Molecular Gas and Dust in Infrared Luminous Galaxies

Abstract

We have carried out observations with the James Clerk Maxwell Telescope (JCMT) of the CO(3–2) and C0(2-l) lines and of the submillimeter continuum of a sample of 7 farinfrared (FIR) luminous galaxies. All of these galaxies possess FIR luminosities higher than 10¹¹ L_⨀. Together with data for the CO(1-0) intensities from the literature, we derived the ratios of the brightness temperatures R ₂₁ = T _b (2 — 1)/T _b (1 — 0) and R ₃₂ = T _b (3 —2)/T _b (2 — 1). The galaxies exhibit, in spite of their similar FIR and CO luminosities, very different excitation characteristics. In some galaxies, both line ratios are around 1, consistent with thermalized, optically thick CO. In other galaxies, however, one or both line ratios are significantly lower than 1, indicating the CO is subthermally excited in regions of only moderate molecular gas density.

The submillimeter continuum emission can be well explained by warm, thermally emitting dust (T _dust = 30 — 40 K). The gas masses estimated from the 800 µm continuum flux, M _g,800, and from the CO emission, Mg,CO, are similar, although M _g,CO is, especially at lower masses, systematically higher than M _g,800.