Emmanuel Durodola

Exploring the AGN Fraction of a Sample of JWST's Little Red Dots at 4 < z < 8:
Overmassive Black Holes Are Strongly Favored

Background: Little Red Dots (LRD) are a class of high redshift (5 < z < 9), optically red, compact galaxies observed by the James Webb Space Telescope. The redshift range occupied by these galaxies makes them valuable to the field of galaxy evolution, as they are likley progenitors of the massive galaxies we observe in the universe today. Various analysis of LRD have revealed some peculiar characteristics, such as high stellar masses given their compact nature, overmassive black holes (in the case where LRD are AGN or host AGN), x-ray quiet, undetected in the infrared, Hα emission (with little to no Hβ emission), and the list goes on. Our work here focuses on the result that suggests LRD host overmassive black holes. We wanted to determine if this result is agnostic to the parameter choice made during SED modeling. Specifically, we wanted to understand how different choices in how the AGN contribution is modeled affects the final black hole mass.

Methodology: Using James Webb Space Telescope photometry presented in Perez-Gonzalez+23, we model the SED of LRD as either AGN, galaxy, or a composite model of both templates to determine the nature of these optically red, compact, high redshift galaxies. SED modelling is a technique where we are able to individually model the different components of a galaxy that come together to form the emission we observe. Depending on the resulting contribution from each component, we can extract the likely nature of the galaxy star forming, AGN or composite. We used the publicly availble CIGALE SED modeling code from Boquien+23, to analyze the Litle Red Dot photometry. We varied the AGN contribution from 0% to maximal 99%, allowing us to test all three scenarios (AGN, galaxy, composite).

Results: Through this analysis, we became the first group to show that the overmassive black hole result from LRD is dependent on the strength of the AGN contribution. For LRD hosting a modest AGN < 20%, the black hole mass is in agreement with the local Reines+Volonteri+15 relationship, while > 20% implies an overmassive black hole. An AGN contribution between 20% and 40% corresponds to a moderately overmassive black hole, while > 40% results in extremely overmassive black holes, favoring the Pacucci+23 high redshift relationship.
There are more interesting results in the paper, which can be found here.