These two images show "stacked" Chandra images for two different classes of distant, massive galaxy detected with Spitzer. To enhance the signal, images of these faint objects are stacked on top of one another. On the left is a stacked Chandra image of the "normal" galaxies seen with Spitzer. On the right, is a stacked Chandra image for galaxies with infrared emission exceeding the levels likely to be caused by star formation. Credit: NASA/CXC/Durham/D.Alexander