An optical study of the possible proto-starburst galaxy VII ZW 31

We present an optical imaging and spectroscopic study of an ultraluminous, gas-rich Infrared Astronomical Satellite (IRAS) galaxy, VII Zw 31. Sage and Solomon proposed earlier that this object may be interpreted as a protogalactic disk at low redshift, on account of its large mass fraction of molecular gas. Our data suggest that the object is a merger-induced starburst, and in that respect similar to many other ultraluminous IRAS galaxies. Asymmetries and distortions are seen in the visible light, and a prominent dust lane is detected near the nucleus. The spectra show strong, low-ionization line emission, consistent with photoionization by young stars, and substantial internal extinction. Absorption spectra suggest a presence of both young and old stellar populations. We interpret the object as an old galaxy, rejuvenated by a recent starburst. Since the large molecular gas content (~ 50% of the total mass) is the main distinguishing characteristic of this object, the starburst must be just in its initial stages. VII Zw 31 may thus be a low-redshift analog of at least some protogalaxies at large redshifts