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 $2\times 10^{-10}$ and $6.5\times 10^{-10}$ and the number of light neutrino species is less than 3.9. Our analysis suggests that the $^4$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