These two images show "stacked" Chandra images for two different classes
of distant, massive galaxy detected with Spitzer. Image stacking is a
procedure used to detect emission from objects that is too faint to be
detected in single images. To enhance the signal, images of these faint
objects are stacked on top of one another.
In both images, low-energy X-rays are shown in orange and high-energy
X-rays in blue, and the stacked object is in the center of the image (the
other sources beyond the center of the image are X-ray sources that were
directly detected and are not part of the source stacking). On the left is
a stacked Chandra image of the "normal" galaxies seen with Spitzer. The
infrared emission for these young, massive galaxies is consistent with
expectations for star formation. The Chandra image shows mainly low-energy
X-ray emission at the center as expected. On the right, is a stacked
Chandra image for galaxies with infrared emission exceeding the levels
likely to be caused by star formation. These galaxies contain active
galactic nuclei, or quasars, in their centers. These are luminous objects
powered by the rapid growth of supermassive black holes. The obscured
quasars show much higher levels of high-energy X-ray emission because the
less energetic X-rays are mostly absorbed by gas.
