HST/STIS Far-UV observations of the central nebulae in the cooling core cluster A 1795

We present HST/STIS FUV images of the Lyα and FUV continuum emission of the luminous emission line nebulae in the cooling core cluster A1795. 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. The correlations between the FUV continuum flux and the Lyα emission line flux implies that the nebulae are mainly ionized locally. We suggest that the FUV knots are star clusters with ongoing star formation with rates of several solar masses per year. It appears that star formation occurs through out the nebula, though it is strongly enhanced along the edges of the radio source. The radio source may play a key role in the ionization and morphology of the nebula, e.g., by snow-plowing the gas, shocking and ionizing the gas, triggering star formation, and destroying clouds that it has engulfed. The observed FUV continuum is consistent with about half the number of hot stars which are required to ionize the nebula. We note that we may be missing some of the FUV flux due to obscuration. Thus, we find that young hot stars (e.g., O5) probably provide the bulk of the photons which ionize the nebula, though other sources of ionization may contribute in selected regions of the nebula. 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.