Are Variable Supermassive Black Holes the Engines of Little Red Dots?

Sammanfattning

Supermassive black holes (SMBHs) can be difficult to identify at high redshift, because many are faint or obscured, which causes some of the traditional Active Galactic Nuclei (AGN) diagnostic methods to fail. Alternatively, photometric variability offers another way to detect accretion onto SMBHs, and has been recently used to identify AGN at high redshifts. At the same time, observations with the James Webb Space Telescope (JWST) have revealed a new population of compact, extremely red systems known as Little Red Dots (LRDs), which have distinctive V-shaped spectral energy distributions (SEDs). Among the several possible scenarios, overmassive AGN at high‑z have been hypothesized to be the engines at the center of LRDs. This thesis investigates whether variability‑selected SMBHs in the Hubble Ultra Deep Field (HUDF) exhibit the same spectral and morphological properties as LRDs, in order to understand the role that SMBHs play in the mysterious LRD population. Using multi‑band photometry from JWST as well as the Hubble Space Telescope (HST), rest‑frame ultraviolet (UV) and optical continuum slopes in Fλ were measured for the 191 variable SMBH candidates in the HUDF sample, and sources with V‑shaped SEDs were classified using similar slope‑slope criteria defined in recent JWST studies. Six out of the 191 AGN in the variability‑selected sample (∼3%) are compatible with a V‑shape SED, while only two (≤1%) exhibit steep UV slopes, rising optical continua and compact morphologies as seen in LRDs, leaving the overlap between the two populations to be small. The results indicate that variability‑selected SMBHs largely occupy the same region of parameter space as star‑forming galaxies, with blue UV and optical continua, while LRDs represent a rare, extreme subset with distinct V‑shaped SEDs. The rarity of LRD‑like sources within variable AGN populations could suggest physical scenarios where LRDs may host dense gas envelopes, or simply be powered by compact starbursts, as suggested in recent literature.