Affiliation: University of Heidelberg

Contribution: Oral

Title: Rest-frame mid-infrared spectra of quasars at z>7 with MIRI-MRS

Abstract: The existence of supermassive black holes (SMBH) heavier than a billion solar masses at redshifts z>7 is an enduring mystery, since they cannot have grown fast enough barring direct collapse and/or super-Eddington accretion. Naturally, early quasars and AGN have been closely scrutinized to ensure that our estimates of black hole mass are reliable, as well as to hunt for any peculiar signs which could hint to their origins. The mysterious “Little Red Dots” discovered by JWST appear to be very peculiar indeed: highly extincted but still leaking UV, with no X-ray and no bright MIR emission, they look unlike any AGN types known at low redshift. 

The same cannot be said of luminous quasars, which show very little (if any) changes between z=2 and z=7.5 at equal mass and accretion rate. Their SEDs, metal enrichment in both alpha and non-alpha elements, jet properties, and more, are identical across time. Only subtle signs of evolution are present, related to more energetic and more frequent accretion-powered outflows. 

In this context, we used the JWST/MIRI instrument to search for more signs of quasar evolution in the rest-frame infrared. I will present observations of 4 quasars at z>7, the only ones observed so far with the MIRI Medium Resolution Spectrograph. Overall, we find that the lack of evolution extends to longer wavelengths. In particular, all 4 quasars already have massive hot tori of dust in place, giving rise to a bright spectral “upturn” in IR emission. The composition of this dust appears normal. The Balmer decrements and other H-line ratios of the quasars are consistent with no extinction of the broad lines nor the accretion disk. Further, the black hole masses obtained from the infrared Paschen-alpha and -beta lines, which are in theory the least affected by extinction, are fully in agreement with ground-based measurements from the rest-frame UV. 

Our results firmly confirm the existence of overly-massive SMBHs at early times. Indeed, the lack of evolution in bright quasars is good reassurance that many lower-redshift “analogues” exist, for which black hole mass scaling relations have been carefully calibrated (or can be). At the same time, early luminous quasars do not share any of the peculiarities of LRDs. While JWST gives us an unprecedented view of early AGN, a consistent evolutionary picture of the first supermassive black holes has yet to emerge.

This contribution can be found here (when available).