Capturing the inside-out quenching by black holes with far-infrared atomic line ratios

TL;DR

This paper proposes using far-infrared atomic line ratios, specifically [OIII] 52 and 88 μm, to observationally trace the early stages of inside-out quenching caused by black hole feedback in galaxies.

Contribution

It introduces a physical model linking far-infrared line ratios to black hole feedback effects, validated through simulations, enabling observational detection of quenching onset.

Findings

Line ratios vary with black hole feedback models in simulations.

Line ratios are insensitive to aperture size, facilitating observations.

Integrated line ratios can indicate the onset of inside-out quenching.

Abstract

We propose to use relative strengths of far-infrared fine structure lines from galaxies to characterise early phases of the inside-out quenching by massive black holes (BHs). The BH feedback is thought to quench star formation by evacuating the ambient gas. In order to quantify the feedback effect on the gas density in the galactic centres, we utilise the outputs of IllustrisTNG and Illustris simulations, which implement different BH feedback models. We devise a physical model of H$_{\rm ~II}$ regions and compute the intensities of [O$_{\rm ~III}$] $52$ and $88~{\rm \mu m}$ lines. The line intensity ratio is sensitive to the local electron density, and thus can be used to measure the strength and physical extent of the BH quenching. If the BH feedback abruptly operates and expel the gas when it grows to a certain mass, as modelled in IllustrisTNG, the low-density gas yields relatively weak [O$_{\rm ~III}$] $52$ line with respect to $88~{\rm \mu m}$. In contrast, if the feedback strength and hence the local gas density are not strongly correlated with the BH mass, as in Illustris, the line ratio is not expected to vary significantly among galaxies with different evolutionary stages. We find these features are reproduced in the simulations. We also show that the line ratios are not sensitive to the aperture size for measurement, and thus observations do not need to resolve the galactic centres. We argue that the integrated line ratios can be used to capture the onset of the inside-out quenching by BHs.