A detailed view of a molecular cloud in the far outer disk of M 33

Molecular cloud formation in M 33^⋆,⋆⋆

¹ Univ. Bordeaux, Laboratoire d’Astrophysique de Bordeaux, 33270 Floirac, France
e-mail: braine@obs.u-bordeaux1.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ IRAM, 300 rue de la Piscine, 38406 St Martin d’Hères, France
⁴ Univ. de Chile, Santiago, Chile

Received: 24 July 2012
Accepted: 26 October 2012

Abstract

The amount of H₂ present in spiral galaxies remains uncertain, particularly in the dim outer regions and in low-metallicity environments. We present high-resolution CO(1–0) observations with the Plateau de Bure interferometer of the most distant molecular cloud in the local group galaxy M 33. The cloud is a single entity rather than a set of smaller clouds within the broad beam of the original single-dish observations. The interferometer and single-dish fluxes are very similar and the line widths are indistinguishable, despite the difference in beamsize. At a spatial resolution of 10 pc, beyond the optical radius of the M 33, the CO brightness temperature reaches 2.4 Kelvins. A virial mass estimate for the cloud yields a mass of 4.3 × 10⁴ M_⊙ and a ratio N(H₂)/I_CO(1−0) ≃ 3.5 × 10²⁰   cm^-2/(K  km  s^-1). While no velocity gradient is seen where the emission is strong, the velocity is redshifted to the extreme SW and blue-shifted to the far NE. If the orientation of the cloud is along the plane of the disk (i.e. not perpendicular), then these velocities correspond to slow infall or accretion. The rather modest infall rate would be about 2 × 10^-4 M_⊙ yr^-1.