A cold metal-poor cloud traced by a weak Mg II absorption at z $\mathsfsl{\simeq}$ 0.45

Abstract

Aims. We present the observations of a weak Mg ii absorption system detected at z ∼ 0.452 in the UVES high-resolution spectrum of the QSO HE0001-2340. The weaker of the two Mg ii components forming the system shows associated absorptions due to Si i, Ca i, and Fe i observed for the first time in a QSO spectrum. We investigate the nature of this absorber by comparing its properties with those of different classes of absorbers and reproducing its ionization conditions with photoionization models.Methods. We measured column densities of Mg i, Mg ii, Si i, Ca i, Ca ii, Mn ii, Fe i, and Fe ii with Voigt profile fitting. Although most of the observed velocity profiles are not resolved in the UVES spectrum, a curve of growth analysis excluded a significant underestimation of the column densities, in particular for the Fe ii and Mg ii multiplets. We compared our measurements with a sample of 28 weak Mg ii systems detected in the interval 0.4 < z < 1.4 in the 18 UVES spectra of the ESO Large Programme, plus 24 weak Mg ii systems in the same redshift range taken from the literature. Then, we performed a comparison with 11 damped systems for which ionic column densities were measured component by component and with 44 Galactic lines of sight with measured Ca i, Ca ii and Fe i. We also ran a grid of photoionization models with Cloudy to reproduce the observed Mg i/Mg ii, Ca i/Ca ii, and Fe i/Fe ii column density ratios for the studied system. Results. The observed absorber belongs to the class of weak Mg ii systems on the basis of its equivalent width, however the relative strength of commonly observed transitions deviates significantly from those of the above mentioned absorbers. A rough estimate of the probability of crossing such a system with a QSO line of sight is P ∼ 0.03. The presence of rare neutral transitions suggests that the cloud is shielded by a large amount of neutral hydrogen. A detailed comparison of the observed column densities with the average properties of damped Lyman-α systems and local interstellar cold clouds shows, in particular, deficient Mg ii/Mg i and Ca ii/Ca i ratios in our cloud. The results of photoionization models indicate that the cloud could be ionized by the UV background.However, a simple model of a single cloud with uniform density cannot reproduce the observed ionic abundance ratios, suggesting a more complex density structure for the absorber. Supposing that ionization corrections are negligible, the most puzzling result is the underabundance of magnesium with respect to iron which is hard to explain both with nucleosynthesis and with differential dust depletion.

Ionization corrections in a multiphase interstellar medium: lessons from a zabs∼ 2 sub-DLA

“…Ellison, Mallen‐Ornelas & Sawicki (2003) previously investigated the complex kinematic structure and ionization structure of this absorber. A HIRES spectrum obtained by one of us (JXP) also exhibits Si i and Fe i , ions also reported by D'Odorico (2007), although neither of these cases has coverage of higher ionization species. The N (H i ) of the z abs = 0.745 absorber towards Q1331+17 is not known, but its D ‐index (Ellison 2006; Ellison et al 2009) indicates that it is likely to be a sub‐DLA.…”

A BARE MOLECULAR CLOUD ATz∼ 0.45

“…Fei, Sii, and Ca i are also detected in this cloud; these neutral states have not been reported to exist in any other extragalactic environment, even in most DLAs, and have only been found in several dense galactic molecular clouds (Welty et al 2003). D'Odorico (2007) notes that the ratios of Mg i/Mg ii and Ca i/Ca ii in this system are orders of magnitude higher than in other absorbers, implying a very low ionization state. She also observes that there is an extreme underabundance of Mg with respect to Fe, which cannot be explained by nucleosynthesis or dust depletion, and cannot be reproduced by photoionization models.…”

First measurement of Mg isotope abundances at high redshifts and accurate estimate of Δα/α

“…This unique systems exhibiting a set of Fe i lines was discovered and first described by D'Odorico (2007). The system consists of two components separated by ≈70 km s −1 (Fig.…”

“…These are the systems at z abs = 0.45207 (D'Odorico 2007), z abs = 1.5864 (Chand et al 2004), z abs = 1.6515 (Paper I), which are suitable for measuring the Mg isotope abundances, and a sub-DLA system at z abs = 2.1871 (Richter et al 2005) that exhibits several sets of absorption lines of the same ion, which can be used to test the local wavelength calibration.…”