Figure 1: This illustration
shows the different features of an active galactic nucleus (AGN). The
extreme luminosity of an AGN is powered by accretion onto a supermassive
black hole. In addition to the accretion disk, models of active
galaxies also include a region of cold gas and dust, the torus. Viewed
edge-on, the torus blocks out the light from the accretion disk and the
system is a Type II AGN. Viewed face-on, the accretion disk dominates
the luminosity and the system is a quasar. Credit: Aurore Simonnet, Sonoma State University.
Previous studies of large AGN samples both a low and at high redshifts
seemed to rule out galaxy mergers as the drivers for black hole growth. A
new technique developed at MPA for selecting a rare type of active
galactic nuclei now show that it is possible to identify a new class of
AGN in which more than 80% of the galaxies turn out to be merging or
interacting systems, with clear indications of an accreting black hole. A
detailed statistical analysis then reveals that mergers drive black
hole formation in the most massive galaxies in the local Universe.
Our understanding of the formation paths of supermassive black holes
is still very sketchy. In 1982, Andrzej Soltan showed that the summed
emission from all observed quasars yields a remarkably accurate estimate
of the total mass of present-day black holes. His argument was based on
the expected conversion efficiency of the rest mass energy of matter in
an accretion disk falling into a black hole at the centre of a quasar -
these distant objects are thus believed to signpost the main sites of
black hole growth across the Universe.
Unfortunately, quasars are not ideal objects to study the mechanisms
by which black holes grow. The emission from the central nucleus is more
luminous than the underlying host galaxy by many orders of magnitude,
making detailed studies of the host system extremely difficult. For this
reason, studies to constrain possible triggering mechanisms for black
hole growth focus on so called Type II active galactic nuclei (AGN). In
these systems, the radiation from the accretion disk is believed to be
blocked by a very dense layer of gas and dust (the so-called torus, see
Figure 1). Large spectroscopic galaxy surveys such as the Sloan Digital
Sky Survey have yielded samples of hundreds of thousands of nearby Type
II AGN, which are selected according to their optical emission line
ratios. At higher redshifts, Type II AGN are commonly selected at X-ray
wavelengths.
Figure 2: SDSS colour images
(g,r,i-band) of typical AGN host galaxies in our sample.
Many are
interacting or have disturbed morphologies. © SDSS
So far, studies of the host galaxies of these systems appear to rule
out theoretical scenarios in which black hole growth occurs when two or
more galaxies merge together. Simulations of the gravitational
interactions and gas dynamics of two merging galaxies show that tidal
torques during the merger cause the gas to shock, lose energy and flow
towards the centre of the merger remnant. Energetic processes that act
on the gas very close to the black hole are, however, very difficult to
simulate in a reliable way.
In recent work at MPA, a new technique combined data from several
observing programmes using the Wide-field Infrared Survey Explorer
satellite, the Very Large Array (VLA) FIRST Survey (Radio Images of the
Sky at Twenty-Centimeters) and the Sloan Digital Sky Survey. This
combination of data permits a more reliable selection of a large sample
of active galaxies where there is strong hot dust emission from a
central torus. The radio data turned out to be a critical element of the
selection technique, because there are large number of galaxies where
the hot dust extends over a large area and is probably not being heated
by the black hole. This had not been accounted for in previous work.
Follow-up work demonstrated that the new selection, which includes
only 1.6% of the sources in previous AGN samples, yields galaxies with
properties that are very different. More than 80% of the galaxies in the
new sample turn out to be merging or interacting systems. For many of
them, their stellar spectra show strong bursts of star formation in
their central regions. The emission lines indicate that the gas is
highly ionized, with the main source of ionization likely being an
accreting black hole rather than the young stars in the nucleus. The
radio emission is usually compact and centrally located and is too
luminous to be explained by the observed young stars in the nucleus.
With 1300 galaxies the new sample is large enough to show conclusively
that these AGN currently signpost the bulk of black hole formation in
the most massive galaxies in the local Universe - with the "normal" AGN
population dominant in lower mass galaxies.
The challenge now is to go back in time to younger galaxies, i.e. to
extend this study to higher redshifts. Pulling similar samples out of
surveys of galaxies at higher redshifts, however, will need comparable
data sets in different wavelength bands. Then we can investigate how the
accreting black holes in these younger systems are influencing the gas
in and around their host galaxies.
Author
Kauffmann, Guinevere
Director
Phone: 2013
Email: gkauffmann@mpa-garching.mpg.de
Room: 121
Original publication
1. Kauffmann, Guinevere
The physical properties of galaxies with unusually red mid-infrared colours
Monthly Notices of the Royal Astronomical Society, Volume 473, Issue 4, p.5210-5220
Source/DOI
