Radio loud far-infrared galaxies

The first results are presented of a multiwavelength study of Infrared Astronomy Satellite (IRAS) galaxies with excess radio emission. The sample was selected by cross correlating the IRAS Faint Source Survey, and the Point Source Catalogue with the Texas radio survey. Recent optical (imaging and spectroscopic) and radio (VLA) observations are discussed. These observations will be used to investigate possible connections between radio galaxy activity, star formation and galaxy interactions

Consequences of Dust in Metal-Rich HII Regions

Dust and associated depletion of heavy elements from the gas phase can modify the thermal properties of HII regions from the dust-free case, with significant consequences for the emergent optical spectrum. We present the results of theoretical calculations illustrating the effects of grains on the spectra of giant, extragalactic HII regions, with emphasis on high metallicity systems (i.e. solar and higher Z). Dust provides a simple explanation for the observational absence of pure Balmer-line spectra that are expected on theoretical grounds for dust-free, chemically enriched nebulae. Grains may also play a role in enhancements of forbidden-line emission observed in HII regions in the enriched nuclei of normal galaxies. In most cases, depletion introduces the strongest perturbations to the optical spectrum. Selective absorption of the ionizing continuum as well as heating by grain photoelectrons are important in some instances, however, and grain heating can be particularly important for enhancing emission in high-ionization lines. Allowing for depletion, the presence of dust is unlikely to introduce large errors in global metallicity indicators, although uncertainties in depletion factors coupled with the sensitivity of infrared cooling to electron density will make accurate calibrations difficult at high Z. The present calculations establish further that previous relative abundance analyses that fail to take into account dust effects in a self-consistent way (grain heating as well as depletion) may overestimate temperature gradients in high-Z nebulae, resulting in errors in relative abundances for different elements.Comment: 21 pages (AASTeX), plus 9 figures (uuencoded, gzipped, tar), to appear in ApJ, December 199

Nebular Properties and the Ionizing Radiation Field in the Galactic Center

Nebulosity in the central parsec of the Milky Way exhibits a low-ionization spectrum that has led previous analyses of this region to conclude that this material is photoionized by a relatively soft continuum. We have reanalyzed the infrared emission-line spectrum of te Galactic center in order to consider whether the data could actually be explained with photoionization by a relatively hard, yet dilute continuum, and additionally to constrain the properties of the reaiating plasma. We conclude that the composite infrared spectrum does not place strong restrictions on the nature of the ionizing continuum and that much of the ionized gas in the Galactic center is probably quite dense (≳ 105 cm-3). If the ionizing spectral energy distribution is, in fact, relatively hard, this material additionally must be highly clumped. Indications that the ionizing continuum is probably fairly soft are provided by radio recombination-line studies, however. Shocks are unlikely to be important for generating the observed nebulosity in light of the large far-infrared continuum luminosity of the central parsec. The infrared fine-structure spectrum provides mixed evidence forenhanced heavy-element abundances, and enrichment is probably limited to ≲ 2 times solar levels. We predict the optical spectrum of the Galactic center and conclude that it probably appears as an H II nucleus to external observers. Comparisons between the nucleus of the Milky Way and nuclei of external galaxies remain uncertain, however, since the parsec-scale metric apertures typically employed for studies of Galactic-center nebular emission are 1-2 orders of magnitude smaller than those in extragalactic measurements

Variability in Quasar Broad Absorption Line Outflows I. Trends in the Short-Term versus Long-Term Data

Broad absorption lines (BALs) in quasar spectra identify high velocity outflows that likely exist in all quasars and could play a major role in feedback to galaxy evolution. The variability of BALs can help us understand the structure, evolution, and basic physical properties of the outflows. Here we report on our first results from an ongoing BAL monitoring campaign of a sample of 24 luminous quasars at redshifts 1.2<z<2.9, focusing on C IV 1549 BAL variability in two different time intervals: 4 to 9 months (short-term) and 3.8 to 7.7 years (long-term) in the quasar rest-frame. We find that 39% (7/18) of the quasars varied in the short-term, whereas 65% (15/23) varied in the long-term, with a larger typical change in strength in the long-term data. The variability occurs typically in only portions of the BAL troughs. The components at higher outflow velocities are more likely to vary than those at lower velocities, and weaker BALs are more likely to vary than stronger BALs. The fractional change in BAL strength correlates inversely with the strength of the BAL feature, but does not correlate with the outflow velocity. Both the short-term and long-term data indicate the same trends. The observed behavior is most readily understood as a result of the movement of clouds across the continuum source. If the crossing speeds do not exceed the local Keplerian velocity, then the observed short-term variations imply that the absorbers are <6 pc from the central quasar.Comment: 14 pages, 7 figures, accepted for publication in MNRA