NGC 1614: A laboratory for starburst evolution

The modest extinction and reasonably face-on viewing geometry make the lumi nous infrared galaxy NGC 1614 an ideal laboratory for the study of a powerf ul starburst. Hubble Space Telescope/near-infrared camera and multiobject s pectrometer (NICMOS) observations show (1) deep CO stellar absorption, trac ing a starburst nucleus about 45 pc in diameter (2) surrounded by a similar to 600 pc diameter ring of supergiant H II regions revealed in Pa alpha li ne emission (3) lying within a molecular ring indicated by its extinction s hadow in H-K, (4) all at the center of a disturbed spiral galaxy. The lumin osities 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, comparab le with 30 Dor, are also found in the spiral arms of the galaxy. Luminous s tellar clusters surround the nucleus and lie in the spiral arms, similar to clusters observed in other infrared luminous and ultraluminous galaxies. T he 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 propag ating outward into the surrounding molecular ring. This hypothesis is suppo rted by evolutionary starburst modeling that shows that the properties of N GC 1614 can be fitted with two short-lived bursts of star formation separat ed by 5 Myr (and by inference by a variety of models with a similar duratio n of star formation). The total dynamical mass of the starburst region of 1 .3 x 10(9) M. is mostly accounted for by the old prestarburst stellar popul ation. Although our starburst models use a modified Salpeter initial mass f unction (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 star s than the Salpeter function. The dynamical mass is nearly 4 times smaller than the mass of molecular gas estimated from the standard ratio of (CO)-C- 12 (1-0) to H-2. A number of arguments place the mass of gas in the starbur st region at similar to 25% of the dynamical mass, nominally about 1/15 and with an upper limit of 1/10 of the amount estimated from (CO)-C-12 and the standard ratio.