70 research outputs found
Discovery of a Metal-Line Absorber Associated with a Local Dwarf Starburst Galaxy
We present optical and near-infrared images, H I 21 cm emission maps, optical
spectroscopy, and Hubble Space Telescope/Space Telescope Imaging Spectrograph
ultraviolet spectroscopy of the QSO/galaxy pair SBS 1122+594/IC 691. The QSO
sight line lies at a position angle of 27 degrees from the minor axis of the
nearby dwarf starburst galaxy IC 691 (cz_gal = 1204+-3 km/s, L_B ~ 0.09 L*,
current star formation rate = 0.08-0.24 solar masses per year) and 33 kpc (6.6
arcmin) from its nucleus. We find that IC 691 has an H I mass of M_HI =
(3.6+-0.1) x 10^8 solar masses and a dynamical mass of M_dyn = (3.1+-0.5) x
10^10 solar masses. The UV spectrum of SBS 1122+594 shows a metal-line
(Ly-alpha + C IV) absorber near the redshift of IC 691 at cz_abs = 1110+-30
km/s. Since IC 691 is a dwarf starburst and the SBS 1122+594 sight line lies in
the expected location for an outflowing wind, we propose that the best model
for producing this metal-line absorber is a starburst wind from IC 691. We
place consistent metallicity limits on IC 691 ([Z/Zsun] ~ -0.7) and the
metal-line absorber ([Z/Zsun] < -0.3). We also find that the galaxy's escape
velocity at the absorber location is v_esc = 80+-10 km/s and derive a wind
velocity of v_w = 160+-50 km/s. Thus, the evidence suggests that IC 691
produces an unbound starburst wind that escapes from its gravitational
potential to transport metals and energy to the surrounding intergalactic
medium.Comment: 31 pages, 8 figures; AJ in press; a version with high resolution
figures can be downloaded from
http://casa.colorado.edu/~keeney/research/papers/IC691.pd
UNION (Reino Unido). Inglaterra. Canal. Canales. 1792?. 1:64400
Comprende parte de los condados ingleses de Leicestershire y NorthamptonshireEscalas grĂĄficas de 9 millas y 72 estadios [= 22,5 cm]OrografĂa a trazosTabla de signos convencionales para indicar el trazado del canal proyectado, distintos tipos de caminos y molinos de aguaConsta una tabla con datos referentes a las dimensiones y capacidad de los tramos de la obra proyectadaForma parte de la ColecciĂłn Mendoz
HI Properties of Low Luminosity Star-Forming Galaxies in the KPNO International Spectroscopic Survey
New HI observations are presented for a complete sample of 109 low luminosity
star-forming galaxies taken from the KPNO International Spectroscopic Survey
(KISS), the first CCD-based wide-field objective-prism survey for emission-line
galaxies. This sample consists of all star-forming galaxies with M_B > -18.0
and cz < 11,000 km/s from the first Halpha-selected survey list. Overall, 97
out of 109 galaxies have been detected in HI. We confirm the weak trend of
increasing gas richness with decreasing luminosity found by previous authors.
Gas richness is also shown to be weakly anti-correlated with metallicity. The
dependence of star formation rates (SFRs) and HI gas depletion timescales on
metallicity is examined. The median solar metallicity based SFR and gas
depletion timescale are 0.1639 M_sun/yr and 5 Gyrs, respectively. Corrections
for variations in metallicity decreases SFRs by about 0.5 dex and increases gas
depletion timescales by an average of about 8 Gyrs. The majority of galaxies in
this sample still have large reservoirs of HI gas, and despite their large
current star formation rates, could have formed stars in a quasi-continuous
manner for a Hubble time. Finally, we present the first HI mass function for
low luminosity star-forming galaxies and show that this subpopulation
contributes 10-15% of the overall HI density in the local universe. We conclude
that if the HI mass function of the Universe does indeed have a steeply rising
low-mass slope as suggested by previous authors, it is not due to the
population of low luminosity star-forming galaxies.Comment: 27 pages, 13 figures, 4 tables. Accepted for publication in the
Astronomical Journa
Ghost of a Shell: Magnetic Fields of Galactic Supershell GSH 006157
We identify a counterpart to a Galactic supershell in diffuse radio
polarisation, and use this to determine the magnetic fields associated with
this object. GSH 006157 has perturbed the polarised emission at
2.3GHz, as observed in the S-band Polarisation All Sky Survey (S-PASS),
acting as a Faraday screen. We model the Faraday rotation over the shell, and
produce a map of Faraday depth over the area across it. Such models require
information about the polarised emission behind the screen, which we obtain
from the Wilkinson Microwave Anisotropy Probe (WMAP), scaled from 23GHz to
2.3GHz, to estimate the synchrotron background behind GSH 006157.
Using the modelled Faraday thickness we determine the magnitude and the
plane-of-the-sky structure of the line-of-sight magnetic field in the shell. We
find a peak line-of-sight field strength of G. Our measurement probes weak magnetic fields
in a low-density regime (number densities of cm) of the ISM,
thus providing crucial information about the magnetic fields in the
partially-ionised phase.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society. 19 pages, 19 figure
Ascospore release and survival in Sclerotinia sclerotiorum
The release and survival of ascospores of a UK Sclerotinia sclerotiorum isolate were studied. Apothecia placed in a spore clock apparatus with different lighting regimes at 15 °C released ascospores continuously with an increasing rate for the duration of experiments (72â84 h). Spore release was not confined to light or dark periods in alternating regimes and occurred in continuous dark or light. Ascospores were released in both saturated air (90â95% rh) and at 65â75% rh. High temperature and rh were detrimental to ascospore survival but spore viability was maintained for longer periods than previously reported. The significance of these results in relation to disease control is discussed
Galactic and Magellanic Evolution with the SKA
As we strive to understand how galaxies evolve it is crucial that we resolve
physical processes and test emerging theories in nearby systems that we can
observe in great detail. Our own Galaxy, the Milky Way, and the nearby
Magellanic Clouds provide unique windows into the evolution of galaxies, each
with its own metallicity and star formation rate. These laboratories allow us
to study with more detail than anywhere else in the Universe how galaxies
acquire fresh gas to fuel their continuing star formation, how they exchange
gas with the surrounding intergalactic medium, and turn warm, diffuse gas into
molecular clouds and ultimately stars. The 21-cm line of atomic
hydrogen (HI) is an excellent tracer of these physical processes. With the SKA
we will finally have the combination of surface brightness sensitivity, point
source sensitivity and angular resolution to transform our understanding of the
evolution of gas in the Milky Way, all the way from the halo down to the
formation of individual molecular clouds.Comment: 25 pages, from "Advancing Astrophysics with the Square Kilometre
Array", to appear in Proceedings of Scienc
Two Large HI Shells in the Outer Galaxy near l=279 degrees
As part of a survey of HI 21-cm emission in the Southern Milky Way, we have
detected two large shells in the interstellar neutral hydrogen near l=279 deg.
The center velocities are +36 and +59 km/s, which puts the shells at kinematic
distances of 7 and 10 kpc. The larger shell is about 610 pc in diameter and
very empty, with density contrast of at least 15 between the middle and the
shell walls. It has expansion velocity of about 20 km/s and swept up mass of
several million solar masses. The energy indicated by the expansion may be as
high as 2.4 X 10^53 ergs. We estimate its age to be 15 to 20 million years. The
smaller shell has diameter of about 400 pc, expansion velocity about 10 km/s
and swept up mass of about 10^6 solar masses.
Morphologically both regions appear to be shells, with high density regions
mostly surrounding the voids, although the first appears to have channels of
low density which connect with the halo above and below the HI layer. They lie
on the edge of the Carina arm, which suggests that they may be expanding
horizontally into the interarm region as well as vertically out of the disk. If
this interpretation is correct, this is the first detection of an HI chimney
which has blown out of both sides of the disk.Comment: 21 pages, 14 jpeg figures, accepted for publication in A
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
Astrophysical and Astrobiological Implications of Gamma-Ray Burst Properties
Combining results for the local cosmic rate and mean peak luminosity of GRBs
with the cosmic history of the star formation rate, we provide estimates for
the local GRB rate per unit blue luminosity in galaxies. We find a typical GRB
rate per unit B luminosity of 2.4x10^-17 h_{70}^2/Lsun/yr. The corresponding
mean rate in the Milky Way is 5.5x10^-7 h_{70}^2/yr. We conclude: 1) the ratio
of supernova rate to isotropic equivalent GRB rate is large: more than 6000
SNIbc per GRB or 30,000 SNII per GRB. GRBs could arise in a large fraction of
black hole-forming events only with collimation in the range 0.01 - 0.001 and a
steep enough slope of the IMF; 2) GRBs cannot account for the majority of large
HI holes observed in galaxies; 3) the probability that the solar system was
exposed to a fluence large enough to melt the chondrules during the first 10^7
yr of solar system history is negligibly small; 4) Even for very opaque
atmospheres, a significant fraction of the GRB energy is transmitted as UV
lines due to excitation by secondary electrons. For eukaryotic-like organisms
in thin atmospheres (e.g. contemporary Mars), or for UV line exposure in thick
atmospheres (e.g. Earth), biologically significant events occur at a rate of
about 100--500 /Gyr. The direct contribution of these "jolts" to mutational
evolution may, however, be negligible because of the short duration of the
GRBs. Evolutionary effects due to partial sterilizations and to longer-lived
disruptions of atmospheric chemistry should be more important. (Abridged)Comment: 36 pages, no figures Accepted by Astrophysical Journal Oct. 2001.
First submitted December,1999. Substantially rewritten discussion of burst
source count distributions and of biological implication
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