Explaining the reportedly over-massive black holes in early-type galaxies with intermediate-scale discs

TL;DR

This paper demonstrates that proper decomposition of early-type galaxies with intermediate-scale discs resolves discrepancies in black hole mass estimates, reaffirming the fundamental correlation between black hole and spheroid mass.

Contribution

It introduces a correct modeling approach for intermediate-scale discs in early-type galaxies, clarifying black hole mass measurements and their relation to spheroid properties.

Findings

Correct decomposition aligns black hole and spheroid mass relations.

Intermediate-scale discs are often misclassified, affecting black hole mass estimates.

Revised models negate the need for alternative galaxy evolution scenarios.

Abstract

The classification "early-type" galaxy includes both elliptically- and lenticular-shaped galaxies. Theoretically, the spheroid-to-disc flux ratio of an early-type galaxy can assume any positive value, but in practice studies often consider only spheroid/disc decompositions in which the disc neatly dominates over the spheroid at large galaxy radii, creating an inner "bulge" as observed in most spiral galaxies. Here we show that decompositions in which the disc remains embedded within the spheroid, labelled by some as "unphysical", correctly reproduce both the photometric and kinematic properties of early-type galaxies with intermediate-scale discs. Intermediate-scale discs have often been confused with large-scale discs and incorrectly modelled as such; when this happens, the spheroid luminosity is considerably underestimated. This has recently led to some surprising conclusions, such as the claim that a number of galaxies with intermediate-scale discs (Mrk 1216, NGC 1277, NGC 1271, and NGC 1332) host a central black hole whose mass is abnormally large compared to expectations from the (underestimated) spheroid luminosity. We show that when these galaxies are correctly modelled, they no longer appear as extreme outliers in the (black hole mass)-(spheroid mass) diagram. This not only nullifies the need for invoking different evolutionary scenarios for these galaxies but it strengthens the significance of the observed (black hole mass)-(spheroid mass) correlation and confirms its importance as a fundamental ingredient for theoretical and semi-analytic models used to describe the coevolution of spheroids and their central supermassive black holes.