Figure 14: The ratio of 24-micron to -band optical flux as a function of color for the sources in the E-CDFS field [75]. Contours show the location of all the Spitzer 24 μm sources. Red points show the location of X-ray sources with hard X-ray spectra (hardness ratio , defined as where and are the background-subtracted counts in the soft and hard bands, resp.), while blue points have . Small black points show the 193 sources in the IR-red excess region that were not detected individually in X-rays.