On the Heterogeneity of Metal-Line and Lyα Absorption in Galaxy "Halos" at z ~ 0.7

Abstract

We examine the properties of two galaxy "halos" at z ~ 0.7 in the Ton 153 (z_(em) = 1.01) quasar field. The first absorber-galaxy pair (G1) is a z = 0.672, L_B = 4.3L*_B, E/S0 galaxy probed at D = 58 kpc. G1 is associated with a remarkable five-component Lyα complex having τ_(LL) ≤ 0.4, W_(r)(Lyα) = 2.8 Å, and a velocity spread of Δv = 1420 km s^(-1). We find no Mg II, C IV, N V, nor O VI absorption in these clouds and infer metallicity upper limits of -3 ≤ log Z/Z_☉ ≤ -1, depending on assumptions of photoionized or collisionally ionized gas. The second absorber-galaxy pair (G2) is a z = 0.661, L_B = 1.8 L*_B, Sab galaxy probed at D = 103 kpc. G2 is associated with metal-enriched (log Z/Z_☉ ≃ -0.4) photoionized gas having N(H I) ≃ 18.3 (cm^(-2)) and a velocity spread of Δv = 200 km s^(-1). The very different G1 and G2 systems both have gas-galaxy properties inconsistent with the standard luminosity-dependent galaxy "halo" model commonly invoked for quasar absorption line surveys. We emphasize that mounting evidence is revealing that extended galactic gaseous envelopes in the regime of D ≤ 100 kpc do not exhibit a level of homogeneity supporting a standardized halo model. Selection effects may have played a central role in the development of a simple model. We discuss the G1 and G2 systems in the context of ΛCDM models of galaxy formation and suggest that the heterogeneous properties of absorber-galaxy pairs is likely related to the range of overdensities from which galaxies and gas structures arise.

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