Nuclear bar catalyzed star formation: (CO)-C-13, (CO)-O-18, and molecular gas properties in the nucleus of Maffei 2

Abstract

We present ~3" resolution maps of CO, its isotopologues, and HCN from in the center of Maffei 2. The J = 1-0 rotational lines of 12CO, 13CO, C18O and HCN, and the J = 2-1 lines of 13CO and C18O were observed with the Owens Valley Radio Observatory ( OVRO) and Berkeley-Illinois- Maryland Association (BIMA) arrays. The lower opacity CO isotopologues give more reliable constraints on H-2 column densities and physical conditions than optically thick 12CO. The J = 2-1/1-0 line ratios of the isotopologues constrain the bulk of the molecular gas to originate in low-excitation, subthermal gas. From large velocity gradient (LVG) modeling, we infer that the central giant molecular clouds (GMCs) have nH2 ~10(2.75) cm^(-3) and T-k ~30 K. Continuum emission at 3.4, 2.7, and 1.4 mm was mapped to determine the distribution and amount of H II regions and dust. Column densities derived from C18O and 1.4 mm dust continuum fluxes indicate the standard Galactic conversion factor overestimates the amount of molecular gas in the center of Maffei 2 by factors of ~2-4. Gas morphology and the clear "parallelogram" in the position-velocity diagram shows that molecular gas orbits within the potential of a nuclear (~220 pc) bar. The nuclear bar is distinct from the bar that governs the large-scale morphology of Maffei 2. Giant molecular clouds in the nucleus are nonspherical and have large line widths, due to tidal effects. Dense gas and star formation are concentrated at the sites of the x(1)-x(2)-orbit intersections of the nuclear bar, suggesting that the starburst is dynamically triggered.

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