My research focusses on the co-evolution of galaxies and their supermassive black holes, in particular the process of quenching whereby galaxies stop forming stars. I wrote a review of the topic in the introduction of the final paper from my thesis, Smethurst et al. (2017) which looked at the environmental dependence of quenching. During my position as research fellow I am focussing on determining whether active black holes (AGN) can cause quenching of star formation in a galaxy due to AGN feedback. I am currently using the SDSS-IV MaNGA survey data to infer the quenching histories of galaxies hosting AGN (selected to be a star formation agnostic sample). The MaNGA survey is an IFU survey which provides spectra in up to 127 mosaic 'spaxels' for each galaxy it targets. Using this data along with a spectral inference code I am currently developing will allow me to determine if AGN are the true cause of quenching in galaxies or their activity is a consequence of another quenching mechanism.

Figure 1: Image of an example target galaxy from SDSS (left) showing the hexagonal fibre bundle coverage (magenta) with corresponding data from the MaNGA survey showing the spectral index D4000 (middle, a tracer of stellar age) and equivalent width of the Hα emission line (right, a tracer of star formation) resolved across the galaxy in each of the spaxels.

This work was inspired by a study I completed in my thesis and now published in Smethurst et al. (2016). Using a code I developed, STARPY I inferred the global quenching histories of over 1000 AGN host galaxies in the SDSS using their optical and NUV photometry. This work showed that across the AGN host galaxy population, there is a higher likelihood for rapid, recent quenching to have occurred than in a mass matched sample of currently inactive galaxies. Whilst this is tempting to claim that this inferred quenching is due to the AGN themselves, we need the follow-up work with the MaNGA survey data in order to confirm this.

Figure 2: The difference in the time (left) and rate (right) of quenching in nearby galaxies that host AGN (black, solid) compared to galaxies with inactive SMBHs (red, dashed). The plots are normalised so that the areas underneath the curves are equal. The peaks in the black, solid lines show that recent (∼ 12 Gyr left panel), rapid (∼ 0.1 Gyr right panel) quenching is occurring in the AGN host galaxy population, which is not seen in the inactive galaxy population (red dashed lines), suggesting this quenching may be caused by AGN feedback. Adapted from Figures 4 and 5 in Smethurst et al. (2016).

In Figure 2, you can also see that some AGN host galaxies have quenched slowly, with rates that are more consistent with secular, non-violent processes. This highlights another area of my research, highlighting the importance of non-merger processes in the contribution to black hole growth. In Simmons, Smethurst Lintott (2017) we showed that a population of bulgeless galaxies had measured supermassive black hole masses 2 orders of magnitude higher than predicted by the black hole mass - bulge mass relation. However, their black hole masses were as expected given their total stellar mass. This work was followed up by Martin et al. (2018) who showed that this was also the case when tracing the black hole masses through the merger trees of the Horizon-AGN simulation.

Using STARPY I have also looked at the visual morphological dependence of quenching across a sample of over 126,000 galaxies from the SDSS, published in Smethurst et al. (2015). In this paper I showed that contrary to previous work the Green Valley is NOT a 'red herring'. More recently I have also looked at the kinematic morphology dependence of quenching using STARPY and have found that slow rotators are more likely to quench at a faster rate than fast rotators. This work was undertaken with the MaNGA survey data and has been published in Smethurst et al. (2018).

With my inference code I have been able to study a broad range of different possible quenching mechanisms in order to help understand the bigger picture of galaxy evolution.

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