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

We study the molecular environment, the onset of starbursts, and secular evolution in the circumnuclear (inner 1–2 kpc) region of barred galaxies using a carefully selected sample of barred non-starbursts and starbursts, along with a panchromatic dataset made of high resolution CO (J=1–0) observations, optical, NIR, Hα, radio continuum, Brγ, and archival HST data. (1) We find that the molecular environment which has built up in the inner kpc of barred galaxies departs markedly from that present in the outer disk of galaxies. It involves molecular gas masses of 3× 10 to 2× 10 M⊙, molecular gas surface densities of 500-3500 M⊙ pc , gas mass fractions of 10 to 30 %, and epicyclic frequencies of several 100 to several 1000 km s kpc. In this environment, gravitational instabilities can set in only at very high gas densities (few 100-1000 M⊙ pc ), but once triggered, they grow on a short timescale of a few Myrs. This high density short timescale ‘burst’ mode may explain why the most intense starbursts tend to be in the central parts of galaxies. The molecular environment in the inner kpc of the ultra luminous infrared galaxy Arp 220 is a scaled up version of the one shown by barred starbursts, suggesting that interactions build up even more extreme conditions. (2) The barred starbursts and non-starbursts have circumnuclear SFRs of 3–11 and 0.1–2 M⊙ yr , respectively, although they host comparable amounts of molecular hydrogen in the inner few kpc. We suggest that some of the large variations in SFR per unit gas mass (SFR/MH2) are tied in to different stages of bar-driven inflow. In non-starbursts which are in the early stages of bar-driven inflow (henceforth type I non-starbursts), a large fraction of the circumnuclear gas is still along the large-scale stellar bar, has large non-circular kinematics and is not forming stars efficiently. In contrast, the starbursts and several non-starbursts (henceforth type II nonstarbursts) seem to be in later stages of bar-driven inflow where their circumnuclear molecular gas is already concentrated within the inner kpc, inside the outer inner Lindblad resonance of the bar, and has predominantly circular motions. The higher SFR/MH2 in the starbursts may be related to the fact that their gas surface densities are larger (1000–3500 M⊙ pc ) and close to the Toomre critical density over a large region. (3) In both barred non-starbursts and starbursts, the Q parameter reaches its minimum value of ∼ 1–2 in the region of star formation, despite an order of magnitude variation in the gas surface density and epicyclic frequency. This remarkable behavior strongly suggests that the onset of gravitational instabilities as characterized by Q plays an important role even in the inner kpc region. (4) We suggest that an episode of bar-driven inflow causes a barred galaxy to go through starburst and non-starburst phases typified by our sample. This may partly explain why no one-to-one correlation is seen between bars and central starbursts, even if a statistical correlation exists at the high luminosity end. The dynamical mass enclosed within the inner kpc radius of the barred galaxies in our sample is 6–30 ×10 M⊙ and can increase by several % over a Gyr for nominal bar-driven inflow rates. We present evidence that the inner kpc of selected sample galaxies are building compact stellar components which have disk-like properties and may belong to the class of pseudo-bulges (Kormendy 1993). We speculate that over its lifetime, a barred galaxy can experience numerous episodes of minor mergers and bar-driven gas inflows which can cause a buildup of the central mass concentration, bulge, and lead to secular evolution along the Hubble sequence.