Infrared emission and excitation in LMC HII regions

The infrared excess (IRE) of the Large Magellanic Cloud (LMC) HII nebulae is found to correlate positively with the temperature of the ambient radiation field or with the He(+)/H(+) abundance ratio. This result is discussed in terms of a selective absorption of the photons in the range 504 to 912 A relative to the He ionizing photons. This interpretation may explain the paradox of finding highly excited nebulae with only relatively moderate equivalent width of their Balmer lines

The molecular spiral arms of NGC 6946

High resolution observations of molecules in external galaxies are essential to understanding physical processes leading to the formation of stars. One question is whether there is a spiral structure in the molecular gas, but it was not possible to resolve the spiral arms of external galaxies until the advent of large millimeter-wave telescopes. With the Institute for Radio Astronomy in the Millimeter Range (IRAM) 30 m telescope, researchers are carrying through the mapping of NGC 6946 in the CO-12(1-0) and (2-1) lines. This galaxy is a large, gas-rich Scd spiral with a strong star formation activity. NGC 6946 is well studied at radio and optical wavelengths, so that it is possible to compare the location of the spiral arms tracers: HI ridge, HII regions and molecular clouds. The disk CO emission is very contrasted (no lines for some positions, 1 K in CO(1-0) for some others) and correlated with the optical spiral arms: this clearly shows up in a figure which presents superimposed contours of CO(2-1) integrated emissivity and of H alpha line emission. The agreement is very good, and there is no displacement across the arm between the CO, HI and H alpha ridges of emission. The arms are barely resolved by the 23 inch beam and the molecular contrast averaged over the map is about 4. The CO(2-1) maxima are closer to the position of the HII regions than those of CO(1-0), which could be due to variations of excitation conditions. The CO excitation in the disk of NGC 6946 is low: when all data are convolved to the same resolution of 23 inches the CO(2-1) lines are about 0.45 times fainter than the CO(1-0) ones, while in the nucleus they have roughly the same intensity. This suggests that in the disk of NGC 6946 most of the CO emission comes from cold optically thick gas located in cloud envelopes rather than from cloud cores. The molecular and atomic component in the observed regions of NGC 6946 seems to be organized in large gaseous complexes

Detection of CO in the inner part of M31's bulge

We report the first detection of CO in M31's bulge. The 12CO (1-0) and (2-1) lines are both detected in the dust complex D395A/393/384, at 1.3" (~0.35 kpc) from the centre. From these data and from visual extinction data, we derive a CO-luminosity to reddening ratio (and a CO-luminosity to H_2 column density ratio) quite similar to that observed in the local Galactic clouds. The (2-1) to (1-0) line intensity ratio points to a CO rotational temperature and a gas kinetic temperature > 10 K. The molecular mass of the complex, inside a 25' (100 pc) region, is 1.5 10^4 Mo.Comment: 5 pages including 4 figures (2 in colour

Global properties of the nearby spiral M101

M101 (NGC 5457) is a classic Sc I spiral galaxy located suffiently nearby, 6.8 Mpc, that its structure can be studied even with the coarse angular resolution of the Infrared Astronomy Satellite (IRAS). The global infrared properties of M101 are addressed including the radial dependence of its infrared emission

A New Probe of the Molecular Gas in Galaxies: Application to M101

Recent studies of nearby spiral galaxies suggest that photodissociation regions (PDRs) are capable of producing much of the observed HI in galaxy disks. In that case, measurements of the HI column density and the far-ultraviolet (FUV) photon flux provide a new probe of the volume density of the local underlying H_2. We develop the method and apply it to the giant Scd spiral M101 (NGC 5457). We find that, after correction for the best-estimate gradient of metallicity in the ISM of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30-1000 cm^-3 is found near star- forming regions at all radii in the disk of M101 out to a distance of 12' (approximately 26 kpc), close to the photometric limit of R_25 = 13.5'. In this picture, the ISM is virtually all molecular in the inner parts of M101. The strong decrease of the HI column density in the inner disk of the galaxy at R_G < 10 kpc is a consequence of a strong increase in the dust-to-gas ratio there, resulting in an increase of the H_2 formation rate on grains and a corresponding disappearance of hydrogen in its atomic form.Comment: accepted for publication in The Astrophysical Journal (1 August 2000); 29 pages including 20 figures (7 gif); AAS LaTex; contact authors for full resolution versions of gif figure

Radio recombination lines from the nuclear regions of starburst galaxies

Using the Very Large Array (VLA) with an angular resolution of 3", we have detected the hydrogen recombination line H92&#945; from the galaxies Arp 220, M83, and NGC 2146. The line emission arises from the nuclear regions with a line-to-continuum ratio of 1% or less. In order to fit both the observed H92&#945; line and continuum data in the nuclear regions, we have considered two types of models. First, we utilize a model with a collection of H ii regions. A large number of compact H ii regions are required in this model. With electron temperatures in the range 5 &#215; 103-1 &#215; 104 K and a range of electron densities, this model can account for both the line intensity and the continuum spectrum. In most cases, the H92&#945; line is dominated by internal stimulated emission due to free-free continuum arising within the H ii regions. In a low-density case (ne = 50 cm-3) for Arp 220, about half the line emission comes from external stimulated emission due to the background nonthermal source. Typical rates of ionizing photons predicted from these models are ~5 &#215; 1052 s-1 for M83, ~4 &#215; 10-53 s-1 for NGC 2146, and 5 &#215; 1054 s-1 for Arp 220. We infer that 105 O5 stars are required in Arp 220, which is an order of magnitude greater than in NGC 2146 and 2 orders of magnitude greater than in M83. Alternatively, several uniform slab models with Te = 5 &#215; 103 and ne in the range of 50 - 1 &#215; 104 cm-3 appear to fit both the H92&#945; line and continuum data of Arp 220 and M83. In the low-density models, stimulated emission by the background nonthermal radiation appears to be dominant at low frequencies, and the lines at higher frequencies arise primarily from spontaneous emission. The uniform slab model requires a higher ionizing photon rate than the H ii region model. No slab models with reasonable Te can fit the data observed in M83 and NGC 2146. Combining previous published data with these new observations, a sample of 13 galaxies has been observed for radio recombination lines (RRLs) with the VLA. Nine out of the 13 galaxies have been detected in the H92&#945; line. While the H92&#945; line luminosity appears to be correlated with the Bra line luminosity, we find that nearly all the RRL galaxies show a significant excess in H92&#945; line compared to the expected LTE value. The excess in the H92&#945; line flux suggests that non-LTE effects are important for the H92&#945; line in these starburst nuclei. A strong correlation between H92&#945; and the molecular lines of HCN/HCO+ is also found, indicating that the RRL emitters may be spatially associated with the dense molecular cores. The inferred high electron density also suggests an intimate relation between the RRLs and the dense molecular medium in these galaxies

High-density, compact H II regions in the starburst galaxies NGC 3628 and IC 694: high-resolution VLA observations of the H92α radio recombination line

Using the Very Large Array (VLA), we have observed the H92&#945; line in the nuclear regions of NGC 3628 and Arp 299 (IC 694 + NGC 3690) with an angular resolution of 1". The radio recombination lines could provide quite important constraints to the physical conditions for the ionized gas in the nuclear regions. For the central nuclear components in NGC 3628, IC 694, and NGC 3690, we find that the electron density of the ionized gas is in the range 5 &#215; 102 to 5 &#215; 104 cm-3, with corresponding geometrical sizes ranging from 25 to 0.01 pc. For the regions with a large line-to-continuum ratio, such as the anomalous kinematic components in IC 694 and NGC 3628, the electron density of the H II regions is well constrained. The derived parameters suggest the existence of ~100 high-density, compact H II regions (ne ~ 104 cm-3 and l ~ 1-2 pc) in these two components. The large number of high-density, compact H II regions deduced for these components can be regarded as evidence for enhanced massive star formation in anomalous kinematic structures that are often produced in interacting systems. Our new H92&#945; line observations reveal the presence of rotating circumnuclear ionized gas disks in both NGC 3628 and IC 694. From the analysis of the nuclear kinematics, we find that a total dynamical mass of 3 &#215; 108 M&#x02299; exists within a radius R = 120 pc in NGC 3628 and 7 &#215; 108 M&#x02299; within R = 200 pc in IC 694. The kinematics of the ionized gas and the models with a collection of H II regions infer that the ratio of H II to dynamical mass in the starburst nuclei varies from 1 &#215; 10-4 (for the high-density, compact H II region model) to 1 &#215; 10-2 (for the low-density, large H II region model)

A GLIMPSE into the Nature of Galactic Mid-IR Excesses

We investigate the nature of the mid-IR excess for 31 intermediate-mass stars that exhibit an 8 micron excess in either the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire or the Mid-Course Space Experiment using high resolution optical spectra to identify stars surrounded by warm circumstellar dust. From these data we determine projected stellar rotational velocities and estimate stellar effective temperatures for the sample. We estimate stellar ages from these temperatures, parallactic distances, and evolutionary models. Using MIPS [24] measurements and stellar parameters we determine the nature of the infrared excess for 19 GLIMPSE stars. We find that 15 stars exhibit Halpha emission and four exhibit Halpha absorption. Assuming that the mid-IR excesses arise in circumstellar disks, we use the Halpha fluxes to model and estimate the relative contributions of dust and free-free emission. Six stars exhibit Halpha fluxes that imply free-free emission can plausibly explain the infrared excess at [24]. These stars are candidate classical Be stars. Nine stars exhibit Halpha emission, but their Halpha fluxes are insufficient to explain the infrared excesses at [24], suggesting the presence of a circumstellar dust component. After the removal of the free-free component in these sources, we determine probable disk dust temperatures of Tdisk~300-800 K and fractional infrared luminosities of L(IR)/L(*)~10^-3. These nine stars may be pre-main-sequence stars with transitional disks undergoing disk clearing. Three of the four sources showing Halpha absorption exhibit circumstellar disk temperatures ~300-400 K, L(IR)/L(*)~10^-3, IR colors K-[24]< 3.3, and are warm debris disk candidates. One of the four Halpha absorption sources has K-[24]> 3.3 implying an optically thick outer disk and is a transition disk candidate.Comment: 17 figures. Accepted for publication in Ap

Observational Constraints to the Evolution of Massive Stars

We consider some aspects of the evolution of massive stars which can only be elucidated by means of "indirect" observations, i.e. measurements of the effects of massive stars on their environments. We discuss in detail the early evolution of massive stars formed in high metallicity regions as inferred from studies of HII regions in external galaxies.Comment: 6 pages, 1 figure; Invited Paper presented at the Roma-Trieste Workshop 1999 "The Chemical Evolution of the Milky Way: Stars versus Clusters", Vulcano Island (ME, Italy), 20-24 September, 1999, eds. F. Giovannelli & F. Matteucci, Kluwer-Holland (in press