IC 630: Piercing the Veil of the Nuclear Gas

IC 630 is a nearby early-type galaxy with a mass of 6 × 1010 M⊙ with an intense burst of recent (6Myr) star formation (SF). It shows strong nebular emission lines, with radio and X-ray emission, which classifies it as an active galactic nucleus (AGN). With VLT-SINFONI and Gemini North-NIFS adaptive optics observations (plus supplementary ANU 2.3m WiFeS optical IFU observations), the excitation diagnostics of the nebular emission species show no sign of standard AGN engine excitation; the stellar velocity dispersion also indicates that a supermassive black hole (if one is present) is small (M⊙ = 2.25 × 10 M⊙ 5 ). The luminosity at all wavelengths is consistent with SF at a rate of about 1-2M⊙ yr-1. We measure gas outflows driven by SF at a rate of 0.18M⊙ yr-1 in a face-on truncated cone geometry. We also observe a nuclear cluster or disk and other clusters. Photoionization from young, hot stars is the main excitation mechanism for [Fe II] and hydrogen, whereas shocks are responsible for the H2 excitation. Our observations are broadly comparable with simulations where a Toomre-unstable, self-gravitating gas disk triggers a burst of SF, peaking after about 30 Myr and possibly cycling with a period of about 200 Myr.