JAXA Japan Aerospace Exploration Agency

Q. Your specialty is X-ray astronomy. What attracted you to it?

The center of the Milky Way galaxy viewed with X-rays. The gas spreading toward the top right and left from the center looks red. Red indicates low energy and blue indicates high energy. (courtesy: NASA/CXC/MIT/F.K.Baganoff et al.)

X-rays are light that is invisible to the human eye. This light can let us see various astronomical phenomena that can’t be seen with optical light – that’s the attraction for me. X-rays provide us with a lot of information, and what is particularly interesting to me is that this includes information about most of the universe. A star emitting optical light is very beautiful, and it’s an important component of the universe as well, but to understand everything about the universe, we have to look at things apart from stars. We need to see various aspects of the universe in various wavelengths, but I am personally interested in high-energy phenomena, so X-rays are perfect for me.

Q. Are X-rays suited to the observation of black holes?

The areas around black holes have very high energy, so they can be observed very well with X-rays. In particular, darker black holes can only be found using X-rays. Brighter black holes can be observed in other wavelengths, such as optical light and infrared rays, so I think it is important to combine these in observations.

Q. A basic question: what is black hole?

Inside the square is a black hole formed after a supernova explosion, which is located on the outer rim of galaxy NGC7793. (courtesy: X-ray (NASA/CXC/Univ of Strasbourg/M. Pakull et al); Optical (ESO/VLT/Univ of Strasbourg/M. Pakull et al); H-alpha (NOAO/AURA/NSF/CTIO 1.5m))

(Top Left) Virgo galaxy cluster observed by XMM-Newton. (Top Right) Image of a black hole with X-ray jets extracted from the top left photo. (Bottom) X-ray jets, indicated in white, superimposed over a radio map of the galaxy. Two lines of radio-wave jets stretching left and below overlap with X-ray jets. (courtesy: E. Belsole, Service d'Astrophysique, CEA Saclay, France)

A black hole is a celestial object that has very high density and a strong gravitational field. Even light cannot escape its gravitational field, so a black hole itself doesn’t release light. Thus, we cannot observe black holes directly. But when gases are sucked into a black hole, they reach ultra-high temperatures and release a lot of energy, flashing brightly in the X-ray spectrum. This proves the existence of a black hole. Most galaxies have a black hole at their center.
To date, two kinds of black holes have been observed. One type is generated after a supernova explosion. When a massive star with more than 30 times the mass of the Sun finishes its life, a supernova explosion occurs, and the center of the star becomes unable to withstand its own gravity, shrinking to the utmost limit to become a black hole. At this point, a black hole has a mass about 10 times greater than the Sun.
Another kind is an enormous black hole with a mass that is millions, or even several hundred million times greater than that of the Sun. It still remains a mystery how such an enormous black hole can be generated.

Q. What is your research specialty? Tell us about your major research achievement?

My major research topic is the formation of galaxies and galaxy clusters. In conjunction with that, I research black holes located at the center of galaxies. As for my major achievement to date, my research on the Virgo galaxy cluster using the European X-ray astronomy satellite XMM-Newton stands out. At the center of this galaxy cluster, there is a big galaxy known as M87, and in its center is an enormous black hole about 100 million times the mass of the Sun. We captured very high-energy gas shooting out from this black hole. A black hole has such strong gravity that it sucks in anything around it, but at the same time, it provides very high energy to areas around it.
Meanwhile, because the gas density in a galaxy cluster is extremely high, X-ray radiation releases very strong energy. Normally, when energy is released, a gas should cool down and stop emitting X-rays. But at the center of a galaxy cluster, X-ray emissions continue. Continuous X-ray emissions need a heat source, and one of the possibilities is the heavy mass of a black hole. It is thought that a huge black hole at the center of a galaxy releases energy to heat the surrounding gas. My research achieved evidence to support this theory.

Q. How does a black hole influence the formation of a galaxy?

Artist’s rendition of a black hole (courtesy: ESA/NASA/AVO/Paolo Padovani)

The arrow points to the supermassive black hole in the dwarf galaxy Henize 2-10, located near the Milky Way. The mass of black holes had been thought to be proportional to the mass of the galaxies they are in, but this black hole’s mass is unexpectedly large in comparison to that of its galaxy. (courtesy: X-ray (NASA/CXC/Virginia/A.Reines et al); Radio (NRAO/AUI/NSF); Optical (NASA/STScI))

Many galaxies are disc-shaped, like convex lenses, and their middle parts are known as bulges. When scientists measured the masses of several enormous black holes and those of their host galaxies’ bulges, they found a proportional relationship between the two. This means that the formation of a massive black hole and its galaxy are very closely related to each other. The galaxy and the black hole probably grew together.
A possible scenario goes like this. About 10 billion years ago, when the universe was one third its current size, a massive black hole was still in its growth process. At some point that black hole became very active and released a lot of energy to promote the formation of stars. When more stars were created, the universe became bigger. As the black hole grew, a galaxy was growing around it at the same time. The jet of energy that shot out as the black hole grew may have blown out gas, which occasionally would have formed a star. The precise mechanism has not been understood yet, but we do presume the co-evolution of galaxies and massive black holes.

Q. Which was created first, a black hole or a galaxy?

That's similar to the question, “which came first, the chicken or the egg,” isn’t it? It’s not understood yet whether a black hole or a galaxy came first. In recent observations, a super-massive black hole was discovered in an early-stage small galaxy. When you look at the correlation between the mass of a black hole and that of a galaxy, as I mentioned before, this black hole is way too big for a galaxy. It is also understood that stars were actively formed inside that galaxy. Thus, there may be some people who think a black hole comes first, but we cannot draw a conclusion from research on just one galaxy. There are some galaxies with no black holes, or others where a black hole has not been detected. Additionally, there are other possibilities, for instance that a black hole was created when stars and galaxies collided and merged together. Discussion on this issue will continue for a while.