The mechanisms that may be involved and their
implications are discussed for (1) low-mass stars, most of which probably form
in binary or multiple systems; (2) massive stars, which typically form in
clusters; and (3) supermassive black holes that form in galactic nuclei. It is
suggested that in all cases, gravitational interactions with other stars or
mass concentrations in a forming system play an important role in
redistributing angular momentum and thereby enabling the formation of a compact
object. If this is true, the formation of stars and black holes must be a more
complex, dynamic, and chaotic process than in standard models. The
gravitational interactions that redistribute angular momentum tend to couple
the mass of a forming object to the mass of the system, and this may have
important implications for mass ratios in binaries, the upper stellar IMF in
clusters, and the masses of supermassive black holes in galaxies.Comment: Accepted by Reports on Progress in Physic
On the Ly-alpha emission from gamma-ray burst host galaxies: evidence for low metallicities
We report on the results of a search for Ly-alpha emission from the host
galaxy of the z=2.140 GRB 011211 and other galaxies in its surrounding field.
We detect Ly-alpha emission from the host as well as from six other galaxies in
the field. The restframe equivalent width of the Ly-alpha line from the GRB
011211 host is about 21 AA. This is the fifth detection of Ly-alpha emission
out of five possible detections from GRB host galaxies, strongly indicating
that GRB hosts, at least at high redshifts, are Ly-alpha emitters. This is
intriguing as only about 25% of the Lyman-Break selected galaxies at similar
redshifts have Ly-alpha emission lines with restframe equivalent width larger
than 20 AA. Possible explanations are i) a preference for GRB progenitors to be
metal-poor as expected in the collapsar model, ii) an optical afterglow
selection bias against dusty hosts, and iii) a higher fraction of Ly-alpha
emitters at the faint end of the luminosity function for high-z galaxies. Of
these, the current evidence seems to favour i).Comment: 5 pages, 3 figures. Accepted for publication in A&A Letter
Benchmarking Materials Property Prediction Methods: The Matbench Test Set and Automatminer Reference Algorithm
We present a benchmark test suite and an automated machine learning procedure
for evaluating supervised machine learning (ML) models for predicting
properties of inorganic bulk materials. The test suite, Matbench, is a set of
13 ML tasks that range in size from 312 to 132k samples and contain data from
10 density functional theory-derived and experimental sources. Tasks include
predicting optical, thermal, electronic, thermodynamic, tensile, and elastic
properties given a materials composition and/or crystal structure. The
reference algorithm, Automatminer, is a highly-extensible, fully-automated ML
pipeline for predicting materials properties from materials primitives (such as
composition and crystal structure) without user intervention or hyperparameter
tuning. We test Automatminer on the Matbench test suite and compare its
predictive power with state-of-the-art crystal graph neural networks and a
traditional descriptor-based Random Forest model. We find Automatminer achieves
the best performance on 8 of 13 tasks in the benchmark. We also show our test
suite is capable of exposing predictive advantages of each algorithm - namely,
that crystal graph methods appear to outperform traditional machine learning
methods given ~10^4 or greater data points. The pre-processed, ready-to-use
Matbench tasks and the Automatminer source code are open source and available
online (http://hackingmaterials.lbl.gov/automatminer/). We encourage evaluating
new materials ML algorithms on the MatBench benchmark and comparing them
against the latest version of Automatminer.Comment: Main text, supplemental inf
Investigating the genetic architecture of dementia with Lewy bodies: a two-stage genome-wide association study
Background Dementia with Lewy bodies is the second most common form of dementia in elderly people but has been overshadowed in the research field, partly because of similarities between dementia with Lewy bodies, Parkinson’s disease, and Alzheimer’s disease. So far, to our knowledge, no large-scale genetic study of dementia with Lewy bodies has been done. To better understand the genetic basis of dementia with Lewy bodies, we have done a genome-wide association study with the aim of identifying genetic risk factors for this disorder.
Methods In this two-stage genome-wide association study, we collected samples from white participants of European ancestry who had been diagnosed with dementia with Lewy bodies according to established clinical or pathological criteria. In the discovery stage (with the case cohort recruited from 22 centres in ten countries and the controls derived from two publicly available database of Genotypes and Phenotypes studies [phs000404.v1.p1 and phs000982.v1.p1] in the USA), we performed genotyping and exploited the recently established Haplotype Reference Consortium panel as the basis for imputation. Pathological samples were ascertained following autopsy in each individual brain bank, whereas clinical samples were collected by clinical teams after clinical examination. There was no specific timeframe for collection of samples. We did association analyses in all participants with dementia with Lewy bodies, and also in only participants with pathological diagnosis. In the replication stage, we performed genotyping of significant and suggestive results from the discovery stage. Lastly, we did a meta-analysis of both stages under a fixed-effects model and used logistic regression to test for association in each stage.
Findings This study included 1743 patients with dementia with Lewy bodies (1324 with pathological diagnosis) and 4454 controls (1216 patients with dementia with Lewy bodies vs 3791 controls in the discovery stage; 527 vs 663 in the replication stage). Results confirm previously reported associations: APOE (rs429358; odds ratio [OR] 2·40, 95% CI 2·14–2·70; p=1·05 × 10–⁴⁸), SNCA (rs7681440; OR 0·73, 0·66–0·81; p=6·39 × 10–¹⁰), and GBA (rs35749011; OR 2·55, 1·88–3·46; p=1·78 × 10–⁹). They also provide some evidence for a novel candidate locus, namely CNTN1 (rs7314908; OR 1·51, 1·27–1·79; p=2·21 × 10–⁶); further replication will be important. Additionally, we estimate the heritable component of dementia with Lewy bodies to be about 36%.
Interpretation Despite the small sample size for a genome-wide association study, and acknowledging the potential biases from ascertaining samples from multiple locations, we present the most comprehensive and well powered genetic study in dementia with Lewy bodies so far. These data show that common genetic variability has a role in the disease
A comprehensive screening of copy number variability in dementia with Lewy bodies
The role of genetic variability in dementia with Lewy bodies (DLB) is now indisputable; however, data regarding copy number variation (CNV) in this disease has been lacking. Here, we used whole-genome genotyping of 1454 DLB cases and 1525 controls to assess copy number variability. We used 2 algorithms to confidently detect CNVs, performed a case-control association analysis, screened for candidate CNVs previously associated with DLB-related diseases, and performed a candidate gene approach to fully explore the data. We identified 5 CNV regions with a significant genome-wide association to DLB; 2 of these were only present in cases and absent from publicly available databases: one of the regions overlapped LAPTM4B, a known lysosomal protein, whereas the other overlapped the NME1 locus and SPAG9. We also identified DLB cases presenting rare CNVs in genes previously associated with DLB or related neurodegenerative diseases, such as SNCA, APP, and MAPT. To our knowledge, this is the first study reporting genome-wide CNVs in a large DLB cohort. These results provide preliminary evidence for the contribution of CNVs in DLB risk.info:eu-repo/semantics/publishedVersio
A comprehensive screening of copy number variability in dementia with Lewy bodies
The role of genetic variability in dementia with Lewy bodies (DLB) is now indisputable; however, data regarding copy number variation (CNV) in this disease has been lacking. Here, we used whole-genome genotyping of 1454 DLB cases and 1525 controls to assess copy number variability. We used 2 algorithms to confidently detect CNVs, performed a case-control association analysis, screened for candidate CNVs previously associated with DLB-related diseases, and performed a candidate gene approach to fully explore the data. We identified 5 CNV regions with a significant genome-wide association to DLB; 2 of these were only present in cases and absent from publicly available databases: one of the regions overlapped LAPTM4B, a known lysosomal protein, whereas the other overlapped the NME1 locus and SPAG9. We also identified DLB cases presenting rare CNVs in genes previously associated with DLB or related neurodegenerative diseases, such as SNCA, APP, and MAPT. To our knowledge, this is the first study reporting genome-wide CNVs in a large DLB cohort. These results provide preliminary evidence for the contribution of CNVs in DLB risk. (C) 2019 Elsevier Inc. All rights reserved.Peer reviewe
- …