Dr Imogen Whittam

Abstract:

Abstract We investigate the relationship between disc winds, radio jets, accretion rates and black hole masses of a sample of ∼100k quasars at z ≈ 2. Combining spectra from the 17th data release of the Sloan Digital Sky Survey (SDSS) with radio fluxes from the 2nd data release of the Low Frequency ARray (LOFAR) Two-Meter Sky Survey (LoTSS), we statistically characterise a radio loud and radio quiet population using a two-component Gaussian Mixture model, and perform population matching in black hole mass and Eddington fraction. We determine how the fraction of radio loud sources changes across this parameter space, finding that jets are most efficiently produced in quasars with either a very massive central black hole (MBH > 109M⊙) or one that is rapidly accreting (λEdd > 0.3). We also show that there are differences in the blueshift of the $\textrm {C}\, \rm \small {IV}$ λ1549Å line and the equivalent width of the $\rm {He}\, \rm \small {II}$ λ1640Å line in radio loud and radio quiet quasars that persist even after accounting for differences in the mass and accretion rate of the central black hole. Generally, we find an anti-correlation between the inferred presence of disc winds and jets, which we suggest is mediated by differences in the quasars’ spectral energy distributions. The latter result is shown through the close coupling between tracers of wind kinematics and the ionising flux– which holds for both radio loud and radio quiet sources, despite differences between their emission line properties– and is hinted at by a different Baldwin effect in the two populations.

Abstract:

Understanding black hole–galaxy co-evolution and the role of active galactic nucleus (AGN) feedback requires complete AGN samples, including heavily obscured systems. Such sources are key to constraining the black hole accretion rate density over cosmic time, yet they are challenging to identify and characterize across most wavelengths. In this work, we present the first ultraviolet (UV) line-selected ([Ne v] Å and C iv Å) sample of obscured AGN with full X-ray-to-radio coverage, assembled by combining data from the Chandra COSMOS Legacy survey, the COSMOS2020 UV–NIR catalogue, mid- and far-IR photometry from XID+, and radio observations from the Very Large Array and MeerKAT International GHz Tiered Extragalactic Exploration Survey (MIGHTEE) surveys. Using cigale to perform spectral energy distribution (SED) fitting, we analyse 184 obscured AGNs at and , enabling detailed measurements of AGN and host-galaxy properties, and direct comparison with simba hydrodynamical simulations. We find that X-ray and radio data are essential for accurate SED fits, with the radio band proving critical when X-ray detections are missing or in cases of poor IR coverage. Comparisons with matched non-active galaxies and simulations suggest that the [Ne v]-selected sources are in a pre-quenching stage, while the C iv-selected ones are likely quenched by AGN activity. Our results indicate that [Ne v] and C iv selections target galaxies in a transient phase of their co-evolution, characterized by intense, obscured accretion, and pave the way for future extensions with upcoming large area high-z spectroscopic surveys.

Abstract:

Radio continuum emission provides a unique opportunity to study star formation unbiased by dust obscuration. However, if radio observations are to be used to accurately trace star formation to high redshifts, it is crucial that the physical processes that affect the radio emission from star-forming galaxies are well understood. While inverse Compton (IC) losses from the cosmic microwave background (CMB) are negligible in the local universe, the rapid increase in the strength of the CMB energy density with redshift [] means that this effect becomes increasingly important at . Using a sample of high-redshift () Lyman-break galaxies selected in the rest-frame ultraviolet (UV), we have stacked radio observations from the MIGHTEE survey to estimate their 1.4-GHz flux densities. We find that for a given rest-frame UV magnitude, the 1.4-GHz flux density and luminosity decrease with redshift. We compare these results to the theoretical predicted effect of energy losses due to IC scattering off the CMB, and find that the observed decrease is consistent with this explanation. We discuss other possible causes for the observed decrease in radio flux density with redshift at a given UV magnitude, such as a top-heavy initial mass function at high redshift or an evolution of the dust properties, but suggest that IC scattering is the most compelling explanation.