Figure 1: An artist's illustration of galaxy distribution, a host galaxy of an active galactic nucleus (AGN), and an active galactic nucleus. An active galactic nucleus is a luminous compact region at the center of the galaxy, powered by the accretion of gas onto the massive black hole. This research reveals that the mass of a massive black hole at the galaxy center is related to the distribution of surrounding galaxies. Enlarge

Massive black holes of more than one million solar masses exist at the center of most galaxies. Some of the massive black holes are observed as active galactic nuclei (AGN) which attract surrounding gas and release huge amounts of energy.

How does a  get "fat"? One possibility is that mutual interaction between galaxies leads to the growth of a black hole. If this theory is correct, there must be some relationship between properties of an  and environment of its host galaxy. Previous studies revealed that radio-loud AGNs are in the overcrowded region. However, it is still not clear that relation between the mass of an central black hole and the environment around an active galaxy (galaxies hosting AGNs). This is why the research team explored the distribution of galaxies surrounding active galaxies.

The research team utilized the "Virtual Observatory" to examine many massive black holes and the environment of active galaxies.

The Virtual Observatory is a system to make integrated use of various astronomical databases around the world via sharing over the Internet. The Astronomy Data Center of NAOJ has been developing an original portal site for the . To begin with this research, the team collected the data on more than 10,000 AGN whose black hole mass had been already measured by spectroscopic observation with SDSS. Next, data on galaxies surrounding active galaxies were gathered from galaxy catalogues of UKIDSS; the number of galaxies reached approximately 70 million. Using the virtual observatory, the team could automatically and efficiently extract only necessary galactic data from the large amount of data sets on those galaxies surrounding . In addition, they developed new analysis method in order to obtain the density distribution of galaxies with a high degree of accuracy.

Fat black holes grown up in cities: 'Observational' result using virtual observatory

Figure 2: Distribution of galaxies surrounding AGN, arranged according to mass of black holes; only the heaviest and lightest groups are described here. This figure makes clear that the density tends to be higher in galaxies closer to AGN, and also higher around active galaxies with heavier black holes.

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