108 research outputs found
Evaporation of Compact Young Clusters near the Galactic Center
We investigate the dynamical evolution of compact young clusters (CYCs) near
the Galactic center (GC) using Fokker-Planck models. CYCs are very young (< 5
Myr), compact (< 1 pc), and only a few tens of pc away from the GC, while they
appear to be as massive as the smallest Galactic globular clusters (~10^4
Msun). A survey of cluster lifetimes for various initial mass functions,
cluster masses, and galactocentric radii is presented. Short relaxation times
due to the compactness of CYCs, and the strong tidal fields near the GC make
clusters evaporate fairly quickly. Depending on cluster parameters, mass
segregation may occur on a time scale shorter than the lifetimes of most
massive stars, which accelerates the cluster's dynamical evolution even more.
When the difference between the upper and lower mass boundaries of the initial
mass function is large enough, strongly selective ejection of lighter stars
makes massive stars dominate even in the outer regions of the cluster, so the
dynamical evolution of those clusters is weakly dependent on the lower mass
boundary. The mass bins for Fokker-Planck simulations were carefully chosen to
properly account for a relatively small number of the most massive stars. We
find that clusters with a mass <~ 2x10^4 Msun evaporate in <~ 10 Myr. A simple
calculation based on the total masses in observed CYCs and the lifetimes
obtained here indicates that the massive CYCs comprise only a fraction of the
star formation rate (SFR) in the inner bulge estimated from Lyman continuum
photons and far-IR observations.Comment: 20 pages in two-column format, accepted for publication in Ap
Ejection of Supermassive Black Holes from Galaxy Cores
[Abridged] Recent numerical relativity simulations have shown that the
emission of gravitational waves during the merger of two supermassive black
holes (SMBHs) delivers a kick to the final hole, with a magnitude as large as
4000 km/s. We study the motion of SMBHs ejected from galaxy cores by such kicks
and the effects on the stellar distribution using high-accuracy direct N-body
simulations. Following the kick, the motion of the SMBH exhibits three distinct
phases. (1) The SMBH oscillates with decreasing amplitude, losing energy via
dynamical friction each time it passes through the core. Chandrasekhar's theory
accurately reproduces the motion of the SMBH in this regime if 2 < ln Lambda <
3 and if the changing core density is taken into account. (2) When the
amplitude of the motion has fallen to roughly the core radius, the SMBH and
core begin to exhibit oscillations about their common center of mass. These
oscillations decay with a time constant that is at least 10 times longer than
would be predicted by naive application of the dynamical friction formula. (3)
Eventually, the SMBH reaches thermal equilibrium with the stars. We estimate
the time for the SMBH's oscillations to damp to the Brownian level in real
galaxies and infer times as long as 1 Gyr in the brightest galaxies. Ejection
of SMBHs also results in a lowered density of stars near the galaxy center;
mass deficits as large as five times the SMBH mass are produced for kick
velocities near the escape velocity. We compare the N-body density profiles
with luminosity profiles of early-type galaxies in Virgo and show that even the
largest observed cores can be reproduced by the kicks, without the need to
postulate hypermassive binary SMBHs. Implications for displaced AGNs and
helical radio structures are discussed.Comment: 18 pages, The Astrophysical Journal, in press. Replaced with revised
versio
Dynamical Processes in Globular Clusters
Globular clusters are among the most congested stellar systems in the
Universe. Internal dynamical evolution drives them toward states of high
central density, while simultaneously concentrating the most massive stars and
binary systems in their cores. As a result, these clusters are expected to be
sites of frequent close encounters and physical collisions between stars and
binaries, making them efficient factories for the production of interesting and
observable astrophysical exotica. I describe some elements of the competition
among stellar dynamics, stellar evolution, and other processes that control
globular cluster dynamics, with particular emphasis on pathways that may lead
to the formation of blue stragglers.Comment: Chapter 10, in Ecology of Blue Straggler Stars, H.M.J. Boffin, G.
Carraro & G. Beccari (Eds), Astrophysics and Space Science Library, Springe
Halo mass - concentration relation from weak lensing
We perform a statistical weak lensing analysis of dark matter profiles around
tracers of halo mass from galactic- to cluster-size halos. In this analysis we
use 170,640 isolated ~L* galaxies split into ellipticals and spirals, 38,236
groups traced by isolated spectroscopic Luminous Red Galaxies (LRGs) and 13,823
MaxBCG clusters from the Sloan Digital Sky Survey (SDSS) covering a wide range
of richness. Together these three samples allow a determination of the density
profiles of dark matter halos over three orders of magnitude in mass, from
10^{12} M_{sun} to 10^{15} M_{sun}. The resulting lensing signal is consistent
with an NFW or Einasto profile on scales outside the central region. We find
that the NFW concentration parameter c_{200b} decreases with halo mass, from
around 10 for galactic halos to 4 for cluster halos. Assuming its dependence on
halo mass in the form of c_{200b} = c_0 [M/(10^{14}M_{sun}/h)]^{\beta}, we find
c_0=4.6 +/- 0.7 (at z=0.22) and \beta=0.13 +/- 0.07, with very similar results
for the Einasto profile. The slope (\beta) is in agreement with theoretical
predictions, while the amplitude is about two standard deviations below the
predictions for this mass and redshift, but we note that the published values
in the literature differ at a level of 10-20% and that for a proper comparison
our analysis should be repeated in simulations. We discuss the implications of
our results for the baryonic effects on the shear power spectrum: since these
are expected to increase the halo concentration, the fact that we see no
evidence of high concentrations on scales above 20% of the virial radius
suggests that baryonic effects are limited to small scales, and are not a
significant source of uncertainty for the current weak lensing measurements of
the dark matter power spectrum. [ABRIDGED]Comment: 17 pages, 5 figures, accepted to JCAP pending minor revisions that
are included in v2 here on arXi
The lives and deaths of star clusters near the Galactic center
We study the evolution and observability of young, compact star clusters near
the Galactic center, such as the Arches and Quintuplet systems. The clusters
are modeled by integrating the equations of motion of all stars while
accounting for the internal evolution of stars and binaries, as well as the
effect of the Galactic tidal field. We find that clusters within 150 pc of the
Galactic center dissolve within ~55 Myr, but their projected densities drop
below the background density in the direction of the Galactic center within
only a few Myr, effectively making these clusters undetectable after that time.
Detailed observations of the Arches cluster, taken at face value, suggest that
its mass function is unusually flat and that the cluster contains an
overabundance of stars more massive than 20 Msun. Our dynamical analysis,
however, shows that the observed characteristics of the Arches cluster are
consistent with a perfectly normal initial mass function. The observed
anomalies are then caused by a combination of observational selection effects
and the dynamical evolution of the cluster. We calibrate the current parameters
of the Arches cluster using a normal initial mass function and conclude that
the cluster is more massive than 40000 Msun, has a half mass radius of about
0.35 pc and is located between 50 and 90 pc from the Galactic center.Comment: 21 pages, to be published on Januari 1, 2001 in ApJ v56
Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA
Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany
Hypermethylation-mediated reduction of WWOX expression in intraductal papillary mucinous neoplasms of the pancreas
We have previously shown that WW domain-containing oxidoreductase (WWOX) has tumour-suppressing effects and that its expression is frequently reduced in pancreatic carcinoma. In this study, we examined WWOX expression in intraductal papillary mucinous neoplasm of the pancreas (IPMN) to assess the function of WWOX in pancreatic duct tumourigenesis using immunohistochemistry and methylation-specific polymerase chain reaction analysis. Among 41 IPMNs including intraductal papillary mucinous adenomas (IPMAs) and intraductal papillary mucinous carcinomas (IPMCs), loss or reduced WWOX immunoreactivity was detected in 3 (15%) of 20 IPMAs and 17 (81%) of 21 IPMCs. In addition, hypermethylation of the WWOX regulatory site was detected in 1 (33%) of 3 WWOX(−) IPMAs and 9 (53%) of 17 WWOX(−) IPMCs, suggesting that hypermethylation may possibly be important in the suppression of WWOX expression. Reduction of WWOX expression was significantly correlated with a higher Ki-67 labelling index but was not correlated with the ssDNA apoptotic body index. Interestingly, decreased WWOX expression was significantly correlated with loss of SMAD4 expression in these IPMNs. The results indicate that downregulation of WWOX expression by the WWOX regulatory region hypermethylation is critical for transformation of pancreatic duct
Cloning and characterization of a 9-lipoxygenase gene induced by pathogen attack from Nicotiana benthamiana for biotechnological application
<p>Abstract</p> <p>Background</p> <p>Plant lipoxygenases (LOXs) have been proposed to form biologically active compounds both during normal developmental stages such as germination or growth as well as during responses to environmental stress such as wounding or pathogen attack. In our previous study, we found that enzyme activity of endogenous 9-LOX in <it>Nicotiana benthamiana </it>was highly induced by agroinfiltration using a tobacco mosaic virus (TMV) based vector system.</p> <p>Results</p> <p>A <it>LOX </it>gene which is expressed after treatment of the viral vectors was isolated from <it>Nicotiana benthamiana</it>. As the encoded LOX has a high amino acid identity to other 9-LOX proteins, the gene was named as <it>Nb-9-LOX</it>. It was heterologously expressed in yeast cells and its enzymatic activity was characterized. The yeast cells expressed large quantities of stable 9-LOX (0.9 U ml<sup>-1 </sup>cell cultures) which can oxygenate linoleic acid resulting in high yields (18 μmol ml<sup>-1 </sup>cell cultures) of hydroperoxy fatty acid. The product specificity of Nb-9-LOX was examined by incubation of linoleic acid and Nb-9-LOX in combination with a 13-hydroperoxide lyase from watermelon (Cl-13-HPL) or a 9/13-hydroperoxide lyase from melon (Cm-9/13-HPL) and by LC-MS analysis. The result showed that Nb-9-LOX possesses both 9- and 13-LOX specificity, with high predominance for the 9-LOX function. The combination of recombinant Nb-9-LOX and recombinant Cm-9/13-HPL produced large amounts of C<sub>9</sub>-aldehydes (3.3 μmol mg<sup>-1 </sup>crude protein). The yield of C<sub>9</sub>-aldehydes from linoleic acid was 64%.</p> <p>Conclusion</p> <p>The yeast expressed Nb-9-LOX can be used to produce C<sub>9</sub>-aldehydes on a large scale in combination with a <it>HPL </it>gene with 9-HPL function, or to effectively produce 9-hydroxy-10(<it>E</it>),12(<it>Z</it>)-octadecadienoic acid in a biocatalytic process in combination with cysteine as a mild reducing agent.</p
Dynamic Chromatin Organization during Foregut Development Mediated by the Organ Selector Gene PHA-4/FoxA
Central regulators of cell fate, or selector genes, establish the identity of cells by direct regulation of large cohorts of genes. In Caenorhabditis elegans, foregut (or pharynx) identity relies on the FoxA transcription factor PHA-4, which activates different sets of target genes at various times and in diverse cellular environments. An outstanding question is how PHA-4 distinguishes between target genes for appropriate transcriptional control. We have used the Nuclear Spot Assay and GFP reporters to examine PHA-4 interactions with target promoters in living embryos and with single cell resolution. While PHA-4 was found throughout the digestive tract, binding and activation of pharyngeally expressed promoters was restricted to a subset of pharyngeal cells and excluded from the intestine. An RNAi screen of candidate nuclear factors identified emerin (emr-1) as a negative regulator of PHA-4 binding within the pharynx, but emr-1 did not modulate PHA-4 binding in the intestine. Upon promoter association, PHA-4 induced large-scale chromatin de-compaction, which, we hypothesize, may facilitate promoter access and productive transcription. Our results reveal two tiers of PHA-4 regulation. PHA-4 binding is prohibited in intestinal cells, preventing target gene expression in that organ. PHA-4 binding within the pharynx is limited by the nuclear lamina component EMR-1/emerin. The data suggest that association of PHA-4 with its targets is a regulated step that contributes to promoter selectivity during organ formation. We speculate that global re-organization of chromatin architecture upon PHA-4 binding promotes competence of pharyngeal gene transcription and, by extension, foregut development
Novel and Conserved Protein Macoilin Is Required for Diverse Neuronal Functions in Caenorhabditis elegans
Neural signals are processed in nervous systems of animals responding to variable environmental stimuli. This study shows that a novel and highly conserved protein, macoilin (MACO-1), plays an essential role in diverse neural functions in Caenorhabditis elegans. maco-1 mutants showed abnormal behaviors, including defective locomotion, thermotaxis, and chemotaxis. Expression of human macoilin in the C. elegans nervous system weakly rescued the abnormal thermotactic phenotype of the maco-1 mutants, suggesting that macoilin is functionally conserved across species. Abnormal thermotaxis may have been caused by impaired locomotion of maco-1 mutants. However, calcium imaging of AFD thermosensory neurons and AIY postsynaptic interneurons of maco-1 mutants suggest that macoilin is required for appropriate responses of AFD and AIY neurons to thermal stimuli. Studies on localization of MACO-1 showed that C. elegans and human macoilins are localized mainly to the rough endoplasmic reticulum. Our results suggest that macoilin is required for various neural events, such as the regulation of neuronal activity
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