Bar‐driven Transport of Molecular Gas to Galactic Centers and Its Consequences

Abstract

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.

Kinematics and dynamics of molecular gas in galactic centers

“…Galaxies are plotted with different symbols for barred and unbarred morphological types (SAB+SB and SB, respectively, in de Vaucouleurs et al 1991). It is evident from the data that barred galaxies have higher degrees of gas concentration to their central kpc (Sakamoto et al 1999b).…”

“…Regarding the formation mechanism of CMZs, statistical evidence has been presented for bar-driven transport of molecular gas toward galactic centers by Sakamoto et al (1999b), Sheth et al (2005), and Kuno et al (2007). Their observations are summarized in Figure 1.…”

“…Middle: A variant of Figure 2 of Sheth et al (2005) plotting the distribution of gas central concentration indices against galaxy morphologies. The data of Sheth et al (2005) and Sakamoto et al (1999b) are shown together. Small symbols with vertical lines are 2σ upper limits.…”

The role of bars

“…There could even be a lower fraction of strong bars in Seyferts (Shlosman et al 2000). The only positive signs of the bar action is the observation of more outer rings in Seyfert galaxies (Hunt & Malkan 1999, and Agüero et al, this meeting), and more gas concentration in barred galaxies (Sakamoto et al 1999). Also, there is always circumnuclear dust in AGN, while not always in normal galaxies (Martini et al 2003).…”

The Fueling and Evolution of AGN: Internal and External Triggers

“…Moderate (4 ′′ ) resolution CO(J=1-0) interferometric surveys of molecular gas in the circumnuclear region of nearby spirals show that the molecular gas central concentration (Fig. 5) is on average higher in barred than in unbarred galaxies (Sakamoto et al 1999). Barred galaxies also show What happens once gas reaches the central kpc region of barred galaxies?…”