A young cluster of galaxies at z=2.5

We recently used narrowband infrared imaging to discover an Her-emitting galaxy at a redshift of z = 2.5 (Malkan, Teplitz, & McLean 1995). Optical spectroscopy with the Keek-I telescope reveals a spectrum of both strong narrow emission lines, especially Ly alpha and the stellar and interstellar absorption lines characteristic of young starbursts, at a redshift of 2.498. This galaxy has a current star formation rate of 10-100 M. yr(-1) (the uncertainty depending mostly on reddening corrections), and should evolve into a normal galaxy with a present-day luminosity of L* or less. Combining deep optical imaging with our previous infrared photometry, we obtain an accurate VIJHK energy distribution which strongly constrains the evolutionary status of this object. The best model is a burst of star formation that started no earlier than z similar to 5.5. Therefore, this young galaxy is probably being seen during its initial episode of star formation, and meets our operational definition of a protogalaxy. In addition, further narrow-band infrared imaging of this region has revealed a close pair of fainter Her-emitting galaxies which lie within 500 kpc of the first galaxy. Their high Her luminosities and bluer colors suggest that they are seen even closer to the peak of their initial epochs of star formation. We propose that all three objects are part of a young galaxy cluster at z = 2.5.