Discovery of optically emitting circumgalactic nebulae around the majority of UV-luminous quasars at intermediate redshift

We report the discovery of large ionized, O II emitting circumgalactic nebulae around the majority of thirty UV luminous quasars at z=0. 4-1. 4 observed with deep, wide-field integral field spectroscopy (IFS) with the Multi-Unit Spectroscopy Explorer (MUSE) by the Cosmic Ultraviolet Baryon Survey (CUBS) and MUSE Quasar Blind Emitters Survey (MUSEQuBES). Among the 30 quasars, seven (23%) exhibit O II emitting nebulae with major axis sizes greater than 100 kpc, twenty greater than 50 kpc (67%), and 27 (90%) greater than 20 kpc. Such large, optically emitting nebulae indicate that cool, dense, and metal-enriched circumgalactic gas is common in the halos of luminous quasars at intermediate redshift. Several of the largest nebulae exhibit morphologies that suggest interaction-related origins. We detect no correlation between the sizes and cosmological dimming corrected surface brightnesses of the nebulae and quasar redshift, luminosity, black hole mass, or radio-loudness, but find a tentative correlation between the nebulae and rest-frame O II equivalent width in the quasar spectra. This potential trend suggests a relationship between ISM content and gas reservoirs on CGM scales. The O II-emitting nebulae around the z1 quasars are smaller and less common than Ly nebulae around z3 quasars. These smaller sizes can be explained if the outer regions of the Ly halos arise from scattering in more neutral gas, by evolution in the cool CGM content of quasar host halos, by lower-than-expected metallicities on 50 kpc scales around z1 quasars, or by changes in quasar episodic lifetimes between z=3 and 1.