CII∗ ABSORPTION IN DAMPED Lyα SYSTEMS: (I) STAR FORMATION RATES IN A TWO-PHASE MEDIUM
نویسنده
چکیده
We describe a technique that for the first time measures star formation rates (SFRs) in damped Lyman alpha systems (DLAs) directly. We assume that massive stars form in DLAs, and that the FUV radiation they emit heats the gas by the grain photoelectric mechanism. We infer the heating rate by equating it to the cooling rate measured by the strength of CII∗ 1335.7 absorption. Since the heating rate is proportional to the product of the dust-to-gas ratio, the grain photoelectric heating efficiency, and the SFR per unit area, we can deduce the SFR per unit area for DLAs in which the cooling rate and the dust-to-gas ratio have been measured. We consider models in which the dust consists of carbonaceous grains and silicate grains. We present two phase models in which the cold neutral medium (CNM) and warm neutral medium (WNM) are in pressure equilibrium. In the CNM model the line of sight passes through CNM and WNM gas, while in the WNM model the line of sight passes only through WNM gas. Since the grain photoelectric heating efficiency is at least an order of magnitude higher in the CNM than in the WNM, most of the CII∗ absorption arises in the CNM in the CNM model. By contrast, in the WNM model all of the CII∗ absorption arises in the WNM. We use the measured CII∗ absorption lines to derive the SFR per unit area for a sample of ≈ 30 DLAs in which the dust-to-gas ratio has been inferred from element depletion patterns. We show that the inferred SFR per unit area corresponds to an average over the star forming volume of the DLA rather than to local star formation along the line of sight. We obtain the average SFR per unit area and show that it equals 10−2.2 M⊙yr −1kpc−2 for the CNM solution and 10−1.3 M⊙yr −1kpc−2 for the WNM solution. Interestingly, the SFR per unit area in the CNM solution is similar to that measured in the Milky Way ISM. Subject headings: cosmology—galaxies: evolution—galaxies: quasars—absorption lines
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