198 research outputs found
Bar imprints on the inner gas kinematics of M33
We present measurements of the stellar and gaseous velocities in the central
5' of the Local Group spiral M33. The data were obtained with the ARC 3.5m
telescope. Blue and red spectra with resolutions from 2 to 4\AA covering the
principal gaseous emission and stellar absorption lines were obtained along the
major and minor axes and six other position angles. The observed radial
velocities of the ionized gas along the photometric major axis of M33 remain
flat at ~22 km s^{-1} all the way into the center, while the stellar velocities
show a gradual rise from zero to 22 km s^{-1} over that same region. The
central star cluster is at or very close to the dynamical center, with a
velocity that is in accordance with M33's systemic velocity to within our
uncertainties. Velocities on the minor axis are non-zero out to about 1' from
the center in both the stars and gas. Together with the major axis velocities,
they point at significant deviations from circular rotation. The most likely
explanation for the bulk of the velocity patterns are streaming motions along a
weak inner bar with a PA close to that of the minor axis, as suggested by
previously published IR photometric images. The presence of bar imprints in M33
implies that all major Local Group galaxies are barred. The non-circular
motions over the inner 200 pc make it difficult to constrain the shape of M33's
inner dark matter halo profile. If the non-circular motions we find in this
nearby Sc galaxy are present in other more distant late-type galaxies, they
might be difficult to recognize.Comment: 20 pages, 12 figures, ApJ in pres
The effects of an ionizing background on the HI column density distribution in the local Universe
Using data on the HI column density distribution in the local Universe,
f(N_{HI}), we show how to determine g(N_{H}), the distribution of the total gas
(HI+HII) column density. A simple power law fit to f(N_{HI}) fails due to
bendings in the distributions when N_{HI}<10^{20} cm^{-2} and H is no longer
fully neutral. If an ultraviolet background is responsible for the gas
ionization, and g(N_{H}) is proportional N_{H}^{-alpha}, we find the values of
alpha and of the intensity of the background radiation which are compatible
with the present data. These best fitting values, however, depend upon the
scaling law of the the gas volume densities with N_{H} and cannot be determined
unambiguously. We examine in detail two models: one in which the average gas
volume density decreases steadily with N_H, while in the other it stays
constant at low column densities. The former model leads to a steep power law
fit for g(N_{H}), with alpha of order 3.3 +- 0.4 and requires an ultraviolet
flux larger than what the QSOs alone produce at z=0. For the latter alpha is
1.5 +- 0.1 and a lower ionizing flux is required. The ambiguities about the
modelling and the resulting steep or shallow N_{H} distribution can be resolved
only if new 21-cm observations and QSOs Lyman limit absorbers searches will
provide more data in the HI-HII transition region at low redshifts. Using the
best fit obtained for higher redshift data we outline two possible scenarios
for the evolution of gaseous structures, compatible with the available data at
z=0.Comment: 12 pages, 2 figures, ApJ in press (2002, vol. 567
Radial HI Profiles at the Periphery of Galactic Disks: The Role of Ionizing Background Radiation
Observations of neutral hydrogen in spiral galaxies reveal a sharp cutoff in
the radial density profile at some distance from the center. Using 22 galaxies
with known HI distributions as an example, we discuss the question of whether
this effect can be associated exclusively with external ionizing radiation, as
is commonly assumed. We show that before the surface density reaches
(the same for
galaxies of different types), it is hard to expect the gas to be fully ionized
by background radiation. For two of 13 galaxies with a sharp drop in the HI
profile, the "steepening" can actually be caused by ionization. At the same
time, for the remaining galaxies, the observed cutoff in the radial HI profile
is closer to the center than if it was a consequence of ionization by
background radiation and, therefore, it should be caused by other factors.Comment: 15 pages, 6 figure
High-resolution imaging of compact high-velocity clouds (II)
We have imaged five compact high-velocity clouds in HI with arcmin angular-
and km/s spectral-resolution using the WSRT. Supplementary total-power data,
which is fully sensitive to both the cool and warm components of HI, is
available for comparison for all the sources, albeit with angular resolutions
that vary from 3' to 36'. The fractional HI flux in compact CNM components
varies from 4% to 16% in our sample. All objects have at least one local peak
in the CNM column which exceeds about 10^19 cm^-2 when observed with arcmin
resolution. It is plausible that a peak column density of 1-2x10^19 cm^-2 is a
prerequisite for the long-term survival of these sources. One object in our
sample, CHVC120-20-443 (Davies' cloud), lies in close projected proximity to
the disk of M31. This object is characterized by exceptionally broad linewidths
in its CNM concentrations (more than 5 times greater than the median value).
These CNM concentrations lie in an arc on the edge of the source facing the M31
disk, while the diffuse HI component of this source has a position offset in
the direction of the disk. All of these attributes suggest that CHVC120-20-443
is in a different evolutionary state than most of the other CHVCs which have
been studied. Similarly broad CNM linewidths have only been detected in one
other object, CHVC111-07-466, which also lies in the Local Group barycenter
direction and has the most extreme radial velocity known. A distinct
possibility for Davies' cloud seems to be physical interaction of some type
with M31. The most likely form of this interaction might be the ram-pressure or
tidal- stripping by either one of M31's visible dwarf companions, M32 or
NGC205, or else by a dark companion with an associated HI condensation.Comment: 12 pages, 11 (low res.) png figs, accepted for pub. in A&
The impact on disability of initial treatment with methotrexate in patients with rheumatoid arthritis: results from the MARI study
The study aimed to assess in a population of subjects with rheumatoid arthritis (RA) treated with methotrexate (MTX) how the initial approach to the treatment influenced subsequent disability. We performed a cross-sectional analysis of data collected during the baseline visit of the MARI study, a multicenter observational study on patients with RA on treatment with MTX for at least 12 months. Subjects who fulfilled the Health Assessment Questionnaire (HAQ) were included in the evaluation. For every patient we retrospectively evaluated the disease duration, the duration of symptoms before the diagnosis, the time elapsed before first MTX treatment, the initial MTX dose, and the concomitant medications in the first six months of therapy. Disability was defined as a DI-HAQ score ≥1. The study population included 1015 subjects. Patients with a DI-HAQ score ≥1 had a longer duration of symptoms before diagnosis, a higher delay in treatment initiation, a lower initial dose of MTX and a more frequent co-treatment with symptomatic drugs. Disability was found less frequently in subjects treated with other concomitant disease modifying anti-rheumatic drugs (DMARDs) but not with biological agents. Logistic regression analysis identified as significant predictors of disability: older age, female sex, a longer time to complete diagnosis, a delay in starting MTX treatment higher than 6 months, and a concomitant treatment with symptomatic drugs, while a combination therapy with other DMARDs was associated with a lower risk of disability. A late diagnosis and a delay in starting a treatment with MTX are associated with poorer functional outcomes in patients with RA
Modeling the ionizing spectra of H ii regions: individual stars versus stellar ensembles
Aims. We study how IMF sampling affects the ionizing flux and emission line
spectra of low mass stellar clusters. Methods. We performed 2 x 10^6 Monte
Carlo simulations of zero-age solar-metallicity stellar clusters covering the
20 - 10^6 Mo mass range. We study the distribution of cluster stellar masses,
Mclus, ionizing fluxes, Q(H0), and effective temperatures, Tclus. We compute
photoionization models that broadly describe the results of the simulations and
compare them with photoionization grids. Results. Our main results are: (a) A
large number of low mass clusters (80% for Mclus = 100 Mo) are unable to form
an H ii region. (b) There are a few overluminous stellar clusters that form H
ii regions. These overluminous clusters preserve statistically the mean value
of obtained by synthesis models, but the mean value cannot be used as a
description of particular clusters. (c) The ionizing continuum of clusters with
Mclus < 10^4 Mo is more accurately described by an individual star with
self-consistent effective temperature(T*) and Q(H0) than by the ensemble of
stars (or a cluster Tclus) produced by synthesis models. (d)Photoionization
grids of stellar clusters can not be used to derive the global properties of
low mass clusters. Conclusions. Although variations in the upper mass limit,
mup, of the IMF would reproduce the effects of IMF sampling, we find that an ad
hoc law that relates mup to Mclus in the modelling of stellar clusters is
useless, since: (a) it does not cover the whole range of possible cases, and
(b) the modelling of stellar clusters with an IMF is motivated by the need to
derive the global properties of the cluster: however, in clusters affected by
sampling effects we have no access to global information of the cluster but
only particular information about a few individual stars.Comment: A&A in pres
A catalogue of damped Lyman alpha absorption systems and radio flux densities of the background quasars
We present a catalogue of the 322 damped Lyman alpha absorbers taken from the
literature. All damped Lyman alpha absorbers are included, with no selection on
redshift or quasar magnitude. Of these, 123 are candidates and await
confirmation using high resolution spectroscopy. For all 322 objects we
catalogue the radio properties of the background quasars, where known. Around
60 quasars have radio flux densities above 0.1 Jy and approximately half of
these have optical magnitudes brighter than V = 18. This compilation should
prove useful in several areas of extragalactic/cosmological research.Comment: 26 Pages, 12 PS tables, 1 embedded table. Accepted by PASA.
Continuously updated online catalogue available at
http://www.phys.unsw.edu.au/~sjc/dl
Gravitational stability and dynamical overheating of stellar disks of galaxies
We use the marginal stability condition for galactic disks and the stellar
velocity dispersion data published by different authors to place upper limits
on the disk local surface density at two radial scalelengths .
Extrapolating these estimates, we constrain the total mass of the disks and
compare these estimates to those based on the photometry and color of stellar
populations. The comparison reveals that the stellar disks of most of spiral
galaxies in our sample cannot be substantially overheated and are therefore
unlikely to have experienced a significant merging event in their history. The
same conclusion applies to some, but not all of the S0 galaxies we consider.
However, a substantial part of the early type galaxies do show the stellar
velocity dispersion well in excess of the gravitational stability threshold
suggesting a major merger event in the past. We find dynamically overheated
disks among both seemingly isolated galaxies and those forming pairs. The ratio
of the marginal stability disk mass estimate to the total galaxy mass within
four radial scalelengths remains within a range of 0.4---0.8. We see no
evidence for a noticeable running of this ratio with either the morphological
type or color index.Comment: 25 pages, 5 figures, accepted to Astronomy Letter
The Pressure of an Equilibrium Interstellar Medium in Galactic Disks
Based on an axisymmetric galactic disk model, we estimate the equilibrium gas
pressure P/k in the disk plane as a function of the galactocentric distance R
for several galaxies (MW, M33, M51, M81, M100, M101, M106, and the SMC). For
this purpose, we solve a self-consistent system of equations by taking into
account the gas self-gravity and the presence of a dark pseudo-isothermal halo.
We assume that the turbulent velocity dispersions of the atomic and molecular
gases are fixed and that the velocity dispersion of the old stellar disk
corresponds to its marginal stability (except for the Galaxy and the SMC). We
also consider a model with a constant disk thickness. Of the listed galaxies,
the SMC and M51 have the highest pressure at a given relative radius R/R_25,
while M81 has the lowest pressure. The pressure dependence of the relative
molecular gas fraction confirms the existence of a positive correlation between
these quantities, but it is not so distinct as that obtained previously when
the pressure was estimated very roughly. This dependence breaks down for the
inner regions of M81 and M106, probably because the gas pressure has been
underestimated in the bulge region. We discuss the possible effects of factors
other than the pressure affecting the relative content of molecular gas in the
galaxies under consideration.Comment: 10 pages, 5 figure
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