Critical Metallicity and Fine-Structure Emission of Primordial Gas   Enriched by the First Stars
  The influence of the first stars on the formation of second-generation objects at high redshift may be determined, in part, by their metal enrichment of surrounding gas. At a critical metallicity, Zcrit, primordial gas cools more efficiently by fine-structure lines of CII (157.74 um), OI (63.18 um, 145.5 um), SiII (34.8 um), and FeII (25.99 um, 35.35 um) than by HI or H2 emission. This cooling may alter the process of fragmentation into smaller units. We study the time-dependent cooling of primordial gas enriched by heavy elements from early massive stars, particularly O, Si, and Fe. We define Zcrit as the point when the total cooling rate by metals plus H2 equals the adiabatic compressional heating. We explore two metallicity scenarios: (1) a single metallicity for all heavy elements; (2) individual metallicities (Z_C, Z_O, Z_Si, Z_Fe) from theoretical supernova yields. For dense gas [n > 10^3 cm^(-3)] with metals in relative solar abundances, fragmentation occurs at Zcrit ~ 10^(-3.5) Z_sun. However, for lower density gas, [n = 1-100 cm^(-3)], particularly in halos enriched in Si, O, and Fe, we find Zcrit = 0.1-1% Zsun. The critical metallicity approaches a minimum value at high-n set by efficient LTE cooling, with thermalized level populations of fine-structure states and H2 rotational states (J = 2 and J = 3). Primordial clouds of 10^8 Msun at 200K are detectable in redshifted fine-structure lines, with far-infrared fluxes between 10^-22 and 10^-21 W/m^2. For metallicities Z_O = 10^(-3) and molecular fractions f(H2) = 10^(-3) the fine-structure emission lines of OI, SiII, and FeII could be 10^2 - 10^3 times stronger than the H2 rotational lines at 28.22 um (J = 2-0) and 17.03 um (J = 3-1). 
Fernando Santoro and J. Michael Shull (Univ. of Colorado)