Thursday, November 29, 2012

An over-massive black hole in the compact lenticular galaxy NGC 1277

A new survey recently reported in Nature found a supermassive black hole (mass~17 billions of solar masses) at the center of a relatively "light" galaxy. This wouldn't be a surprise if the mass of the black hole wasn't more than half the mass of the buldge of the hosting galaxy.

Figure 3: The correlation between black-hole mass and near-infrared bulge luminosity.
The black line shows the mass–luminosity relation for galaxies with a directly measured black-hole mass.
NGC 1277 is a significant positive outlier.
[taken from Nature]

Indeed, we already know that most galaxies -- including our own Milky Way -- host supermassive black holes which lurk at the galactic center. Also, the mass of the black hole is believed to be tightly connected with the properties of the hosting galaxy. Several models of galaxy dynamics and mergers predict a black hole mass VS bulge luminosity relation similar to that shown in the Figure above and this has important implications in the understanding of the galaxy evolution and of black hole population models. Typically, the mass of the black hole is about 0.1 per cent of the mass of the stellar bulge of the galaxy and the maximum mass fraction observed so far was about 10%.

The discovery of NGC 1277,  a compact, lenticular galaxy with a mass of roughly 1.2x10^11 solar masses, is particularly interesting because this galaxy hosts a black hole of mass about 1.7x10^10 solar masses, that is, roughly 59% of the total bulge mass. Indeed, it's evident in the Figure above how NGC 1277 deviates from the expected empirical behavior.

This discovery seems confirmed by other observations of galaxies that host oversized black holes and it might suggest a failure (or the need of some improvement) in current models.

See also the BBC press coverage.

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