876 research outputs found
Stability of an Ultra-Relativistic Blast Wave in an External Medium with a Steep Power-Law Density Profile
We examine the stability of self-similar solutions for an accelerating
relativistic blast wave which is generated by a point explosion in an external
medium with a steep radial density profile of a power-law index > 4.134. These
accelerating solutions apply, for example, to the breakout of a gamma-ray burst
outflow from the boundary of a massive star, as assumed in the popular
collapsar model. We show that short wavelength perturbations may grow but only
by a modest factor <~ 10.Comment: 12 pages, 3 figures, submitted to Physical Review
Observational Evidence for an Age Dependence of Halo Bias
We study the dependence of the cross-correlation between galaxies and galaxy
groups on group properties. Confirming previous results, we find that the
correlation strength is stronger for more massive groups, in good agreement
with the expected mass dependence of halo bias. We also find, however, that for
groups of the same mass, the correlation strength depends on the star formation
rate (SFR) of the central galaxy: at fixed mass, the bias of galaxy groups
decreases as the SFR of the central galaxy increases. We discuss these findings
in light of the recent findings by Gao et al (2005) that halo bias depends on
halo formation time, in that halos that assemble earlier are more strongly
biased. We also discuss the implication for galaxy formation, and address a
possible link to galaxy conformity, the observed correlation between the
properties of satellite galaxies and those of their central galaxy.Comment: 4 pages, 4 figures, Accepted for publication in ApJ Letters. Figures
3 and 4 replaced. The bias dependence on the central galaxy luminosity is
omitted due to its sensitivity to the mass mode
Short-Sale Constraints and Corporate Investment
In a sample of non-U.S. regulatory regime shifts, we find that expanded short selling is associated with stock price declines, reductions in capital expenditure, and lower asset growth. In a reversal of results found for U.S. stocks in a study of Regulation SHO by Grullon, Michenaud, and Weston (2015), our results are stronger for large firms than for small firms. We also show that this investment effect is stronger for firms that previously relied on outside financing. Our results suggest that short-sale policies affect corporate investment and that this effect is not driven by capital constraints
Galaxy Groups in the SDSS DR4: II. halo occupation statistics
We investigate various halo occupation statistics using a large galaxy group
catalogue constructed from the SDSS DR4 with an adaptive halo-based group
finder. The conditional luminosity function (CLF) is measured separately for
all, red and blue galaxies, as well as in terms of central and satellite
galaxies. The CLFs for central and satellite galaxies can be well modelled with
a log-normal distribution and a modified Schechter form, respectively. About
85% of the central galaxies and about 80% of the satellite galaxies in halos
with masses M_h\ga 10^{14}\msunh are red galaxies. These numbers decrease to
50% and 40%, respectively, in halos with M_h \sim 10^{12}\msunh. For halos of
a given mass, the distribution of the luminosities of central galaxies, ,
has a dispersion of about 0.15 dex. The mean luminosity (stellar mass) of the
central galaxies scales with halo mass as
() for halos with masses M\gg 10^{12.5}\msunh, and
both relations are significantly steeper for less massive halos. We also
measure the luminosity (stellar mass) gap between the first and second
brightest (most massive) member galaxies, (). These gap statistics, especially in halos with M_h \la
10^{14.0}\msunh, indicate that the luminosities of central galaxies are
clearly distinct from those of their satellites. The fraction of fossil groups,
defined as those groups with , ranges from for groups with M_h\sim 10^{14}\msunh to 18-60% for groups with
M_h\sim 10^{13}\msunh. Finally, we measure the fraction of satellites, which
changes from for galaxies with \rmag\sim -22.0 to for
galaxies with \rmag\sim -17.0. (abridged)Comment: 16 pages, 11 figures. Accepted for publication in Ap
Genomic profiling of tumor initiating prostatospheres
<p>Abstract</p> <p>Background</p> <p>The cancer stem cell (CSC) hypothesis proposes that a population of tumor cells bearing stem cell properties is responsible for the origin and maintenance of tumors. Normal and cancer stem cells possess the ability to grow in vitro as self-renewing spheres, but the molecular basis of this phenotype remains largely unknown. We intended to establish a comprehensive culture system to grow prostatospheres (PSs) from both cancer cell lines and patient tumors. We then used gene expression microarrays to gain insight on the molecular pathways that sustain the PS tumor initiating cell (TIC) phenotype.</p> <p>Results</p> <p>Traditional stem cell medium (SCM) supplemented with Knockout™SR (KO) allows the propagation of monoclonal PSs from cell lines and primary cells. PSs display gene expression and tumorigenicity hallmarks of TICs. Gene expression analysis defined a gene signature composed of 66 genes that characterize LNCaP and patient PSs. This set includes novel prostate TIC growth factors (NRP1, GDF1, JAG1), proteins implicated in cell adhesion and cytoskeletal maintenance, transcriptional regulators (MYCBP, MYBL1, ID1, ID3, FOS, ELF3, ELF4, KLF2, KLF5) and factors involved in protein biosynthesis and metabolism. Meta-analysis in Oncomine reveals that some of these genes correlate with prostate cancer status and/or progression. Reporter genes and inhibitors indicate that the Notch pathway contributes to prostatosphere growth.</p> <p>Conclusions</p> <p>We have developed a model for the culture of PSs, and provide a genomic profile that support CSCs identity. This signature identifies novel markers and pathways that are predicted to correlate with prostate cancer evolution.</p
The clustering of SDSS galaxy groups: mass and color dependence
We use a sample of galaxy groups selected from the SDSS DR 4 with an adaptive
halo-based group finder to probe how the clustering strength of groups depends
on their masses and colors. In particular, we determine the relative biases of
groups of different masses, as well as that of groups with the same mass but
with different colors. In agreement with previous studies, we find that more
massive groups are more strongly clustered, and the inferred mass dependence of
the halo bias is in good agreement with predictions for the CDM
cosmology. Regarding the color dependence, we find that groups with red
centrals are more strongly clustered than groups of the same mass but with blue
centrals. Similar results are obtained when the color of a group is defined to
be the total color of its member galaxies. The color dependence is more
prominent in less massive groups and becomes insignificant in groups with
masses \gta 10^{14}\msunh. We construct a mock galaxy redshift survey
constructed from the large Millenium simulation that is populated with galaxies
according to the semi-analytical model of Croton et al. Applying our group
finder to this mock survey, and analyzing the mock data in exactly the same way
as the true data, we are able to accurately recover the intrinsic mass and
color dependencies of the halo bias in the model. This suggests that our group
finding algorithm and our method of assigning group masses do not induce
spurious mass and/or color dependencies in the group-galaxy correlation
function. The semi-analytical model reveals the same color dependence of the
halo bias as we find in our group catalogue. In halos with M\sim
10^{12}\msunh, though, the strength of the color dependence is much stronger
in the model than in the data.Comment: 16 pages, 14 figures, Accepted for publication in ApJ. In the new
version, we add the bias of the shuffled galaxy sample. The errors are
estimated according to the covariance matrix of the GGCCF, which is then
diagonalize
Three Different Types of Galaxy Alignment within Dark Matter Halos
Using a large galaxy group catalogue based on the Sloan Digital Sky Survey
Data Release 4 we measure three different types of intrinsic galaxy alignment
within groups: halo alignment between the orientation of the brightest group
galaxies (BGG) and the distribution of its satellite galaxies, radial alignment
between the orientation of a satellite galaxy and the direction towards its
BGG, and direct alignment between the orientation of the BGG and that of its
satellites. In agreement with previous studies we find that satellite galaxies
are preferentially located along the major axis. In addition, on scales r < 0.7
Rvir we find that red satellites are preferentially aligned radially with the
direction to the BGG. The orientations of blue satellites, however, are
perfectly consistent with being isotropic. Finally, on scales r < 0.1 \Rvir, we
find a weak but significant indication for direct alignment between satellites
and BGGs. We briefly discuss the implications for weak lensing measurements.Comment: 4 pages, 4 figures, ApJL accepte
Contribution of an SFK-Mediated Signaling Pathway in the Dorsal Hippocampus to Cocaine-Memory Reconsolidation in Rats
Environmentally induced relapse to cocaine seeking requires the retrieval of context–response–cocaine associative memories. These memories become labile when retrieved and must undergo reconsolidation into long-term memory storage to be maintained. Identification of the molecular underpinnings of cocaine-memory reconsolidation will likely facilitate the development of treatments that mitigate the impact of cocaine memories on relapse vulnerability. Here, we used the rat extinction-reinstatement procedure to test the hypothesis that the Src family of tyrosine kinases (SFK) in the dorsal hippocampus (DH) critically controls contextual cocaine-memory reconsolidation. To this end, we evaluated the effects of bilateral intra-DH microinfusions of the SFK inhibitor, PP2 (62.5 ng per 0.5 μl per hemisphere), following re-exposure to a cocaine-associated (cocaine-memory reactivation) or an unpaired context (no memory reactivation) on subsequent drug context-induced instrumental cocaine-seeking behavior. We also assessed alterations in the phosphorylation state of SFK targets, including GluN2A and GluN2B N-methyl-D-aspartate (NMDA) and GluA2 α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunits at the putative time of memory restabilization and following PP2 treatment. Finally, we evaluated the effects of intra-DH PEAQX (2.5 μg per 0.5 μl per hemisphere), a GluN2A-subunit-selective NMDAR antagonist, following, or in the absence of, cocaine-memory reactivation on subsequent drug context-induced cocaine-seeking behavior. GluN2A phosphorylation increased in the DH during putative memory restabilization, and intra-DH PP2 treatment inhibited this effect. Furthermore, PP2—as well as PEAQX—attenuated subsequent drug context-induced cocaine-seeking behavior, in a memory reactivation-dependent manner, relative to VEH. These findings suggest that hippocampal SFKs contribute to the long-term stability of cocaine-related memories that underlie contextual stimulus control over cocaine-seeking behavior
Probing the Intrinsic Shape and Alignment of Dark Matter Haloes using SDSS Galaxy Groups
We study the three-dimensional and projected shapes of galaxy groups in the
Sloan Digital Sky Survey Data Release 4, and examine the alignment between the
orientation of the central galaxy and the spatial distribution of satellite
galaxies. The projected ellipticity of a group is measured using the moments of
the discrete distribution of its member galaxies. We infer the
three-dimensional and projected axis ratios of their dark matter haloes by
comparing the measured ellipticity distributions with those obtained from Monte
Carlo simulations of projected, triaxial dark matter haloes with different axis
ratios. We find that the halo shape has a strong dependence on the halo mass.
While the haloes of low-mass groups are nearly spherical, those of massive
groups tend to be prolate. For groups containing at least four members, the
statistical distribution of their measured ellipticities does not have a strong
dependence on the colors of their central galaxies. Our analysis further shows
that the average three-dimensional axis ratio for haloes with is about , resulting in a
projected axis ratio of . Our results for the alignment between the
orientation of the central galaxy of a group and the distribution of their
satellite galaxies are in broad agreement with those obtained by Yang et al.
The distribution of satellite galaxies preferentially aligns with the major
axis of the central galaxy, with a clear dependence on both halo mass and
galaxy colors. (abridged)Comment: 13 pages, 10 figures and 2 tables. Accepted for publication in MNRA
The Architecture of CCAN Proteins Creates a Structural Integrity to Resist Spindle Forces and Achieve Proper Intrakinetochore Stretch
Constitutive Centromere Associated Network (CCAN) proteins, particularly CENP-C, CENP-T and the CENP-H/-I complex, mechanically link CENP-A-containing centromeric chromatin within the inner kinetochore to outer kinetochore proteins, like the Ndc80 complex, that bind kinetochore microtubules. Accuracy of chromosome segregation depends critically upon Aurora B phosphorylation of Ndc80/Hec1. To determine how CCAN protein architecture mechanically constrains intrakinetochore stretch between CENP-A and Ndc80/Hec1 for proper Ndc80/Hec1 phosphorylation, we used super-resolution fluorescence microscopy and selective protein depletion. We found that at bi-oriented chromosomes in late prometaphase cells, CENP-T is stretched ~16 nm to the inner end of Ndc80/Hec1, much less than expected for full-length CENP-T. Depletion of various CCAN linker proteins induced hyper-intrakinetochore stretch (an additional 20-60 nm) with corresponding significant decreases in Aurora B phosphorylation of Ndc80/Hec1. Thus, proper intrakinetochore stretch is required for normal kinetochore function and depends critically on all the CCAN mechanical linkers to the Ndc80 complex
- …