4,489 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
An HST Search for Lyman Continuum Emission From Galaxies at z=1.1--1.4
If enough of their Lyman limit continuum escapes, star-forming galaxies could
be significant contributors to the cosmic background of ionizing photons. To
investigate this possibility, we obtained the first deep imaging in the far
ultraviolet of eleven bright blue galaxies at intermediate redshift
(z=1.1--1.4). NO Lyman continuum emission was detected. Sensitive,
model-independent, upper limits of typically 2 x 10**-19 erg/sec/cm2/Ang were
obtained for the ionizing flux escaping from these normal galaxies. This
corresponds to lower limits on the observed ratio of 1500 to 700Ang flux of 150
up to 1000. Based on a wide range of stellar synthesis models, this suggests
that less than 6%, down to less than 1%, of the available ionizing flux emitted
by hot stars is escaping these galaxies. The magnitude of this spectral break
at the Lyman l imit confirms that the basic premise of `Lyman break' searches
for galaxies at high redshift can also be applied at intermediate redshifts.
This implies that the integrated contribution of galaxies to the UV cosmic
background at z around 1.2 is less than 15%, and may be less than 2%.Comment: 20 manuscript pages, which includes two tables and two figures. To be
published in 1 December 2003 issue of The Astrophysical Journa
Giant HII Regions in NGC 7479 & NGC 6070
We present new results from our search for Giant H\,{\sc ii} Regions in
galaxies visible from the southern hemisphere. In this work we study two
galaxies: NGC\,7479 and NGC\,6070. Using high-resolution spectra, obtained with
different instruments at Las Campanas Observatory, we are able to resolve the
emission-line profile widths and determine the intrinsic velocity dispersion of
the ionised gas. We detect profile widths corresponding to supersonic velocity
dispersions in the six observed H\,{\sc ii} regions. We find that all of them
show at least two distinct kinematical components: a relatively narrow feature
(between ~11 and ~22\kms) and a broader (between ~31 and ~77\kms) component.
Two of the regions show a complex narrow profile in all ion lines, which can be
further split into two components with different radial velocities. Whereas the
wing broadening of the overall profile can be fitted with a low-intensity broad
component for almost all profiles, in one region it was better reproduced by
two separate shell-like wings. We have analysed the impact that the presence of
multiple components has on the location of the H{\sc ii} regions in the
plane. Although the overall distribution confirms the
presence of a regression, the precise location of the regions in the plane is
strongly dependent on the components derived from the profile fitting.Comment: This manuscript has been accepted for publication in Monthly Notices
of the Royal Astronomical Society Main Journal. 16 pages, 11 figure
The H alpha Galaxy Survey VI. Star-forming companions of nearby field galaxies
We present a search for star-forming satellite galaxies that are close enough
to their parent galaxies to be considered analogues of the Magellanic Clouds.
Our search technique relied on the detection of the satellites in
continuum-subtracted narrow-band H alpha imaging of the central galaxies, which
removes most of the background and foreground line-of-sight companions, thus
giving a high probability that we are detecting true satellites. The search was
performed for 119 central galaxies at distances between 20 and 40 Mpc, although
spatial incompleteness means that we have effectively searched 53 full
satellite-containing volumes. We find only 9 probable star-forming satellites,
around 9 different central galaxies, and 2 possible satellites. After
incompleteness correction, this is equivalent to 0.17/0.21 satellites per
central galaxy. The Small Magellanic Cloud is just below the median values of
both star formation rate and R-band luminosity of the 9 probable satellites.
The Large Magellanic Cloud, however, has a higher R-band luminosity than any of
the 9 and is only exceeded in star formation rate by the one satellite that
appears to be undergoing a tidally-induced starburst. Thus the Milky Way
appears to be quite unusual, both in having two star-forming satellite galaxies
and in the high luminosity of the Large Magellanic Cloud.Comment: 13 pages, accepted for publication in A&
Dense Molecular Gas Associated with the Circumnuclear Star Forming Ring in the Barred Spiral Galaxy NGC 6951
We present high resolution (3" - 5") observations of CO(1-0) and HCN(1-0)
emission from the circumnuclear star forming ring in the barred spiral galaxy
NGC 6951, a host of a type-2 Seyfert, using the Nobeyama Millimeter Array and
45 m telescope. We find that most of the HCN emission is associated with the
circumnuclear ring, where vigorous star formation occurs. The HCN to CO
integrated intensity ratio is also enhanced in the star forming ring; the peak
value of HCN/CO ratio is 0.18, which is comparable to the ratio in the
starbursts NGC 253 and M82. The formation mechanism of dense molecular gas has
been investigated. We find that the shocks along the orbit crowding do not
promote the formation of the dense molecular gas effectively but enhance the
presence of low density GMCs. Instead, gravitational instabilities of the gas
can account for the dense molecular gas formation. The HCN/CO ratio toward the
Seyfert nucleus of NGC 6951 is a rather normal value (0.086), in contrast with
other Seyferts NGC 1068 and M51 where extremely high HCN/CO value of ~ 0.5 have
been reported.Comment: 33 pages, 17 figures, to appear in the Astrophysical Journa
Quantum cosmic models and thermodynamics
The current accelerating phase of the evolution of the universe is considered
by constructing most economical cosmic models that use just general relativity
and some dominating quantum effects associated with the probabilistic
description of quantum physics. Two of such models are explicitly analyzed.
They are based on the existence of a sub-quantum potential and correspond to a
generalization of the spatially flat exponential model of de Sitter space. The
thermodynamics of these two cosmic solutions is discussed, using the second
principle as a guide to choose which among the two is more feasible. The paper
also discusses the relativistic physics on which the models are based, their
holographic description, some implications from the classical energy
conditions, and an interpretation of dark energy in terms of the entangled
energy of the universe.Comment: 15 pages, 1 figure, accepted for publication in Class. Quantum Gra
A multi-transition HCN and HCO+ study of 12 nearby active galaxies: AGN versus SB environments
Recent studies have indicated that the HCN-to-CO(J=1-0) and
HCO+-to-HCN(J=1-0) ratios are significantly different between galaxies with AGN
(active galactic nucleus) and SB (starburst) signatures. In order to study the
molecular gas properties in active galaxies and search for differences between
AGN and SB environments, we observed the HCN(J=1-0), (J=2-1), (J=3-2),
HCO+(J=1-0) and HCO+(J=3-2), emission with the IRAM 30m in the centre of 12
nearby active galaxies which either exhibit nuclear SB and/or AGN signatures.
Consistent with previous results, we find a significant difference of the
HCN(J=2-1)-to-HCN(J=1-0), HCN(J=3-2)-to-HCN(J=1-0), HCO+(J=3-2)-to-HCO+(J=3-2),
and HCO+-to-HCN intensity ratios between the sources dominated by an AGN and
those with an additional or pure central SB: the HCN, HCO+ and HCO+-to-HCN
intensity ratios tend to be higher in the galaxies of our sample with a central
SB as opposed to the pure AGN cases which show rather low intensity ratios.
Based on an LVG analysis of these data, i.e., assuming purely collisional
excitation, the (average) molecular gas densities in the SB dominated sources
of our sample seem to be systematically higher than in the AGN sources. The LVG
analysis seems to further support systematically higher HCN and/or lower HCO+
abundances as well as similar or higher gas temperatures in AGN compared to the
SB sources of our sample. Also, we find that the HCN-to-CO ratios decrease with
increasing rotational number J for the AGN while they stay mostly constant for
the SB sources.Comment: accepted for publication in ApJ; 20 pages, 7 figures; in emulateApJ
forma
Combined effects of sprayâdrying conditions and postdrying storage time and temperature on Salmonella choleraesuis and Salmonella typhimurium survival when inoculated in liquid porcine plasma
The objective of this study was to determine the effectiveness of the spray-drying process on the inactivation of Salmonella choleraesuis and Salmonella typhimurium spiked in liquid porcine plasma and to test the additive effect of immediate postdrying storage. Commercial spray-dried porcine plasma was sterilized by irradiation and then reconstituted (1:9) with sterile water. Aliquots of reconstituted plasma were inoculated with either S. choleraesuis or S. typhimurium, subjected to spray-drying at an inlet temperature of 200°C and an outlet temperature of either 71 or 80°C, and each spray-drying temperature combinations were subjected to either 0, 30 or 60 s of residence time (RT) as a simulation of residence time typical of commercial dryers. Spray-dried samples were stored at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days. Bacterial counts of each Salmonella spp., were completed for all samples. For both Salmonella spp., spray-drying at both outlet temperatures reduced bacterial counts about 3 logs at RT 0 s, while there was about a 5·5 log reduction at RT 60 s. Storage of all dried samples at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days eliminate all detectable bacterial counts of both Salmonella spp.info:eu-repo/semantics/publishedVersio
Combined effects of spray-drying conditions and postdrying storage time and temperature on Salmonella choleraesuis and Salmonella typhimurium survival when inoculated in liquid porcine plasma
Altres ajuts: GC/2014DI066The objective of this study was to determine the effectiveness of the spray-drying process on the inactivation of Salmonella choleraesuis and Salmonella typhimurium spiked in liquid porcine plasma and to test the additive effect of immediate postdrying storage. Commercial spray-dried porcine plasma was sterilized by irradiation and then reconstituted (1:9) with sterile water. Aliquots of reconstituted plasma were inoculated with either S. choleraesuis or S. typhimurium, subjected to spray-drying at an inlet temperature of 200°C and an outlet temperature of either 71 or 80°C, and each spray-drying temperature combinations were subjected to either 0, 30 or 60 s of residence time () as a simulation of residence time typical of commercial dryers. Spray-dried samples were stored at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days. Bacterial counts of each Salmonella spp., were completed for all samples. For both Salmonella spp., spray-drying at both outlet temperatures reduced bacterial counts about 3 logs at 0 s, while there was about a 5·5 log reduction at 60 s. Storage of all dried samples at either 4·0 ± 3·0°C or 23·0 ± 0·3°C for 15 days eliminate all detectable bacterial counts of both Salmonella spp. Safety of raw materials from animal origin like spray-dried porcine plasma (SDPP) may be a concern for the swine industry. Spray-drying process and postdrying storage are good inactivation steps to reduce the bacterial load of Salmonella choleraesuis and Salmonella typhimurium. For both Salmonella spp., spray-drying at 71°C or 80°C outlet temperatures reduced bacterial counts about 3 log at residence time (RT) 0 s, while there was about a 5.5 log reduction at RT 60 s. Storage of all dried samples at either 4.0 ± 3.0°C or 23.0 ± 0.3°C for 15 days was effective for eliminating detectable bacterial counts of both Salmonella spp. Significance and Impact of the Study: Safety of raw materials from animal origin like spray-dried porcine plasma (SDPP) may be a concern for the swine industry. Spray-drying process and postdrying storage are good inactivation steps to reduce the bacterial load of Salmonella choleraesuis and Salmonella typhimurium. For both Salmonella spp., spray-drying at 71°C or 80°C outlet temperatures reduced bacterial counts about 3 log at residence time (RT) 0 s, while there was about a 5.5 log reduction at RT 60 s. Storage of all dried samples at either 4.0 ± 3.0°C or 23.0 ± 0.3°C for 15 days was effective for eliminating detectable bacterial counts of both Salmonella spp
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