NGC 1614: A laboratory for starburst evolution

The modest extinction and reasonably face-on viewing geometry make the luminous infrared galaxy NGC 1614 an ideal laboratory for the study of a powerful starburst. Hubble Space Telescope/near-infrared camera and multiobject spectrometer (NICMOS) observations show (1) deep CO stellar absorption, tracing a starburst nucleus about 45 pc in diameter (2) surrounded by a ∼600 pc diameter ring of supergiant H II regions revealed in Paα line emission (3) lying within a molecular ring indicated by its extinction shadow in H - K, (4) all at the center of a disturbed spiral galaxy. The luminosities of the giant H II regions in the ring are extremely high, an order of magnitude brighter than 30 Doradus; very luminous H II regions, comparable with 30 Dor, are also found in the spiral arms of the galaxy. Luminous stellar clusters surround the nucleus and lie in the spiral arms, similar to clusters observed in other infrared luminous and ultraluminous galaxies. The star-forming activity may have been initiated by a merger between a disk galaxy and a companion satellite whose nucleus appears in projection about 300 pc to the northeast of the nucleus of the primary galaxy. The relation of deep stellar CO bands to surrounding ionized gas rings to molecular gas indicates that the luminous starburst started in the nucleus and is propagating outward into the surrounding molecular ring. This hypothesis is supported by evolutionary starburst modeling that shows that the properties of NGC 1614 can be fitted with two short-lived bursts of star formation separated by 5 Myr (and by inference by a variety of models with a similar duration of star formation). The total dynamical mass of the starburst region of 1.3 x 10⁹ M⊙ is mostly accounted for by the old prestarburst stellar population. Although our starburst models use a modified Salpeter initial mass function (IMF) (turning over near 1 M⊙), the tight mass budget suggests that the IMF may contain relatively more 10-30 M⊙ stars and fewer low-mass stars than the Salpeter function. The dynamical mass is nearly 4 times smaller than the mass of molecular gas estimated from the standard ratio of ¹²CO (1-0) to H₂. A number of arguments place the mass of gas in the starburst region at ∼25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from ¹²CO and the standard ratio.