Relations between interstellar medium tracers in galaxies

Aims:We study the relations existing between fluxes emitted at the CO(1-0) line, 60 and 100 μm wavelengths, and the B and soft X-ray wavebands for galaxies of all morphological types. The large set of data that we created allowed us to revisit some of the already known relations between the different tracers of the interstellar medium (ISM): the link between the FIR flux and the CO line emission, the relation between X-ray emission in non-active galaxies and the blue or FIR luminosity. Methods: Using both catalogues of galaxies and works in the literature, we collected fluxes in the FIR, 21 cm, and CO(1-0) lines and in the soft X-ray for two samples, consisting of normal and interacting galaxies, respectively. By joining these samples, we have data for a total of 2953 galaxies, not all observed in these four wavebands. Results: All the relations found are discussed in the frame of the star formation activity that is the link for most of them. We note that when an active star formation is present, it may link the galaxy fluxes at almost all wavelengths, from X to microwaves. In contrast, in early-type galaxies where the current star formation rate has faded, the X-FIR fluxes link disappears. This result for early-type galaxies is discussed and explained in detail in the frame of a suitable theoretical model, obtained by coupling chemo-dynamical N-body simulations with a dusty spectrophotometric code of population synthesis.