Kerrison et al. report the ASKAP Flash discovery of an intervening neutral hydrogen (HI) 21 cm absorption line at a redshift of z = 0.882 towards the z = 1.284 quasar PKS 0405−385. A follow-up optical spectrum, obtained with the 8.1 m Gemini South telescope, revealed absorption lines associated with the intervening galaxy detected in FLASH data (blue, dot-dashed), and additionally two previously unidentified intervening galaxies. But which galaxies are these?

The image above left is a three colour image from the Legacy Survey of a region centred on PKS 0405−385. Five nearby galaxies visible in the image are identified as A–E. Galaxies A–D all have photometric redshifts within the range 0.8 ~ 1.2, with Galaxy A closest to the redshift of the FLASH detection at z = 0.8 ± 0.3. The image above right is the three colour Gemini image obtained as part of follow up on this source. The white rectangle indicates the positioning of the slit used to obtain spectroscopy, aligned to span both PKS 0405−385 and Galaxy A, and coincidentally also spanning Galaxy D. However, it was not possible to extract any identifiable spectral features at the location of Galaxy A, and so the team were unable to determine whether it is the host of the intervening H I. Integral field spectroscopy spanning PKS 0405−385 and galaxies A–E, along with deeper optical imaging, will be key to securely identifying the intervening systems seen in absorption against PKS 0405−385.