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 ≲ 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 ≳ 17.2 over a total redshift survey path-length of Δz_LL = 9.3, and a number density of n(z) = 0.43⁺₋⁰₀²⁶₁₈. Considering all absorbers with log N(H I)/cm⁻² > 16.5 leads to n(z) = 1.08⁺₋⁰₀³¹₂₅ 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.