85 research outputs found
On the origin of the neutral hydrogen supershells: the ionized progenitors and the limitations of the multiple supernovae hypothesis
Here we address the question whether the ionized shells associated with giant
HII regions can be progenitors of the larger HI shell-like objects found in the
Milky Way and other spiral and dwarf irregular galaxies. We use for our
analysis a sample of 12 HII shells presented recently by Rela\~no et al. (2005,
2007). We calculate the evolutionary tracks that these shells would have if
their expansion is driven by multiple supernovae explosions from the parental
stellar clusters. We find, contrary to Rela\~no et al. (2007), that the
evolutionary tracks of their sample HII shells are inconsistent with the
observed parameters of the largest and most massive neutral hydrogen
supershells. We conclude that HII shells found inside giant HII regions may
represent the progenitors of small or intermediate HI shells, however they
cannot evolve into the largest HI objects unless, aside from the multiple
supernovae explosions, an additional energy source contributes to their
expansion.Comment: Accepted for publication in ApJ, tentatively scheduled for the ApJ
July 1, 2008, v681n1 issue. 19 pages, 4 figure
Spherically-symmetric Accretion onto a Black Hole at the Center of a Young Stellar Cluster
We present a self-consistent, bimodal stationary solution for spherically
symmetric flows driven by young massive stellar clusters with a central
supermassive black hole. We demonstrate that the hydrodynamic regime of the
flow depends on the location of the cluster in the 3D (star cluster mechanical
luminosity - BH mass - star cluster radius) parameter space. We show that a
threshold mechanical luminosity (L_crit) separates clusters which evolve in the
BH dominated regime frome those whose internal structure is strongly affected
by the radiative cooling. In the first case(below the threshold energy) gravity
of the BH separates the flow into two distinct zones: the inner accretion zone
and the outer zone where the star cluster wind is formed. In the second case
(above the critical luminosity), catastrophic cooling sets in inside the
cluster. In this case the injected plasma becomes thermally unstable that
inhibits a complete stationary solution. We compared the calculated accretion
rates and the BH luminosities with those predicted by the classic Bondi
accretion theory and found that Bondi's theory is in good agreement with our
results in the case of low mass clusters. However, it substantially
underestimates the accretion rates and BH luminosities if the star cluster
mechanical luminosity, L_sc, approaches the threshold value (L_sc > 0.1
L_crit).Comment: 23 pages, 9 figures, accepted for publication in the Astrophysical
Journa
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
The Fine-Scale Structure of the neutral Interstellar Medium in nearby Galaxies
We present an analysis of the properties of HI holes detected in 20 galaxies
that are part of "The HI Nearby Galaxy Survey" (THINGS). We detected more than
1000 holes in total in the sampled galaxies. Where they can be measured, their
sizes range from about 100 pc (our resolution limit) to about 2 kpc, their
expansion velocities range from 4 to 36 km/s, and their ages are estimated to
range between 3 and 150 Myr. The holes are found throughout the disks of the
galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find
that shear limits the age of holes in spirals (shear is less important in dwarf
galaxies) which explains why HI holes in dwarfs are rounder, on average than in
spirals. Shear, which is particularly strong in the inner part of spiral
galaxies, also explains why we find that holes outside R25 are larger and
older. We derive the scale height of the HI disk as a function of
galactocentric radius and find that the disk flares up in all galaxies. We
proceed to derive the surface and volume porosity (Q2D and Q3D) and find that
this correlates with the type of the host galaxy: later Hubble types tend to be
more porous. The size distribution of the holes in our sample follows a power
law with a slope of a ~ -2.9. Assuming that the holes are the result of massive
star formation, we derive values for the supernova rate (SNR) and star
formation rate (SFR) which scales with the SFR derived based on other tracers.
If we extrapolate the observed number of holes to include those that fall below
our resolution limit, down to holes created by a single supernova, we find that
our results are compatible with the hypothesis that HI holes result from star
formation.Comment: 142 pages, 55 figures, accepted for publication in the Astronomical
Journa
Numerical simulations of expanding supershells in dwarf irregular galaxies. I. Application to Holmberg I
Numerical hydrodynamical modelling of supernova-driven shell formation is
performed with a purpose to reproduce a giant HI ring (diameter 1.7 kpc) in the
dwarf irregular galaxy Holmberg I (Ho I). We find that the contrast in HI
surface density between the central HI depression and the ring is sensitive to
the shape of the gravitational potential. This circumstance can be used to
constrain the total mass (including the dark matter halo) of nearly face-on
dwarf irregulars. We consider two models of Ho I, which differ by an assumed
mass of the dark matter halo M_h. The contrast in HI surface density between
the central HI depression and the ring, as well as the lack of gas expansion in
the central hole, are better reproduced by the model with a massive halo of
M_h=6.0*10^9 M_sun than by that with a small halo of M_h=4.0*10^8 M_sun,
implying that Ho I is halo-dominated. Assuming the halo mass of 6.0*10^9 M_sum,
we determine the mechanical energy required to form the observed ring equal to
(3.0 +- 0.5)*10^53 ergs, equivalent 300+-50 Type II supernovae. The inclination
of Ho I is constrained to 15-20 degrees by comparing the modelled HI spectrum
and channel maps with those observed.Comment: 11 pages, 11 figures, accepted for publication in Astronomy and
Astrophysic
Unveiling the nature of the "Green Pea" galaxies
We review recent results on the oxygen and nitrogen chemical abundances in
extremely compact, low-mass starburst galaxies at redshifts between 0.1-0.3
recently named to as "Green Pea" galaxies. These galaxies are genuine
metal-poor galaxies ( one fifth solar) with N/O ratios unusually high for
galaxies of the same metallicity. In combination with their known general
properties, i.e., size, stellar mass and star-formation rate, these findings
suggest that these objects could be experiencing a short and extreme phase in
their evolution. The possible action of both recent and massive inflow of gas,
as well as stellar feedback mechanisms are discussed here as main drivers of
the starburst activity and their oxygen and nitrogen abundances.Comment: To appear in JENAM Symposium "Dwarf Galaxies: Keys to Galaxy
Formation and Evolution", P. Papaderos, G. Hensler, S. Recchi (eds.). Lisbon,
September 2010, Springer Verlag, in pres
Probing Star Formation at Low Metallicity: The Radio Emission of Super Star Clusters in SBS0335-052
We present high-resolution radio continuum observations of the nascent
starburst in the metal-poor galaxy SBS 0335-052. These radio data were taken
with the Very Large Array and include observations at 0.7cm, 1.3cm, 2cm, 3.6cm,
and 6cm. These observations enable us to probe the thermal radio nebulae
associated with the extremely young star-forming regions in this galaxy. Two
discrete and luminous star-forming regions are detected in the south of the
galaxy that appear to be associated with massive star clusters previously
identified at optical wavelengths. However, the remaining optically-identified
massive star clusters are not clearly associated with radio emission (either
thermal or non-thermal) down to the sensitivity limits of these radio data. The
spectral energy distributions of the two radio-detected clusters are consistent
with being purely thermal, and the entire region has an inferred ionizing flux
of ~1.2 x 10^ 53 s^-1, which is equivalent to ~12,000 "typical" O-type stars
(type O7.5 V). The observations presented here have resolved out a significant
contribution from diffuse non-thermal emission detected previously, implying a
previous episode of significant star formation. The current star formation rate
(SFR) for this southern region alone is ~1.3 M_sun yr^-1, or ~ 23M_sun yr^-1
kpc^-2. This SFR derived from thermal radio emission also suggests that
previous optical recombination line studies are not detecting a significant
fraction of the current star formation in SBS 0335-052. From model fits to the
radio spectral energy distribution, we infer a global mean density in the two
youngest clusters of n_e > 10^3-10^4 cm^-3. In addition, a comparison between
the compact and diffuse radio emission indicates that up to ~50% of the
ionizing flux could be leaking out of the compact HII regions.Comment: accepted AJ, 14 pages, 5 figure
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