The relationship between the density wave, molecular gas and star formation in M51

Citation

Rand, Richard J. (1991) The relationship between the density wave, molecular gas and star formation in M51. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/c2j4-qh42. https://resolver.caltech.edu/CaltechETD:etd-09162008-105121

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. A study is presented of the relationship between the density wave, molecular gas and star formation in the molecule-rich, grand-design spiral galaxy M51. An interferometric map of CO emission covering most of the disk of M51 at 8" resolution is presented. A narrow, two-arm spiral pattern is seen in the molecular gas. Streaming motions indicating the presence of a density wave can clearly be seen. Giant Molecular Associations (GMAs) of typical mass 3 x 10[superscript 7] M[...], first reported by Vogel, Kulkarni and Scoville, are seen in abundance along the arms. The virial theorem indicates that these GMAs are roughly gravitationally bound. A few unbound GMAs are also seen between the arms. There is evidence that these interarm GMAs form in a secondary compression of the density wave. Substructure in the GMAs is indicated by their CO spectra, which typically reveal a few discrete velocity components. Higher (2.5") resolution maps of a small area south of the nucleus of M51 reveal patchy substructure along the southern molecular spiral arm and in the GMAs. The above-mentioned streaming motions are confirmed at this resolution. The one-dimensional velocity dispersion along the arms at this resolution is roughly 10 km s[superscript -1]. A more careful virial analysis confirms that the on-arm GMAs are bound while the interarm GMAs are unbound. The on-arm GMAs are shown to be stable against galactic tidal forces, while the interarm GMAs are close to the point of marginal stability. Along the spiral arm in this map, the detailed agreement between CO and Hα luminosity is poor. There are several possible explanations for this. Two consequences of star formation on the state of the ISM of M51 are discussed. Using the new, 8"-resolution VLA map of 21-cm emission, we find much new evidence in support of the idea of Tilanus and Allen that the HI is predominantly a product of dissociation in star-forming regions. The possibility is suggested that HI emission may be used as an indicator of star formation efficiency in molecule-rich galaxies. IRAS pointed observations of M51 are used to probe the origins of the infrared emission. Two independent tests indicate that a substantial fraction of the infrared emission originates in a cirrus component, and not due to dust heating by star-forming regions. One consequence of the existence of the cirrus component is that infrared emission cannot be used in a straightforward way to test the hypothesis of the triggering of star formation by a density wave. A kinematic study of the old-disk stars, and the molecular, neutral and ionized gas is presented. The old-disk stars, since they comprise most of the mass of the galaxy, trace the underlying gravitational perturbation. Long-slit spectroscopy of the absorption lines provides some evidence for radial streaming motions in the massive old-disk population. The amplitude of the motion at 85" from the nucleus is greater than that expected from linear density wave theory. A possible explanation from nonlinear theory is given. Long-slit spectroscopy of the [...] and [NII] lines, along with kinematic information from our CO map and the high-resolution VLA map of 21-cm emission, clearly reveal streaming motions in the molecular, neutral and ionized gas. These motions are compared with the predictions of nonlinear density wave models. A quantitative analysis of possible GMA formation mechanisms, which draws on our morphological and kinematic observations of the molecular gas, is presented. Gravitational instability and collisional agglomeration are both viable mechanisms for GMA formation on the arms. The interarm gas is close to the point of marginal stability, and the postulated secondary compression of the density wave may be sufficient to cause either weak instabilities in the gas, or low-level orbit crowding, thereby explaining the unbound interarm GMAs. The relationship between GMA formation and the triggering of star formation is briefly addressed. Problems with the conclusion that the on-arm GMAs are gravitationally bound are discussed. Disruption of bound on-arm GMAs must be rapid since they are confined to narrow spiral arms. Although the arguments are by no means conclusive, the input power of star formation is a plausible mechanism for GMA disruption.

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