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Before starting my PhD I spend some time at the European Southern Observatory on a research project, working with Dr. Chris Harrison. This work involved investigating the relationship between the size of the radio emission from AGN with the prevalence of extreme ionised outflows. We found that extreme ionised outflows are more common when the radio emission is compact in AGN host galaxies (Molyneux et al. 2019).
Understanding the physical processes that connect galaxy growth and the growth of their central supermassive black holes remains one of the biggest outstanding problems of galaxy evolution research. There is now a growing body of evidence that there is a strong connection between the presence of ionised outflows and the radio emission in AGN host galaxies.
To investigate this, I worked on a research project at the European Southern Observatory under the supervision of Dr. Chris Harrison. I led a study on a sample of 2922 z < 0.2, spectroscopically-identified Active Galactic Nuclei (AGN) where we explored the relationship between radio size and the prevalence of extreme ionised outflows, as traced using broad [O III] emission-line profiles in Sloan Digital Sky Survey spectra. We determined which sources were extended or compact in the radio using a combination of direct radio size measurements and morphological classifications, and searched for ionised outflows using the [O III] emission line.
Example 1.4 GHz images (≈ 5 arcsec resolution from FIRST) and, where available, overlaid green contours from ≈ 1 arcsec resolution 1.4 GHz images (from our follow up VLA observations on a subset of targets). The insets show ≈ 0.3 arcsec resolution 6 GHz images (with blue contours). Synthesised beams are represented by appropriately coloured ellipses. Compact radio sources (top row) have their radio emission dominated within the SDSS fibre extent (magenta circles), whilst extended sources (bottom row) show significant emission outside of this. Right: Cumulative fraction of AGN with [O III] emission-line components with FWHMB greater than a given value, for: all sources (blue curve), compact radio sources (orange curve) and extended radio sources (green curve).
We found that when the radio emission is compact in AGN host galaxies, extreme ionised outflows are more common. This result was also most enhanced for radio luminosities where AGN dominate the radio emission. Follow-up high resolution observations of a subset of targets imply that compact, low-power radio jets may be responsible for the high-velocity ionised gas, in line with some recent model predictions. This reveals a strong connection between radio emission and ionised outflows — possibly due to compact radio jets which are interacting with the host galaxy.
On-going and future, deep and large-area multi-frequency radio surveys such as VLASS, LOFAR and eventually SKA, that are combined with spectroscopic information, will be crucial to unravelling a complete picture of the origin of radio emission in AGN and to further establish the physical processes behind AGN–host galaxy interactions.
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