413 research outputs found
Events: Modellierungen und Schnittstellen
Abstract fĂŒr das Konferenzpanel "Events: Modellierungen und Schnittstellen", DHd202
Keck Spectroscopy of Candidate Proto-globular Clusters in NGC 1275
Keck spectroscopy of 5 proto-globular cluster candidates in NGC 1275 has been
combined with HST WFPC2 photometry to explore the nature and origin of these
objects and discriminate between merger and cooling flow scenarios for globular
cluster formation. The objects we have studied are not HII regions, but rather
star clusters, yet their integrated spectral properties do not resemble young
or intermediate age Magellanic Cloud clusters or Milky Way open clusters. The
clusters' Balmer absorption appears to be too strong to be consistent with any
of the standard Bruzual & Charlot evolutionary models at any metallicity. If
these models are adopted, an IMF which is skewed to high masses provides a
better fit to the data. A truncated IMF with a mass range of 2-3 Mo reproduces
the observed Balmer equivalent widths and colors at about 450 Myr. Formation in
a continuous cooling flow appears to be ruled out since the age of the clusters
is much larger than the cooling time, the spatial scale of the clusters is much
smaller than the cooling flow radius, and the deduced star formation rate in
the cooling flow favors a steep rather than a flat IMF. A merger would have to
produce clusters only in the central few kpc, presumably from gas in the
merging galaxies which was channeled rapidly to the center. Widespread shocks
in merging galaxies cannot have produced these clusters. If these objects are
confirmed to have a relatively flat, or truncated, IMF it is unclear whether or
not they will evolve into objects we would regard as bona fide globular
clusters.Comment: 30 pages (AAS two column style, including 9 tables and 7 figures) to
appear in the AJ (August issue), also available at
http://www.ucolick.org/~mkissler/Sages/sages.html (with a full resolution
Fig.1) Revised Version: previous posted version was an uncorrect ealier
iteration, parts of the text, tables and figures changed. The overall
conclusions remain unchange
Giant H II regions in the merging system NGC 3256: Are they the birthplaces of globular clusters?
CCD images and spectra of ionized hydrogen in the merging system NGC3256 were
acquired as part of a kinematic study to investigate the formation of globular
clusters (GC) during the interactions and mergers of disk galaxies. This paper
focuses on the proposition that giant H II regions, with an H_alpha luminosity
> 1.5 x 10^{40} erg s^{-1}, are birthplaces of young populous clusters (YPC).
We estimate 900 30-Doradus-like H II regions formed during the galaxy-galaxy
interaction which created NGC 3256. A specific frequency argument for forming a
sufficient number of GC, assuming that the NGC3256 merger remnant will be an
elliptical galaxy, only requires that about 10% of these H II regions harbor
YPC which survive several Gyr and have masses at least as large as the cluster
R136 in 30 Doradus.Comment: Jpeg images; colour images are meant to only appear on monitors, not
printout. 19 pages, 3 figures, and 3 tables. To be published in Astronomical
Journal Mar. 200
The Mid-IR Properties of Starburst Galaxies from Spitzer-IRS Spectroscopy
We present 5-38um mid-infrared spectra at a spectral resolution of R~65-130
of a large sample of 22 starburst nuclei taken with the Infrared Spectrograph
IRS on board the Spitzer Space Telescope. The spectra show a vast range in
starburst SEDs. The silicate absorption ranges from essentially no absorption
to heavily obscured systems with an optical depth of tau(9.8um)~5. The spectral
slopes can be used to discriminate between starburst and AGN powered sources.
The monochromatic continuum fluxes at 15um and 30um enable a remarkably
accurate estimate of the total infrared luminosity of the starburst. We find
that the PAH equivalent width is independent of the total starburst luminosity
L_IR as both continuum and PAH feature scale proportionally. However, the
luminosity of the 6.2um feature scales with L_IR and can be used to approximate
the total infrared luminosity of the starburst. Although our starburst sample
covers about a factor of ten difference in the [NeIII]/[NeII] ratio, we found
no systematic correlation between the radiation field hardness and the PAH
equivalent width or the 7.7um/11.3um PAH ratio. These results are based on
spatially integrated diagnostics over an entire starburst region, and local
variations may be ``averaged out''. It is presumably due to this effect that
unresolved starburst nuclei with significantly different global properties
appear spectrally as rather similar members of one class of objects.Comment: 22 pages, accepted for publication in ApJ, a high-resolution version
is available from http://www.strw.leidenuniv.nl/~brandl/IRS_starbursts.pd
The Antennae Galaxies (NGC 4038/4039) Revisited: ACS and NICMOS Observations of a Prototypical Merger
The ACS and NICMOS have been used to obtain new HST images of NGC 4038/4039
("The Antennae"). These new observations allow us to better differentiate
compact star clusters from individual stars, based on both size and color. We
use this ability to extend the cluster luminosity function by approximately two
magnitudes over our previous WFPC2 results, and find that it continues as a
single power law, dN/dL propto L^alpha with alpha=-2.13+/-0.07, down to the
observational limit of Mv~-7. Similarly, the mass function is a single power
law dN/dM propto M^beta with beta=-2.10+/-0.20 for clusters with ages t<3x10^8
yr, corresponding to lower mass limits that range from 10^4 to 10^5 Msun,
depending on the age range of the subsample. Hence the power law indices for
the luminosity and mass functions are essentially the same. The luminosity
function for intermediate-age clusters (i.e., ~100-300 Myr old objects found in
the loops, tails, and outer areas) shows no bend or turnover down to Mv~-6,
consistent with relaxation-driven cluster disruption models which predict the
turnover should not be observed until Mv~-4. An analysis of individual ~0.5-kpc
sized areas over diverse environments shows good agreement between values of
alpha and beta, similar to the results for the total population of clusters in
the system. Several of the areas studied show evidence for age gradients, with
somewhat older clusters appearing to have triggered the formation of younger
clusters. The area around Knot B is a particularly interesting example, with an
~10-50 Myr old cluster of estimated mass ~10^6 Msun having apparently triggered
the formation of several younger, more massive (up to 5x10^6 Msun) clusters
along a dust lane.Comment: 84 pages, 9 tables, 31 figures; ApJ accepte
Exact Fermi-edge singularity exponent in a Luttinger liquid
We report the exact calculation of the Fermi-edge singularity exponent for
correlated electrons in one dimension (Luttinger liquid). Focusing on the
special interaction parameter g=1/2, the asymptotic long-time behavior can be
obtained using the Wiener-Hopf method. The result confirms the previous
assumption of an open boundary fixed point. In addition, a dynamic k-channel
Kondo impurity is studied via Abelian bosonization for k=2 and k=4. It is shown
that the corresponding orthogonality exponents are related to the orthogonality
exponent in a Luttinger liquid.Comment: 8 Pages RevTeX, no figure
Photoluminescence investigations of 2D hole Landau levels in p-type single Al_{x}Ga_{1-x}As/GaAs heterostructures
We study the energy structure of two-dimensional holes in p-type single
Al_{1-x}Ga_{x}As/GaAs heterojunctions under a perpendicular magnetic field.
Photoluminescence measurments with low densities of excitation power reveal
rich spectra containing both free and bound-carrier transitions. The
experimental results are compared with energies of valence-subband Landau
levels calculated using a new numerical procedure and a good agreement is
achieved. Additional lines observed in the energy range of free-carrier
recombinations are attributed to excitonic transitions. We also consider the
role of many-body effects in photoluminescence spectra.Comment: 13 pages, 10 figures, accepted to Physical Review
Modulation of food intake by mTOR signalling in the dorsal motor nucleus of the vagus in male rats: focus on ghrelin and nesfatinâ1
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/101864/1/expphysiol.2013.074930.pd
Ankyrin repeat and SOCS box containing protein 4 (Asb-4) colocalizes with insulin receptor substrate 4 (IRS4) in the hypothalamic neurons and mediates IRS4 degradation
Abstract Background The arcuate nucleus of the hypothalamus regulates food intake. Ankyrin repeat and SOCS box containing protein 4 (Asb-4) is expressed in neuropeptide Y and proopiomelanocortin (POMC) neurons in the arcuate nucleus, target neurons in the regulation of food intake and metabolism by insulin and leptin. However, the target protein(s) of Asb-4 in these neurons remains unknown. Insulin receptor substrate 4 (IRS4) is an adaptor molecule involved in the signal transduction by both insulin and leptin. In the present study we examined the colocalization and interaction of Asb-4 with IRS4 and the involvement of Asb-4 in insulin signaling. Results In situ hybridization showed that the expression pattern of Asb-4 was consistent with that of IRS4 in the rat brain. Double in situ hybridization showed that IRS4 colocalized with Asb-4, and both Asb-4 and IRS4 mRNA were expressed in proopiomelanocortin (POMC) and neuropeptide Y (NPY) neurons within the arcuate nucleus of the hypothalamus. In HEK293 cells co-transfected with Myc-tagged Asb-4 and Flag-tagged IRS4, Asb-4 co-immunoprecipitated with IRS4; In these cells endogenous IRS4 also co-immunoprecipitated with transfected Myc-Asb-4; Furthermore, Asb-4 co-immunoprecipitated with IRS4 in rat hypothalamic extracts. In HEK293 cells over expression of Asb-4 decreased IRS4 protein levels and deletion of the SOCS box abolished this effect. Asb-4 increased the ubiquitination of IRS4; Deletion of SOCS box abolished this effect. Expression of Asb-4 decreased both basal and insulin-stimulated phosphorylation of AKT at Thr308. Conclusions These data demonstrated that Asb-4 co-localizes and interacts with IRS4 in hypothalamic neurons. The interaction of Asb-4 with IRS4 in cell lines mediates the degradation of IRS4 and decreases insulin signaling
Gravitational waves from coalescing binaries and Doppler experiments
Doppler tracking of interplanetary spacecraft provides the only method
presently available for broad-band searches of low frequency gravitational
waves. The instruments have a peak sensitivity around the reciprocal of the
round-trip light-time T of the radio link connecting the Earth to the
space-probe and therefore are particularly suitable to search for coalescing
binaries containing massive black holes in galactic nuclei. A number of Doppler
experiments -- the most recent involving the probes ULYSSES, GALILEO and MARS
OBSERVER -- have been carried out so far; moreover, in 2002-2004 the CASSINI
spacecraft will perform three 40 days data acquisition runs with expected
sensitivity about twenty times better than that achieved so far. Central aims
of this paper are: (i) to explore, as a function of the relevant instrumental
and astrophysical parameters, the Doppler output produced by in-spiral signals
-- sinusoids of increasing frequency and amplitude (the so-called chirp); (ii)
to identify the most important parameter regions where to concentrate intense
and dedicated data analysis; (iii) to analyze the all-sky and all-frequency
sensitivity of the CASSINI's experiments, with particular emphasis on possible
astrophysical targets, such as our Galactic Centre and the Virgo Cluster.Comment: 52 pages, LaTeX, 19 Postscript Figures, submitted to Phys. Rev.
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