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

Identification of dusty massive stars in star-forming dwarf irregular galaxies in the Local Group with mid-IR photometry

Increasing the statistics of spectroscopically confirmed evolved massive stars in the Local Group enables the investigation of the mass loss phenomena that occur in these stars in the late stages of their evolution. Aims. We aim to complete the census of luminous mid-IR sources in star-forming dwarf irregular (dIrr) galaxies of the Local Group. To achieve this we employed mid-IR photometric selection criteria to identify evolved massive stars, such as red supergiants (RSGs) and luminous blue variables (LBVs), by using the fact that these types of stars have infrared excess due to dust. Methods. The method is based on 3.6 μm and 4.5 μm photometry from archival Spitzer Space Telescope images of nearby galaxies. We applied our criteria to four dIrr galaxies: Pegasus, Phoenix, Sextans A, and WLM, selecting 79 point sources that we observed with the VLT/FORS2 spectrograph in multi-object spectroscopy mode. Results. We identified 13 RSGs, of which 6 are new discoveries, as well as two new emission line stars, and one candidate yellow supergiant. Among the other observed objects we identified carbon stars, foreground giants, and background objects, such as a quasar and an early-type galaxy that contaminate our survey. We use the results of our spectroscopic survey to revise the mid-IR and optical selection criteria for identifying RSGs from photometric measurements. The optical selection criteria are more efficient in separating extragalactic RSGs from foreground giants than mid-IR selection criteria, but the mid-IR selection criteria are useful for identifying dusty stars in the Local Group. This work serves as a basis for further investigation of the newly discovered dusty massive stars and their host galaxies. © 2015 ESO

Determining the Timescale over Which Stellar Feedback Drives Turbulence in the Interstellar Medium: A Study of Four Nearby Dwarf Irregular Galaxies

Stellar feedback is fundamental to the modeling of galaxy evolution, as it drives turbulence and outflows in galaxies. Understanding the timescales involved are critical for constraining the impact of stellar feedback on the interstellar medium. We analyzed the resolved star formation histories along with the spatial distribution and kinematics of the atomic and ionized gas of four nearby star-forming dwarf galaxies (NGC 4068, NGC 4163, NGC 6789, and UGC 9128) to determine the timescales over which stellar feedback drives turbulence. The four galaxies are within 5 Mpc and have a range of properties including current star formation rates of 0.0005-0.01 Myr-1, log(M ∗/M) between 7.2 and 8.2, and log(M H i /M) between 7.2 and 8.3. Their color-magnitude diagram derived star formation histories over the past 500 Myr were compared to their atomic and ionized gas velocity dispersion and H i energy surface densities as indicators of turbulence. The Spearman's rank correlation coefficient was used to identify any correlations between their current turbulence and their past star formation activity on local scales (∼400 pc). The strongest correlation found was between the H i turbulence measures and the star formation rate 100-200 Myr ago. This suggests a coupling between the star formation activity and atomic gas on this timescale. No strong correlation between the ionized gas velocity dispersion and the star formation activity between 5 and 500 Myr ago was found. The sample and analysis are the foundation of a larger program aimed at understanding the timescales over which stellar feedback drives turbulence. © 2022. The Author(s). Published by the American Astronomical Society.Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Galaxy Properties at the Faint End of the H i Mass Function

The Survey of H i in Extremely Low-mass Dwarfs (SHIELD) includes a volumetrically complete sample of 82 gas-rich dwarfs with {M}_{{\rm{H}}\,{\rm\small{I}}}\lesssim {10}^{7.2} {M}_{\odot } selected from the ALFALFA survey. We are obtaining extensive follow-up observations of the SHIELD galaxies to study their gas, stellar, and chemical content, and to better understand galaxy evolution at the faint end of the H i mass function. Here, we investigate the properties of 30 SHIELD galaxies using Hubble Space Telescope imaging of their resolved stars and Westerbork Synthesis Radio Telescope observations of their neutral hydrogen. We measure tip of the red giant branch (TRGB) distances, star formation activity, and gas properties. The TRGB distances are up to 4 greater than estimates from flow models, highlighting the importance of velocity-independent distance indicators in the nearby universe. The SHIELD galaxies are in underdense regions, with 23% located in voids; one galaxy appears paired with a more massive dwarf. We quantify galaxy properties at low masses including stellar and H i masses, star formation rate (SFRs), specific SFRs, star formation efficiencies, birth-rate parameters, and gas fractions. The lowest-mass systems lie below the mass thresholds where stellar mass assembly is predicted to be impacted by reionization. Even so, we find the star formation properties follow the same trends as higher-mass gas-rich systems, albeit with a different normalization. The H i disks are small (\langle r \rangle \lt 0.7\,{\rm{kpc}}), making it difficult to measure the H i rotation using standard techniques; we develop a new methodology and report the velocity extent, and its associated spatial extent, with robust uncertainties. © 2021. The American Astronomical Society. All rights reserved.Immediate accessThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Comparing m31 and milky way satellites: The extended star formation histories of andromeda

We present the first comparison between the lifetime star formation histories (SFHs) of M31 and Milky Way (MW) satellites. Using the Advanced Camera for Surveys on board the Hubble Space Telescope, we obtained deep optical imaging of Andromeda II (And II; MV = -12.0; log(M /M 99) 3c 6.7) and Andromeda XVI (And XVI; MV = -7.5; log(M /M 99) 3c 4.9) yielding color-magnitude diagrams that extend at least 1 mag below the oldest main-sequence turnoff, and are similar in quality to those available for the MW companions. And II and And XVI show strikingly similar SFHs: both formed 50%-70% of their total stellar mass between 12.5 and 5 Gyr ago (z 3c 5-0.5) and both were abruptly quenched 3c5 Gyr ago (z 3c 0.5). The predominance of intermediate age populations in And XVI makes it qualitatively different from faint companions of the MW and clearly not a pre-reionization fossil. Neither And II nor And XVI appears to have a clear analog among MW companions, and the degree of similarity in the SFHs of And II and And XVI is not seen among comparably faint-luminous pairs of MW satellites. These findings provide hints that satellite galaxy evolution may vary substantially among hosts of similar stellar mass. Although comparably deep observations of more M31 satellites are needed to further explore this hypothesis, our results underline the need for caution when interpreting satellite galaxies of an individual system in a broader cosmological context. \ua9 2014. The American Astronomical Society. All rights reserved.Peer reviewed: YesNRC publication: Ye