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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.
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
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.