7,268 research outputs found
The Search for Intergalactic Hydrogen Clouds in Voids
I present the results of a search for intergalactic hydrogen clouds in voids.
Clouds are detected by their HI LyA absorption lines in the HST spectra of
low-redshift AGN. The parameter with which the environments of clouds are
characterized is the tidal field, which places a lower limit on the cloud
mass-density which is dynamically stable against disruption. Galaxy redshift
catalogs are used to sum the tidal fields along the lines of sight, sorting
clouds according to tidal field upper, or lower limits. The analytical
methodology employed is designed to detect gas clouds whose expansion following
reionization is restrained by dark matter perturbations. End-products are the
cloud equivalent width distribution functions (EWDF) of catalogs formed by
sorting clouds according to various tidal field upper, or lower limits.
Cumulative EWDFs are steep in voids (S ~ -1.5 \pm 0.2), but flatter in high
tidal field zones (S ~ -0.5 \pm 0.1). Most probable cloud Doppler parameters
are ~30 km/s in voids and ~60 km/s in proximity to galaxies. In voids, the
cumulative line density at low EW (~ 15 mA) is ~ 500 per unit redshift. The
void filling factor is found to be 0.87 <= f_v <= 0.94. The void EWDF is
remarkably uniform over this volume, with a possible tendency for more massive
clouds to be in void centers. The size and nature of the void cloud population
suggested by this study is completely unanticipated by the results of published
3-D simulations, which predict that most clouds are in filamentary structures
around galaxy concentrations, and that very few observable absorbers would lie
in voids. Strategies for modeling this population are briefly discussed.Comment: 21 pages, 19 figures, apjemulate style, to appear in ApJ vol. 57
Rotational Widths for Use in the Tully-Fisher Relation. II. The Impact of Surface Brightness
Using a large sample of spiral galaxies for which 21 cm single-dish and/or
long-slit optical spectra are available, we make a detailed comparison between
various estimates of rotational widths. Different optical width estimators are
considered and their limitations discussed, with emphasis on biases associated
with rotation curve properties (shape and extent) and disk central surface
brightness. The best match with HI rotational velocities is obtained with
Polyex widths, which are measured at the optical radius (encompassing a fixed
fraction of the total light of the galaxy) from a model fit to the rotation
curve. In contrast with Polyex widths, optical rotational velocities measured
at 2.15 disk scale lengths r_d deviate from HI widths by an amount that
correlates with the central surface brightness of the disk. This bias occurs
because the rotation curves of galaxies are in general still rising at 2.15
r_d, and the fraction of total mass contained within this radius decreases with
increasing disk surface brightness. Statistical corrections, parameterized by
the radial extent of the observed rotation curve, are provided to reduce Polyex
and HI width measurements into a homogeneous system. This yields a single
robust estimate of rotational velocity to be used for applications of disk
scaling relations.Comment: 13 pages, 8 figures. To appear in the Astronomical Journal (August
2007
No Hubble Bubble in the Local Universe
Zehavi et al. (1998) have suggested that the Hubble flow within 70/h Mpc may
be accelerated by the existence of a void centered on the Local Group. Its
underdensity would be ~20 %, which would result in a local Hubble distortion of
about 6.5 %. We have combined the peculiar velocity data of two samples of
clusters of galaxies, SCI and SCII, to investigate the amplitude of Hubble
distortions to 200/h Mpc. Our results are not supportive of that conclusion.
The amplitude of a possible distortion in the Hubble flow within 70/h Mpc in
the SCI+SCII merged data is 0.010\pm0.022. The largest, and still quite
marginal, geocentric deviation from smooth Hubble flow consistent with that
data set is a shell with (Delta H)/H =0.027\pm0.023, centered at hd = 101 Mpc
and extending over some 30/h Mpc. Our results are thus consistent with a Hubble
flow that, on distances in excess of about 50/h Mpc, is remarkably smooth.Comment: 11 pages, 1 tables, 1 figure; uses AAS LaTex; to appear in ApJ Nov 9
A Digital Archive of HI 21 cm Line Spectra of Optically-targeted Galaxies
We present a homogeneous compilation of HI spectral parameters extracted from
global 21 cm line spectra for some 9000 galaxies in the local universe
(heliocentric velocity -200 < V_Sun < 28,000 km/s) obtained with a variety of
large single dish radio telescopes but reanalyzed using a single set of
parameter extraction algorithms. Corrections to the observed HI line flux for
source extent and pointing offsets and to the HI line widths for instrumental
broadening and smoothing are applied according to model estimates to produce a
homogenous catalog of derived properties with quantitative error estimates.
Where the redshift is available from optical studies, we also provide flux
measurements for an additional 156 galaxies classified as marginal HI
detections and rms noise limits for 494 galaxies classified as nondetections.
Given the diverse nature of the observing programs contributing to it, the
characteristics of the combined dataset are heterogeneous, and as such, the
compilation is neither integrated HI line flux nor peak flux limited. However,
because of the large statistical base and homogenous reprocessing, the spectra
and spectral parameters of galaxies in this optically targeted sample can be
used to complement data obtained at other wavelengths to characterize the
properties of galaxies in the local universe and to explore the large scale
structures in which they reside.Comment: 13 pages, 9 figures, 3 external online tables, accepted for
publication in ApJ
Development of large radii half-wave plates for CMB satellite missions
The successful European Space Agency (ESA) Planck mission has mapped the
Cosmic Microwave Background (CMB) temperature anisotropy with unprecedented
accuracy. However, Planck was not designed to detect the polarised components
of the CMB with comparable precision. The BICEP2 collaboration has recently
reported the first detection of the B-mode polarisation. ESA is funding the
development of critical enabling technologies associated with B-mode
polarisation detection, one of these being large diameter half-wave plates. We
compare different polarisation modulators and discuss their respective
trade-offs in terms of manufacturing, RF performance and thermo-mechanical
properties. We then select the most appropriate solution for future satellite
missions, optimized for the detection of B-modes.Comment: 16 page
Hydroxyurea and zileuton differentially modulate cell proliferation and interleukin-2 secretion by murine spleen cells: Possible implication on the immune function and risk of pain crisis in patients with sickle cell disease
Background: Hydroxyurea (HU) reduces major complications associated with sickle cell disease in part because of the induction of fetal hemoglobin. However, because of its antiproliferative property, its long-term use may impair immunity. Zileuton, a derivative of HU, also induces fetal hemoglobin and has antiinflammatory properties, a feature that can reduce the risk of sickling. Our goal was to investigate the capacity of both drugs to modulate the secretion of interleukin-2 (IL-2), a regulatory cytokine for immune responses. Methods: Spleen cells obtained from 11 4-month-old C57BL/6 female mice were incubated without and with 10 µ/mL HU or zileuton, 2.5 µ/mL concanavalin A (ConA), 20 µ/mL phytohemagglutinin (PHA), and 50 ng/mL anti-CD3 antibody for 12-48 h. IL-2 was measured in the supernatant by enzyme-linked immunosorbent assay and cell proliferation byH-thymidine uptake. Results: While HU reduced lymphocyte proliferation in response to mitogens (
Kinematic Evidence of Minor Mergers in Normal Sa Galaxies: NGC3626, NGC3900, NGC4772 and NGC5854
BVRI and H-alpha imaging and long-slit optical spectroscopic data are
presented for four morphologically normal and relatively isolated Sa galaxies,
NGC3626, NGC3900, NGC4772 and NGC5854. VLA HI synthesis imaging is presented
for the first 3 objects. In all 4 galaxies, evidence of kinematic decoupling of
ionized gas components is found; the degree and circumstances of the distinct
kinematics vary from complete counterrotation of all of the gas from all of the
stars (NGC3626) to nuclear gas disks decoupled from the stars (NGC5854) to
anomalous velocity central gas components (NGC3900 and NGC4772). In the 3
objects mapped in HI, the neutral gas extends far beyond the optical radius,
R_HI/R_25 > 2. In general, the HI surface density is very low and the outer HI
is patchy and asymmetric or found in a distinct ring, exterior to the optical
edge. While the overall HI velocity fields are dominated by circular motions,
strong warps are suggested in the outer regions. Optical imaging is also
presented for NGC 4138 previously reported by Jore et al. (1996) to show
counterrotating stellar components. The multiwavelength evidence is interpreted
in terms of the kinematic "memory" of past minor mergers in objects that
otherwise exhibit no morphological signs of interaction.Comment: 26 pages, 15 figures, accepted for publication in Astron. J.,
postscript figures available at
ftp://culebra.tn.cornell.edu/pub/haynes/figures.tar.g
The Far-Infrared/Radio Correlation in Nearby Abell Clusters
A comprehensive study of the effect of the cluster environment on the far-
infrared (FIR)/radio correlation in nearby Abell clusters is presented. Using
the cluster radio galaxy database from Miller & Owen (2001) and optical
spectroscopy and high resolution radio images to remove AGN, we assess the
FIR/radio correlation of cluster galaxies from the centers of the clusters out
well past the classical Abell radius. The FIR/radio correlation is shown to
hold quite well for star forming galaxies, and the FIR and radio fluxes for
cluster AGN are also well correlated. In the case of AGN, the relative radio-
to-FIR fluxes are greater and the scatter in the correlation is larger than
those seen for star forming galaxies. We also find that there is a rare but
statistically significant excess of star forming galaxies with enhanced radio
emission in the centers of the clusters, and that the degree of this
enhancement is typically a factor of two or three. The FIR/radio correlation
for cluster star forming galaxies is also tested against line-of-sight velocity
relative to the cluster systemic velocities, but no significant correlation is
found. While the radial dependence of the FIR/radio correlation is consistent
with the model wherein ram pressure increases the cluster galaxies' magnetic
field strengths through compression, the velocity data do not confirm this
model. Although a contribution from ram pressure can not be ruled out, the
thermal pressure due to the ICM alone is an equally viable alternative. The
high resolution radio images largely reject the hypothesis that the increased
radio emission arises from an AGN component, strengthening the claim that the
change in the correlation is caused by a change in the environment of the
galaxies.Comment: 26 pages, including 5 figures (uses AASTeX 5.0). Accepted for
publication in the Astronomical Journa
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