A B S T R A C TIn a recent paper, Elmegreen has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical time-scales. If so, this has important implications for the mechanisms by which molecular clouds form. In particular, we consider the hypothesis that molecular clouds may form not by in situ cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium, much, if not most, of which is already in molecular form.

Dust extinction can be determined from the number of distant field galaxies seen through a spiral disk. To calibrate this number for the crowding and confusion introduced by the foreground image, González et al. (1998) andHolwerda et al. (2005a) developed the "Synthetic Field Method" (SFM), which analyses synthetic fields constructed by adding various deep exposures of unobstructed background fields to the candidate foreground galaxy field.The advantage of the SFM is that it gives the average opacity for area of galaxy disk without assumptions about either the distribution of absorbers or of the disk starlight. However it is limited by low statistics of the surviving field galaxies, hence the need to combine a larger sample of fields. This paper presents the first results for a sample of 32 deep HST/WFPC2 archival fields of 29 spirals.The radial profiles of average dust extinction in spiral galaxies based on calibrated counts of distant field galaxies is presented here, both for individual galaxies as well as for composites from our sample. The effects of inclination, spiral arms and Hubble type on the radial extinction profile are discussed.The dust opacity of the disk apparently arises from two distinct components; an optically thicker (A I = 0.5 − 4 mag) but radially dependent component associated with the spiral arms and a relatively constant optically thinner disk (A I ≈ 0.5 mag.). These results are completely in agreement with earlier work on occulted galaxies. The early

We describe a new, direct method for determining the opacity of foreground galaxies which does not require any a priori assumptions about the spatial distribution or the reddening law of the obscuring material. The method is to measure the colors and counts of background galaxies which can be identified through the foreground system. The method is calibrated, and the effects of confusion and obscuration are decoupled by adding various versions of a suitable deep reference frame containing only field galaxies with known properties into the image of the foreground galaxy, and analyzing these "synthetic field" images in the same way as the real images.We test the method on HST WFPC2 archived images of two galaxies which are quite different: NGC 4536 is a large Sc spiral, and NGC 3664 is a small Magellanic irregular. The reference frames are taken from the Hubble Deep Field.From the background galaxy counts, NGC 4536 shows an extinction A I ∼ 1 mag in the northwestern arm region, and lower than 0.5 mag in the corresponding interarm region (no correction for inclination has been attempted). However, from the galaxy colors, the same reddening of E(V − I) ∼ 0.2 is observed in both the arm and the interarm regions. In the interarm region, the combination of extinction and reddening can be explained by a diffuse component with a Galactic reddening law (R V ≈ 3). In the spiral arm, however, the same diffuse, low opacity component seems to coexist with regions of much higher opacity. Since the exposures are shorter the results for NGC 3664 are less clear, but also appear to be consistent with a two component distribution.

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We present optical emission-line spectra for outlying HII regions in the extended neutral gas disk surrounding the blue compact dwarf galaxy NGC 2915. Using a combination of strong-line R23 and direct oxygen abundance measurements, we report a flat, possibly increasing, metallicity gradient out to 1.2 times the Holmberg radius. We find the outer-disk of NGC 2915 to be enriched to a metallicity of 0.4 Z ⊙ . An analysis of the metal yields shows that the outer disk of NGC 2915 is overabundant for its gas fraction, while the central star-foming core is similarly under-abundant for its gas fraction. Star formation rates derived from very deep ∼ 14 ks GALEX FUV exposures indicate that the lowlevel of star formation observed at large radii is not sufficient to have produced the measured oxygen abundances at these galactocentric distances. We consider 3 plausible mechanisms that may explain the metal-enriched outer gaseous disk of NGC 2915: radial redistribution of centrally generated metals, strong galactic winds with subsequent fallback, and galaxy accretion. Our results have implications for the physical origin of the mass-metallicity relation for gas-rich dwarf galaxies.

The gas at the surfaces of molecular clouds in galaxies is heated and dissociated by photons from young stars both near and far. H i resulting from the dissociation of molecular hydrogen H 2 emits hyperfine line emission at 21 cm, and warmed CO emits dipole rotational lines such as the 2.6 mm line of CO (1-0). We use previously developed models for photodissociation regions (PDRs) to compute the intensities of these H i and CO (1-0) lines as a function of the total volume density n in the cloud and the far-ultraviolet (FUV) flux G 0 incident on it and present the results in units familiar to observers. The intensities of these two lines behave differently with changing physical conditions in the PDR, and taken together, the two lines can provide a ground-based radio astronomy diagnostic for determining n and G 0 separately in distant molecular clouds. This diagnostic is particularly useful in the range G 0 P100, 10 cm À3 P n P10 5 cm À3 , which applies to a large fraction of the volume of the interstellar medium in galaxies. If the molecular cloud is located near discrete sources of far-UV (FUV) emission, the PDR-generated H i and CO (1-0) emission on the cloud surface can be more easily identified, appearing as layered ''blankets'' or ''blisters'' on the side of the cloud nearest the FUV source. As an illustration, we consider the Galactic object G216À2.5, i.e., ''Maddalena's Cloud,'' which has been previously identified as a large PDR in the Galaxy. We determine that this cloud has n % 200 cm À3 and G 0 % 0:8, consistent with other data.

Using the Canada-France-Hawaii Telescope we have obtained deep high-resolution CCD images in V and I of a 28 ′ × 28 ′ field in the outer disk of M 31 at ≈ 116 ′ from the center along the major axis to the south-west, and covering a range of projected galactocentric distance from about 23 to 33 kpc. The field was chosen to correspond with extended H I features recorded near the H I edge of the galaxy.The many tens of thousands of objects detected in this large field have been classified using an automatic algorithm which distinguishes unresolved from resolved structures and provides photometry on them. For the most part the unresolved objects are stars in M 31. The V-I colors of these stars are highly correlated with the column density of H I in the field. Assuming a Galactic extinction law, this yields a minimum extinction/atomic-gas ratio about 1/3 of that in the Solar neighbourhood. The ISM in this outer disk of M 31 therefore contains substantial amounts of dust.We have identified a population of B stars in the field whose distribution is also well correlated with the extended H I distribution. Evidently, star formation is both ongoing and wide spread in the outer disk of M 31. According to the current view of the star formation process, molecular gas is therefore also expected to be present.The objects classified as "resolved" turn out to be a mix of background galaxies and overlapping images of foreground stars in M 31. The counts and colors of the slightly-resolved objects in these ground-based CCD images therefore cannot be used for a reliable determination of the total extinction and reddening by the interstellar medium in M 31. However, the larger background galaxies are easily recognizable, and their surface density above a specific magnitude limit is anticorrelated with the H I column density, confirming that a relatively large amount of extinction is closely associated with the H I gas.

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