1,129 research outputs found
High resolution spectroscopy of H II Galaxies: Structure and Supersonic line widths
We present high resolution echelle spectroscopy of a sample of H II galaxies.
In all galaxies we identify different H(alpha) emitting knots along the slit
crossing the nucleus. All of these have been isolated and separately analyzed
through luminosity and size vs diagnosis plots. We find that in all cases, for
a particular galaxy, the bulk of emission comes from their main knot and
therefore, at least for the compact class galaxies we are dealing with,
luminosity and sigma values measured using single aperture observations would
provide similar results to what is obtained with spatially resolved
spectroscopy. In the size vs plots as expected there is a shift in the
correlations depending on whether we are including all emission in a single
point or we split it in its different emitting knots. The problem of a proper
determination of the size of the emitting region so that it can be used to
determine the mass of the system remains open. From the data set gathered,
using the highest surface brightness points as recently proposed by
Fuentes-Masip et al. (2000), the best luminosity vs correlation turns out to be
consistent with a Virial model.Comment: 14 pages, 4 figures, ApJ accepted. Also available from
http://www.daf.on.br/~etelles/papers/wht.ps.g
Initial Ionization of Compressible Turbulence
We study the effects of the initial conditions of turbulent molecular clouds
on the ionization structure in newly formed H_{ii} regions, using
three-dimensional, photon-conserving radiative transfer in a pre-computed
density field from three-dimensional compressible turbulence. Our results show
that the initial density structure of the gas cloud can play an important role
in the resulting structure of the H_{ii} region. The propagation of the
ionization fronts, the shape of the resulting H_{ii} region, and the total mass
ionized depend on the properties of the turbulent density field. Cuts through
the ionized regions generally show ``butterfly'' shapes rather than spherical
ones, while emission measure maps are more spherical if the turbulence is
driven on scales small compared to the size of the H_{ii} region. The
ionization structure can be described by an effective clumping factor , where is number density of the gas. The larger
the value of , the less mass is ionized, and the more irregular the
H_{ii} region shapes. Because we do not follow dynamics, our results apply only
to the early stage of ionization when the speed of the ionization fronts
remains much larger than the sound speed of the ionized gas, or Alfv\'en speed
in magnetized clouds if it is larger, so that the dynamical effects can be
negligible.Comment: 9 pages, 10 figures, version with high quality color images can be
found in http://research.amnh.org/~yuexing/astro-ph/0407249.pd
Thermal Emission from HII Galaxies: Discovering the Youngest Systems
We studied the radio properties of very young massive regions of star
formation in HII galaxies, with the aim of detecting episodes of recent star
formation in an early phase of evolution where the first supernovae start to
appear. Our sample consists of 31 HII galaxies, characterized by strong
Hydrogen emission lines, for which low resolution VLA 3.5cm and 6cm
observations were obtained. The radio spectral energy distribution has a range
of behaviours; 1) there are galaxies where the SED is characterized by a
synchrotron-type slope, 2) galaxies with a thermal slope, and, 3) galaxies with
possible free-free absorption at long wavelengths. The latter SEDs were found
in a few galaxies and represent a signature of heavily embedded massive star
clusters closely related to the early stages of massive star formation. Based
on the comparison of the star formation rates determined from the recombination
lines and those determined from the radio emission we find that SFR(Ha) is on
average five times higher than SFR(1.4GHz). We confirm this tendency by
comparing the ratio between the observed flux at 20 cm and the expected one,
calculated based on the Ha star formation rates, both for the galaxies in our
sample and for normal ones. This analysis shows that this ratio is a factor of
2 smaller in our galaxies than in normal ones, indicating that they fall below
the FIR/radio correlation. These results suggest that the emission of these
galaxies is dominated by a recent and massive star formation event in which the
first supernovae (SN) just started to explode. We conclude that the systematic
lack of synchrotron emission in those systems with the largest equivalent width
of Hb can only be explained if those are young starbursts of less than 3.5Myr
of age.Comment: Accepted for publication in Ap
Environment, Ram Pressure, and Shell Formation in HoII
Neutral hydrogen VLA D-array observations of the dwarf irregular galaxy HoII,
a prototype galaxy for studies of shell formation, are presented. HI is
detected to radii over 16' or 4 R_25, and M_HI=6.44x10^8 M_sun. The total HI
map has a comet-like appearance suggesting that HoII is affected by ram
pressure from an intragroup medium (IGM). A rotation curve corrected for
asymmetric drift was derived and an analysis of the mass distribution yields a
total mass 6.3x10^9 M_sun, of which about 80% is dark. HoII lies northeast of
the M81 group's core, along with Kar52 (M81dwA) and UGC4483. No signs of
interaction are observed and it is argued that HoII is part of the NGC2403
subgroup, infalling towards M81. A case is made for ram pressure stripping and
an IGM in the M81 group. Stripping of the disk outer parts would require an IGM
density n_IGM>=4.0x10^-6 atoms/cm^3 at the location of HoII. This corresponds
to 1% of the virial mass of the group uniformly distributed over a volume just
enclosing HoII and is consistent with the X-ray properties of small groups. It
is argued that existing observations of HoII do not support self-propagating
star formation scenarios, whereby the HI holes and shells are created by
supernova explosions and stellar winds. Many HI holes are located in low
surface density regions of the disk, where no star formation is expected or
observed. Ram pressure has the capacity to enlarge preexisting holes and lower
their creation energies, helping to bridge the gap between the observed star
formation rate and that required to create the holes. (abridged)Comment: 43 pages, including 7 figures. 4 figures available as JPEG only.
Complete manuscript including full resolution figures available at
http://www.strw.leidenuniv.nl/~bureau/pub_list.html . Accepted for
publication in The Astronomical Journa
The LBT Panoramic View on the Recent Star-Formation Activity in IC2574
We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81
group taken with the Large Binocular Telescope in order to study in detail the
recent star-formation history of this galaxy and to constrain the stellar
feedback on its HI gas. We identify the star-forming areas in the galaxy by
removing a smooth disk component from the optical images. We construct
pixel-by-pixel maps of stellar age and stellar mass surface density in these
regions by comparing their observed colors with simple stellar populations
synthesized with STARBURST99. We find that an older burst occurred about 100
Myr ago within the inner 4 kpc and that a younger burst happened in the last 10
Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar
populations residing in the known HI holes of IC2574. Our results indicate
that, even at the remarkable photometric depth of the LBT data, there is no
clear one-to-one association between the observed HI holes and the most recent
bursts of star formation in IC2574. The stellar populations formed during the
younger burst are usually located at the periphery of the HI holes and are seen
to be younger than the holes dynamical age. The kinetic energy of the holes
expansion is found to be on average 10% of the total stellar energy released by
the stellar winds and supernova explosions of the young stellar populations
within the holes. With the help of control apertures distributed across the
galaxy we estimate that the kinetic energy stored in the HI gas in the form of
its local velocity dispersion is about 35% of the total stellar energy.Comment: 16 pages, 14 figures, accepted for publication in Ap
A Test of the Standard Hypothesis for the Origin of the HI Holes in Holmberg II
The nearby irregular galaxy Holmberg II has been extensively mapped in HI
using the Very Large Array (VLA), revealing intricate structure in its
interstellar gas component (Puche et al. 1992). An analysis of these structures
shows the neutral gas to contain a number of expanding HI holes. The formation
of the HI holes has been attributed to multiple supernova events occurring
within wind-blown shells around young, massive star clusters, with as many as
10-200 supernovae required to produce many of the holes. From the sizes and
expansion velocities of the holes, Puche et al. assigned ages of ~10^7 to 10^8
years. If the supernova scenario for the formation of the HI holes is correct,
it implies the existence of star clusters with a substantial population of
late-B, A and F main sequence stars at the centers of the holes. Many of these
clusters should be detectable in deep ground-based CCD images of the galaxy. In
order to test the supernova hypothesis for the formation of the HI holes, we
have obtained and analyzed deep broad-band BVR and narrow-band H-alpha images
of Ho II. We compare the optical and HI data and search for evidence of the
expected star clusters in and around the HI holes. We also use the HI data to
constrain models of the expected remnant stellar population. We show that in
several of the holes the observed upper limits for the remnant cluster
brightness are strongly inconsistent with the SNe hypothesis described in Puche
et al. Moreover, many of the HI holes are located in regions of very low
optical surface brightness which show no indication of recent star formation.
Here we present our findings and explore possible alternative explanations for
the existence of the HI holes in Ho II, including the suggestion that some of
the holes were produced by Gamma-ray burst events.Comment: 30 pages, including 6 tables and 3 images. To appear in Astron.
Journal (June 1999
Generalized strongly increasing semigroups
In this work we present a new class of numerical semigroups called
GSI-semigroups. We see the relations between them and others families of
semigroups and we give explicitly their set of gaps. Moreover, an algorithm to
obtain all the GSI-semigroups up to a given Frobenius number is provided and
the realization of positive integers as Frobenius numbers of GSI-semigroups is
studied
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
Superbubble evolution including the star-forming clouds: Is it possible to reconcile LMC observations with model predictions?
Here we present a possible solution to the apparent discrepancy between the
observed properties of LMC bubbles and the standard, constant density bubble
model. A two-dimensional model of a wind-driven bubble expanding from a
flattened giant molecular cloud is examined. We conclude that the expansion
velocities derived from spherically symmetric models are not always applicable
to elongated young bubbles seen almost face-on due to the LMC orientation. In
addition, an observational test to differentiate between spherical and
elongated bubbles seen face-on is discussed.Comment: 25 pages, 7 figures, accepted to ApJ (September, 1999 issue
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