931 research outputs found
Three-dimensional modeling of the HI kinematics of NGC 2915
The nearby blue compact dwarf, NGC 2915, has its stellar disc embedded in a
large, extended (~ 22 B-band scale-lengths) HI disc. New high-resolution HI
synthesis observations of NGC 2915 have been obtained with the Australia
Telescope Compact Array. These observations provide evidence of extremely
complex HI kinematics within the immediate vicinity of the galaxy's
star-forming core. We identify and quantify double-peaked HI line profiles near
the centre of the galaxy and show that the HI energetics can be accounted for
by the mechanical energy output of the central high-mass stellar population
within time-scales of 10^6-10^7 yr. Full three-dimensional models of the HI
data cube are generated and compared to the observations to test various
physical scenarios associated with the high-mass star-forming core of NGC 2915.
Purely circular HI kinematics are ruled out together with the possibility of a
high-velocity-dispersion inter-stellar medium at inner radii. Radial velocities
of ~ 30 km/s are required to describe the central-most HI kinematics of the
system. Our results lend themselves to the simple physical scenario in which
the young stellar core of the galaxy expels the gas outwards from the centre of
the disc, thereby creating a central HI under-density. These kinematics should
be thought of as being linked to a central HI outflow rather than a large-scale
galactic blow-out or wind.Comment: 11 pages, 6 figures, accepted for publication in MNRA
The Rich Globular Cluster System of Abell 1689 and the Radial Dependence of the Globular Cluster Formation Efficiency
We study the rich globular cluster (GC) system in the center of the massive
cluster of galaxies Abell 1689 (z=0.18), one of the most powerful gravitational
lenses known. With 28 HST/ACS orbits in the F814W bandpass, we reach magnitude
I_814=29 with >90% completeness and sample the brightest ~5% of the GC system.
Assuming the well-known Gaussian form of the GC luminosity function (GCLF), we
estimate a total population of N(GC_total) = 162,850 GCs within a projected
radius of 400kpc. As many as half may comprise an intracluster component. Even
with the sizable uncertainties, which mainly result from the uncertain GCLF
parameters, this is by far the largest GC system studied to date. The specific
frequency S_N is high, but not uncommon for central galaxies in massive
clusters, rising from S_N~5 near the center to ~12 at large radii. Passive
galaxy fading would increase S_N by ~20% at z=0. We construct the radial mass
profiles of the GCs, stars, intracluster gas, and lensing-derived total mass,
and we compare the mass fractions as a function of radius. The estimated mass
in GCs, M(GC_total)=3.9x10^10 Msun, is comparable to ~80% of the total stellar
mass of the Milky Way. The shape of the GC mass profile appears intermediate
between those of the stellar light and total cluster mass. Despite the extreme
nature of this system, the ratios of the GC mass to the baryonic and total
masses, and thus the GC formation efficiency, are typical of those in other
rich clusters when comparing at the same physical radii. The GC formation
efficiency is not constant, but varies with radius, in a manner that appears
similar for different clusters; we speculate on the reasons for this similarity
in profile.Comment: 13 pages, 11 figures; accepted for publication in Ap
The dark matter content of the blue compact dwarf NGC 2915
NGC 2915 is a nearby blue compact dwarf with the HI properties of a late-type
spiral. Its large, rotating HI disk (extending out to R ~ 22 B-band scale
lengths) and apparent lack of stars in the outer HI disk make it a useful
candidate for dark matter studies. New HI synthesis observations of NGC 2915
have been obtained using the Australian Telescope Compact Array. These data are
combined with high-quality 3.6 m imaging from the Spitzer Infrared Nearby
Galaxies Survey. The central regions of the HI disk are shown to consist of two
distinct HI concentrations with significantly non-Gaussian line profiles. We
fit a tilted ring model to the HI velocity field to derive a rotation curve.
This is used as input for mass models that determine the contributions from the
stellar and gas disks as well as the dark matter halo. The galaxy is
dark-matter-dominated at nearly all radii. At the last measured point of the
rotation curve, the total mass to blue light ratio is ~ 140 times solar, making
NGC 2915 one of the darkest galaxies known. We show that the stellar disk
cannot account for the steeply-rising portion of the observed rotation curve.
The best-fitting dark matter halo is a pseudo-isothermal sphere with a core
density \msun pc and a core radius kpc.Comment: MNRAS in press. 17 pages, 15 figure
Cluster Analysis and Comparison of Various Chloroplast Transcriptomes and Genes in Arabidopsis thaliana
Chloroplast RNA metabolism is integrated into wider gene regulatory networks. To explore how, we performed a chloroplast genome-wide expression analysis on numerous nuclear Arabidopsis mutants affected in diverse chloroplast functions and wild-type plants subjected to various stresses and conditions. On the basis of clustering analysis, plastid genes could be divided into two oppositely regulated clusters, largely congruent with known targets of nucleus- and plastid-encoded RNA polymerases, respectively. Further eight sub-clusters contained co-transcribed and functionally tightly associated genes. The chloroplast transcriptomes could also be classified into two major groups comprising mutants preferentially affected in general plastid gene expression and other chloroplast functions, respectively. Deviations from characteristic expression profiles of transcriptomes served to identify novel mutants impaired in accumulation and/or processing of specific plastid RNAs. Expression profiles were useful to distinguish albino mutants affected in plastid gene expression from those with defects in other plastid functions. Remarkably, biotic and abiotic stressors did not define transcriptionally determined clusters indicating that post-transcriptional regulation of plastid gene expression becomes more important under changing environmental conditions. Overall, the identification of sets of co-regulated genes provides insights into the integration of plastid gene expression into common pathways that ensures a coordinated response
Evolution of the Lyman-alpha Halos around High-Redshift Radio Galaxies
We have obtained the first constraints on extended Ly-alpha emission at z ~ 1
in a sample of five radio galaxies. We detect Ly-alpha emission from four of
the five galaxies. The Ly-alpha luminosities range from 0.1 - 4 times 10^43
erg/s and are much smaller than those observed for halos around higher redshift
radio galaxies. If the z ~ 1 radio galaxies are the descendents the z >~ 2
radio galaxies, then their Ly-alpha luminosities evolve strongly with redshift
as ~(1+z)^5. There do not appear to be strong correlations between other
parameters, such as radio power, suggesting that this observed evolution is
real and not an observational artifact or secondary correlation. We speculate
that this evolution of luminous halos may be due to gas depletion (as gas
cools, settles, and forms stars) accompanied by an overall rise in the mean gas
temperature and a decrease in specific star-formation rate in and around these
massive galaxies.Comment: 5 pages, 4 figures, published in ApJ Letters, 694, L31-35 March 20
200
The Shape and Figure Rotation of NGC 2915's Dark Halo
NGC 2915 is a blue compact dwarf galaxy with a very extended HI disk showing
a short central bar and extended spiral arms, both reaching far beyond the
optical component. We use Tremaine & Weinberg (1984) method to measure the
pattern speed of the bar from HI radio synthesis data. Our measurements yield a
pattern speed of 0.21+/-0.06 km/s/arcsec (8.0+/-2.4 km/s/kpc for D=5.3 Mpc), in
disagreement with the general view that corotation in barred disks lies just
outside the end of the bar, but consistent with recent models of barred
galaxies with dense dark matter halos. Our adopted bar semi-length puts
corotation at more than 1.7 bar radii. The existence of the pattern is also
problematic. Because NGC 2915 is isolated, interactions cannot account for the
structure observed in the HI disk. We also demonstrate that the low observed
disk surface density and the location of the pseudo-rings make it unlikely that
swing amplification or bar-driven spiral arms could explain the bar and spiral
pattern.
Based on the similarity of the dark matter and HI surface density profiles,
we discuss the possibility of dark matter distributed in a disk and following
closely the HI distribution. The disk then becomes unstable and can naturally
form a bar and spiral pattern. However, this explanation is hard to reconcile
with some properties of NGC 2915. We also consider the effect of a massive and
extended triaxial dark matter halo with a rotating figure. The existence of
such halos is supported by CDM simulations showing strongly triaxial dark halos
with slow figure rotation. The observed structure of the HI disk can then arise
through forcing by the rotating triaxial figure. We associate the measured
pattern speed in NGC 2915 with the figure rotation of its dark halo.Comment: 37 pages, including 8 figures and 2 tables (AASTeX, aaspp4.sty).
Fig.1 and 2 available as jpg. Accepted for publication in The Astronomical
Journal. Online manuscript with PostScript figures available at:
http://www.strw.leidenuniv.nl/~bureau/pub_list.htm
Coulombically Interacting Electrons in a One-dimensional Quantum Dot
The spectral properties of up to four interacting electrons confined within a
quasi one--dimensional system of finite length are determined by numerical
diagonalization including the spin degree of freedom. The ground state energy
is investigated as a function of the electron number and of the system length.
The limitations of a description in terms of a capacitance are demonstrated.
The energetically lowest lying excitations are physically explained as
vibrational and tunneling modes. The limits of a dilute, Wigner-type
arrangement of the electrons, and a dense, more homogeneous charge distribution
are discussed.Comment: 10 pages (excl. Figures), Figures added in POSTSCRIPT, LaTe
ACS Observations of a Strongly Lensed Arc in a Field Elliptical
We report the discovery of a strongly lensed arc system around a field
elliptical galaxy in Hubble Space Telescope (HST) Advanced Camera for Surveys
(ACS) images of a parallel field observed during NICMOS observations of the HST
Ultra-Deep Field. The ACS parallel data comprise deep imaging in the F435W,
F606W, F775W, and F850LP bandpasses. The main arc is at a radius of 1.6 arcsec
from the galaxy center and subtends about 120 deg. Spectroscopic follow-up at
Magellan Observatory yields a redshift z=0.6174 for the lensing galaxy, and we
photometrically estimate z_phot = 2.4\pm0.3 for the arc. We also identify a
likely counter-arc at a radius of 0.6 arcsec, which shows structure similar to
that seen in the main arc. We model this system and find a good fit to an
elliptical isothermal potential of velocity dispersion
\kms, the value expected from the fundamental plane, and some external shear.
Several other galaxies in the field have colors similar to the lensing galaxy
and likely make up a small group.Comment: Accepted for publication in ApJ Letters. 10 pages, 3 figures. Figures
have been degraded to meet size limit; a higher resolution version and
addtional pictures available at http://acs.pha.jhu.edu/~jpb/UDFparc
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