280 research outputs found
Dynamics of blue compact galaxies, as revealed by their H-alpha velocity fields I. The data, velocity fields and rotation curves
Observations of six luminous blue compact galaxies (BCGs) and two star
forming companion galaxies were carried out with the CIGALE scanning
Fabry-Perot interferometer attached to the ESO 3.6m telescope on La Silla. The
observations were made in the H-alpha emission line which is prominent in BCGs.
A velocity sampling of 5 km/s and a pixel size of 0.9 arcseconds were used. In
this paper we present the observations and the data together with the velocity
fields and the derived rotation curves. In addition we provide rough estimates
of the total dynamical mass and of the ionised gas mass for each galaxy. All
galaxies display rotation, but while the companion galaxies have regular
velocity fields, those of the BCGs are complex and appear perturbed. This is
the most extensive study to date of the optical velocity fields of BCGs. The
interpretation of these results will be presented in a forthcoming paper (Paper
II).Comment: 26 pages, 14 figures. Accepted for publication in A&AS. The paper
(with figures in slightly higher resolution) and an electronic table is also
available at ftp://ftp.iap.fr/pub/from_users/ostlin/Articles/ . Replaced
version, figure captions fixe
Thermal Transport Properties of New Coatings on Steels for Supercritical Steam Power Plants
International audienc
An AUC-based Permutation Variable Importance Measure for Random Forests
The random forest (RF) method is a commonly used tool for classification with high dimensional data as well as for ranking candidate predictors based on the so-called random forest variable importance measures (VIMs). However the classification performance of RF is known to be suboptimal in case of strongly unbalanced data, i.e. data where response class sizes differ considerably. Suggestions were made to obtain better classification performance based either on sampling procedures or on cost sensitivity analyses. However to our knowledge the performance of the VIMs has not yet been examined in the case of unbalanced response classes. In this paper we explore the performance of the permutation VIM for unbalanced data settings and introduce an alternative permutation VIM based on the area under the curve (AUC) that is expected to be more robust towards class imbalance. We investigated the performance of the standard permutation VIM and of our novel AUC-based permutation VIM for different class imbalance levels using simulated data and real data. The results suggest that the standard permutation VIM loses its ability to discriminate between associated predictors and predictors not associated with the response for increasing class imbalance. It is outperformed by our new AUC-based permutation VIM for unbalanced data settings, while the performance of both VIMs is very similar in the case of balanced classes. The new AUC-based VIM is implemented in the R package party for the unbiased RF variant based on conditional inference trees. The codes implementing our study are available from the companion website: http://www.ibe.med.uni-muenchen.de/organisation/mitarbeiter/070_drittmittel/janitza/index.html
Clusters of Extragalactic Ultra Compact HII Regions
We report on the detection of optically thick free-free radio sources in the
galaxies M33, NGC 253, and NGC 6946 using data in the literature. We interpret
these sources as being young, embedded star birth regions, which are likely to
be clusters of ultracompact HII regions. All 35 of the sources presented in
this article have positive radio spectral indices alpha>0 suggesting an
optically thick thermal bremsstrahlung emission arising in the HII region
surrounding hot stars. Energy requirements indicate a range of a several to
>500 O7V star equivalents powering each HII region. Assuming a Salpeter IMF,
this corresponds to integrated stellar masses of 0.1--60,000 Msun. For roughly
half of the sources in our sample, there is no obvious optical counterpart,
giving further support for their deeply embedded nature. Their luminosities and
radio spectral energy distributions are consistent with HII regions having
electron densities from 1500 cm^-3 to 15000 cm^-3 and radii of 1 - 7 pc. We
suggest that the less luminous of these sources are extragalactic ultracompact
HII region complexes, those of intermediate luminosity are similar to W49 in
the Galaxy, while the brightest will be counterparts to 30 Doradus. These
objects constitute the lower mass range of extragalactic ``ultradense HII
regions'' which we argue are the youngest stages of massive star cluster
formation yet observed. This sample is beginning to fill in the continuum of
objects between small associations of ultracompact HII regions and the massive
extragalactic clusters that may evolve into globular clusters.Comment: 37 pages, uses AASTeX; scheduled to appear in ApJ v. 559 October
2001. Full postscript version available from
http://www.astro.wisc.edu/~chip/Papers/Johnson_Kobulnicky_etal_ApJ559.ps.g
Giant Molecular Clouds in M33 - I. BIMA All Disk Survey
We present the first interferometric CO(J=1->0) map of the entire H-alpha
disk of M33. The 13" diameter synthesized beam corresponds to a linear
resolution of 50 pc, sufficient to distinguish individual giant molecular
clouds (GMCs). From these data we generated a catalog of 148 GMCs with an
expectation that no more than 15 of the sources are spurious. The catalog is
complete down to GMC masses of 1.5 X 10^5 M_sun and contains a total mass of
2.3 X 10^7 M_sun. Single dish observations of CO in selected fields imply that
our survey detects ~50% of the CO flux, hence that the total molecular mass of
M33 is 4.5 X 10^7 M_sun, approximately 2% of the HI mass. The GMCs in our
catalog are confined largely to the central region (R < 4 kpc). They show a
remarkable spatial and kinematic correlation with overdense HI filaments; the
geometry suggests that the formation of GMCs follows that of the filaments. The
GMCs exhibit a mass spectrum dN/dM ~ M^(-2.6 +/- 0.3), considerably steeper
than that found in the Milky Way and in the LMC. Combined with the total mass,
this steep function implies that the GMCs in M33 form with a characteristic
mass of 7 X 10^4 M_sun. More than 2/3 of the GMCs have associated HII regions,
implying that the GMCs have a short quiescent period. Our results suggest the
rapid assembly of molecular clouds from atomic gas, with prompt onset of
massive star formation.Comment: 19 pages, Accepted for Publication in the Astrophysical Journal
Supplemen
The M33 Metallicity Project: Resolving the Abundance Gradient Discrepancies in M33
We present a new determination of the metallicity gradient in M33, based on
Keck/LRIS measurements of oxygen abundances using the temperature-sensitive
emission line [OIII] 4363 A in 61 HII regions. These data approximately triple
the sample of direct oxygen abundances in M33. We find a central abundance of
12 + log(O/H) = 8.36+/-0.04 and a slope of -0.027+/-0.012 dex/kpc, in agreement
with infrared measurements of the neon abundance gradient but much shallower
than most previous oxygen gradient measurements. There is substantial intrinsic
scatter of 0.11 dex in the metallicity at any given radius in M33, which
imposes a fundamental limit on the accuracy of gradient measurements that rely
on small samples of objects. We also show that the ionization state of neon
does not follow the ionization state of oxygen as is commonly assumed,
suggesting that neon abundance measurements from optical emission lines require
careful treatment of the ionization corrections.Comment: 10 pages, Accepted to Ap
The kinematics of the warm gas in Hickson compact group of galaxies HCG 90
We present kinematic observations of H emission for two early-type
galaxies and one disk system, members of the Hickson compact group 90 (HCG 90)
obtained with a scanning Fabry-Perot interferometer and samplings of 16
and 1\arcsec. Mapping of the gas kinematics was possible to 2
r for the disk galaxy N7174 and to 1.3 r and 1.7
r for the early-type galaxies N7176 and N7173 respectively. Evidence
for ongoing interaction was found in the properties of the warm gas of the
three galaxies, some of which do not have stellar counterparts.
We suggest the following evolutionary scenario for the system. H90d is the
warm gas reservoir of the group in process of fueling H90b with gas. H90c and d
have experienced past interaction with gas exchange. The gas acquired by H90c
has already settled and relaxed but the effects of the interaction can still be
visible in the morphology of the two galaxies and their stellar kinematics.
This process will possibly result in a major merger.Comment: 32 pages - 10 figures, Accepted for publication in Astronomical
Journa
Spiral inflow feeding the nuclear starburst in M83, observed in H-alpha emission with the GHAFAS Fabry-Perot interferometer
We present observations of the nearby barred starburst galaxy, M83 (NGC5236),
with the new Fabry-Perot interferometer GHAFAS mounted on the 4.2 meter William
Herschel Telescope on La Palma. The unprecedented high resolution observations,
of 16 pc/FWHM, of the H-alpha-emitting gas cover the central two kpc of the
galaxy. The velocity field displays the dominant disk rotation with signatures
of gas inflow from kpc scales down to the nuclear regions. At the inner Inner
Lindblad Resonance radius of the main bar and centerd at the dynamical center
of the main galaxy disk, a nuclear rapidly
rotating disk with scale length of pc has formed. The nuclear
starburst is found in the vicinity as well as inside this nuclear disk, and our
observations confirm that gas spirals in from the outer parts to feed the
nuclear starburst, giving rise to several star formation events at different
epochs, within the central 100 pc radius of M83.Comment: Accepted for publication in ApJ Letters. High-resolution version can
be found at http://www.astro.su.se/~kambiz/DOC/paper-M83.pd
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