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
zabs<1. We report five new LLSs of log N(HI)/cm^-2 >~ 17.2 over a total
redshift survey pathlength of dz=9.3, and a number density of n(z)=0.43 (-0.18,
+0.26). Considering all absorbers with log N(HI)/cm^-2 > 16.5 leads to
n(z)=1.08 (-0.25, +0.31) at z<1. All LLSs exhibit a multi-component structure
and associated metal transitions from multiple ionization states such as CII,
CIII, MgII, SiII, SiIII, 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<~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/disk, 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 ~0.01L* to ~3L*. Our study shows
that LLSs originate in a variety of galaxy environments and trace gaseous
structures with a broad range of metallicities.
