Probing feedback in protogalaxies : multiphase gas in a DLA at z ≈ 2 . 4

We investigate the physical processes occurring in the multiphase gas of a damped Lyα absorber (DLA). We base our analysis on a high-quality Keck High-Resolution Echelle Spectrometer (HIRES) spectrum of the QSO J1211+0422 in which a DLA is detected at z ! 2.377. There is little contamination of the high-ion (O VI, N V, C IV, Si IV) absorption, allowing us to explore the properties of the highly ionized gas and its connection to other gas phases. The metallicity ([Z/H] = −1.41 ± 0.08), H I column density [log N (H I) = 20.80 ± 0.10], full-width velocity (#vneut ! 70 km s−1) and relative abundances ([Si/Fe] = +0.23 ± 0.05 and [N/Si] =−0.88 ± 0.07) of this DLA are not unusual. However, we derive the lowest C II∗ cooling rate in a DLA, lc < 10−27.8 erg s−1 per H atom (3σ ). Using this stringent limit, we show that the neutral gas (confined at |v| < +39 km s−1) must be warm and the star formation rate is <7.1 × 10−3 M% yr−1 kpc−2. Surprisingly, the gas shows strong, complex absorption profiles from highly ionized gas whose kinematics appear connected to each other and the low ions. The total amount of highly and weakly ionized gas is very large with Ntot(H II)/Ntot(H I) ! 1.5. At |v| ! +39 km s−1, the gas is fully and highly ionized [H+/H ∼ 1, N(C IV) ' N (C II), N(Si IV) ' N (Si II)]. Based on ionization models, O VI and N V are generally difficult to produce by hard photons, while Si IV and C IV can be photoionized to a large extent. There is, however, no evidence of O VI-bearing gas at T ∼ 106 K associated with this DLA. In contrast, there is some evidence for narrow O VI, N V and C IV components (unexplained by photoionization), implying too low temperatures (T < 105 K) for simple collisional ionization models to produce their observed column densities. Stellar feedback is a possible source for producing the high ions, but we cannot rule out accretion of non-pristine material on to the protogalaxy.