Dynamical Mechanism of Episodic Gas Accretion to the Central Region of Spiral Galaxies

Abstract

We performed \textit{N}-body/SPH simulations of isolated spiral galaxies with various bulge-to-disk mass ratios ($M_{\rm bulge}/M_{\rm disk}$) from 0.02 to 0.2 to investigate mass transport from galactic scales (10 kpc) down to circumnuclear disk scales ($\lesssim$ 100 pc). Our analysis revealed these main findings, (1) Gravitational torque from stellar spiral arms causes gas accretion with $\sim1$ $M_\odot$ ${\rm yr}^{-1}$ along the gas spiral arms from a few kpc to a few 100 pc scale. The density of accreting gas is a few 100 ${\rm cm^{-3}}$, comparable to the gas arms. The pressure gradient force is over an order of magnitude weaker than the stellar gravitational torque. (2) Gravitational torque from barred structure causes episodic gas clump accretion with $\sim1$ $M_\odot$ ${\rm yr}^{-1}$ on timescales of 10 Myr from kpc to a few 100 pc scale. The densities of these clumps exceed 700 ${\rm cm^{-3}}$, and this accretion occurs along elliptical orbits with a delayed phase relative to the bar potential \citep{wada1994}. (3) Episodic gas clumpy accretion is important for galactic center instability, confirmed by $M_{\rm bulge}/M_{\rm disk}$ = 0.02 but not by $M_{\rm bulge}/M_{\rm disk}$ = 0.1 and 0.2. This difference occurs because in the bulge-dominated potentials, bar instability is suppressed, and rapid gas clumps accretion does not occur. These findings suggest that gas clump accretion events driven by bars could be a source of high-density gas to the galactic center of the spiral galaxy, potentially promoting temporary activity in the galactic center.