An Energetic AGN Outburst Powered by a Rapidly Spinning Supermassive Black Hole or an Accreting Ultramassive Black Hole

Powering the 10⁶² erg nuclear outburst in the MS0735.6+7421 cluster central galaxy by accretion with a 10% mass-to-energy conversion efficiency implies that its putative supermassive black hole (SMBH) grew by ~6 × 10⁸ M _sun over the past 100 Myr. Guided by data at several wavelengths, we place upper limits on the amount of cold gas and star formation near the nucleus of <10⁹ M _sun and <2 M _sun yr^-1, respectively. These limits imply that an implausibly large fraction of the preexisting cold gas in the inner several kpc must have been consumed by its SMBH at the rate of ~3-5 M _sun yr^-1 during the past 100 Myr while leaving no trace of star formation. Such a high accretion rate would be difficult to maintain by stellar accretion or the Bondi mechanism, unless the black hole mass approaches 10¹¹ M _sun. Furthermore, its feeble nuclear luminosities in the UV, I, and X-ray bands compared to its enormous mechanical power are inconsistent with rapid accretion onto a ~5 × 10⁹ M _sun black hole. We suggest instead that the active galactic nucleus (AGN) outburst is powered by angular momentum released from a rapidly spinning black hole. The rotational energy and power available from a spinning black hole are consistent with the cavity and shock energetics inferred from X-ray observations. A maximally spinning 10⁹ M _sun black hole contains enough rotational energy, ~10⁶² erg, to quench a cooling flow over its lifetime and to contribute significantly to the excess entropy found in the hot atmospheres of groups and clusters. Two modes of AGN feedback may be quenching star formation in elliptical galaxies centered in cooling halos at late times. An accretion mode that operates in gas-rich systems, and a spin mode operating at modest accretion rates. The spin conjecture may be avoided in MS0735 by appealing to Bondi accretion onto a central black hole whose mass greatly exceeds 10¹⁰ M _sun. The host galaxy's unusually large 3.8 kpc stellar core radius (light deficit) may witness the presence of an ultramassive black hole.