Wednesday, July 9, 2014

The growth of black holes in the Galaxy Zoo

The growth of the black holes happens over a large fraction of the age of the age of the universe (z = 1 - 3; from when the universe is 2 billion years old to 6 billion years old). Black holes grow through accretion (gathering stars, dust and other material from around them) and by merging with one other to become the supermassive black holes we observe today. Their mass today is millions to billions of solar masses. Our own supermassive black hole at the centre of the Milky Way has about 4 million solar masses. Most galactic black holes are dormant. However, the ones that are accreting and growing in the process are very luminous objects,  and can be, in principle, observed and understood. They are also very powerful, e.g., the matter funnelled towards the black hole creates enough energy to stop a star from motion.

Why do we care about observing the growth of black holes? Understanding how black holes grow, and how they interact with their host galaxies is an important piece of the puzzle of the evolution of the universe. Meg Urry and her collaborators (including one of her former postdocs Kevin Schawinski, who is now a professor at ETH-Zurich) studied the dependence of black hole growth on the host galaxy morphology.  Kevin S. and Chris Lintott co-founded the Galaxy Zoo, where the public can classify different images of galaxies.  For  example, they separated active galaxies, which are galaxies that each hold a growing supermassive black hole that produces lots of non-stellar light, by morphology. They found that black holes grow differently in elliptical galaxies than in spiral galaxies. It seems that most high mass black holes grow in spiral galaxies and low mass black holes in elliptical galaxies. In elliptical galaxies there may be more mergers, and so this may show that mergers are more important in the growth of low mass black holes than high mass black holes. The Galaxy Zoo facilitated this science. This catalogue is built by volunteers who work on classifying galaxies because they are just too many images for scientists to do all this work. Also, when many people are involved the errors average out and are smaller than when one over-worked postdoc does everything.

How does the universe evolve from a really hot uniform soup to the complicated structure we have today?

The first light
The oldest light we can see is the light from the Cosmic Microwave Background (z ~ 1100; the universe was about 380, 000 years old), which is the thermal radiation assumed to be left over from the Big Bang. We have instruments both on Earth and in space that are powerful enough to detect fluctuations in the temperature of this light, which are very small - of the order of 1 part per million. These tiny thermal fluctuations are believed to trace slight density fluctuations. The CMB radiation is the first light that can be seen. The universe was not transparent before this time and so we cannot see light that came from an earlier time.

The story of the first black holes
Over time, the amplitude of density fluctuations is amplified by gravity. So, the slightly over-dense regions from the early universe became denser and denser.  Eventually, the dark matter in these regions collapses under its own weight into dark matter halos. These halos then attract H and He gas, which form the first stars (z ~ 15; the universe was 200-300 million years old). The first stars are believed to be different from those we see today in that they are heavy (believed to be over 100 times bigger than our Sun) and almost entirely lack heavy elements. This is all inferred from simulations. The stars eventually collapse to black holes of about 100 solar mass, which grow. These early black holes are much lighter than the supermassive black holes we see today.

I've been referring to z a lot. So, what is z , i.e, the Redshift? 
Most of our instruments detect light, and hence are only sensitive to visible matter. Cosmologists quantify the age of an object through its redshift called z. When light travels from a distant object to us its frequency lowers (lower energy per photon) and its wavelength increases moving towards the red end of the spectrum of visible light and beyond as the light frequency continues to decrease. Today is defined as redshift z = 0. The universe is measured by Planck and other satellites to be about 13.8 billion years old.

Note: This post was inspired by Meg Urry's science talk from April.  I was waiting for ETH to post a video of it so that I can link to it and also clarify some of my own misunderstandings. But there is no video that I can find. The talk and the science she does is very interesting. Of course, I forgot a lot of the things she said by now and got distracted by numerous other things since then. But I thought that the topic was interesting enough to finish the post.