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We have observed emission lines of [S iv] 10.51, H(7–6) 12.37, [Ne ii] 12.81, [Ne iii] 15.56 and [S iii] 18.71 μm in a number of extragalactic H ii regions with the Spitzer Space Telescope. A previous paper presented our data and analysis for the substantially face-on spiral galaxy M83. Here we report our results for the Local Group spiral galaxy M33. The nebulae selected cover a wide range of galactocentric radii (RG). The observations were made with the Infrared Spectrograph with the short wavelength, high-resolution module. The above set of five lines is observed cospatially, thus permitting a reliable comparison of the fluxes. From the measured fluxes, we determine the ionic abundance ratios including Ne++/Ne+, S3+/S++, and S++/Ne+ and find that there is a correlation of increasingly higher ionization with larger RG. By sampling the dominant ionization states of Ne (Ne+, Ne++) and S (S++, S3+) for H ii regions, we can estimate the Ne/H, S/H and Ne/S ratios. We find from linear least-squares fits that there is a decrease in metallicity with increasing RG: d log (Ne/H)/dRG=−0.058 ± 0.014 and d log (S/H)/dRG=−0.052 ± 0.021 dex kpc−1. There is no apparent variation in the Ne/S ratio with RG. Unlike our previous similar study of M83, where we conjectured that this ratio was an upper limit, for M33 the derived ratios are likely a robust indication of Ne/S. This occurs because the H ii regions have lower metallicity and higher ionization than those in M83. Both Ne and S are primary elements produced in α-chain reactions, following C and O burning in stars, making their yields depend very little on the stellar metallicity. Thus, it is expected that the Ne/S ratio remains relatively constant throughout a galaxy. The median (average) Ne/S ratio derived for H ii regions in M33 is 16.3 (16.9), just slightly higher than the Orion Nebula value of 14.3. The same methodology is applied to Spitzer observations recently published for three massive H ii regions: NGC 3603 (Milky Way), 30 Dor (LMC) and N 66 (SMC) as well as for a group of blue compact dwarf galaxies. We find median Ne/S values of 14.6, 11.4, 10.1, and 14.0, respectively. All of these values are in sharp contrast with the much lower ‘canonical’, but controversial, solar value of ∼5. A recent nucleosynthesis, galactic chemical evolution model predicts an Ne/S abundance of ∼9. Our observations may also be used to test the predicted ionizing spectral energy distribution of various stellar atmosphere models. We compare the ratio of fractional ionizations 〈Ne++〉/〈S++〉, 〈Ne++〉/〈S3+〉, and 〈Ne++〉/〈Ne+〉 versus 〈S3+〉/〈S++〉 with predictions made from our photoionization models using several of the state-of-the-art stellar atmosphere model grids. The trends of the ionic ratios established from the prior M83 study are remarkably similar, but continued to higher ionization with the present M33 objects.

We present Infrared Space Observatory (ISO) measurements of the far-infrared (FIR) fine-structure lines [S IV] (10.5 μm), [S III] (33.5 μm), [O III] (51.8 and 88.4 μm), [N III] (57.3 μm), [N II] (121.9 μm), [O I] (63.2 and 145.5 μm), and [C II] (157.7 μm) at five positions on the thermal ``Arched'' Filaments in the Galactic center region in order to investigate the influence of the massive, young Arches cluster on the interstellar medium. Serendipitous measurements of OH (119.3 μm) are also presented. Radio recombination line and ISO spectroscopic measurements provide strong evidence that the Arches cluster is responsible for the excitation of the Arched Filaments. The observations reported here were selected to provide measurements close to the Arches cluster and at uniformly increasing projected distances over the range 0-9 pc. Within the measurement uncertainties, the diagnostic line ratios most sensitive to excitation and least sensitive to density fluctuations, [O III]/[S III] and [S IV]/[S III], appear to decrease with projected distance from the Arches cluster. In agreement with other studies, this provides additional evidence that the cluster is the primary ionization source for the Arched Filaments. The total FIR luminosity also generally falls with projected distance from the Arches cluster. The best predictions from photoionization models for the rapid decrease in the measured line flux ratios with projected distance from the Arches cluster, however, are only accurate to a factor of 1.5, including allowance for additional offset along the line of sight. The measurements of lines formed predominantly in photodissociation regions are not well matched by current models. High-resolution spectra indicate gas moving at -70 km s-1 only at the location of the Arches cluster, significantly different from the predominant -30 km s-1 gas seen in the underlying molecular cloud.

We present J (1.2 microns) H (1-6 microns) K' (2.11 microns) and Br(gamma) (2.166 microns) images from four large regions within the central 40 pc of the Galaxy. Localized variations in the extinction, as determined by observations of the stellar population, are examined using the median H-K' color as a function of position within each region. As the value of the derived extinction from the stars is dependent upon the intrinsic magnitude of the assumed stellar type, the J-H vs. H-K' diagrams are first used to investigate the distribution of stellar types in the four regions. We find that there is a distinct OB population, contrary to earlier assumptions, with the ratio of K and M giants and supergiants to OB stars approximately twice that of the solar neighborhood. Although the on the scale of approx. 1 min. fluctuations in the extinction are on the order of A(sub V) approx. greater than 2, throughout the entire region the extinction varies from A(sub V) approx. greater than 25 to A(sub V) approx. less than 40. We also examine whether there is any variation in the extinction and stellar number density relative to the unusual radio features in these regions and do not find a significant correlation. Spectral imaging in Br(gamma) 2.166 microns emission shows a strong morphological correspondence between the 6 cm radio images and the diffuse Br(gamma) emission. By comparing the theoretical Br(gamma) flux derived from the radio flux using recombination theory, with our measured Br(gamma) flux, we obtain a second, independent, estimate of the extinction. We compare the two data sets and find that the extinction as derived from the stars is consistently greater, sometimes by a factor of two, than the value of the extinction derived from the Br(gamma) images. The derived extinction in various regions, however, is insufficient for any of these regions to be located behind the molecular clouds as previously observed in the Galactic Center, consistent with the theory that the observed radio emission is produced on the foreground surface of these clouds.

Although variations in elemental abundance ratios in the Milky Way Galaxy certainly exist, details remain uncertain, particularly in the inner Galaxy, where stars and H II regions in the Galactic plane are obscured optically. In this paper we revisit two previously studied, inner Galaxy H II regions: G333.6-0.2 and W43. We observed three new positions in G333.6-0.2 with the Kuiper Airborne Observatory and reobserved the central position with the Infrared Space Observatory's Long Wavelength Spectrometer in far-infrared lines of S++, N++, N+, and O++. We also added the N+ lines at 122 and 205 micron to the suite of lines measured in W43 by Simpson et al. (1995). The measured electron densities range from ~40 to over 4000 cm-3 in a single H II region, indicating that abundance analyses must consider density variations, since the critical densities of the observed lines range from 40 to 9000 cm-3. We propose a method to handle density variations and make new estimates of the S/H and N/H abundance ratios. We find that our sulfur abundance estimates for G333.6-0.2 and W43 agree with the S/H abundance ratios expected for the S/H abundance gradient previously reported by Simpson et al. The estimated N/H, S/H, and N/S ratios are the most reliable because of their small corrections for unseen ionization states (< 10%). We compute models of the two H II regions to estimate corrections for the other unseen ionization states. We discuss these predictions and conclude that only a few of the latest models adequately reproduce H II region observations, including the well-known, relatively-large observed Ne++/O++ ratios in low- and moderate-excitation H II regions., 29 pages, 6 figures, ApJ in press

Far-infrared lines of (N III) (57 microns), (O III) (52, 88 microns), (Ne III) (36 microns), and (S III) (19, 33 microns) have been measured in the H II regions G1.13 - 0.11, W31B, G23.95 + 0.15, G25.38 - 0.18, G29.96 - 0.02, W43, W51e, S156, S158, NGC 3576, NGC 3603, and G298.22-0.34. These observations were made with the facility Cryogenic Grating Spectrometer on the Kuiper Airborne Observatory to examine variations in abundances throughout the Galaxy. Previously published observations of G0.095 + 0.012, G333.60 - 0.21, G45.13 + 0.14A, K3-50, and M17 are also discussed. The giant H II region 30 Doradus in the Large Magellanic Cloud (LMC) was observed for comparison. Fluxes for (Ne II) (12.8 microns), (S IV) (10.5 microns), and the radio free-free continuum were collected from the literature for those sources. Electron densities were estimated from FIR line-pair ratios, and ionic abundances were estimated from the FIR line and radio fluxes. The excitation was estimated from the O(2+)/S(2+) ratio. Corrections for unseen ionization stages were calculated with the use of constnat-density H II region models. The validity and range of applicability of such semiempirical ionization correction schemes are discussed. The abundances with respect to hydrogen exhibit gradients with R(sub G) comparable to those previously measured for our Galaxy and for other galaxies. The overall gradients are d (log N/H)/dR = -0.10 +/- 0.02 dex/kpc, d(log Ne/H)/dR = -0.08 +/- 0.02 dex/kpc and d(log S/H)/dR = 0.07 +/- 0.02 dex/kpc. Compared to the Orion Nebula, the intermediate R(sub G) H II regions with 6 is less than R(sub G) is less than 11 kpc have similar or lower S/H and N/O ratios. The N/O ratios in the inner Galaxy are more than twice those observed in the Orion Nebula and intermediate R(sub G) H II regions. In fact, all the abundance ratios are as well or better fitted by a step fit with two levels than by a linear gradient. As has been noted in previous studies, the N/O ratio estimated from infrared observations of the doubly ionized N and O lines in H II regions is larger than the ratio estimated from optical observations of the singly ionized N and O lines. The Ne(2+)/O(2+) ratio is observed to be essentially constant over a wide range of excitation. This contradicts predictions of model H II regions calculated with the use of Local Thermodynamic Equilibrium (LTE) model stellar atmospheres. We conclude that these stellar atmospheres significantly underestimate the actual emergent fluxes for energies greater than 41 eV.

Detection of the 205 micrometer fine structure line of N II in the nearby starburst galaxy M82 is reported. The intensity wihin a 54 sec Full width at Half Maximum (FWHM) beam is (7.1 +/- 1.2) x 10(exp -19) W cm(exp -2). The ratio of the intensity of the recently detected 122 micrometer line to that of the 2.5 micrometer lines is = (4.2) (sup =1.6) (sub -1.2), significantly larger than the corresponding Galactic value of 1.6 +/- 0.3, reflecting higher electron densities within the central 850 pc of M82 in comparison to the Cosmic Background Explorer (COBE) Galactic average. The 2.5 micrometer line profile is consistent with other far-infrared fine-structure line profiles observed in M82. The observations are interpreted in the context of a two-component model of the ionized medium in M82. We find that a component of density as low as approximately 50 cm(exp -3) can comprise up to 70% of the total mass of warm ionized gas within the beam. The balance of the ionized mass is comprised of a component of density approximately greater than 100 cm(exp -3). A model is explored in which the dneser ionized medium constitute the boundaries of neutral surfaces which border the expanding hot plasma from the nuclear region.

Measurements of far-infrared lines and continuum from GO.095 + 0.012 and the E2 thermal 'arched' radio filament near the Galactic center are well explained by numerous embedded stars with T(sub eff) approximately 35,000 K. The structure of the filament and the apparent absence of hotter stars are qualitatively difficult to reconcile with this idea.

Systematic emission and absorption surveys have been conducted for high-velocity H I in the direction of 63 bright continuum sources. High-velocity emisison is detected along 18 of these 63 directions with a threshold about a factor of 2 better than previous, much more extensive surveys. No high-velocity absorption was found in any of the directions. The sky distribution of the high-velocity emission and the column density distribution agree with larger surveys. Detailed emission maps in the vicinity of each continuum source show that the variations in column density within a high-velocity cloud are similar to the variations between high-velocity clouds. A previously discovered high-velocity cloud at high galactic latitude, in the direction of part of the Virgo Cluster of galaxies, was mapped. This small cloud is moving away from the galactic plane at v greater than 30 km/s and is probably not self-gravitating. 36 refs.

From an analysis of galactic H I spectra acquired over a period of seven months, we investigate the accuracy of H I profiles obtained with the dual circular feed at Arecibo Observatory. The H I stray radiation contribution is estimated from a comparison of H I spectra toward the moon with H I spectra in the same direction when the moon is absent. The profile areas of corrected H I spectra are accurate to 3%, while individual features are accurate to 15%. At velocities where the H I emission is less than 3 K, the observed stray radiation is less than 0.5 K.

Observations of OH emission/absorption toward 19 low-Galactic-latitude background-continuum sources are reported. Data obtained at 1.665 and 1.667 GHz using the 12-m radio telescope at Arecibo Observatory during February-March 1984, are presented in extensive tables and graphs and characterized in detail, with reference to theoretical models. A total of 11 OH absorption features were detected toward nine of the sources, with an average of one feature of optical depth 0.02 or greater per 5 kpc of path length and an average column density of 1.6 x 10 to the 12th/sq cm (corrected to b = 90 deg).