We are conducting a detailed study of the nearby, face-on,local-group galaxy M33. M33 is ideally suited to study the dependence of the star formation characteristics on the physical conditions across the spatially resolved galactic disk. Our Herschel open time key project has been granted almost 200 hours of observing time by ESA. We will survey the major gas cooling lines, notably C+, H2O, O, N+, and N++, as well as the dust spectral energy distribution (SED) in M33. We will be using all three instruments HIFI, PACS, SPIRE.
At a distance of 840 kpc, M33 is the only nearby, gas rich disk galaxy that allows a coherent survey at high spatial resolution. It lacks the Milky Way distance ambiguity, and it is not as extended and inclined as the Andromeda galaxy. M33 is a regular, mostly unperturbed disk galaxy, as opposed to the nearer Magellanic Clouds, which are highly disturbed irregular dwarf galaxies.
Observing a deep, extended strip along the major axis of our template galaxy M33, will allow us to study the ionized, atomic, and molecular phases of the interstellar medium, its life cycle and thermal balance, tracing the formation of molecular clouds and of stars. Aside of insights related to the local processes in the galaxy itself, the mapped source will set a standard, providing a basis for the interpretation of phenomena encountered in other targets of the Local Group and in more distant galaxies.
Key science topics
(A) Study the phases of the ISM via their most important cooling lines at a wide range of galactocentric radii out to the edge of the optical disk. Derive the life cycle of the interstellar gas as a function of galactic radius. Derive the contribution of [CII] emission from the various phases and the carbon budget through [CII] and dust observations, complemented by [CI] and CO observations from the ground.
(B) Determine the energy balance of the interstellar medium as a function of galactic environment. Measure the ratio of dust continuum cooling vs. gas cooling, and the relative importance of the various gas coolants and relevant heating processes.
(C) Use [NII] in conjunction with [CII] as extinction free tracers of star formation. Determine the parameters controlling the star formation rate. Calculate the star formation efficiency or gas depletion time scales as a function of galactocentric radius. Study stellar feedback and the disruption of molecular clouds. Investigate how the interstellar magnetic field affects or controls the star formation rate or efficiency.
(D) Combine FIR line data with existing HI data to study the formation of molecular clouds from the diffuse atomic medium. To determine the details of the evolution from atomic to molecular gas and ultimately star formation itself.