Hubble Space TelescopeSTIS Far‐Ultraviolet Observations of the Central Nebulae in the Cooling‐Core Clusters A1795 and A2597

Abstract

We present Hubble Space Telescope STIS FUV images of the Lyα and FUV continuum emission of the luminous emission-line nebulae in two cooling-core clusters, A1795 and A2597. The Lyα and FUV continuum emission consist of a diffuse component (~60%) and more compact features (knots and filaments), which lie preferentially along the radio source edges. There are correlations between the FUV continuum flux and the emission-line fluxes of Lyα and Hα that imply that the brighter parts of the nebulae are ionized locally. We suggest that the FUV knots are star clusters with ongoing star formation of several solar masses per year. The bolometric luminosity of such a starburst (if absorbed by dust) would be detectable with the Spitzer Space Telescope MIPS. It appears that star formation occurs throughout the nebula, although it is strongly enhanced along the edges of the radio source. We find that young hot stars (e.g., O5) probably provide the bulk of the photons that ionize the nebula, although other sources of ionization may contribute in selected regions of the nebula. Constraints on diagnostic UV emission lines are consistent with photoionization from a starburst population and perhaps marginally with intermediate-velocity shocks, ~400 km s-1. We suggest that the mass accretion rates are comparable to the star formation rates (of order 10 M☉ yr-1). This is consistent with the lack of intermediate-temperature gas (<1 keV) being due to energy input to the cooling gas rather than to hiding the cooling gas.