2,443 research outputs found
On the massive star content of the nearby dwarf irregular Wolf-Rayet galaxy IC 4662
Aims. We investigate the massive stellar content of the nearby dwarf irregular Wolf-Rayet galaxy IC 4662, and consider its global star forming properties in the context of other metal-poor galaxies, the SMC, IC 10 and NGC 1569.
Methods. Very Large Telescope/FORS2 imaging and spectroscopy plus archival Hubble Space Telescope/ACS imaging datasets permit us to spatially identify the location, number and probable subtypes of Wolf-Rayet stars within this galaxy. We also investigate suggestions that a significant fraction of the ionizing photons of the two giant H II regions A1 and A2 lie deeply embedded within
these regions.
Results. Wolf-Rayet stars are associated with a number of sources within IC 4662-A1 and A2, plus a third compact H II region to the north west of A1 (A1-NW). Several sources appear to be isolated, single (or binary) luminous nitrogen sequence WR stars, while extended sources are clusters whose masses exceed the Orion Nebula Cluster by, at most, a factor of two. IC 4662 lacks optically
visible young massive, compact clusters that are common in other nearby dwarf irregular galaxies. A comparison between radio and H-derived ionizing fluxes of A1 and A2 suggests that 30–50% of their total Lyman continuum fluxes lie deeply embedded within these regions.
Conclusions. The star formation surface density of IC 4662 is insufficient for this galaxy to qualify as a starburst galaxy, based upon its photometric radius, R25. If instead, we were to adopt the
The JCMT Nearby Galaxies Legacy Survey VII: H\alpha{} imaging and massive star formation properties
We present H\alpha{} fluxes, star formation rates (SFRs) and equivalent
widths (EWs) for a sample of 156 nearby galaxies observed in the 12CO J=3-2
line as part of the James Clerk Maxwell Telescope Nearby Galaxies Legacy
Survey. These are derived from images and values in the literature and from new
H\alpha{} images for 72 galaxies which we publish here. We describe the sample,
observations and procedures to extract the H\alpha{} fluxes and related
quantities. We discuss the SFR properties of our sample and confirm the
well-known correlation with galaxy luminosity, albeit with high dispersion. Our
SFRs range from 0.1 to 11 Msun yr-1 with a median SFR value for the complete
sample of 0.2 Msun yr-1. This median values is somewhat lower than similar
published measurements, which we attribute, in part, to our sample being
HI-selected and, thus, not biased towards high SFRs as has frequently been the
case in previous studies. Additionally, we calculate internal absorptions for
the H\alpha{} line, A(H\alpha{}), which are lower than many of those used in
previous studies. Our derived EWs, which range from 1 to 880\AA{} with a median
value of 27\AA{}, show little dependence with luminosity but rise by a factor
of five from early- to late-type galaxies. This paper is the first in a series
aimed at comparing SFRs obtained from H\alpha{} imaging of galaxies with
information derived from other tracers of star formation and atomic and
molecular gas.Comment: Accepted for publication in MNRAS. 47 pages, 18 figure
Molecular hydrogen deficiency in HI-poor galaxies and its implications for star formation
We use a sample of 47 homogeneous and high sensitivity CO images taken from
the Nobeyama and BIMA surveys to demonstrate that, contrary to common belief, a
significant number (~40%) of HI-deficient nearby spiral galaxies are also
depleted in molecular hydrogen. While HI-deficiency by itself is not a
sufficient condition for molecular gas depletion, we find that H2 reduction is
associated with the removal of HI inside the galaxy optical disk. Those
HI-deficient galaxies with normal H2 content have lost HI mainly from outside
their optical disks, where the H2 content is low in all galaxies. This finding
is consistent with theoretical models in which the molecular fraction in a
galaxy is determined primarily by its gas column density. Our result is
supported by indirect evidence that molecular deficient galaxies form stars at
a lower rate or have dimmer far infrared fluxes than gas rich galaxies, as
expected if the star formation rate is determined by the molecular hydrogen
content. Our result is consistent with a scenario in which, when the atomic gas
column density is lowered inside the optical disk below the critical value
required to form molecular hydrogen and stars, spirals become quiescent and
passive evolving systems. We speculate that this process would act on the
time-scale set by the gas depletion rate and might be a first step for the
transition between the blue and red sequence observed in the color-magnitude
diagram.Comment: 12 pages, 9 figures, accepted for publication in Ap
Measuring the Hubble Constant with the Hubble Space Telescope
Ten years ago our team completed the Hubble Space Telescope Key Project on
the extragalactic distance scale. Cepheids were detected in some 25 galaxies
and used to calibrate four secondary distance indicators that reach out into
the expansion field beyond the noise of galaxy peculiar velocities. The result
was H_0 = 72 +/- 8 km/sec/Mpc and put an end to galaxy distances uncertain by a
factor of two. This work has been awarded the Gruber Prize in Cosmology for
2009.Comment: Gruber Prize Lecture to be published in Transactions of the IA
Molecular gas in late-type galaxies
We present CO(J=1--0) line observations of 22 low-luminosity spiral
galaxies in the Virgo cluster. These data, together with 244 others available
in the literature, allow us to build a large sample that we use to study the
molecular gas properties of galaxies spanning a large range of morphological
types and luminosities and belonging to different environments (clusters -
field). The molecular gas content of the target galaxies is estimated using a
luminosity-dependent X = conversion factor that has been
calibrated on a sample of nearby galaxies. spans from 10 mol
cm (K km s in giant spirals to 10 mol
cm (K km s in dwarf irregulars. The value of the
conversion factor is found consistent with a value derived independently from
dust masses estimated from FIR fluxes, with a metallicity-dependent dust to gas
ratio. The relationships between X and the UV radiation field (as traced by the
), the metallicity and the H band luminosity are analysed.
We show that the molecular gas contained in molecular clouds or complexes is of
the order of 15% of the total gas on average whatever the luminosity or the
Hubble type of the galaxies. We discuss the relation between the star formation
rate and the molecular gas content and estimate the average star formation
efficiency of late-type galaxies.Comment: accepted for publication on Astronomy and Astrophysic
CDM, Feedback and the Hubble Sequence
We have performed TreeSPH simulations of galaxy formation in a standard LCDM
cosmology, including effects of star formation, energetic stellar feedback
processes and a meta-galactic UV field, and obtain a mix of disk, lenticular
and elliptical galaxies. The disk galaxies are deficient in angular momentum by
only about a factor of two compared to observed disk galaxies. The stellar
disks have approximately exponential surface density profiles, and those of the
bulges range from exponential to r^{1/4}, as observed. The bulge-to-disk ratios
of the disk galaxies are consistent with observations and likewise are their
integrated B-V colours, which have been calculated using stellar population
synthesis techniques. Furthermore, we can match the observed I-band
Tully-Fisher (TF) relation, provided that the mass-to-light ratio of disk
galaxies, (M/L_I), is about 0.8. The ellipticals and lenticulars have
approximately r^{1/4} stellar surface density profiles, are dominated by
non-disklike kinematics and flattened due to non-isotropic stellar velocity
distributions, again consistent with observations.Comment: 6 pages, incl. 4 figs. To appear in the proceedings of the
EuroConference "The Evolution of Galaxies: II - Basic Building Blocks", Ile
de La Reunion (France), 16-21 October 2001 (Slightly updated version). A much
more comprehensive paper about this work with links to pictures of some of
the galaxies can be found at http://babbage.sissa.it/abs/astro-ph/020436
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Star cluster catalogues for the LEGUS dwarf galaxies
We present the star cluster catalogues for 17 dwarf and irregular galaxies in the HST Treasury Program 'Legacy ExtraGalactic UV Survey' (LEGUS). Cluster identification and photometry in this sub-sample are similar to that of the entire LEGUS sample, but special methods were developed to provide robust catalogues with accurate fluxes due to low cluster statistics. The colours and ages are largely consistent for two widely used aperture corrections, but a significant fraction of the clusters are more compact than the average training cluster. However, the ensemble luminosity, mass, and age distributions are consistent suggesting that the systematics between the two methods are less than the random errors. When compared with the clusters from previous dwarf galaxy samples, we find that the LEGUS catalogues are more complete and provide more accurate total fluxes. Combining all clusters into a composite dwarf galaxy, we find that the luminosity and mass functions can be described by a power law with the canonical index of -2 independent of age and global SFR binning. The age distribution declines as a power law, with an index of approximate to -0.80 +/- 0.15, independent of cluster mass and global SFR binning. This decline of clusters is dominated by cluster disruption since the combined star formation histories and integrated-light SFRs are both approximately constant over the last few hundred Myr. Finally, we find little evidence for an upper-mass cut-off (< 2 sigma) in the composite cluster mass function, and can rule out a truncation mass below approximate to 10(4.5)M(circle dot) but cannot rule out the existence of a truncation at higher masses.NASA [NAS 5-26555]; NASA; Swedish Research Council (Vetenskapsradet); Swedish National Space Board (SNSB); German Aerospace Center (DLR); Federal Ministry for EconomicAffairs and Energy (BMWi) [50OR1801]; [13364]This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]
HST Survey of Clusters in Nearby Galaxies. II. Statistical Analysis of Cluster Populations
We present a statistical system that can be used in the study of cluster
populations. The basis of our approach is the construction of synthetic cluster
color-magnitude-radius diagrams (CMRDs), which we compare with the observed
data using a maximum likelihood calculation. This approach permits a relatively
easy incorporation of incompleteness (a function of not only magnitude and
color, but also radius), photometry errors and biases, and a variety of other
complex effects into the calculation, instead of the more common procedure of
attempting to correct for those effects.
We then apply this procedure to our NGC 3627 data from Paper I. We find that
we are able to successfully model the observed CMRD and constrain a number of
parameters of the cluster population. We measure a power law mass function
slope of alpha = -1.50 +/- 0.07, and a distribution of core radii centered at
r_c = 1.53 +/- 0.15 pc. Although the extinction distribution is less
constrained, we measured a value for the mean extinction consistent with that
determined in Paper I from the Cepheids.Comment: 21 pages, 3 figures accepted for publication by A
A survey of the Wolf-Rayet population of the barred, spiral galaxy NGC 1313
We present a VLT/FORS1 survey of Wolf-Rayet (WR) stars in the spiral galaxy
NGC 1313. In total, 94 WR candidate sources have been identified from
narrow-band imaging. Of these, 82 have been spectroscopically observed, for
which WR emission features are confirmed in 70 cases, one of which also
exhibits strong nebular HeII 4686 emission. We also detect strong nebular HeII
4686 emission within two other regions of NGC 1313, one of which is a possible
supernova remnant. Nebular properties confirm that NGC 1313 has a metal-content
log(O/H)+12=8.23+/-0.06, in good agreement with previous studies. From
continuum subtracted Halpha images we infer a global star formation rate of 0.6
Msun/yr. Using template LMC WR stars, spectroscopy reveals that NGC 1313 hosts
a minimum of 84 WR stars. Our census comprises 51 WN stars, including a rare
WN/C transition star plus 32 WC stars. In addition, we identify one WO star
which represents the first such case identified beyond the Local Group. The
bright giant HII region PES 1, comparable in Halpha luminosity to NGC 595 in M
33, is found to host a minimum of 17 WR stars. The remaining photometric
candidates generally display photometric properties consistent with WN stars,
such that we expect a global WR population of ~115 stars with N(WR)/N(O)~0.01
and N(WC)/N(WN)~0.4.Comment: Accepted for publication in MNRAS. Finding charts omitted, full
version available by anonymous ftp (ftp:
hydra.shef.ac.uk/pub/lh/ngc1313-fullversion.pd
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