The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular Evolution

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  • 15 February 2004
Abstract
We present results based on panchromatic CO(1-0), optical, NIR, and radio continuum observations of a sample of barred non-starbursts and starbursts. (1) The molecular environment which has built up in the inner kpc of barred galaxies departs markedly from that in the outer disk. It involves gas surface densities of 500-3500 Msun pc-2, gas mass fractions of 10-30 %, and epicyclic frequencies of several (100-1000) km s-1 kpc-1. In this environment gravitational instabilities can only set in at high gas densities (several 100-1000 Msun pc-2), but once triggered, they grow on a short timescale (a few Myr). This `burst' mode may explain why the most intense starbursts tend to be in the center of galaxies. (2) Why do barred galaxies with comparable amounts of circumnuclear molecular hydrogen [M(H2)] show vastly different SFR/[M(H2)]? We present three explanations. Some non-starbursts are in the early stages of bar-driven inflow where large amounts of gas with large non-circular kinematics still exist along the bar and cannot form stars efficiently. The starbursts and a second class of non-starbursts both appear to be a later stage of bar-driven inflow with large gas masses already inside the Outer Inner Lindblad resonance of the bar. However, in the starbursts, the gas densities are highest (1000-3500 Msun pc-2) and remarkably close to the Toomre critical density over a wide region.(3) The dynamical mass enclosed within R=1 kpc (6-30 x 10^9 Msun) can increase significantly over a Gyr for nominal bar-driven inflow rates. We present evidence that the inner kpc of sample galaxies host compact stellar components which have disk-like properties and may represent pseudo-bulges (Kormendy 1993). Implications for secular evolution along the Hubble sequence are discussed.

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