ALMA observations of massive molecular gas reservoirs in dusty early-type galaxies

Abstract

Unresolved gas and dust observations show a surprising diversity in the amount of interstellar matter in early-type galaxies. Using Atacama Large Millimetre/submillimeter Array (ALMA) observations we resolve the ISM in z ∼ 0.05 early-type galaxies. From a large sample of early-type galaxies detected in the Herschel Astrophysical Terahertz Large Area Survey (H-ATLAS), we selected five of the dustiest cases, with dust masses M d ∼several × 10 7 M, with the aim of mapping their submillimetre continuum and 12CO(2–1) line emission distributions. These observations reveal molecular gas discs. There is a lack of associated, extended continuum emission in these ALMA observations, most likely because it is resolved out or surface brightness limited, if the dust distribution is as extended as the CO gas. However, two galaxies have central continuum ALMA detections. An additional, slightly offset, continuum source is revealed in one case, which may have contributed to confusion in the Herschel fluxes. Serendipitous continuum detections further away in the ALMA field are found in another case. Large and massive rotating molecular gas discs are mapped in three of our targets, reaching a few ×10 9 M. One of these shows evidence of kinematic deviations from a pure rotating disc. The fields of our two remaining targets contain only smaller, weak CO sources, slightly offset from the optical galaxy centres. These may be companion galaxies seen in ALMA observations or background objects. These heterogeneous findings in a small sample of dusty early-type galaxies reveal the need for more such high spatial resolution studies to understand statistically how dust and gas are related in early-type galaxies.