Multiple-scale gas infall through gravity torques on Milky Way twins

TL;DR

This study investigates how gravitational torques from bars and spirals in Milky Way-like galaxies facilitate gas inflow towards the center, crucial for feeding supermassive black holes, by analyzing multi-scale observations of nearby barred and unbarred galaxies.

Contribution

It quantifies the efficiency of bars and spirals in driving gas inward at different scales, using multi-resolution data from various telescopes on a sample of Milky Way analogues.

Findings

Barred galaxies show negative torques at kiloparsec scales, indicating gas inflow.

Inside the inner Lindblad resonance, torques are negative in some cases, suggesting inward gas movement.

Unbarred galaxies exhibit positive torques, indicating outward gas movement or lack of inflow.

Abstract

One of the main problems raised by the feeding of supermassive black holes (SMBHs) at the centres of galaxies is the huge angular momentum of the circumnuclear gas and of the gas reservoir in the galaxy disk. Because viscous torques are not efficient at kiloparsec or 100 pc scales, the angular momentum must be exchanged through gravity torques that arise from the non-axisymmetric patterns in the disks. Our goal here is to quantify the efficiency of bars and spirals in driving the gas towards the centre at different scales in galaxies. We selected a sample of nearby galaxies considered to be analogues of the Milky Way, that is, galaxies of late morphological type Sbc. Their bar strength was variable, either SB, or SAB, or SA, so that we were able to quantify the influence of the bar. The gravitational potential was computed from deprojected red images, either from Hubble Space Telescope or Legacy survey, depending on the spatial resolution and field of view considered. The torques were computed on the gas through CO emission maps from ALMA at different resolutions. H$\alpha$ maps from MUSE were used, when available. Eight out of ten galaxies are barred. The torques are found to be negative in the eight barred objects at kiloparsec scales, between corotation and the inner Lindblad resonance (ILR), with a loss of angular momentum in a few rotations. Inside the ILR, the torques are negative in only five cases, with a timescale of one to two rotations. The torques are positive for the galaxies without bars. The torques applied on the ionized gas are comparable to what is deduced from molecular gas. The bars are confirmed to be the essential pattern in the SMBH feeding at kiloparsec and 100 pc scales; higher-resolution gas maps are required to explore scales of 10 pc.