INTEGRAL probes large scale structure of the local Universe

Matter is distributed inhomogeneously in the Universe. On scales smaller than 100-200 Mpc there are large concentrations of mass and voids (Large Scale Structure). Scientists at the Max-Planck Institute for Astrophysics found clear signatures of the local Large Scale Structure in the spatial distribution of nearby X-ray luminous active galactic nuclei detected by the INTEGRAL observatory during an all-sky survey.

Fig. 1: Mass concentrations in the Local Universe. Dashed circle denotes ~70 Mpc radius around the Sun (illustration is adapted from Hudson 1993, MNRAS, 265, 43)

Fig. 2: Space density of hard X-ray bright AGNs in different directions of the sky.

We know from optical and infrared galaxy surveys that the distribution of visible mass in the local Universe (within a few hundred Mpc from us) is far from uniform. The number density of galaxies may vary by an order of magnitude between galaxy superclusters and voids. The highest known mass agglomerations (M > 1015 Msun) in the local Universe include the nearby Virgo cluster and the more distant Great Attractor and Perseus-Pisces supercluster.

It is now generally accepted that practically every galaxy in the local Universe has a supermassive black hole in its center and some of these black holes are active galactic nuclei (AGNs). It is therefore natural to anticipate that the space density of AGNs should be proportional to the space density of ordinary galaxies.

A recent hard X-ray survey of the whole sky performed by the INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) (linkPfeilExtern.gif Krivonos et al., 2007; astro-ph/0701836) provided an unbiased census of AGNs in the local Universe. Thanks to the hard X-ray selection, the obtained sample includes many dust-enshrouded AGNs which are not visible at optical and soft X-ray wavelegths. The effective depth of the survey allows one to effectively probe the nearby Universe out to distances of order 100 Mpc.

Figure 2 demonstrates the space density of hard X-ray bright AGNs in different directions of the sky. Each pixel in the map represents the number density of sources within a cone of space with a half-opening angle of 45 degrees. The image demonstrates that the distribution of nearby AGNs is indeed strongly anisotropic, as expected. For comparison, green contours show a usual tracer of mass concentrations in the local Universe -- the surface density of IRAS PSCz galaxies at distances smaller than 70 Mpc. The large-scale feature in the north-east direction is consistent with the projected position of the highest mass concentrations in the local Universe -- the Virgo cluster and the Great Attractor. The south-western structure is consistent with the Perseus-Pisces supercluster.

Krivonos R., Revnivtsev M., Sazonov S., Churazov E., Sunyaev R.