The evolution of blue and red quasars at cosmic noon and beyond

Abstract

Observations show that allmassive galaxies contain a supermassive black hole (SMBH) at their centre and,despite their vastly different spacial scales, SMBHs and their host galaxiesappear to co-evolve. While most descriptions of this phenomenon invoke energyfeed back from the SMBH during an active accretion phase, the exact nature ofthese feedback mechanisms are not fully understood. Accreting SMBHs or activegalactic nuclei (AGN) come in many flavours - ranging in mass, luminosity, accretionrate and colour. The most luminous AGN are known as quasars, and given theirimmense power, quasars are the ideal laboratory for studying the role of AGN feedbackin massive galaxy assembly. The primary aim of this thesis is to understand therole of energetic quasar-driven winds in depositing energy into their hosts,and hence, how these winds impact galaxy evolution at the peak epoch of bothSMBH and galaxy growth - 1.0 ≲ zsys ≲ 4.0. Byharnessing the power of large statistical samples from the Sloan Digital SkySurvey, I therefore investigate the redshift evolution of the outflowproperties of luminous blue quasars. I leverage advanced spectroscopic analysistechniques to show that the primary driver of quasar-driven outflows is the massand the accretion rate of the SMBH. The strongest outflows occur inquasars with MBH > 109 M⊙ and L/LEdd > 0.2 at allredshifts, hence, there is no evidence to suggest a redshift evolution in theoutflow properties of luminous blue quasars. This thesis also addresses thecrucial question of whether dust enhances the efficiency of AGN feedback. Iconduct a detailed investigation of the rest-UV/optical emission from aluminous, heavily reddened quasar (HRQ) at cosmic noon, with bolometric luminosityLBol = 1048.16 erg s−1 and E(B − V)= 1.55 mag. The HRQ hosts strong multi-phase, multi-scale winds consistent withthe trends identified in blue quasars between SMBH mass, accretion rate andoutflow velocity - making it unclear whether dusty quasars host stronger windsthan their blue counterparts. However, a full rest-UV to infrared SED analysisof 60 additional HRQs shows that their hot dust emission is suppressed. Thissuggests that strong AGN feedback processes are active in the red quasar phaseand have cleared the inner regions of dust. Given also that excess rest-UVemission - likely scattered quasar light - is very common in HRQs, they likelymark a key evolutionary stage in SMBH and host galaxy growth. The results of thisthesis cement the fact that studying AGN feedback is crucial to ourunderstanding of galaxy evolution and highlight that only by studying diversepopulations can we gain a complete understanding of the physics that drivesthese phenomena.

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