2020MNRAS.497..498C

We present initial results from the Cosmic Ultraviolet Baryon Survey (CUBS). CUBS is designed to map diffuse baryonic structures at redshift z <= 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 logN( H I)/ cm^–2≳17.2 over a total redshift survey path-length of Δz_ LL_=9.3, and a number density of n(z)=0.43_–0.18^+0.26. Considering all absorbers with logN( 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, C III, Mg II, Si II, Si III, and O VI 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 0.1 L_* at d <= 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 ~0.01 L_* to ~3 L_*. Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities.