Dynamically Driven Evolution of the Interstellar Medium in M51

TL;DR

This study uses high-resolution CO observations of M51 to show how large molecular clouds form, evolve, and fragment under the influence of galactic spiral arm dynamics, highlighting the role of shear and streaming motions.

Contribution

It provides new insights into GMC evolution driven by large-scale galactic dynamics, challenging previous feedback-based models.

Findings

GMCs assemble into GMAs within spiral arms

GMAs fragment into smaller GMCs upon leaving arms

Most gas remains molecular across spiral arm regions

Abstract

We report the highest-fidelity observations of the spiral galaxy M51 in CO emission, revealing the evolution of giant molecular clouds (GMCs) vis-a-vis the large-scale galactic structure and dynamics. The most massive GMCs (so-called GMAs) are first assembled and then broken up as the gas flow through the spiral arms. The GMAs and their H2 molecules are not fully dissociated into atomic gas as predicted in stellar feedback scenarios, but are fragmented into smaller GMCs upon leaving the spiral arms. The remnants of GMAs are detected as the chains of GMCs that emerge from the spiral arms into interarm regions. The kinematic shear within the spiral arms is sufficient to unbind the GMAs against self-gravity. We conclude that the evolution of GMCs is driven by large-scale galactic dynamics --their coagulation into GMAs is due to spiral arm streaming motions upon entering the arms, followed by fragmentation due to shear as they leave the arms on the downstream side. In M51, the majority of the gas remains molecular from arm entry through the inter-arm region and into the next spiral arm passage.