arXiv subjects

Ronald J. Allen

Publications and source records attributed to Ronald J. Allen.

40 records · Page 3Linked to original sources

arXiv2001

The Formation of Molecular Clouds

In a recent paper, Elmegreen (2000) has made a cogent case, from an observational point of view, that the lifetimes of molecular clouds are comparable to their dynamical timescales. 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 {\it in situ} cooling of atomic gas, but rather by the agglomeration of the dense phase of the interstellar medium (ISM), much, if not most, of which is already in molecular form.

astro-ph↗

arXiv2001

Cold Molecular Gas, PDRs, and the Origin of HI in Galaxies

In the currently-accepted model for star formation out of the interstellar gas in galaxies, the basic construction material is assumed to be large clouds of atomic hydrogen (HI). These clouds are thought to form higher-density complexes of gas and dust, and turn molecular (H2). Stars then form out of this molecular gas. In this paper arguments are advanced for a contrary view, in which the basic construction material is cold molecular gas out of which the stars form directly. HI appears in the region when the leftover H2 is illuminated with UV photons from nearby young stars. The physics of photodissociation regions provides a natural and quantitative explanation for the appearance of HI envelopes around the clouds, and for CO(1-0) emission from the higher-density parts of their surfaces. In this picture, much of the HI in a galaxy is a product of the star formation process, not a precursor to it.

astro-ph↗

arXiv2001

Gas, Dust, and Young Stars in the Outer Disk of M 31

Using the Canada-France-Hawaii Telescope we have obtained deep high-resolution CCD images in V and I of a 28'x28' 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 HI features recorded near the HI 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 HI 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 HI 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.

astro-ph↗

arXiv2000

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.

astro-ph↗