O VI Emission in Starbursting BCGs: An HST-COS Extension of "Reigniting the FUSE"

Abstract

In this extension of our "Reigniting the FUSE" series, we have obtained new far-ultraviolet spectroscopy of 3 starbursting brightest cluster galaxies with the Cosmic Origins Spectrograph (COS) aboard HST. With these new data, in combination with archival COS data of 2 additional systems, we measure star formation rates (SFRs) from the young stellar continuum and cooling rates through $10^{5.5}$ K gas with the O VI $1032,1038\,\mathring{\rm A}$ doublet ($\dot{M}_{\rm O\,VI}$). We use these measurements to increase the size of the "Reigniting the FUSE" sample, which itself contains high resolution far UV spectroscopy for 29 nearby massive elliptical galaxies from FUSE, and extend it 1 dex upwards in SFR and cooling rate. Our derived SFRs agree well with photometric GALEX$+$WISE SFRs from the literature. The new data also reinforce the previously observed trend in cooling efficiency ($\dot{M}_{\rm O\,VI}/\dot{M}_{\rm max}$; where $\dot{M}_{\rm max}$ is the X-ray luminosity-based cooling rate) versus mass, with groups cooling more efficiently than clusters by a factor of $\sim$4-5. When we isolate a sample of the most star-forming central galaxies, we find that the star formation rates, X-ray spectroscopic cooling rates, and UV spectroscopic cooling rates are all consistent with one another, on average, while retaining substantial system-to-system scatter, reflecting time-depenent multiphase condensation and cold gas reservoir evolution. The remainder of the sample, in contrast, overwhelmingly shows ratios of ${\rm SFR}/\dot{M}_{\rm O\,VI} \sim {\rm SFR}/\dot{M}_{\rm X,spec} \sim 10\%$.