IRAC color–color plot for sources in the FIDEL catalog (small gray dots). The sources lie in two partially overlapping clouds. The black circles represent the colors of an SBc galaxy template (Devriendt et al. 1999, VCC 1987) observed at a range of redshifts. The circles are linearly spaced in lookback time; the small circles are spaced by 0.1 Gyr and the large circles by 1 Gyr. A range of redshifts are also indicated by the numbers along the track. The dashed line is the track traced out by the star-bursting galaxy M82. The diamonds indicate intervals of 1 Gyr, equivalent to the large circles for the SBc template. The solid line indicates the color cut used to separate high- and low-redshift sources—sources above the line are mostly at z > 1.2 while sources below the line are mostly at z < 1.2. The choice of this line is described in Devlin et al. (2009, supplement). We use H ₀ = 70.5 km s ⁻¹ Mpc ⁻¹, Ω _M = 0.274, and Ω _Λ = 0.726 (Hinshaw et al. 2009) to tabulate the lookback times.