Bar-driven transport of molecular gas to galactic centers and its consequences

We study the characteristics of molecular gas in the central regions of spi ral galaxies on the basis of our CO(J = 1-0) imaging survey of 20 nearby sp iral galaxies using the NRO and OVRO millimeter arrays. Condensations of mo lecular gas at galactic centers with size scales less than or similar to 1 kpc and GO-derived masses M-gas(R < 500 pc) similar to 10(8)-10(9) M., are found to be prevalent in the gas-rich similar to L* galaxies. Moreover, the degree of gas concentration to the central kiloparsec is found to be highe r in barred systems than in unbarred galaxies. This is the first statistica l evidence for the higher central concentration of molecular gas in barred galaxies, and it strongly supports the theory of bar-driven gas transport. It is most likely that more than half of molecular gas within the central k iloparsec of a barred galaxy was transported there from outside by the bar. The supply of gas has exceeded the consumption of gas by star formation in the central kiloparsec, resulting in the excess gas in the centers of barr ed systems. The mean rate of gas inflow is statistically estimated to be la rger than 0.1-1 M. yr(-1). There is no clear correlation between gas mass i n the central kiloparsec and the type of nuclear spectrum (H II, LINER, or Seyfert), suggesting that the amount of gas at this scale does not determin e the nature of the nuclear activity. There is, however, a clear correlatio n for galaxies with larger gas-to-dynamical mass ratios to have H II nuclea r spectra, while galaxies with smaller ratios show spectra indicating activ e galactic nuclei (AGNs). This trend may well be related to the gravitation al stability of the nuclear gas disk, which is generally lower for larger g as mass fractions. It is therefore possible that all galaxies have active n uclei, but that dwarf AGNs are overwhelmed by the surrounding star formatio n when the nuclear molecular gas disk is massive and unstable. The theoreti cal prediction of bar dissolution by condensation of gas to galactic center s is observationally tested by comparing gas concentration in barred and un barred galaxies. If a bar is to be destroyed so abruptly that the gas conde nsation at the nucleus does not have enough time to be consumed, then there would be currently unbarred but previously barred galaxies with high gas c oncentrations. The lack of such galaxies in our sample, together with the c urrent rates of gas consumption at the galactic centers, suggests that the timescale for bar dissolution is larger than 10(8)-10(10) yr or a bar in a L* galaxy is not destroyed by a condensation of 108-109 Mg gas in the centr al kiloparsec.