Dwarf AGNs from Variability for the Origins of Seeds (DAVOS): Intermediate-mass black hole demographics from optical synoptic surveys

TL;DR

This paper develops a Monte Carlo model to forecast the population of optically variable active galactic nuclei in dwarf galaxies, helping to understand intermediate-mass black hole demographics and informing future survey strategies.

Contribution

It introduces a novel modeling approach that accounts for spectral and variability properties of IMBHs, enabling predictions of AGN demographics and biases in optical surveys.

Findings

Occupation fractions distinguishable at 2-3 sigma with Rubin Observatory data.

Selection bias causes IMBH mass overestimation relative to host galaxy scaling.

Hourly cadence observations can tightly constrain IMBH masses.

Abstract

We present a phenomenological forward Monte Carlo model for forecasting the population of active galactic nuclei (AGNs) in dwarf galaxies observable via their optical variability. Our model accounts for expected changes in the spectral energy distribution of AGNs in the intermediate-mass black hole (IMBH) mass range and uses observational constraints on optical variability as a function of black hole (BH) mass to generate mock light curves. Adopting several different models for the BH occupation function, including one for off-nuclear IMBHs, we quantify differences in the predicted local AGN mass and luminosity functions in dwarf galaxies. As a result, we are able to model the variable fraction of AGNs as a function of physical host properties, such as host galaxy stellar mass, in the presence of complex selection effects. We find that our adopted occupation fractions for the "heavy" and "light" initial BH seeding scenarios can be distinguished with variability data at the $2-3 \sigma$ level for galaxy host stellar masses below $\sim 10^8 M_\odot$ with the Vera C. Rubin Observatory. We demonstrate the prevalence of a selection bias whereby recovered IMBH masses fall, on average, above the predicted value from the local host galaxy - BH mass scaling relation with the strength of the bias dependent on the survey sensitivity. The methodology developed in this work can be used more broadly to forecast and correct for selection effects for AGN demographic studies in synoptic surveys. Finally, we show that a targeted $\sim$ hourly cadence program over a few nights with the Rubin Observatory can provide strong constraints on IMBH masses given their expected rapid variability timescales.