This effect makes the relation in Figure 2 steeper than it would be if the hot gas were perfectly self-similar. The new measurements of X-ray luminosity over a broad range of masses gives a powerful clue to help understand AGN feedback. Comparing these measurements against predictions from numerical simulations, the MPA team showed that gentle, 'self-regulated' AGN feedback is preferred over more violent input of energy.
Detailed comparison with previous measurements show that the new results are perfectly consistent with previously measured scaling relations for galaxies, as well as with scaling relations measured for optically selected samples of galaxy clusters. This suggests that a single relation can indeed describe both types of object. Studies of scaling relations for galaxy clusters selected by their X-ray properties have typically shown a similar slope but a systematically higher mean brightness at given total mass. This is most likely a reflection of the diversity of X-ray properties among clusters of a given total mass, which may have been underestimated in earlier work.
Finally, this work complements a similar analysis performed for the same galaxies and galaxy clusters using data from the Planck satellite. That analysis used the shadows which hot gas atmospheres cast on the cosmic microwave background to measure the total thermal energy of the hot gas, as opposed to its X-ray luminosity, finding this to scale with mass self-similarly. Combining these two results implies that a large reservoir of hot gas surrounds galaxies, but is too rarefied as a result of AGN feedback to emit strongly in X-rays. This would resolve the long-standing problem of the location of the baryons which "should" be associated with the galaxies but had not previously been detected directly.
Mike Anderson, Massimo Gaspari, Simon White (MPA), Wenting Wang (Institute for Computational Cosmology, University of Durham), Xinyu Dai (Department of Physics and Astronomy, University of Oklahoma)