The phases and mass budget of the interstellar medium in outer disks
Résumé
We would like to determine how much of each of the gas phases – molecular, atomic, and ionized – is present in the outer disks of todays spirals and understand why. Since it is clear that in spiral galaxies the neutral interstellar medium (ISM) dominates in mass although it is in fact difficult to measure that of the ionized gas, I will focus on the neutral ISM. Star formation takes place in the neutral interstellar medium so the properties of the atomic and molecular gas must be an important part of understanding whether and why star formation is different at higher redshift. The ratio of atomic to molecular gas is much higher in the outer disk than in the inner parts, even at equivalent H column density. Why? The atomic component has long been observed in outer disks and the atomic gas column density can be reasonably reliably derived from the 21cm HI line. Only recently, however, has it been possible to observe the molecular gas far out in disks. It is clearly associated with the atomic component yet for a given galaxy the HI column density does not provide a reliable prediction of the molecular gas content. I will present unpublished data on the molecular components of several nearby spiral galaxies in order to better understand the relationship between the atomic and molecular gas and star formation in the outer disks of spirals and what the differences are with respect to inner disks. The long term goal is to test whether some local galaxies share the high starforming efficiencies of intermediate redshift galaxies, determine which local galaxies, and understand what is different about their molecular ISM that results in a high SFE.