George Helou (PI), David Hollenbach, Harley Thronson, Gordon Stacey, Robert Rubin, Harriet Dinerstein, Steven Lord, Michael Werner, Deidre Hunter, K.Y. Lo, Charles Beichman

We propose to observe an ensemble of atomic and ionic fine structure lines and the infrared continuum, to characterize the interstellar medium in a broad sample of normal disk galaxies. Uniquely suited to the Infrared Space Observatory (ISO), these observations will probe the birth-sites of stars, yield the physical properties of the gas, dust and radiation field, and refine our picture of the global energetics in galaxies. We request 80 hours of ISO integration time to study 60 unresolved galaxies and selected regions of 20 nearby galaxies. Statistical and physical interpretation of the resulting data will lead to new insights into the fundamental agent of cosmic evolution in the local Universe -- star formation in galaxies.

The infrared offers a variety of robust diagnostics of physical parameters such as density, temperature and radiation hardness in the interstellar medium (ISM). ISO allows us, because of its unprecedented sensitivity to low surface brightness, to apply these diagnostics for the first time to normal galaxies. We propose using the LWS to probe the warm neutral phase with the [C II] line at 158 microns, and [OI] at 63 microns, and to probe HII regions with the [OIII] lines at 52 and 88 microns, [NIII] at 57 microns, and [NII] at 122 microns. From the neutral line flux ratios, line to continuum ratios, and models of photo-dissociation regions (PDRs), we would derive the gas temperature, density and pressure, the UV intensity, and the filling factor of PDR clouds. From the HII region lines, we will determine the N and O abundances, and the effective temperature of the ionizing stars. We propose measuring the spectral energy distribution of our sources from 3 to 200microns, and obtaining their images at 7 and 12 microns, using ISOPHOT and ISOCAM. From these data we will estimate the total luminosity and the dust mass at various temperatures. ISO and ground-based data combined will give accurate measures of the ionized and neutral mass fractions in the ISM, and of the luminosity of each phase in different types of galaxies.

With additional model assumptions and data (HI and CO, radio continuum, IRAS, optical photometry, H$\alpha$ and Br$\gamma$) we will deduce relative numbers, sizes and masses of molecular clouds in the ISM, and evaluate their collision rates. We propose then to study how these parameters relate to the global properties of galaxies -- morphology and kinematics, environment, gas content, metallicity, and star formation efficiency. The ultimate goal of this program is to constrain the evolutionary processes in galaxies, discriminating for example between mechanisms for global star formation activation (cloud-cloud collisions or gravitational instability), and suppression (spatial dispersal or photodissociation and heating).

Normal galaxies account for most of the luminous mass in the local Universe. The sample we propose to study represents their great diversity by sampling the wide ranges they offer in total luminosity ($10^8-10^{10} L_sun), infrared-to-blue luminosity ratio (0.2-10), morphology (S0 to Im), and relative gas content. We exclude galaxies with active nuclei or starbursts since they are well studied in the ISO Central Programme. We stress however that as a vital part of understanding global properties, we need to study spatially resolved, well-defined components such as HII complexes, spiral arms, bars and inter-arm disk regions in nearby galaxies.