Starbursts in cooling flows: blue continua and emission-line nebulae in central cluster galaxies

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Abstract
Optical spectroscopy of X-ray-luminous clusters of galaxies detected in the ROSAT All-Sky Survey provides new evidence for ongoing star formation in the central cluster galaxies (CCGs) of clusters with cooling flows. The CCG spectra, corrected for intrinsic reddening of 0.1≲E(B – V≲0.6, exhibit strong UV/blue continua. In general, these continua are better described by young stars than by power-law emission models. The use of simple stellar population models shows ≳106 O stars to be present in the most luminous systems. Large B, A and F star populations are also inferred. Photoionization models show that the observed O stars may be responsible for much of the strong, nebular line emission seen in the galaxies. This is supported by a linear correlation between the number of O stars and the Hα luminosities of the CCGs. No correlation between the emission-line luminosity and radio power is observed. The data presented here are amongst the most compelling yet for massive star formation in cooling flows and suggest that the dominant source of the spatially extended excess UV/blue continua, frequently observed in CCGs, is young stars rather than scattered non-thermal emission from hidden active nuclei.

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