The Cosmic Ultraviolet Baryon Survey (CUBS) - I. Overview and the diverse environments of Lyman limit systems at z < 1

We present initial results from the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS is designed to map diffuse baryonic structures at redshift z less than or similar to 1 using absorption-line spectroscopy of 15 UV-bright QSOs with matching deep galaxy survey data. CUBS QSOs are selected based on their NUV brightness to avoid biases against the presence of intervening Lyman limit systems (LLSs) at z(abs) < 1. We report five new LLSs of log N(H I)/cm(-2) greater than or similar to 17.2 over a total redshift survey path-length of Delta z(LL) = 9.3, and a number density of n(z) = 0.43(-0.18)(+0.26) . Considering all absorbers with log N(H I)/cm(-2) > 16.5 leads to n(z) = 1.08(-0.25)(+0.31) at z(abs) < 1. All LLSs exhibit a multicomponent structure and associated metal transitions from multiple ionization states such as C II, CIII, MgII, Si II, Si III, and OVI absorption. Differential chemical enrichment levels as well as ionization states are directly observed across individual components in three LLSs. We present deep galaxy survey data obtained using the VLT-MUSE integral field spectrograph and the Magellan Telescopes, reaching sensitivities necessary for detecting galaxies fainter than 0.1L(*) at d less than or similar to 300 physical kpc (pkpc) in all five fields. A diverse range of galaxy properties is seen around these LLSs, from a low-mass dwarf galaxy pair, a co-rotating gaseous halo/disc, a star-forming galaxy, a massive quiescent galaxy, to a galaxy group. The closest galaxies have projected distances ranging from d = 15 to 72 pkpc and intrinsic luminosities from approximate to 0.01L(*) to approximate to 3L(*). Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities.