Millimetre and Submillimetre continuum imaging studies of nearby active galaxies

Abstract

The thesis presents millimetre (mm) and submm continuum imaging observations of the
four, nearby active galaxies Cygnus A, NCC 4374, Centaurus A and M 82. These were
undertaken using the Submillimetre Common-User Bolometer Array (SCUBA) on the
James Clerk Maxwell Telescope in Hawaii. The study focused on the properties of the
cool to cold dust (~ 80 to 10K) and its associations with star formation and nuclear
activity in the galaxies.

The observations of Cygnus A constrained the galaxy's cold dust temperature between
37 and 85K, corresponding to dust masses between 1.0 x 10 8M® and 1.4 x 106M®
respectively. Based on the dust spatial and mass limits, it is possible the heating of the
cold dust in Cygnus A is dominated by high-energy nuclear activity or star formation.
Detailed photometric observations of the core and two major hotspots (A and D) in
Cygnus A showed that their mm to submm spectral indices extend smoothly to about
450μm (677GHz), with no evidence of spectral steepening. The results allow tight determinations
for model parameters of synchrotron electron lifetimes in these components.

In pilot survey observations of the elliptical galaxy NGC 4374, a mass upper-limit
Of ~ 2 x 107M® (i.e. comparable to the dust contents of luminous spiral galaxies) was
placed on the diffuse dust that, if it exists, would affect colour and metallicity studies of elliptical galaxies. Following the pilot observations, a strategy was devised to select a good sample of ellipticals in which to search for this low-level, very cold dust.

For the first time ever, the observations of Centaurus A delineated the galaxy's submm
unresolved core, an inner-jet interacting with gas in the dust lane, and extended emission
from multi-temperature cold dust at ~ 40, 30 and 12K. Also for the first time ever, a
submm high surface brightness ringed, spiral structure of radii ~ 90", that coincides with
the regions of intense star formation and mid-infrared continuum, was revealed in the
optical dust-lane, confirming predictions by a geometric warped disc model consisting of
tilted rings. From the spatial associations in the galaxy, star formation seems to dominate
at lease 50% of the heating of the cold dust, while the rest appears to be heated by diluted
stellar radiation.

In M82, the 850μm continuum has been detected out to 1.5kpc, at least 10% farther
in radius than previous detections in this starburst galaxy. The overall submm morphology
and nuclear spectral index distribution have a general north-south asymmetry similar
to that of H. and X-ray winds, supporting the association of the extended continuum
with outflows of dust grains from the nucleus into the halo. The work of M 82 raises many
interesting points about the origin and structure of the submm emission, however the
results are displayed here still under investigation and therefore are rather preliminary.

In short, the submm continuum observations of Cygnus A, NGC 4374, CentaurusA
and M 82 are presented in this thesis that constrain the physical properties of cold dust
in these active galaxies and elucidate the associations of the dust with star formation,
nuclear activity and stellar components in these galaxies in particular, and other similar
galaxies in general.