Two Blacks Holes At The Centre of Galaxy NGC 3393

Merging galaxies give us some of the most spectacular astronomical images such as this one of NGC 4676, aka The Mice Galaxies, from the Hubble telescope.

NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA

Here we see two spiral galaxies of similar size which have been unable to escape the inevitable effects of gravity.  Eventually these two systems will merge to form one large elliptical galaxy in what is called a major merger.  You can see by the bright blueish colours in the image that this merger is creating a great deal of disruption and creating new stars as the gas and dust is pulled in different directions.

Eventually, even the supermassive black holes at the centre of each of the galaxies will merge after they spiral in towards each other.  We have seen this double nucleus in galaxies such as NGC 6240 below.  It appears as 1 large, irregular galaxy, but zooming in on the central region and looking with the Chandra X-ray telescope shows two distinct regions of host gas being sucked in by two separate black holes

NASA

Eventually, this system will settle down and the star formation will cease.  It will become a large, red elliptical galaxy.

This is the theory of how spiral galaxies merge to form a larger galaxy, but these events are comparatively rare, although, our own Milky Way galaxy will merge with the giant Andromeda spiral in 4.5 billion years.

If galaxy mergers are common, and our theories say they ought to be, what we should also see are the mergers of one large spiral galaxies with a much smaller system.  In this case, because of the difference in size between the two galaxies  you will not have the spectacular tails of stars in the Mice Galaxies or the huge amounts of rapid star formation.  Instead of forming an elliptical galaxy, what would be left is a spiral galaxy like the original but with a double nucleus and the smaller black hole is absorbed by the larger one.  However, because these events aren’t as spectacular and don’t have huge amounts of star formation, or create irregular galaxies, candidates for these minor mergers have been difficult to find.

Results published in this most recent issue of Nature show for the first time evidence of just this event happening.  Again, the researchers have used the Chandra X-ray telescope to look for the gas as it is heated and sucked into the black hole or holes.  You can see in the inset image seen below that there are two distinct regions at the centre of this galaxy.  Here the X-ray data is shown in blue.

NASA/CXC/SAO/G.Fabbiano et al

How do the authors know that this is a case of a minor merger between two unequally matched galaxies?  Well in this case, we switch to looking in the optical part of the spectrum.  This is shown as the gold parts of the image above.  The central region of the NGC 3393 does not show any signs of recent star-formation which is what you would expect from a major merger.  The stars in the central bulge of the galaxy still seem to be quite old.

The next step is to observe the centre of NGC 3393 further so we can measure the masses of each of the black holes to really test whether one is much larger than the other.  The data at the moment aren’t enough to be able to do this accurately, but they do tell us that each of the black holes has a mass of at least 1 million times that of our Sun!