45 research outputs found
Assessing Big-Bang Nucleosynthesis
Systematic uncertainties in the light-element abundances and their evolution
make a rigorous statistical assessment difficult. However, using Bayesian
methods we show that the following statement is robust: the predicted and
measured abundances are consistent with 95\% credibility only if the
baryon-to-photon ratio is between and
and the number of light neutrino species is less than 3.9. Our analysis
suggests that the He abundance may have been systematically underestimated.Comment: 7 pages, LaTeX(2.09), 6 postscript figures (attached). A postscript
version with figures can be found at
ftp://astro.uchicago.edu/pub/astro/copi/assessing_BBN . (See the README file
for details
Efficiency of the dynamical mechanism
The most extreme starbursts occur in galaxy mergers, and it is now
acknowledged that dynamical triggering has a primary importance in star
formation. This triggering is due partly to the enhanced velocity dispersion
provided by gravitational instabilities, such as density waves and bars, but
mainly to the radial gas flows they drive, allowing large amounts of gas to
condense towards nuclear regions in a small time scale. Numerical simulations
with several gas phases, taking into account the feedback to regulate star
formation, have explored the various processes, using recipes like the Schmidt
law, moderated by the gas instability criterion. May be the most fundamental
parameter in starbursts is the availability of gas: this sheds light on the
amount of external gas accretion in galaxy evolution. The detailed mechanisms
governing gas infall in the inner parts of galaxy disks are discussed.Comment: 6 pages, 3 figures, to be published in "Starbursts - From 30 Doradus
to Lyman break galaxies", ed. R. de Grijs and R. Gonzalez-Delgad
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
Toward an internally consistent astronomical distance scale
Accurate astronomical distance determination is crucial for all fields in
astrophysics, from Galactic to cosmological scales. Despite, or perhaps because
of, significant efforts to determine accurate distances, using a wide range of
methods, tracers, and techniques, an internally consistent astronomical
distance framework has not yet been established. We review current efforts to
homogenize the Local Group's distance framework, with particular emphasis on
the potential of RR Lyrae stars as distance indicators, and attempt to extend
this in an internally consistent manner to cosmological distances. Calibration
based on Type Ia supernovae and distance determinations based on gravitational
lensing represent particularly promising approaches. We provide a positive
outlook to improvements to the status quo expected from future surveys,
missions, and facilities. Astronomical distance determination has clearly
reached maturity and near-consistency.Comment: Review article, 59 pages (4 figures); Space Science Reviews, in press
(chapter 8 of a special collection resulting from the May 2016 ISSI-BJ
workshop on Astronomical Distance Determination in the Space Age
HI properties of Low Surface Brightness dwarf and blue compact dwarf galaxies
Original paper can be found at: http://www.astrosociety.org/pubs/cs/156-194.html--Copyright Astronomical Society of the PacificWe present high resolution (~15") VLA HI observations of one LSB dwarf galaxy and 5 Blue Compact Dwarfs. Previous works have emphasized the difference in the peak column density of the HI between the two types of galaxy. However, the peak column density is typically only a factor of 2-4 greater in BCDs than in LSB dwarfs, suggesting that the difference between the two is one of degree, not quality. The HI morphologies of the two types are often quite similar
Gas and Star Formation in Dwarf Galaxies
Resolved observations of nearby galaxies help us to understand the relationship between gas and star formation in dwarf galaxies. The star formation histories recovered from their fossil records place constraints on how much gas dwarf galaxies could have acquired at early times. Are dwarf galaxies able to accrete a significant amount of gas at later times, or is the bulk of the gas in place once the dwarf galaxy establishes its gravitational identity? In the current epoch, we have a wealth of observations which allow us to study the process of star formation. Spatially resolved, recent star formation histories allow us to ask fundamental questions such as: What conditions are required for star formation? What is the impact of the star formation on the structure of the ISM? Does star formation heat the ISM and consequently suppress future star formation, or does it compress the ISM and lead to more star formation? Does understanding star formation at z = 0 help us to understand star formation at higher redshifts
Brackett gamma observations and extinction in giant H II regions in M 101
Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe
A survey of H II galaxies to detect H I companion objects
Original article can be found at: http://adsabs.harvard.edu/abs/ Copyright American Astronomical Society [Full text of this article is not available in the UHRA]Peer reviewe
A Survey of Low Surface Brightness Dwarf Galaxies to Detect H i-rich Companions
Original article can be found at: http://adsabs.harvard.edu/ Copyright American Astronomical Society. DOI: 10.1086/192358 [Full text of this article is not available in the UHRA]We have conducted a survey for H I-rich companions in the vicinity of low surface brightness (LSB) dwarf galaxies, objects that are characterized by low rates of massive star formation. This survey complements our earlier survey of dwarf galaxies exhibiting high star formation rates (H II galaxies). Four of the 17 LSB dwarfs in the sample have nearby, H I-rich companions, and a total of five companions were detected. The companion frequency is therefore {rho} = 0.24, and we determine a lower limit on the true rate of 0.08. This is a much lower rate of occurrence than was detected around the H II galaxies, {rho} = 0.57, which implies a lower limit of {rho} > 0.37. Because the two surveys were carried out under nearly identical circumstances, this difference is genuine and not the result of any relative bias between the two samples. That H II galaxies have companions more than twice as often as LSB dwarfs provides evidence that the bursts of massive star formation in H II galaxies are being triggered by galaxy-galaxy interactions.Peer reviewe