The inner disk of NGC 253

Abstract

The stellar and interstellar matter distributions in the luminous Sc galaxy NGC 253 are analyzed using maps of the near-infrared continuum (λ = 1-2 μm) and the millimeter CO emission line. The stellar disk traced out in the near-infrared exhibits two components: a nuclear peak of diameter ~20" (~300 pc) and an extended, inner disk of diameter ~360" (~6 kpc). Similar components are also evident in the molecular gas distribution. The 2 μm brightness distribution mapped over the full extent of the inner disk is dominated by a barlike feature at position angle 17° east of the major axis extending to ± 120" on opposite sides of the nucleus. The CO distribution exhibits a weak secondary peak just outside the bar. 
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\nThe derived abundance of molecular gas in the inner disk exceeds that of H I by a factor of 10 and exhibits a much steeper falloff with radius. The total mass of H_2 inside R = 4 kpc is ~2 x 10^9 M_☉, which is ~7% of the dynamical mass at the same radius. If the observed far-infrared luminosity in the central R ≤ 500 pc is produced by young stars, the minimum rate of OBA star formation is 2.3 M_☉ yr^(-1), and the observed molecular clouds will be cycled into stars in ≾3 x 10^8 yr. It is speculated that the bar, detected in the near-infrared map, could feed fresh molecular gas from the outer disk to the active, star-forming nucleus.