The diversity of atomic hydrogen in slow rotator early-type galaxies

Abstract

We present interferometric observations of HI in nine slow rotator early-type galaxies of the Atlas3D sample. With these data, we now have sensitive HI searches in 34 of the 36 slow rotators. The aggregate detection rate is 32\% $\pm$ 8\%, consistent with previous work; however, we find two detections with extremely high HI masses, whose gas kinematics are substantially different from what was previously known about HI in slow rotators. These two cases (NGC 1222 and NGC 4191) broaden the known diversity of HI properties in slow rotators. NGC 1222 is a merger remnant with prolate-like rotation and, if it is indeed prolate in shape, an equatorial gas disc; NGC 4191 has two counterrotating stellar discs and an unusually large HI disc. We comment on the implications of this disc for the formation of $2\sigma$ galaxies. In general, the HI detection rate, the incidence of relaxed HI discs, and the HI/stellar mass ratios of slow rotators are indistinguishable from those of fast rotators. These broad similarities suggest that the HI we are detecting now is unrelated to the galaxies' formation processes and was often acquired after their stars were mostly in place. We also discuss the HI nondetections; some of these galaxies that are undetected in HI or CO are detected in other tracers (e.g. FIR fine structure lines and dust). The question of whether there is cold gas in massive galaxies' scoured nuclear cores still needs work. Finally, we discuss an unusual isolated HI cloud with a surprisingly faint (undetected) optical counterpart.