VLT/VIMOS Observations of an Occulting Galaxy Pair: Redshifts and Effective Extinction Curve

We present VLT/VIMOS IFU observations of an occulting galaxy pair previously discovered in HST observations. The foreground galaxy is a low-inclination spiral disk, which causes clear attenuation features seen against the bright bulge and disk of the background galaxy. We find redshifts of $z=0.064 \pm0.003$ and z=0.065 for the foreground and background galaxy respectively. This relatively small difference does not rule out gravitational interaction between the two galaxies. Emission line ratios point to a star-forming, not AGN-dominated foreground galaxy. We fit the Cardelli, Clayton & Mathis (CCM) extinction law to the spectra of individual fibres to derive slope ($R_V$) and normalization ($A_V$). The normalization agrees with the HST attenuation map and the slope is lower than the Milky Way relation ($R_V<3.1$), which is likely linked to the spatial sampling of the disk. We speculate that the values of $R_V$ point to either coherent ISM structures in the disk larger than usual ($\sim9$ kpc) or higher starting values of $R_V$, indicative of recent processing of the dust. The foreground galaxy is a low stellar mass spiral ($M_* \sim 3 \times 10^9 M_\odot$) with a high dust content ($M_{\rm dust} \sim 0.5 \times 10^6 M_\odot$). The dust disk geometry visible in the HST image would explain the observed SED properties of smaller galaxies: a lower mean dust temperature, a high dust-to-stellar mass ratio but relatively little optical attenuation. Ongoing efforts to find occulting pairs with a small foreground galaxies will show how common this geometry is.Comment: 16 pages, 3 tables, 13 figures, accepted for publication in MNRA

Increasing Doctoral Student Completion Rates Within the College of Humanities and Sciences at Virginia Commonwealth University

Research on doctoral education primarily focuses on broad fields of study and general attrition. Often overlooked, is an examination of the student at each stage of their program and the challenges they face that can potentially lead to stopping out. This capstone project explored the doctoral student experience within the Department of Humanities and Sciences at Virginia Commonwealth University. A mixed method study via an online survey was utilized to gather data about the student experience on their path towards candidacy. Quantitative and qualitative data was analyzed through Chi-square tests and thematic coding to identify answers to each research question. This paper aims to inform students, faculty, and administrators, of common attrition points on a doctoral student’s path to completion. In addition, this research aims to use the data to identify interventions that will support doctoral students in their journey and increase retention rates

The Relationship of Hard X-ray and Optical Line Emission in Low Redshift Active Galactic Nuclei

In this paper we assess the relationship of the population of Active Galactic Nuclei (AGN) selected by hard X-rays to the traditional population of AGN with strong optical emission lines. First, we study the emission-line properties of a new hard X-ray selected sample of 47 local AGN (classified optically as both Type 1 and Type 2 AGN). We find that the hard X- ray (3-20 keV) and [OIII]$\lambda$5007 optical emission-line luminosities are well-correlated over a range of about four orders-of-magnitude in luminosity (mean luminosity ratio 2.15 dex with a standard deviation of $\sigma$ = 0.51 dex). Second, we study the hard X-ray properties of a sample of 55 local AGN selected from the literature on the basis of the flux in the [OIII] line. The correlation between the hard X-ray (2-10 keV) and [OIII] luminosity for the Type 1 AGN is consistent with what is seen in the hard X-ray selected sample. However, the Type 2 AGN have a much larger range in the luminosity ratio, and many are very weak in hard X-rays (as expected for heavily absorbed AGN). We then compare the hard X-ray (3-20 keV) and [OIII] luminosity functions of AGN in the local universe. These have similar faint-end slopes with a luminosity ratio of 1.60 dex (0.55 dex smaller than the mean value for individual hard X-ray selected AGN). We conclude that at low redshift, selection by narrow optical emission- lines will recover most AGN selected by hard X-rays (with the exception of BL Lac objects). However, selection by hard X-rays misses a significant fraction of the local AGN population with strong emission lines

Induced star formation in interacting galaxies

Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems

Structure and Evolution of the Opacity of Spiral Disks

The opacity of a spiral disk due to dust absorption influences every measurement we make of it in the UV and optical. Two separate techniques directly measure the total absorption by dust in the disk: calibrated distant galaxy counts and overlapping galaxy pairs. The main results from both so far are a semi-transparent disk with more opaque arms and a relation between surface brightness and disk opacity. In the Spitzer era, spectral energy distribution models of spiral disks add a new perspective on the role of dust in spiral disks. Combined with the overall opacity from galaxy counts, we can derive a typical optical depth of the dusty interstellar medium clouds: 0.4 which implies a size of ∼ 60 pc. Current work on galaxy counts is in the ACS fields of M 51, M 101 and M 81. Occulting galaxies offer the possibility of probing the history of disk opacity from higher redshift occulting pairs. Evolution in disk opacity could influence distance measurements (SN1a, Tully-Fisher relation). Here, we present first results from spectroscopically selected occulting pairs in the Sloan Digital Sky Survey. The redshift range for this sample is limited but does offer a first insight into disk opacity evolution as well as a reference for higher redshift measurements

An Atlas of Warm AGN and Starbursts from the IRAS Deep Fields

We present 180 AGN candidates based on color selection from the IRAS slow-scan deep observations, with color criteria broadened from the initial Point-Source Catalog samples to include similar objects with redshifts up to z=1 and allowing for two-band detections. Spectroscopic identifications have been obtained for 80 (44%); some additional ones are secure based on radio detections or optical morphology, although yet unobserved spectroscopically. These spectroscopic identifications include 13 Sy 1 galaxies, 17 Sy 2 Seyferts, 29 starbursts, 7 LINER systems, and 13 emission-line galaxies so heavily reddened as to remain of ambiguous classification. The optical magnitudes range from R=12.0-20.5; counts suggest that incompleteness is important fainter than R=15.5. Redshifts extend to z=0.51, with a significant part of the sample at z>0.2. The sample includes slightly more AGN than star-forming systems among those where the spectra contain enough diagnostic feature to make the distinction. The active nuclei include several broad-line objects with strong Fe II emission, and composite objects with the absorption-line signatures of fading starbursts. These AGN with warm far-IR colors have little overlap with the "red AGN" identified with 2MASS; only a single Sy 1 was detected by 2MASS with J-K > 2. Some reliable IRAS detections have either very faint optical counterparts or only absorption-line galaxies, potentially being deeply obscured AGN. The IRAS detections include a newly identified symbiotic star, and several possible examples of the "Vega phenomenon", including dwarfs as cool as type K. Appendices detail these candidate stars, and the optical-identification content of a particularly deep set of high-latitude IRAS scans (probing the limits of optical identification from IRAS data alone).Comment: ApJ Suppl, in press. Figures converted to JPEG/GIF for better compression; PDF with full-resolution figures available before publication at http://www.astr.ua.edu/keel/aoagn.pd

Gene expression differs in susceptible and resistant amphibians exposed to Batrachochytrium dendrobatidis.

Chytridiomycosis, the disease caused by the fungal pathogen Batrachochytrium dendrobatidis (Bd), has devastated global amphibian biodiversity. Nevertheless, some hosts avoid disease after Bd exposure even as others experience near-complete extirpation. It remains unclear whether the amphibian adaptive immune system plays a role in Bd defence. Here, we describe gene expression in two host species-one susceptible to chytridiomycosis and one resistant-following exposure to two Bd isolates that differ in virulence. Susceptible wood frogs (Rana sylvatica) had high infection loads and mortality when exposed to the more virulent Bd isolate but lower infection loads and no fatal disease when exposed to the less virulent isolate. Resistant American bullfrogs (R. catesbeiana) had high survival across treatments and rapidly cleared Bd infection or avoided infection entirely. We found widespread upregulation of adaptive immune genes and downregulation of important metabolic and cellular maintenance components in wood frogs after Bd exposure, whereas American bullfrogs showed little gene expression change and no evidence of an adaptive immune response. Wood frog responses suggest that adaptive immune defences may be ineffective against virulent Bd isolates that can cause rapid physiological dysfunction. By contrast, American bullfrogs exhibited robust resistance to Bd that is likely attributable, at least in part, to their continued upkeep of metabolic and skin integrity pathways as well as greater antimicrobial peptide expression compared to wood frogs, regardless of exposure. Greater understanding of these defences will ultimately help conservationists manage chytridiomycosis

II Zwicky 23 and Family

II Zwicky 23 (UGC 3179) is a luminous, nearby compact narrow emission line starburst galaxy with blue optical colors and strong emission lines. We present a photometric and morphological study of II Zw 23 and its interacting companions using data obtained with the WIYN 3.5-m telescope in Kitt Peak, Arizona. II Zwicky 23 has a highly disturbed outer structure with long trails of debris that may be feeding tidal dwarfs. Its central regions appear disky, a structure that is consistent with the overall rotation pattern observed in the H-alpha velocity field measured from Densepak observations obtained with WIYN. We discuss the structure of II Zwicky 23 and its set of companions and possible scenarios of debris formation in this system.Comment: 5 pages, 2 figures. To appear in the proceedings of ESO Astrophysics Symposia: "Groups of Galaxies in the Nearby Universe", eds. I. Saviane, V. Ivanov, J. Burissova (Springer