STScI Preprint #1281

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

Observations at the Palomar Observatory were made as part of a continuing collaborative agreement between the California Institute of Technology and Cornell University.

Only two of the twenty highly luminous starburst galaxies analyzed by Smith et al. exhibit circumnuclear rings of star formation. These galaxies provide a link between ~ 10¹¹L 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 ~ 35000 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 10⁷M, 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 non-ringed 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 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.

Status:

Appeared in: The Astrophysical Journal, 510:669-686, 1999 January 10

Affiliations:

¹)Space Telescope Science Institute, 3700 San Martin Drive Baltimore, MD 21218
²)Center for Radiophysics and Space Research, Space Sciences Building, Cornell University, Ithaca, NY 14853
³)NASA Goddard Space Flight Center, Code 681 Greenbelt, MD20771