Astronomy and Astrophysics – Astronomy
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........23m&link_type=abstract
Thesis (PH.D.)--RENSSELAER POLYTECHNIC INSTITUTE, 1995.Source: Dissertation Abstracts International, Volume: 56-11, Section: B,
Astronomy and Astrophysics
Astronomy
15
Molecular Clouds
Scientific paper
A study of the molecular cloud ensemble in a segment of a spiral arm in the nearby spiral galaxy Messier 31 is presented. I have used the 12-meter millimeter-wave radio telescope of the National Radio Astronomy Observatory to survey a 2' x 3' section of the 10 kiloparsec arm on the southwest side of the galaxy. The J = 2 --> 1 rotational transition of carbon monoxide, at a wavelength of 1.3 mm, was used as a tracer of molecular hydrogen, giving an angular resolution of 30" or 100 parsecs (pc) at the 690 kiloparsec (kpc) distance (from us) of M31. The CO emission is largely confined to the spiral arm, in the vicinity of the HII regions (regions of ionized hydrogen around young, massive stars) that define the optical arm. Downstream of the spiral arm, the emission drops below the detection limit within about 100 pc, indicating that giant molecular clouds do not survive arm passage. Upstream, emission persists well into the interarm region suggesting that a population of possibly smaller molecular clouds has survived a previous arm passage. On the arm, bright narrow spectral lines are seen near the HII regions, indicating the presence of GMCs. The strongest emission, however, containing about 75% of the CO luminosity in the arm region, is in a broad peak offset about 100 pc upstream of the HII regions. The line profiles for these spectra indicate they consist of ensembles of smaller clouds. These spectra, and some of the bright narrow lines near the HII regions, are observed at about 15 kms^{-1} more negative velocity from the interarm emission, suggesting that a kinematic effect of the spiral arm passage is observed. It is argued, however, that the spectral profiles around the velocity transition indicate that this phenomenon may be better explained by assuming two populations of molecular clouds at different heights in the disk, one being the inter-arm clouds, and the other an ensemble of clouds condensed from shocked neutral atomic hydrogen entering the arm.
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