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 |$\log \, N({\mathrm{ H} \,{\small I}})/{{\rm cm^{-2}}}\gtrsim 17.2$| over a total redshift survey path-length of |$\Delta \, z_{\mathrm{ LL}}=9.3$|⁠ , and a number density of |$n(z)=0.43_{-0.18}^{+0.26}$|⁠. Considering all absorbers with |$\log \, N({{\mathrm{ H} \,{\small I}}})/{{\rm cm^{-2}}}\gt 16.5$| 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 |${\approx} 0.01\, L_*$| to |${\approx} 3\, L_*$|⁠. Our study shows that LLSs originate in a variety of galaxy environments and trace gaseous structures with a broad range of metallicities. [ABSTRACT FROM AUTHOR]