Star Formation in M51 Triggered by Galaxy Interaction

We have mapped the inner 360'' regions of M51 in the 158 μm [C II] line at 55'' spatial resolution using the far-infrared imaging Fabry-Perot interferometer (FIFI) on the Kuiper Airborne Observatory (KAO). The emission is peaked at the nucleus but is detectable over the entire region mapped, which covers much of the optical disk of the galaxy. There are also two strong secondary peaks at ~43%-70% of the nuclear value located roughly 120'' to the northeast and southwest of the nucleus. These secondary peaks are at the same distance from the nucleus as the corotation radius of the density wave pattern. The density wave also terminates at this location, and the outlying spiral structure is attributed to material clumping due to the interaction between M51 and NGC 5195. This orbit crowding results in cloud-cloud collisions, stimulating star formation, that we see as enhanced [C II] line emission. The [C II] emission at the peaks originates mainly from photodissociation regions (PDRs) formed on the surfaces of molecular clouds that are exposed to OB starlight, so that these [C II] peaks trace star formation peaks in M51. The total mass of [C II]-emitting photodissociated gas is ~2.6 × 10⁸ M_☉, or about 2% of the molecular gas as estimated from its CO (1-0) line emission. At the peak [C II] positions, the PDR gas mass to total gas mass fraction is somewhat higher, 3%-17%, and at the secondary peaks the mass fraction of the [C II]-emitting photodissociated gas can be as high as 72% of the molecular mass. Using PDR models, we estimate that the far-UV field intensities are a few hundred times the local Galactic interstellar radiation field, similar to that found near OB star-forming giant molecular clouds in the Milky Way. The density solution is degenerate, with both a low- (n ~ 10²-10³ cm^-3) and a high-density (n ~ 10³-10⁶ cm^-3) solution. Our analysis shows that a substantial amount of the observed [C II] emission from the galaxy as a whole can arise from the ionized medium and that the contribution from the cold neutral medium (CNM) is not negligible. At the [C II] peaks, probably ~7%-36% of the [C II] emission arises from the CNM, while northwest of the nucleus, most of the observed emission may arise from the CNM.