We study the characteristics of molecular gas in the central regions of spiral galaxies on the basis of our CO(J=1-0) imaging survey of 20 nearby spiral galaxies using the NRO and OVRO millimeter arrays. Condensations of molecular gas at galactic centers with sizescales ∼ < 1 kpc and CO-derived masses M gas (R < 500pc) ∼ 10 8 -10 9 M ⊙ are found to be prevalent in the gas-rich ∼ L * galaxies. Moreover, the degree of gas concentration to the central kpc is found to be higher in barred systems than in unbarred galaxies. This is the first statistical 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 kpc 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 kpc, resulting in the excess gas in the centers of barred systems. The mean rate of gas inflow is statistically estimated to be larger than 0.1 -1 M ⊙ yr −1 .There is no clear correlation between gas mass in the central kpc and the type of nuclear spectrum (HII, LINER, or Seyfert), suggesting that the amount of gas at this scale does not determine the nature of the nuclear activity. There is, however, a clear correlation for galaxies with larger gas-to-dynamical mass ratios to have HII nuclear spectra, while galaxies with smaller ratios show spectra indicating AGN. This trend may well be related to the gravitational stability of the nuclear gas disk, which is generally lower for larger gas mass fractions. It is therefore possible that all galaxies have active nuclei, but that dwarf AGN are overwhelmed by the surrounding star formation when the nuclear molecular gas disk is massive and unstable.The theoretical prediction of bar-dissolution by condensation of gas to galactic centers is observationally tested by comparing gas concentration in barred and unbarred galaxies. If a bar is to be destroyed so abruptly that the gas condensation at the nucleus does not have enough time to be consumed, then there would be currently unbarred but previously barred galaxies with high gas concentrations. The lack of such galaxies in our sample, together with the current 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 10 8 -10 9 M ⊙ gas in the central kpc.

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The central kpc region of the low luminosity type-1 Seyfert galaxy NGC 1097 has been observed in the CO(1-0) and HCN(1-0) lines with the Nobeyama Millimeter Array and the NRO 45 m telescope. We find a striking enhancement of the HCN emission toward the active nucleus of NGC 1097; a large fraction of the CO emission comes from the circumnuclear starburst ring (r ∼ 10 ′′ or 700 pc at D = 14.5 Mpc), whereas the HCN emission is dominated by a strong unresolved peak at the nucleus. The HCN-to-CO integrated intensity ratio in brightness temperature scale, R HCN/CO , is about 0.11 in the aperture of the 45 m telescope beams (r < 510 pc for CO and r < 640 pc for HCN), but it reaches 0.34 in the smaller aperture (within the central r < 350 × 150 pc region). These CO and HCN properties in NGC 1097 are similar to those in the type-2 Seyfert galaxies NGC 1068 and NGC 5194 (M 51).

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