The Luminous Starburst Ring in NGC 7771: Sequential Star Formation?

Abstract

Only two of the 20 highly luminous starburst galaxies analyzed by Smith et al. exhibit circumnuclear rings of star formation. These galaxies provide a link between ~1011 L☉ luminosity class systems and classical, less-luminous ringed systems. In this paper, we report the discovery of a near-infrared counterpart to the 1.6 kpc diameter nuclear ring of radio emission in NGC 7771 (UGC 12815). The ring contains ≈10 radio-bright clumps and ≈10 near-infrared-bright clumps. A displacement between the peaks of the radio and the near-infrared emission indicates the presence of multiple generations of star formation. The estimated thermal emission from each radio source is equivalent to that of ~35,000 O6 stars. Each near-infrared-bright knot contains ~5000 red supergiants, on average. In the case that the radio-bright knots are 4 Myr old and the near-infrared-bright knots are ≈10 Myr old, each knot is characterized by a stellar mass of 107 M☉, and the implied time-averaged star formation rate is ~40 M☉ yr-1. Several similarities are found between the properties of this system and other ringed and nonringed starbursts. Morphological differences between NGC 7771 and the starburst + Seyfert 1 galaxy NGC 7469 (UGC 12332) suggest that NGC 7771 may not be old enough to fuel an active galactic nucleus (AGN), or may not be capable of fueling an AGN. Alternatively, the differences may be unrelated to the presence or absence of an AGN and may simply reflect the possibility that star formation in rings is episodic.