Extraplanar Dust in the Edge-On Spiral NGC 891

We present high-resolution (<0.65") optical broad-band images of the edge-on Sb galaxy NGC 891 obtained with the WIYN 3.5-m telescope. These BVR images reveal a complex network of hundreds of dust absorbing structures far from the mid-plane of the galaxy. The dust structures have a wide range of morphologies and are clearly visible to |z|<1.5 kpc from the mid-plane. In this paper we discuss the general characteristics of the population of absorbing structures, as well as physical properties of 12 individual features. These 12 structures are characterised by N_H >10^21 cm^-2, with masses estimated to be more than 2x10^5 - 5x10^6 solar masses, assuming Galactic gas-to-dust relationships. The gravitational potential energies of the individual dust structures, given their observed heights and derived masses, lie in the range of 20-200x10^51 ergs. Rough number counts of extraplanar dust features suggest the mass of high-z gas associated with extraplanar dust in NGC 891 likely exceeds 2x10^8 solar masses, or ~2% of the total neutral ISM mass of the galaxy. We discuss several mechanisms which may produce high-z dust structures such as those seen in the images presented here. It is not yet known which of these mechanisms are primarily responsible for the extensive extraplanar dust structures seen in our images. The data presented are part of a larger program to search for and characterize off-plane dust structures in edge-on systems. (Abstract Abridged)Comment: To appear in the Astronomical Journal: 37 pages, Latex; 9 separate figures; the paper and high-resolution figures are also available at http://www.astro.wisc.edu/~howk/Papers/papers.htm

A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo

We describe a new method for determining total gas-phase abundances for the Galactic ISM with minimal ionization uncertainties. For sight lines toward globular clusters containing both UV-bright stars and radio pulsars, one can measure column densities of HI and several metal ions using UV absorption measurements and of H II using radio dispersion measurements, thereby minimizing ionization uncertainties. We apply this method to the globular cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total along this sight line, the highest measured fraction along a high-latitude pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) = -4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar system abundance, consistent with significant depletion. Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher, respectively, than our measurements. Ionization corrections to the gas-phase abundances measured in the standard way are, therefore, significant compared with the measurement uncertainties along this sight line. The systematic uncertainties associated with the uncertain contribution to the electron column density from ionized helium could raise these abundances by <+0.03 dex (+7%). [Abridged]Comment: To appear in the ApJ. 25 pages, including figures and tex

Evidence for Correlated Titanium and Deuterium Depletion in the Galactic ISM

Current measurements indicate that the deuterium abundance in diffuse interstellar gas varies spatially by a factor of ~4 among sightlines extending beyond the Local Bubble. One plausible explanation for the scatter is the variable depletion of D onto dust grains. To test this scenario, we have obtained high signal-to-noise, high resolution profiles of the refractory ion TiII along seven Galactic sightlines with D/H ranging from 0.65 to 2.1x10^-5. These measurements, acquired with the recently upgraded Keck/HIRES spectrometer, indicate a correlation between Ti/H and D/H at the >95% c.l. Therefore, our observations support the interpretation that D/H scatter is associated with differential depletion. We note, however, that Ti/H values taken from the literature do not uniformly show the correlation. Finally, we identify significant component-to-component variations in the depletion levels among individual sightlines and discuss complications arising from this behavior.Comment: 4 pages; Accepted to Astrophysical Journal Letter

Detection of Interstellar C_2 and C_3 in the Small Magellanic Cloud

We report the detection of absorption from interstellar C_2 and C_3 toward the moderately reddened star Sk 143, located in the near 'wing' region of the SMC, in optical spectra obtained with the ESO VLT/UVES. These detections of C_2 (rotational levels J=0-8) and C_3 (J=0-12) absorption in the SMC are the first beyond our Galaxy. The total abundances of C_2 and C_3 (relative to H_2) are similar to those found in diffuse Galactic molecular clouds -- as previously found for CH and CN -- despite the significantly lower average metallicity of the SMC. Analysis of the rotational excitation of C_2 yields an estimated kinetic temperature T_k ~ 25 K and a moderately high total hydrogen density n_H ~ 870 cm^-3 -- compared to the T_01 ~ 45 K and n_H ~ 85-300 cm^-3 obtained from H_2. The populations of the lower rotational levels of C_3 are consistent with an excitation temperature of about 34 K.Comment: accepted to MNRAS; 10 pages, 6 figure