Understanding X-ray emission from galaxies and galaxy clusters

While galaxies, with their billions of stars, may seem unfathomably large, the Universe contains even bigger objects. Clusters of galaxies are the largest known equilibrium structures. They can contain many hundreds of galaxies and a total mass thousands of times that of the Milky Way system. Galaxies and clusters appear very different when viewed in optical light (see Fig. 0), but computer simulations such as the Millennium Simulation suggest that their dark matter distributions should look very similar. The technical term for this is 'self-similarity', which in this context means that the dark matter halos of galaxy clusters are more or less just scaled-up versions of those which surround galaxies.

Both galaxies and galaxy clusters (and their dark matter halos) are expected to be suffused with hot gas as well. This gas, which is heated to temperatures of millions of Kelvin, emits high-energy radiation and can be studied with X-ray telescopes like ROSAT and XMM-Newton. Studies of dozens of galaxy clusters show that the X-ray luminosity of the hot gas increases with the total mass of the cluster. Independently, studies of dozens of elliptical galaxies have shown that the X-ray luminosity of their hot gas increases with the stellar mass of the galaxy. These two correlations connect X-ray luminosity to two different quantities (total mass for clusters, stellar mass for galaxies), and have typically been measured in different ways for the different types of object.

A team at MPA has now combined these two relations using an archived X-ray map of the whole sky. They analysed emission around a sample of 250,000 galaxies in the ROSAT All-Sky Survey - more than a thousand times the number used in any previous galaxy study - and carefully combined the X-ray emission from several thousand similar mass galaxies into a set of average images in a process known as "stacking". Example stacked images are shown in Figure 1 for two different stellar masses. By eye, the distribution of the hot gas in galaxy clusters looks just like a scaled-up version of that around much smaller galaxies. The full results are shown in Figure 2, which shows the relation between mean X-ray luminosity and stellar mass. This relation follows a straight line all the way from the individual galaxy regime (small masses) up to the rich cluster regime.

However, a more detailed analysis shows that the slope of this line is steeper than would be expected if the hot gas were perfectly self-similar. This is probably due to a combination of effects, with a major contribution coming from heating by supermassive black holes at the centres of galaxies. As gas falls into a supermassive black hole, it loses large amounts of energy which are pumped into the hot gas atmosphere surrounding the galaxy. This is known "active galactic nucleus (AGN) feedback" and is thought to be important in the formation of both galaxies and galaxy clusters. AGN feedback has a bigger effect on less massive systems, lowering the X-ray luminosity of galaxies much more than that of clusters.

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)