LABOCA observations of nearby, active galaxies

Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: aweiss@mpifr-bonn.mpg.de

Received: 4 April 2008
Accepted: 17 August 2008

Abstract

We present large scale 870 μm maps of the nearby starburst galaxies NGC 253 and NGC 4945 as well as the nearest giant elliptical radio galaxy Centaurus A (NGC 5128) obtained with the newly commissioned Large Apex Bolometer Camera (LABOCA) operated at the Atacama Pathfinder Experiment telescope. Our continuum images reveal for the first time the distribution of cold dust at a angular resolution of 20'' across the entire optical disks of NGC 253 and NGC 4945 out to a radial distance of (7.5 kpc). In NGC 5128 our LABOCA image also shows, for the first time at submillimeter wavelengths, the synchrotron emission associated with the radio jet and the inner radio lobes. From an analysis of the 870 μm emission in conjunction with ISO-LWS, IRAS and long wavelengths radio data we find temperatures for the cold dust in the disks of all three galaxies of 17-20 K, comparable to the dust temperatures in the disk of the Milky Way. The total gas mass in the three galaxies is determined to be 2.1, 4.2 and 2.8  for NGC 253, NGC 4945 and NGC 5128, respectively. The mass of the warmer (30-40 K) gas associated with the central starburst regions in NGC 253 and NGC 4945 only accounts for ~ of the total gas mass. A detailed comparison between the gas masses derived from the dust continuum and the integrated CO(1-0) intensity in NGC 253 suggests that changes of the CO luminosity to molecular mass conversion factor are mainly driven by a metallicity gradient and only to a lesser degree by variations of the CO excitation. An analysis of the synchrotron spectrum in the northern radio lobe of NGC 5128 shows that the synchrotron emission from radio to the ultraviolet (UV) wavelengths is well described by a broken power law and that the break frequency is a function of the distance from the radio core as expected for aging electrons. We derive an outflow speed of ~0.5 c at a distance of 2.6 kpc from the center, consistent with the speed derived in the vicinity of the nucleus.