Gas cooling within the diffuse ISM of late-type galaxies^*

¹ Ritter Astrophysical Research Center, The University of Toledo, 2801 West Bancroft, Toledo, OH 43606, USA e-mail: pierini@ancona.astro.utoledo.edu
² Max-Planck-Institut für Kernphysik, Saupfercheckweg 1, 69117 Heidelberg, Germany e-mail: Heinrich.Voelk@mpi-hd.mpg.de
³ DEMIRM, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: James.Lequeux@obspm.fr
⁴ Laboratoire d'Astronomie Spatiale, BP 8, Traverse du Syphon, 13376 Marseille, France e-mail: Alessandro.Boselli@astrsp-mrs.fr
⁵ ISO Data Center, Astrophysics Division, ESA Space Science Dept., PO Box 50727, 28080 Madrid, Spain e-mail: kleech@iso.vilspa.esa.es

D. Pierini

Received: 24 January 2000
Accepted: 26 April 2001

Abstract

We combine observations of spiral galaxies in the [CII] line at 158 μm, made with the Long Wavelength Spectrometer aboard ISO, with previous data from the Kuiper Airborne Observatory to study the origin of this line, which is the main coolant of the interstellar medium at relatively low temperatures. We also use HI and CO() observations of these galaxies and estimate the respective line fluxes in the same beam as the [CII] observations. We confirm the existence of a linear relation between the [CII] line intensity and the CO() line intensity, that we extend to intrinsically fainter galaxies. The dispersion around this relation is significant and due to variations in the far-UV flux, thus in the star formation rate. We find that for the least active galaxies of our sample, in terms of star formation, the rate of [CII] line emission per interstellar hydrogen atom is similar to that in the Solar neighbourhood. For those galaxies, most of the [CII] line emission comes probably from the diffuse cold atomic medium. In more active galaxies, considered globally, the average [CII] line emission is dominated by dense photodissociation regions and to some extent by the warm ionized diffuse medium. This is true in the central regions of many spiral galaxies, and probably even in the interarm regions of the most actively star-forming ones.