Accreting supermassive black holes in the early universe

October 27, 2014, Harvard-Smithsonian Center for Astrophysics
A multicolor image of galaxies in the field of the Chandra Cosmic Evolution Survey. A large, new study of 209 galaxies in the early universe with X-ray bright supermassive black holes finds that more modest AGN tend to peak later in cosmic history, and that obscured and unobscured AGN evolve in similar ways. Credit: X-ray: NASA/CXC/SAO/F.Civano et al. Optical: NASA/STScI

Supermassive black holes containing millions or even billions of solar-masses of material are found at the nuclei of galaxies. Our Milky Way, for example, has a nucleus with a black hole with about four million solar masses of material. Around the black hole, according to theories, is a torus of dust and gas, and when material falls toward the black hole (a process called accretion) the inner edge of the disk can be heated to millions of degrees. Such accretion heating can power dramatic phenomena like bipolar jets of rapidly moving charged particles. Such actively accreting supermassive black holes in galaxies are called active galactic nuclei (AGN).

The evolution of AGN in cosmic time provides a picture of their role in the formation and co-evolution of galaxies. Recently, for example, there has been some evidence that AGN with more modest luminosities and accretion rates (compared to the most dramatic cases) developed later in cosmic history (dubbed "downsizing"), although the reasons for and implications of this effect are debated. CfA astronomers Eleni Kalfontzou, Francesca Civano, Martin Elvis and Paul Green and a colleague have just published the largest study of X-ray selected AGN in the universe from the time when it was only 2.5 billion years old, with the most distant AGN in their sample dating from when the universe was about 1.2 billion years old.

The astronomers studied 209 AGN detected with the Chandra X-ray Observatory. They note that the X-ray observations are less contaminated by host galaxy emission than optical surveys, and consequently that they span a wider, more representative range of physical conditions. The team's analysis confirms the proposed trend towards downsizing, while it also can effectively rule out some alternative proposals. The scientists also find, among other things, that this sample of AGN represents nuclei with a wide range of molecular gas and dust extinction. Combined with the range of AGN dates, this result enables them to conclude that obscured and unobscured phases of AGN evolve in similar ways.

Explore further: DRAGNs in the sky

More information: "The largest X-ray-selected sample of z > 3 AGNs: C-COSMOS and ChaMP," E. Kalfountzou, F. Civano, M. Elvis, M. Trichas, and P. Green, MNRAS, 445, 1430, 2014.

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5 comments

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Tuxford
1 / 5 (6) Oct 27, 2014
'Around the black hole, according to theories...'

Are merger maniacs now casting doubt???

'Supermassive black holes containing millions or even billions of solar-masses of material are found at the nuclei of galaxies'

Simple chicken and egg problem...
mscheue1
5 / 5 (1) Oct 27, 2014
I was hoping the article would explain how these early supermassive black holes came into existence. No such luck. If they were formed by massive amounts of hydrogen in the early universe (with a much higher density then today), one would think they would still have to go through the stellar nuclear fusion stage where the outward solar wind would disperse a lot of the surrounding mass.

It's hard to imagine how even a few black holes could collect up so much matter, let alone one per galaxy.

Of course, if they formed from dark matter then nuclear fusion was not an issue.
Da Schneib
5 / 5 (1) Oct 27, 2014
Understanding AGN will significantly advance our models of galaxy formation and the evolution of our universe since the Big Bang. This is a real advance; we now know that AGN we can't see in the visible spectrum due to gas and dust light extinction develop along the same lines as the ones we can see. This reduces the uncertainty in our models, as well as making them more accurate in the first place. Great research, and a great outcome.

mscheue, good question, but with a very simple answer: we don't know. AGN were more common in the early universe than they are today, which means there aren't many close to us to study; and that the ones we *can* study are very far away, limiting how much we can see. Add in gas and dust extinction, and the plane of the Milky Way (which hides just about everything behind it), and it means we haven't had a really good look until now. This is an area of astrophysics where there is a lot yet to be discovered.
tritace
Oct 27, 2014
This comment has been removed by a moderator.
theon
not rated yet Oct 28, 2014
There is a theory for fast BH growth: it eats from bypassing matter, a little when it is small, a lot when it is large. In between, there is a maximum in the BH growth rate. This is related to a maximum in the star formation rate since matter only agitated by the BH will form stars.
EPL 97, 39001, 2012. When the BH develops late, it all happens later, the downsizing.

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