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The kinematics of molecular gas and dust in the nearby galaxies Centaurus A and M82

Citation

Quillen, Alice C. (1993) The kinematics of molecular gas and dust in the nearby galaxies Centaurus A and M82. Dissertation (Ph.D.), California Institute of Technology. http://resolver.caltech.edu/CaltechTHESIS:01042013-155931345

Abstract

This thesis presents a kinematical study of the molecular gas and dust in the nearby radio galaxy Centaurus A (NGC 5128) and the nearby starburst M82.

The CO (2-1) emission along the inner dust lane of Centaurus A, observed with the Caltech Submillimeter Observatory on Mauna Kea, shows the molecular gas to be in a thin disk, with a velocity dispersion of only about 10 km s^(-1) . The observed line profiles are broadened considerably due to beam smearing of the gas velocity field. The profile shapes are inconsistent with planar circular and noncircular motion. However, a warped disk in a prolate potential provides a good fit to the profile shapes. The morphology and kinematics of the molecular gas is similar to that of the ionized material, seen in Ha. The best fitting warped disk model not only matches the optical appearance of the dustlane, but also agrees with the large scale map of the CO emission, and is consistent with HI measurements at larger radii.

We present infrared images of Cen A (NGC5128) in the J,H, and K bands observed with the 1.5 m telescope at CTIO. The infrared morphology is primarily determined by the presence of a thin absorptive warped disk. By integrating the light of the underlying prolate galaxy through such a disk, we construct models which we compare with infrared and X-ray data. The geometry of the warped disk needed to fit the IR data is consistent with a warped disk which has evolved as a result of differential precession in a prolate potential. The disk has an inclination, with respect to the principal axis of the underlying elliptical galaxy, that is higher at large radii than in the inner region.

A scenario is proposed where a small gas rich galaxy infalling under the force of dynamical friction is tidally stripped. Stripping occurs at different times during its infall. The orientation of the resulting gas disk depends upon the angular momentum of the infalling galaxy. We find that the resulting precession angle of the disk is well described by the precession model, but that the inclination angle may vary as a function of radius. We propose an orbit for the infalling galaxy that is consistent with the geometry of the warped disk needed to fit our infrared data, rotation observed in the outer part of the galaxy and the location of the stellar shells in the same region.

We model the kinematics of the molecular gas in the nearly edge-on disk m M82, by considering velocity and surface density perturbations caused by a possible rotating kpc long bar. A model with a bar that has an Inner Linblad Resonance at r ~ 10" ~ 150 pc fits the molecular observations of the inner torus. This model is consistent with the angle of the bar inferred from the K (2.2µm) isophotes. The clouds have a cloud-cloud velocity dispersion of ~ 30 km s^(-1) implying that the disk is unstable to short timescale axisymmetric perturbations. This is consistent with the hypothesis that the high star formation efficiencies in starbursts are due to the the short timescales of gravitational instability. It is likely that the bar has mediated the starburst.

There are serious deviations from our model at large radii. It is likely that there are two components of molecular material which were not considered by our model: (i) a component at large radii that is in the galactic plane and has low line-of-sight velocities due to a larger scale bar or due to the fact that there is a lack of molecular gas over a large range of radius (perhaps due to a previous interaction which caused a large fraction of the gas to sink into the nucleus), and (ii) a molecular wind with velocities of the order of the observed line widths (80 - 120 km s^(-1)). While dense gas can be accelerated in a galactic superwind to velocities of this order of magnitude, it is unclear how this gas interacts with the superwind.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Physics
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Physics
Thesis Availability:Restricted to Caltech community only
Research Advisor(s):
  • unknown, unknown
Group:TAPIR
Thesis Committee:
  • unknown, unknown
Defense Date:3 May 1993
Record Number:CaltechTHESIS:01042013-155931345
Persistent URL:http://resolver.caltech.edu/CaltechTHESIS:01042013-155931345
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7371
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:05 Jan 2013 00:32
Last Modified:18 Aug 2017 20:41

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