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The Host Galaxies of Luminous Reddened Quasars at z~2



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Wethers, Clare 


The work in this thesis concerns the host galaxies of a class of luminous, yet heavily-obscured, quasars at z$\sim$2 - a peak epoch of both star formation and black hole accretion. Here, we seek to characterise the star-forming properties of these obscured quasars to improve our understanding of galaxy-quasar coevolution.

A key issue facing host galaxy studies among populations of the most luminous quasars is being able to disentangle the galaxy emission from that of the quasar. With combined observations from the Dark Energy Survey (DES), the VISTA Hemisphere Survey (VHS) and the UKIDSS Large Area Survey (ULAS), we exploit the quasar dust extinction in our sample to demonstrate that the quasar and galaxy emission can be separated via SED-fitting in these systems. By isolating the galaxy emission in this way, we estimate instantaneous SFRs for the galaxies in our sample, based on the restframe UV emission. In general, we find obscured quasars to reside in prodigiously star forming hosts with 25 SFRUV 365 Myr−1. Furthermore, we show that the most luminous quasars reside in the most actively star-forming galaxies, potentially indicating the same gas supply is fuelling both star formation and accretion on to the black hole.

Having isolated the galaxy emission via SED-fitting, we test our ability to model the restframe-UV emission of obscured z$\sim2quasarhostsin2D.Untilnow,morphologicalstudiesofluminousquasarhostshavetypicallybeenlimitedtolowredshiftsorreliedonspacebasedimaging.Bymakinguseofamultibandmodellingcodehowever,wedemonstratethatitispossibletoaccuratelyinferseveralgalaxyproperties(i.e.thepositionofthegalaxyintheimage,(X,Y),itsradius,R{\rm{eff}},axisratio,q{\rm{GAL}}$, angle of orientation, θ, and S'{e}rsic index, n), based on current ground-based imaging, accross the full range of galaxy and quasar luminosities considered in our sample. This potentially opens the door to future ground-based morphological studies of obscured quasars at high redshift.

At sub-mm wavelengths, thermal emission from cold dust peaks, meaning these wavelengths can be used to probe the dust heating by star formation, effectively giving a measure of the obscured star formation in the galaxy. Using targeted observations from SCUBA-2, we trace the 850μm emission in a sample of obscured quasars, finding evidence for prodigious star formation > 2400 Myr−1 in three of the 19 quasars in our sample. The detection rate of our obscured quasar sample is found to be consistent with that of both more heavily-obscured Hot-DOGs and UV-luminous quasars, once the samples have been matched in luminosity and redshift. Furthermore, we find evidence that several of the obscured quasars lie in overdense regions of the sky ( 3 times denser than sub-mm blank fields).





Banerji, Manda
Hewett, Paul


Galaxy quasar coevolution, High redshift, Galaxy evolution, Reddened Quasars, Star formation, Multi-wavelength


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge