Revising claims of ubiquitous molecular outflows in the early universe

by Mr. James Nianias

University of Hong Kong
at Astrophysics and Cosmology Seminar

Wed, 24 May 2023, 12:10
Sacta-Rashi Building for Physics (54), room 207


Feedback from star formation is held responsible for dispersing gas and thus regulating the stellar growth of galaxies. In the local universe, galactic-scale outflows comprising multiphase gas and dust are seen from starburst galaxies. At high redshifts, outflows are inferred ubiquitously from the spectral profiles of Lyman-α emission, whereas evidence for outflows in molecular gas -- the reservoir for star formation -- is comparatively scarce. Recently, Spilker et al. (2018, 2020) reported the detection of ubiquitous outflows from gravitationally-lensed dusty star-forming galaxies at z > 4 based on a comparison of the spectral profiles of OH absorption at 119um and [CII] emission at 158um; with [CII] emission tracing gas in the galactic discs, any OH absorption blueshifted beyond the [CII] emission is argued to originate from outflows. Redshifted to sub-mm wavelengths and observed by ALMA, such measurements are complicated by both broad and narrow atmospheric absorption lines that require careful correction as well as determination of measurement uncertainties over the spectral profiles. Here, we re-analyse the sources examined by Spilker et al., giving due consideration to the spectral uncertainties and the correction of atmospheric features in the bandpasses. We show that in their spatially-integrated spectra, statistically significant OH absorption blueshifted beyond the [CII] emission is present in only one out of four sources claimed to show definitive signs of outflows by Spilker et al. Furthermore, in channel maps where each galaxy is lensed into an Einstein ring, we find all confidently-detected OH absorption to be spatially coincident with [CII] emission in all but one source, thus providing no strong evidence for a separate outflow component. Based on these sobering results, we propose observing strategies to mitigate atmospheric absorption and emphasise the importance of accurately measuring the continuum level when studying weak spectral lines at sub-mm wavelengths with ALMA.

Created on 22-05-2023 by Zitrin, Adi (zitrin)
Updaded on 22-05-2023 by Zitrin, Adi (zitrin)