Constraints from dwarf galaxies on black hole seeding and growth models with current and future surveys

Abstract

Dwarf galaxies are considered to be potential ideal test-beds for constraining models of the seeding and tracing of the growth of supermassive and intermediate mass black holes (MBH) via their black hole occupation fraction (BHOF). Disentangling seeding from the confounding effects of mass assembly is, however, challenging. In this work, we use semi-analytical models (SAMs) to probe how various surveys perform at teasing apart different seed and growth scenarios. We check for differences in the measured BHOF given various cuts to black hole mass and AGN luminosity and develop a scheme to robustly compare SAMs, with their intrinsic uncertainties, to X-ray observations. We demonstrate that to tell seeding models apart, we need to detect or model all AGN brighter than $10^{37}\ \rm{erg \ s^{-1}}$ in galaxies of $M_* \sim 10^{8-10} \ \rm{M_{\odot}}$ Shallower surveys, like eRASS, cannot distinguish between seed models even with the compensation of a much larger survey volume. We show that the AMUSE survey strongly favours heavy seed models, growing with empirically motivated growth models either a power-law Eddington Ratio Distribution Function (ERDF) or one in which black hole accretion is tagged to the star-formation rate (AGN-MS). These two growth channels in turn can then be distinguished by the AGN luminosity function at $< 10^{44}\ \rm{erg \ s^{-1}}$. The different models also predict different radio scaling relations, which we quantify using the fundamental plane of black hole activity. We close with recommendations for the design of upcoming multi-wavelength campaigns that can optimally detect MBHs in dwarf galaxies.Comment: Submitted to AAS Journal