Probing the cold and warm molecular gas in the Whirlpool Galaxy: Herschel SPIRE-FTS Observations of the central region of M51 (NGC 5194)

TL;DR

This study uses Herschel SPIRE-FTS and ground-based observations to analyze the molecular gas in M51, revealing both cold and warm components with distinct physical conditions, and comparing these to other star-forming galaxies.

Contribution

First detailed multi-transition CO and CI analysis of M51's central region, identifying two distinct molecular gas components and their likely heating mechanisms.

Findings

Cold gas has T_kin~10-20 K, moderate density, in PDRs with G_o~10^2.

Warm gas has T_kin~300-3000 K, low density, likely heated by supernovae, stellar winds, or shocks.

M51's warm component has the lowest density among compared galaxies but a high warm gas mass fraction.

Abstract

We present Herschel SPIRE-FTS intermediate-sampled mapping observations of the central ~8 kpc (~150") of M51, with a spatial resolution of 40". We detect 4 12CO transitions (J=4-3 to J=7-6) and the CI 3P2-3P1 and 3P1-3P0 transitions. We supplement these observations with ground based observations of 12CO J=1-0 to J=3-2 and perform a two-component non-LTE analysis. We find that the molecular gas in the nucleus and centre regions has a cool component (T_kin~10-20 K) with a moderate but poorly constrained density (n(H2)~10^3-10^6 cm^-3), as well as significant molecular gas in a warmer (T_kin~300-3000 K), lower density (n(H2)~10^1.6-10^2.5 cm^-3) component. We compare our CO line ratios and calculated densities along with ratios of CO to total infrared luminosity to a grid of photon dominated region (PDR) models and find that the cold molecular gas likely resides in PDRs with a field strength of G_o~10^2. The warm component likely requires an additional source of mechanical heating, from supernovae and stellar winds or possibly shocks produced in the strong spiral density wave. When compared to similar two-component models of other star-forming galaxies published as part of the Very Nearby Galaxies Survey (Arp 220, M82 and NGC 4038/39), M51 has the lowest density for the warm component, while having a warm gas mass fraction that is comparable to those of Arp 220 and M82, and significantly higher than that of NGC 4038/39.