Extended H-alpha emission from IRAS F10214 + 4724 - Starburst or active galactic nucleus?

We present a narrow-band (lambda/DELTAlambda = 1025) image of Halpha emission from the ultraluminous galaxy IRAS F10214 + 4724. The Halpha-emitting region is extended over 1.8 arcsec (approximately 10 kpc). From a comparison with other data, we conclude that the FWHM of the bulk of the Halpha is at most approximately 450 km s-1, though we cannot rule out a much weaker broad component. These results suggest the presence of a global starburst in IRAS F10214 + 4724. The Halpha, radio continuum and bolometric luminosity can be explained by a starburst with a normal IMF but an enormous star formation rate. The inferred burst duration, total mass of stars formed during the burst and measured extent of the Halpha emission support the view of IRAS F10214 + 4724 as a primeval galaxy undergoing its initial starburst. We also examine the implications of our results for AGN interpretations of IRAS F10214 + 4724. In this case, IRAS F10214 + 4724 provides evidence for the continuity in properties of AGN as the luminosity increases from Seyfert to QSO. In this model, IRAS F10214 + 4724 could be interpreted as a QSO with an obscured broad-line region. The high Halpha/radio flux ratio of IRAS F10214 + 4724 is used to argue in favour of the starburst interpretation, but the presence of an AGN is not ruled out.