Bar-formation as driver of gas inflows in isolated disc galaxies

TL;DR

This study uses high-resolution simulations to show that gas inflows in isolated disc galaxies are most significant during the transient phase of bar formation, not when the bar is fully established, challenging previous assumptions.

Contribution

It reveals that the peak gas inflow occurs during bar formation, highlighting the transient phase as crucial for fueling galactic nuclei, which has been overlooked in prior research.

Findings

Gas inflow peaks during bar formation, not after.

A 'dead zone' inhibits later gas inflows.

Transient bars are more effective in fueling AGNs.

Abstract

Stellar bars are a common feature in massive disc galaxies. On a theoretical ground, the response of gas to a bar is generally thought to cause nuclear starbursts and, possibly, AGN activity once the perturbed gas reaches the central super-massive black hole. By means of high resolution numerical simulations we detail the purely dynamical effects that a forming bar exerts on the gas of an isolated disc galaxy. The galaxy is initially unstable to the formation of non-axisymmetric structures, and within 1 Gyr it develops spiral arms that eventually evolve into a central stellar bar on kpc scale. A first major episode of gas inflow occurs during the formation of the spiral arms while at later times, when the stellar bar is establishing, a low density region is carved between the bar co-rotational and inner Lindblad resonance radii. The development of such "dead zone" inhibits further massive gas inflows. Indeed, the gas inflow reaches its maximum during the relatively fast bar formation phase and not, as often assumed, when the bar is fully formed. We conclude that the low efficiency of long-lived, evolved bars in driving gas toward galactic nuclei is the reason why observational studies have failed to establish an indisputable link between bars and AGNs. On the other hand, the high efficiency in driving strong gas inflows of the intrinsically transient process of bar formation suggests that the importance of bars as drivers of AGN activity in disc galaxies has been overlooked so far. We finally prove that our conclusions are robust against different numerical implementations of the hydrodynamics routinely used in galaxy evolution studies.