Browsing by Author "Tadhunter, C. N."

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    Do AGN triggering mechanisms vary with radio power? - I. Optical morphologies of radio-intermediate HERGs
    (2019) Pierce, J. C. S.; Tadhunter, C. N.; Ramos Almeida, C.; Bessiere, P. S.; Rose, M.
    Active galactic nuclei (AGNs) with intermediate radio powers are capable of driving multiphase outflows in galaxy bulges, and are also more common than their high-radio-power counterparts. In-depth characterization of the typical host galaxies and likely triggering mechanisms for this population is therefore required in order to better understand the role of radio AGN feedback in galaxy evolution. Here, we use deep optical imaging data to study the detailed host morphologies of a complete sample of 30 local radio AGNs with high-excitation optical emission (HERG) spectra and intermediate radio powers [ z < 0.1; 22.5 < log() < 24.0WHz(-1)]. The fraction of hosts with morphological signatures of mergers and interactions is greatly reduced compared to the 2Jy radio-powerful galaxies [log() > 25.0WHz(-1)] with strong optical emission lines: 53 +/- 9 per cent compared with 94 +/- 4 per cent. In addition, the most radio-powerful half of the sample has a higher frequency of morphological disturbance than the least radio-powerful half (67 +/- 12 per cent and 40 +/- 13 per cent, respectively), including the eight most highly disturbed galaxies. This suggests that the importance of triggering nuclear activity in high-excitation radio galaxies (HERGs) through mergers and interactions reduces with radio power. Both visual inspection and detailed light profile modelling reveal a mixed population of early-type and late-type morphologies, contrary to the massive elliptical galaxy hosts of radio-powerful AGNs. The prevalence of late-type hosts could suggest that triggering via secular, disc-based processes has increased importance for HERGs with lower radio powers (e.g. disc instabilities and large-scale bars).
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    Young stellar populations in type II quasars: timing the onset of star formation and nuclear activity
    (OXFORD UNIV PRESS, 2017) Bessiere, P. S.; Tadhunter, C. N.; Ramos Almeida, C.; Villar Martin, M.; Cabrera Lavers, A.
    Despite the emerging morphological evidence that luminous quasar-like active galactic nuclei (AGN) are triggered in galaxy mergers, the natures of the triggering mergers and the order of events in the triggering sequence remain uncertain. In this work, we present a detailed study of the stellar populations of the host galaxies of 21 type II quasars, with the aim of understanding the sequence of events between the onset of the merger, the triggering of the associated starburst and the initiation of the quasar activity. To this end, we model high-quality, wide spectral coverage, intermediate-resolution optical spectra of the type II quasars. We find that of the 21 objects, the higher order Balmer absorption lines, characteristic of young stellar populations (YSPs), are directly detected in similar to 62 per cent of the sample. We also fit these spectra using a number of combinations of stellar and/or power-law components, representative of viable formation histories, as well as including the possibility of scattered AGN light. We find that similar to 90 per cent of the type II quasar host galaxies require the inclusion of a YSP to adequately model their spectra, whilst 71 per cent of the sample require the inclusion of a YSP with age <100 Myr. Since the ages of the YSP in most type II quasar host galaxies are comparable with the expected lifetimes of the AGN activity, these results provide strong evidence that the quasars are triggered close to the peaks of the merger-induced starbursts.