Searching for AGN-associated HI-21cm absorption with the MIGHTEE survey

Charlton, Kathleen

Searching for AGN-associated HI-21cm absorption with the MIGHTEE survey

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Astronomy

Faculty

Faculty of Science

Abstract

We present a targeted search for AGN-associated 21-cm HI absorption in the COSMOS field up to z = 0.48, using HI and continuum data from the MeerKAT International GHz Tiered Extragalactic Exploration (MIGHTEE). MIGHTEE is a galaxy evolution survey with the SKA precursor telescope MeerKAT. The MIGHTEE data has a high spectral resolution of 26.1 kHz (→ 8 km s→1) and has excellent sensitivity, allowing us to probe HI absorption against continuum fluxes down to S1.28 GHz → 40 μJy beam→1, far fainter than previously possible. Our primary goal is to identify inflows and outflows of gas associated with the radio galaxy itself, to examine the interplay between gas, AGN fuelling, AGN feedback, and the impact of these on star formation and overall evolution of the host galaxies. We search for absorption against 111 radio AGN previously identified in the MIGHTEE Early Science field using the automated linefinder flashfinder. Using the linefinder, we identify 12 spectra as having potential spectral lines. We confirm four candidates using the Mahalanobis Distance, a multivariate generalisation of the Z-score which quantifies the o!set between a data point and a reference distribution. We report HI absorption detections against three resolved continuum sources: J100028.27 +024103.3, J100043.19 +014607.8 and J100131.14+022924.6, as well as against one unre- solved continuum source J095927.25+014634.8. With large velocity o!sets from the back- ground hosts and full width half maxima below 200 km s→1, it is likely that we are probing gas in the outer intergalactic medium, rather than close to the supermassive black hole. We employ the extensive multi-wavelength information in the COSMOS field to examine the HI detections as well as the host galaxy properties and environments. We detect two absorbers against the resolved radio galaxy J100028.27+024103.3, hosted by the BCG of cluster WHLJ100028.3 +024103. We suggest that the HI is stripped gas from nearby galaxy zCOSMOS 84586, which if confirmed would make it the highest redshift detection of HI cluster dynamics ever observed. The absorbing HI associated with J100043.19+014607.8, J100131.14+022924.6 and J095927.25+014634.8 is inferred to be part of large-scale HI disks or gas shells in the outer halo. The detections display a range of peak optical depths 0.19+0.04 →0.04 % < ωpeak < 81+10 →11 %. We find that J095927.25+014634.8 is the lowest luminosity radio source with detected HI in absorption at a distance greater than z > 0.1 to date. These results help us probe the current limitations of low-luminosity HI absorption studies and look to what will be achieved with the SKA.

Keywords

AGN-associated

MIGHTEE

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