The Cosmic Ultraviolet Baryon Survey (CUBS) IV: The Complex Multiphase
  Circumgalactic Medium as Revealed by Partial Lyman Limit Systems
  We present a detailed study of two partial Lyman limit systems (pLLSs) of
neutral hydrogen column density
$N_\mathrm{H\,I}\approx(1-3)\times10^{16}\,\mathrm{cm}^{-2}$ discovered at
$z=0.5$ in the Cosmic Ultraviolet Baryon Survey (CUBS). Available
far-ultraviolet spectra from the Hubble Space Telescope Cosmic Origins
Spectrograph and optical echelle spectra from MIKE on the Magellan Telescopes
enable a comprehensive ionization analysis of diffuse circumgalactic gas based
on resolved kinematics and abundance ratios of atomic species spanning five
different ionization stages. These data provide unambiguous evidence of
kinematically aligned multi-phase gas that masquerades as a single-phase
structure and can only be resolved by simultaneous accounting of the full range
of observed ionic species. Both systems are resolved into multiple components
with inferred $\alpha$-element abundance varying from
$[\alpha/\text{H}]\approx\!{-0.8}$ to near solar and densities spanning over
two decades from $\log n_\mathrm{H}\mathrm{cm}^{-3}\approx\!-2.2$ to $<-4.3$.
Available deep galaxy survey data from the CUBS program taken with VLT/MUSE,
Magellan/LDSS3-C and Magellan/IMACS reveal that the $z=0.47$ system is located
55 kpc from a star-forming galaxy with prominent Balmer absorption of stellar
mass $M_\star\approx2\times10^{10}M_\odot$, while the $z=0.54$ system resides
in an over-dense environment of 11 galaxies within 750 kpc in projected
distance, with the most massive being a luminous red galaxy of
$M_\star\approx2\times10^{11}M_\odot$ at 375 kpc. The study of these two pLLSs
adds to an emerging picture of the complex, multiphase circumgalactic gas that
varies in chemical abundances and density on small spatial scales in diverse
galaxy environments. The inhomogeneous nature of metal enrichment and density
revealed in observations must be taken into account in theoretical models of
diffuse halo gas.
