[3.6]−[4.5] color as a function of the redshift bin considered for our stacked Spitzer/IRAC images of sources with −20.5 <  M_UV <  −19.5. The best-fit color model is shown in blue. The color model is the convolution of the 3.6 μm and 4.5 μm transmission curves with a simple model spectrum, which consists of a power-law continuum, an Hα line, an [S III] 9068.6 Å line, and five secondary lines whose strengths are fixed relative to that of the Hα line (Sect. 4.2).