Discovery of extremely low-metallicity circumgalactic gas at z = 0.5 towards Q0454-220

We have obtained new observations of the absorption system at z_abs = 0.48 towards QSO Q0454-220, which we use to constrain its chemical and physical conditions. The system features metal-enriched gas and previously unknown low-metallicity gas detected ${\sim} 200 \, \mathrm{km \, s^{-1}}$ blueward of the metal-enriched gas. The low-metallicity gas is detected in multiple Lyman series lines but is not detected in any metal lines. Our analysis includes low-ionization (e.g. Fe II, Mg II) metal lines, high-ionization (e.g. C IV, O VI, N V) metal lines, and several Lyman series lines. We use new ultraviolet (UV) spectra taken with the Hubble Space Telescope (HST)/Cosmic Origins Spectrograph (COS) along with data taken from HST/Space Telescope Imaging Spectrograph (STIS), Keck/High Resolution Echelle Spectrometer (HIRES), and Very Large Telescope(VLT)/Ultraviolet and Visual Echelle Spectrograph (UVES). We find that the absorption system can be explained with a photoionized low-ionization phase with [Fe/H] ~ -0.5 and $n_\mathrm{H} \sim 10^{-2.3} \, \mathrm{cm}^{-3}$, a photoionized high-ionization phase with a conservative lower limit of -3.3 < [Fe/H] and $n_\mathrm{H} \sim 10^{-3.8} \, \mathrm{cm}^{-3}$, and a low-metallicity component with a conservative upper limit of [Fe/H] < -2.5 that may be photoionized or collisionally ionized. We suggest that the low-ionization phase may be due to cold-flow accretion via large-scale filamentary structure or due to recycled accretion, while the high-ionization phase is the result of ancient outflowing material from a nearby galaxy. The low-metallicity component may come from pristine accretion. The velocity spread and disparate conditions among the absorption system's components suggest a combination of gas arising near galaxies along with gas arising from intergroup material.