Robert H. Rubin

Far-infrared spectroscopy of normal galaxies: Physical conditions in the interstellar medium

The most important cooling lines of the neutral interstellar medium (ISM) lie in the far-infrared (FIR). We present measurements by the Infrared Space Observatory Long Wavelength Spectrometer of seven lines from neutral and ionized ISM of 60 normal, star-forming galaxies. The galaxy sample spans a range in properties such as morphology, FIR colors (indicating dust temperature), and FIR/blue ratios (indicating star formation activity and optical depth). In two-thirds of the galaxies in this sample, the [C II] line flux is proportional to FIR dust continuum. The other one-third show a smooth decline in L[C II]/LFIR with increasing Fν(60 μm)/Fν(100 μm) and LFIR/LB, spanning a range of a factor of more than 50. Two galaxies at the warm and active extreme of the range have L[C II]/LFIR < 2 × 10-4 (3 σ upper limit). This is due to increased positive grain charge in the warmer and more active galaxies, which leads to less efficient heating by photoelectrons from dust grains. The ratio of the two principal photodissociation region (PDR) cooling lines L[O I]/L[C II] shows a tight correlation with Fν(60 μm)/Fν(100 μm), indicating that both gas and dust temperatures increase together. We derive a theoretical scaling between [N II] (122 μm) and [C II] from ionized gas and use it to separate [C II] emission from neutral PDRs and ionized gas. Comparison of PDR models of Kaufman et al. with observed ratios of (1) L[O I]/L[C II] and (L[C II] + L[O I])/LFIR and (2) L[O I]/LFIR and Fν(60 μm)/Fν(100 μm) yields far-UV flux G0 and gas density n. The G0 and n values estimated from the two methods agree to better than a factor of 2 and 1.5, respectively, in more than half the sources. The derived G0 and n correlate with each other, and G0 increases with n as G0 ∝ nα, where α ≈ 1.4 . We interpret this correlation as arising from Stromgren sphere scalings if much of the line and continuum luminosity arises near star-forming regions. The high values of PDR surface temperature (270-900 K) and pressure (6 × 104-1.5 × 107 K cm-3) derived also support the view that a significant part of grain and gas heating in the galaxies occurs very close to star-forming regions. The differences in G0 and n from galaxy to galaxy may be due to differences in the physical properties of the star-forming clouds. Galaxies with higher G0 and n have larger and/or denser star-forming clouds.

Far-infrared lines from H II regions: Abundance variations in the galaxy

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.

Infrared Space Observatory Measurements of [C II] Line Variations in Galaxies

We report measurements of the [C II] fine-structure line at 157.714 ?m in 30 normal star-forming galaxies with the Long Wavelength Spectrometer (LWS) on the Infrared Space Observatory (ISO). The ratio of the line to total far-infrared (FIR) luminosity, LC II/LFIR, measures the ratio of the cooling of gas to that of dust, and thus the efficiency of the grain photoelectric heating process. This ratio varies by more than a factor of 40 in the current sample. About two-thirds of the galaxies have LC II/LFIR ratios in the narrow range of (2-7) ? 10 -->?3. The other one-third show trends of decreasing LC II/LFIR with increasing dust temperature, as measured by the flux ratio of infrared emission at 60 and 100 ?m, F?(60 ?m)/F?(100 ?m), and with increasing star formation activity, measured by the ratio of FIR and blue-band luminosity, LFIR/L -->B. We also find three FIR-bright galaxies that are deficient in the [C II] line, which is undetected with 3 ? upper limits of LC II/LFIR ?4. The trend in the LC II/LFIR ratio with the temperature of dust and with star formation activity may be due to decreased efficiency of photoelectric heating of gas at high UV radiation intensity as dust grains become positively charged, decreasing the yield and the energy of the photoelectrons. The three galaxies with no observed photodissociation region lines have among the highest LFIR/L -->B and F?(60 ?m)/F?(100 ?m) ratios. Their lack of [C II] lines may be due to a continuing trend of decreasing LC II/LFIR with increasing star formation activity and dust temperature seen in one-third of the sample with warm IRAS colors. In that case, the upper limits on LC II/LFIR imply a ratio of UV flux to gas density of G -->0/n>10 cm -->3 (where G -->0 is in units of the local average interstellar field). The low LC II/LFIR ratio could also be due to either weak [C II], owing to self-absorption, or a strong FIR continuum from regions weak in [C II], such as dense H II regions or plasma ionized by hard radiation of active galactic nuclei. The mid-infrared and radio images of these galaxies show that most of the emission comes from a compact nucleus. CO and H I are detected in these galaxies, with H I seen in absorption toward the nucleus.

Nebular properties from far-infrared spectrosopy

We describe a semiempirical methodology-based on measurements of far-infrared (FIR) lines-that yields information on electron densities in regions where various ionic species exist, effective temperatures (T(sub eff)) for stars ionizing H II regions, and gas-phase heavy element abundances. Although this capability has long been available via optical data, the special features of FIR lines-relative insensitivity to extinction and electron temperature variations-extend the analysis ability. Several line ratios serve as diagnostics of electron density, N(sub e), probing different ionization conditions and different density regimes. The more N(sub e)-diagnostic observations made, the more reliable will be the deciphering of the actual variation in density throughout a nebula. A method to estimate T(sub eff) from the FIR (N III)/(N II) line ratio requires that the nebula be ionization bounded and that substantially all of the flux from the revevant lines be observed. However, to estimate T(sub eff) by a second method that uses the ratio of FIR (S III)/(O III) lines, an ionization-bounded nebula is a sufficient, but not necessary, condition. These restrictions are unnecessary for estimating densities and heavy element abundances. We show that a fairly general determination of metallicity, via the S/H ratio, may be made for H II regions with observations of just two lines-(S III) 19 micron and a hydrogen recombination line (or appropriate substitute). These techniques are applied to recent FIR data for the G333.6-0.2 H II region, including application to the recently measured (N II) 122 and 205 micron lines.

ISO Mid-Infrared Observations of Normal Star-forming Galaxies: The Key Project Sample*

We present mid-infrared maps and preliminary analysis for 61 galaxies observed with the ISOCAM instrument aboard the Infrared Space Observatory. Many of the general features of galaxies observed at optical wavelengths?spiral arms, disks, rings, and bright knots of emission?are also seen in the mid-infrared, except the prominent optical bulges are absent at 6.75 and 15 ?m. In addition, the maps are quite similar at 6.75 and 15 ?m, except for a few cases where a central starburst leads to lower I?(6.75 ?m)/I?(15 ?m) ratios in the inner region. We also present infrared flux densities and mid-infrared sizes for these galaxies. The mid-infrared color I?(6.75 ?m)/I?(15 ?m) shows a distinct trend with the far-infrared color I?(60 ?m)/I?(100 ?m). The quiescent galaxies in our sample [I?(60 ?m)/I?(100 ?m) 0.6] show I?(6.75 ?m)/I?(15 ?m) near unity, whereas this ratio drops significantly for galaxies with higher global heating intensity levels. Azimuthally averaged surface brightness profiles indicate the extent to which the mid-infrared flux is centrally concentrated, and provide information on the radial dependence of mid-infrared colors. The galaxies are mostly well resolved in these maps: almost half of them have <10% of their flux in the central resolution element. A comparison of optical and mid-infrared isophotal profiles indicates that the flux at 4400 ? near the optical outskirts of the galaxies is approximately 8 (7) times that at 6.75 ?m (15 ?m), comparable to observations of the diffuse quiescent regions of the Milky Way.

The Interstellar Medium of Star-forming Irregular Galaxies: The View with ISO

We present mid-infrared imaging and far-infrared (FIR) spectroscopy of five IBm galaxies observed by ISO as part of our larger study of the interstellar medium of galaxies. Most of the irregulars in our sample are very actively forming stars, and one is a starburst system. Thus, most are not typical Im galaxies. The mid-infrared imaging was in a band centered at 6.75 μm that is dominated by polycyclic aromatic hydrocarbons (PAHs) and in a band centered at 15 μm that is dominated by small dust grains. The spectroscopy of three of the galaxies includes [C II] λ158 μm and [O I] λ63 μm, important coolants of photodissociation regions (PDRs), and [O III] λ88 μm and [N II] λ122 μm, which come from ionized gas. [O I] λ145 μm and [O III] λ52 μm were measured in one galaxy as well. These data are combined with PDR and H II region models to deduce properties of the interstellar medium of these galaxies. We find a decrease in PAH emission in our irregulars relative to small grain, FIR, and Hα emissions for increasing FIR color temperature, which we interpret as an increase in the radiation field due to star formation resulting in a decrease in PAH emission. The f15/fHα ratio is constant for our irregulars, and we suggest that the 15 μm emission in these irregulars is being generated by the transient heating of small dust grains by single-photon events, possibly Lyα photons trapped in H II regions. The low f15/fHα ratio, as well as the high f/f15 ratio, in our irregulars compared to spirals may be due to the lower overall dust content, resulting in fewer dust grains being, on average, near heating sources. We find that, as in spirals, a large fraction of the [C II] emission comes from PDRs. This is partly a consequence of the high average stellar effective temperatures in these irregulars. However, our irregulars have high [C II] emission relative to FIR, PAH, and small grain emission compared to spirals. If the PAHs that produce the 6.75 μm emission and the PAHs that heat the PDR are the same, then the much higher f/f6.75 ratio in irregulars would require that the PAHs in irregulars produce several times more heat than the PAHs in spirals. Alternatively, the carrier of the 6.75 μm feature tracks, but contributes only a part of, the PDR heating, that is due mostly to small grains or other PAHs. In that case, our irregulars would have a higher proportion of the PAHs that heat the PDRs compared to the PAHs that produce the 6.75 μm feature. The high f/f ratio may indicate a smaller solid angle of optically thick PDRs outside the H II regions compared to spirals. The very high L/LCO ratios among our sample of irregulars could be accounted for by a very thick [C II] shell around a tiny CO core in irregulars, and PDR models for one galaxy are consistent with this. The average densities of the PDRs and far-ultraviolet stellar radiation fields hitting the PDRs are much higher in two of our irregulars than in most normal spirals; the third irregular has properties like those in typical spirals. We deduce the presence of several molecular clouds in each galaxy with masses much larger than typical GMCs.

The [Fe IV] discrepancy : Constraining the iron abundances in nebulae

We study the current discrepancy between the model-predicted and measured concentrations of Fe++ and Fe+3 in ionized nebulae. We calculate a set of photoionization models, updated with the atomic data relevant to the problem, and compare their results with those derived for the available nebulae where both [Fe III] and [Fe IV] lines have been measured. Our new model results are closer to the measured values than the results of previous calculations, but a discrepancy remains. This discrepancy translates into an uncertainty in the derived Fe abundances of a factor of up to ~4. We explore the possible causes of this discrepancy and find that errors in the Fe atomic data may be the most likely explanation. The discrepancy can be fully accounted for by any of the following changes: (1) an increase by a factor of ~10 in the recombination rate (radiative plus dielectronic, or charge transfer) for Fe+3, (2) an increase by a factor of 2-3 in the effective collision strengths for Fe++, or (3) a decrease by a factor of 2-3 in the effective collision strengths for Fe+3. We derive the Fe abundances implied by these three explanations and use the results to constrain the degree of depletion of Fe in our sample nebulae. The Galactic H II regions and planetary nebulae are found to have high depletion factors, with less than 5% of their Fe atoms in the gas phase. The extragalactic H II regions (LMC 30 Doradus, SMC N88A, and SBS 0335-052) have somewhat lower depletions. The metal-deficient blue compact galaxy SBS 0335-052 could have from 13% to 40% of Fe in the gas phase. The depletions derived for the different objects define a trend of increasing depletion at higher metallicities.

High-Resolution Spectroscopy of Faint Emission Lines in the Orion Nebula

We present high-resolution spectrophotometric observations of the Orion Nebula, made with the Cassegrain echelle spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). The resolution and signal-to-noise ratio make it possible to identify 444 emission lines in the 3498-7468 ? range, down to 104 times fainter than H?. We present a detailed atlas of these emission lines along with an analysis of the associated errors. This data set is used to study the velocity field in the Orion Nebula. The forbidden lines split into two distinct groups. The low-ionization group has ions with an ionization potential less than 20 eV. Lines of these ions, [O I], [N I], [Ni II], and [Fe II], have recession velocities, relative to the hydrogen lines, of +10 to +15 km s-1. There is a sharp change to the second, high-ionization group, which includes lines of ions with ionization potentials larger than 20 eV, namely, [S II], [O II], [N II], and [Fe III]. These lines have velocities around +3 km s-1, with a slight trend of decreasing velocity with the increasing ionization potential. This is consistent with previously proposed dynamical models in which lines of ions with different ionization potentials originate at different distances from the ionizing stars. Significant acceleration appears to take place across the narrow region where Fe2+ exists. Across this region the gas receives an acceleration of ~ 2.5 ? 10-5 cm s-2. This provides a constraint on hydrodynamical models. We set a limit He II 4686/H? < 7 ? 10-5, which in turn sets a limit to the intensity of the ionizing continuum at energies higher than 54 eV. Modern stellar atmospheres predict a continuum that is far stronger than is present in the region near ?1 Ori C.

Temperature variations from Hubble Space Telescope spectroscopy of the Orion Nebula

We present HST/STIS long-slit spectroscopy of NGC 1976. Our goal is to measure the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the fractional mean-square Te variation (t 2 ) across the nebula. We also measure the intrinsic line ratio [N II] 5756/6585 in order to estimate Te and t 2 in the N + region. The interpretation of the [N II] data is not as clear cut as the [O III] data because of a higher sensitivity to knowledge of the electron density as well as a possible contribution to the [N II] 5756 emission by recombination (and cascading). We present results from binning the data along the various slits into tiles that are 0.5 ′′ square (matching the slit width). The average [O III] temperature for our four HST/STIS slits varies from 7678 K to 8358 K; t 2 varies from 0.00682 to at most 0.0176. For our preferred solution, the average [N II] temperature for each of the four slits varies from 9133 K to 10232 K; t 2 varies from 0.00584 to 0.0175. The measurements of Te reported here are an average along each line of sight. Therefore, despite finding remarkably low t 2 , we cannot rule out significantly larger temperature fluctuations along the line of sight. The result that the average [N II] Te exceeds the average [O III] Te confirms what has been previously found for Orion and what is expected on theoretical grounds. Observations of the proplyd P159-350 indicate: large local extinction associated; ionization stratification consistent with external ionization by θ 1 Ori C; and indirectly, evidence of high electron density.

Physical Conditions in Low-Ionization Regions of the Orion Nebula

ABSTRACTWe reexamine the spectroscopic underpinnings of recent suggestions that [O I ] and[Fe II ] lines from the Orion H region are produced in gas where the iron-carryinggrains have been destroyed and the electron density is surprisingly high. Our newobservations show that previous detections of [O I ] 5577 were dominated by telluricemission. Our limits are consistent with a moderate density (≈ 10 4 cm −3 ) photoionizedgas. We show that a previously proposed model of the Orion H II region reproducesthe observed [O I ] and [Fe II ] spectrum. These lines are fully consistent with formationin a dusty region of moderate density.Subject headings: ISM: H II regions — ISM: abundances — ISM: atoms — ISM:individual (Orion Nebula)1. IntroductionThe Orion Nebula is the defining blister H II region (Zuckerman 1973; Balick, Gammon, &Hjellming 1974). A star cluster ionizes the skin of the molecular cloud, causing an expansion away 1 Based in part on observations made with the NASA/ESAHubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555