Arpita Ganguly   (India)

 a.ganguly @ thphys.uni-heidelberg.de

Hot dust around the first supermassive black holes: Probing the Torus Evolution of Luminous Quasars with SPHEREx

My research focuses on the dusty environments surrounding luminous quasars, in particular the hot dust torus around accreting supermassive black holes. These complex dusty structures surround the accretion disk, reprocess energy from the central engine, and emit it in infrared wavelengths. A key signature of this hot dust is an upturn in the rest-frame spectrum at wavelengths ≳ 1 μm, closely resembling black-body emission with temperatures of order 1200– 1400 K.
Recent JWST observations of quasars at z > 7 have revealed very hot and massive dust tori and radiative-transfer modelling suggests that these structures may contain 0.2–7% of the total dust mass of their host galaxies (Bosman+25). This raises a fundamental question: are such extreme dusty environments unique to the earliest quasars, or are they common among quasars across all redshifts?
To address this, I use SPHEREx, a new full-sky near-infrared spectro-photometric survey, in synergy with UV/optical spectra from SDSS. SPHEREx provides continuous wavelength coverage that directly captures the hot-dust upturn around rest-frame 1 μm, enabling a homogeneous study of torus emission in more than 10,000 low-redshift quasars. I also aim to extend this work to higher redshift quasar candidates at z > 1 using JWST MIRI or NIRSpec data. The long-term goal of this project is to constrain how the physical properties of hot dusty tori—including their geometry, dust content, temperature, and mass— evolve with redshift, thus contributing to the bigger picture of quasar evolution through cosmic history.

Supervisor:    Sarah Bosman   (ITP/MPIA)