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29th of August 2019
ASKAP and ALMA observations of PKS 1740-517
by Allison et al.
Cold neutral gas is a key ingredient for growing the stellar and central black hole mass in galaxies throughout cosmic history. Allison et al. have used the Atacama Large Millimetre Array (ALMA) to detect a rare example of redshifted 12CO (2–1) absorption in PKS B1740–517, a young, luminous radio galaxy at a redshift of 0.44 that is undergoing a tidal interaction with at least one lower mass companion. The coincident ASKAP neutral hydrogen 21-cm and ALMA molecular absorption features have very similar broad line profiles and reveal a reservoir of cold gas that was possibly tidally stripped from the companion.

The figure above is the ALMA spectrum of PKS B1740–517 showing 12CO absorption towards the radio-loud AGN. Superposed is the average ASKAP 21-cm spectrum showing H I absorption. Although 12CO absorption is clearly detected, there is a noticeable absence of molecular gas detected at the velocity of the narrower, deeper H I component, which is consistent with a single diffuse cold atomic structure. The absence of molecular gas suggests that either this structure is much smaller than 100 pc along our line of sight and/or that it arises from lower metallicity gas, perhaps from a companion galaxy. More details are given in the paper by Allison et al. published in Monthly Notices of the Royal Astronomical Society.

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