Some WISDOM on the Molecular Cloud Properties of Early-Type Galaxies

In order to understand how star-formation takes place, it is vital to first understand the properties of the molecular clouds that these stars are born in. Given the typically small sizes of giant molecular clouds (GMCs), studies of these objects have tended to be confined to the most nearby galaxies. The pioneering work of Larson (1981) showed that, at least in the Milky Way, GMCs tended to have reasonably constant surface densities, and appear to be virially bound. Extending to nearby galaxies in our Local Group, and now further with projects like PHANGS-ALMA, we can now probe a significant number of galaxies outside of our own. Work beyond our Galaxy has shown that GMCs in fact span a range of surface densities, but a very narrow dynamic range of virial parameter — however dense these clouds are, they appear to be bound (e.g. Heyer et al, 2009; Leroy et al 2015; Sun et al 2018). These studies, however, have targeted star-forming, late-type spiral galaxies; early-type galaxies present an interesting contrast to these systems. ETGs tend to be much more quickly rotating, and the dynamics of these systems are significantly different to those of LTGs (e.g. Cappellari, 2016). These galaxies also tend to be suppressed on the classical ‘Kennicutt-Schmidt' relation; despite their often quite significant molecular gas content, they are forming stars at a lower efficiency than LTGs (e.g. Davis et al 2014).

I will present a study of the GMC content of several ETGs, taken as part of the WISDOM survey with ALMA. With resolutions of typically 10s of parsec, we can well probe the GMC content of these galaxies, and assess the boundedness of these structures. I will show that the clouds in ETGs span a narrow range of surface densities, but a large dynamic range of virial parameter. This appears to vary systematically with the global properties of the galaxy in question, and so we see no universal “Larson’s Laws” in these galaxies. I argue that it is the dynamical state of these clouds that is suppressing the star-formation compared to star-forming, spiral galaxies.