Abstract

Contributed Talk - Splinter SMBHs

Thursday, 10 September 2026, 16:51   (MW-0250)

Hot Dust Across Cosmic History: Probing the Torus Evolution of Luminous Quasars with SPHEREx

Arpita Ganguly, Sarah E.I. Bosman
University of Heidelberg

Dusty tori play an essential role in the structure of quasars, reprocessing radiation from the accreting supermassive black hole and mediating the interaction between the central engine and the surrounding interstellar medium. Despite their importance, the physical properties of these structures–such as their geometry, mass, and dust content–remain poorly constrained, particularly in the most luminous systems. A major challenge is that the bulk of the torus emission emerges in the infrared, a wavelength regime that has historically been only partially accessible from the ground, leaving key aspects of the dusty environment around quasars largely unexplored. Recent mid-infrared spectroscopy with the James Webb Space Telescope (JWST) has provided the first direct constraints on the hot-dust component of luminous quasars at very high redshifts. Observations of four quasars at z > 7 reveal prominent dust emission consistent with compact, massive tori at temperatures exceeding 1400K (Bosman et al. 2025). Radiative-transfer modeling provides further insight into their geometries and dust masses, which can reach 0.2–7% of the total dust mass of their host galaxies. At Eddington accretion rates, such reservoirs would be depleted within only a few million years, implying rapid dust formation and efficient replenishment mechanisms in the early Universe. However, the small size of this sample leaves open a fundamental question: are such extreme torus properties unique to the earliest quasars, or are they also present in luminous quasars at later cosmic times? In this talk, I will present my work addressing this question by leveraging the recently launched all-sky spectrophotometric survey of the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer (SPHEREx). With continuous coverage from 0.75μm to 5μm, SPHEREx enables direct measurements of hot-dust emission, characterised by a change in spectral slope around 1μm in the rest-frame spectra of luminous quasars out to (z ≃ 1). In synergy with the large spectroscopic samples from the Sloan Digital Sky Survey (SDSS), these data will enable the first homogeneous study of hot-dust emission in luminous quasars across cosmic time. Using more than 10,000 combined SPHEREx and SDSS spectra at (z ≤ 1), this work will provide the first statistical constraints on the physical properties of dusty tori in bright, low-redshift quasars. The methodology also has strong potential for application to other high-z infrared data from JWST/NIRSpec and MIRI observations, where combined photometric and spectroscopic data can reveal so much about the torus. This benchmark will be crucial for determining whether the extreme dust structures observed at the highest redshifts are a unique phase of quasar evolution or a common feature of luminous quasars.