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

Abstract

We have obtained new observations of the absorption system at $z_{\mathrm{abs}}=0.48$ toward 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 $\sim 200\mathrm{km}{\mathrm{s}}^{-1}$ 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 UV spectra taken with HST/COS along with data taken from HST/STIS, Keck/HIRES, and VLT/UVES. We find that the absorption system can be explained with a photoionized low-ionization phase with $[Fe/H]\sim -0.5$ 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.

arXiv comment18 pages, 9 figures; updated author affiliation

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