Warm dust as a tracer of galaxies with gaseous halos

¹ Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands ( ESO, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile.)
² Astrophysics Department, School of Physics A28, University of Sydney, NSW 2006, Australia
³ Australia Telescope National Facility, PO Box 76, Epping, NSW 2121, Australia
⁴ XMM-Newton Science Operations Centre, Apartado 50727, 28080 Madrid, Spain
⁵ Astrophysics Division, Space Science Department of ESA, ESTEC, 2200 AG Noordwijk, The Netherlands
⁶ IRAM, Avenida Divina Pastora 7, NC, 18012 Granada, Spain

Corresponding author: M. Dahlem, mdahlem@eso.org

Received: 20 December 2000
Accepted: 1 May 2001

Abstract

We present radio continuum observations conducted with the VLA and ATCA of a sample of 15 edge-on spiral galaxies. 11 of these galaxies, with inclination angles of and neither active galactic nuclei nor nearby interaction partners, are suitable for studies of halo properties in relation to the level of star formation in their disks. In 6 of these 11 galaxies radio halos were detected at the angular resolution of the current data. In the remaining cases the presence of halo emission could not be proven unambiguously, partly due to relatively low angular resolution. A clear trend was found that galaxies with radio halos are those with the highest far-infrared 60 μm to 100 μm flux ratios. This shows the suitability of high ratios of ≥ 0.4 as a reliable tracer of galaxies with high star formation rates and related disk-halo interactions, leading to the presence of extraplanar emission, e.g. from cosmic ray electrons. The measured exponential scale heights of those 6 radio halos that were clearly detected range from about 1.4 to 3.1 kpc. All 4 physically small galaxies in our sample do show extraplanar synchrotron radio emission, indicating that their more shallow gravitational potential compared to normal-sized spirals might facilitate the escape of cosmic-ray electrons from the sites of star formation in their disks. Although the galaxies with the highest energy input rates into the ISM of their disks are those that have the most prominent radio halos, there is no direct relation between the halo scale heights and the energy input rates. Instead, the scale heights of the radio halos are dominated by the energy losses of the cosmic ray electrons on their way out of the galaxy disks.