The central region of barred galaxies: Molecular environment, starbursts, and secular evolution

Stellar bars drive gas into the circumnuclear (CN) region of galaxies. To investigate the fate of the CN gas and star formation ( SF), we study a sample of barred nonstarbursts and starbursts with high-resolution CO, optical, H alpha, radio continuum, Br gamma, and HST data, and find the following. ( 1) The inner kiloparsec of bars differs markedly from the outer disk. It hosts molecular gas surface densities Sigma gas-m of 500-3500 M-circle dot pc(-2), gas mass fractions of 10%-30%, and epicyclic frequencies of several 100-1000 km s(-1) kpc(-1). Consequently, in the CN region gravitational instabilities can only grow at high gas densities and on short timescales, explaining in part why powerful starbursts reside there. (2) Across the sample, we find bar pattern speeds with upper limits of 43 - 115 km s(-1) pc(-1) and outer inner Lindblad resonance radii of > 500 pc. (3) Barred starbursts and nonstarbursts have CN SF rates of 3-11 and 0.1-2 M-circle dot yr(-1), despite similar CN gas masses. The Sigma(gas-m) value in the starbursts is larger ( 1000 - 3500 M circle dot pc(-2)) and close to the Toomre critical density over a large region. (4) Molecular gas makes up 10%-30% of the CN dynamical mass and fuels large CN SF rates in the starbursts, building young, massive, high-V/sigma components. Implications for secular evolution along the Hubble sequence are discussed.