Multi frequency interferometer studies of active and starburst galaxies

Doctoral thesis, 2009

We report the results of a multi-frequency study of three interacting galaxies: NGC 5218, NGC 1614 and NGC 4194 (the Medusa merger). These sources belong to different classes of active and starburst galaxies. NGC 5218 is a spiral in an early phase of its gravitational interaction with a nearby elliptical galaxy, and is classified as a pure LINER (Low Ionization Nuclear Emission-line Region). NGC 1614 is more luminous and classified as both starburst and LINER where the starburst activity takes place in a nuclear ring. It is in a later stage of its interaction than NGC 5218 and is suggested to be a merger between two unequal-mass spirals. The Medusa (NGC 4194) is a merger of one spiral and one elliptical galaxy. It exhibits intense star formation activity, and is classified as a starburst. In contrast to NGC 1614 the starburst in the Medusa occurs on scales of several kpc. We have used interferometric techniques at millimeter and decimeter wavelengths to image the molecular and atomic components in these sources. The 3mm J=1-0 transition of CO was imaged in emission and the 21cm atomic hydrogen line was imaged in absorption toward the radio continuum source. Radio continuum observations were also used to map the synchrotron (and thermal) emission associated with the activity - and to attempt to establish the nature of the nuclear activity: starburst or AGN (Active Galactic Nucleus). In particular, the goal has been to establish whether the LINER characteristics of the two first objects, NGC 5218 and NGC 1614 are caused by starburst or AGN activity. Sources with LINER activity make up a rather inhomogeneous, and not yet well understood class, although LINER activity is found in one third of the galaxies in the local bright galaxy population. The LINER spectrum at optical wavelengths has similarities with spectra from both Starburst galaxies and galaxies with AGNs, which raises the question whether LINERs are typically driven by starbursts, AGN, a combination, or something else. We also report single dish observations of CO, HCN and HCO+ in NGC 5218 and interpret this in terms of dense gas content and nuclear activity. With our multi-frequency observations we have studied the distribution and dynamics of the neutral gas on scales ranging from several kiloparsecs into the central few hundred parsecs. In the central regions, we have detected significant deviations from regular circular rotation, caused by supernova-driven large-scale outflows and/or nuclear spiral arms and bars. We present detailed atomic and molecular maps combined with optical images to show that these velocity deviations are associated with regions of intense star formation, past- or ongoing. In general we find that, on the observed scales, the molecular and atomic gas phases are well mixed and trace the same dynamics. The lack of unresolved, high-brightness radio cores indicate that the two LINER sources are starburst driven.