By estimating the mass and
energy released by the outflows, the astronomers have
identified them as major agents in the feedback processes
required by models of galactic evolution to explain the
observed correlation between the mass of black holes and the
stellar content of their host galaxies.
What determines the mass of
galaxies and of the supermassive black holes residing at
their cores? Is there a universal physical mechanism that
regulates the growth of both components?
These are among the most hotly
debated topics concerning galactic evolution scenarios, and
a new study, reporting on the detection of massive outflows
streaming away from the centres of galaxies, is shedding new
light on these issues.
In recent years, astronomers
have noticed a strong correlation between the mass of a
black hole and the stellar content of its host galaxy: more
massive black holes appear to reside in galaxies whose
bulges contain more stars that also move faster, on average.
These empirical relations are
quite puzzling, given that they connect two extremely
different scales - the close environment of the black hole
and the entire extent of the galaxy that harbours it. In
fact, the enormous gravitational attraction exerted by the
black hole is only effective in the vicinity of the black
hole and has hardly any impact on the galaxy at large.
To explain these observed
relations astronomers often invoke some sort of feedback: a
matter remixing mechanism that originates in the accretion
disc feeding the black hole and then propagates throughout
the entire galaxy.
It had been thought that
relativistic jets of highly energetic particles, such as
those observed streaming from the centres of almost all
active galaxies at radio wavelengths, could play this role.
However, simulations show that
these jets, which are strongly collimated, cannot provide
enough feedback and only have a major impact on the
outskirts of their host galaxy; neither is the radiative
feedback produced by the intense luminosity of the active
galactic nucleus (AGN) sufficient to regulate global
The results of simulations seem
to point to the need for an additional feedback agent
besides these two: wide-angle galactic outflows that arise
from ionised material in the accretion disc.
Such outflows can be observed
by detecting blue-shifts in the absorption lines of the
galaxy spectra. These lines arise from highly ionised iron
atoms in clouds located in the very centre of a galaxy, in
the immediate surroundings of the black hole. In recent
years, astronomers have identified such outflows in a few
galaxies, but the observations have thus far been too sparse
to allow a quantitative analysis of the phenomenon.
Now, the first systematic
scrutiny of these outflows in a sample of 42 nearby
AGN-hosting galaxies has been performed using data from
ESA's XMM-Newton X-ray Observatory. The study, led by
Francesco Tombesi from NASA's Goddard Space Flight Center in
Greenbelt, USA, demonstrates for the first time that these
outflows have the 'right' characteristics to produce the
feedback effects required to reconcile observations with the
predictions from simulations.
"We have seen these outflows in
40 per cent of the galaxies in our sample, thus
demonstrating that they are quite a common phenomenon in
these sources," comments Tombesi, lead author of the three
papers reporting the results.
Analysing the spectroscopic
data and comparing them to models of the inner regions of
AGN, Tombesi and his colleagues estimated the physical
parameters of the outflows: velocity, density and ionisation
"We call them ultra-fast
outflows, or UFOs, because their velocities are very large -
between 10 000 and 100 000 kilometres per second. With these
mildly-relativistic velocities, UFOs are much faster, hence
much more powerful, than other, ordinary galactic outflows,
although they are still slower than relativistic jets," he
The analysis shows that these
UFOs consist of highly ionised plasma, which locates their
origin extremely close to the black hole as the material
must have been exposed to the intense radiation emitted by
the accretion disc in order to reach such high levels of
The high column density of the
outflowing material, on the other hand, suggests that the
quantities of matter released via the UFOs are substantial,
up to one solar mass per year.
"We then used the velocity,
density and ionisation level of the outflows estimated from
the data to assess the strength of their impact on the host
galaxies," notes co-author Massimo Cappi from Istituto
Nazionale di Astrofisica - Istituto di Astrofisica Spaziale
e Fisica Cosmica in Bologna, Italy.
The kinetic power of the UFOs
appears to be a few per cent of the total luminosity of the
AGN, which is enough to exert sufficient feedback on the
host galaxy, as simulations suggest. Moreover, by comparing
the density and velocity of the outflowing material, the
astronomers have determined the mass loss rate caused by the
"Interestingly, the rate at
which mass is released in the outflows is of the same order
as the black-hole accretion rate, and might even exceed it
in some cases," Cappi adds.
The result demonstrates, for
the first time, that a major mass recycling process is
taking place between the dense galactic centres and the
diffuse interstellar medium of the host galaxies. This
synergy between accretion and ejection processes suggests
that something does link AGN and galactic processes on much
"The outflows studied in our work exert
a more intense feedback on the host galaxy than do jets.
Since they are more massive, slower and have wider opening
angles, they are bound to interact more significantly with
the interstellar medium," explains Tombesi.
These feedback mechanisms may
be able to quench star formation in the bulge and the growth
of the black hole at the same time, thus contributing to
establishing the observed correlations between the
properties of these two components.
"Astronomers have been using
XMM-Newton to study these outflows since their earliest
detections," comments Norbert Schartel, XMM-Newton ProjectScientist
at ESA. The observatory's unprecedented spectral resolution
is instrumental in measuring with great precision the
blue-shifted lines caused by outflows.
"It is very satisfying to see
how the accumulation of such a large sample has provided the
body of data needed to establish the role of these outflows
in the context of cosmological feedback," he adds.
After having demonstrated that
these outflows are common in active galaxies and that they
can have a major role in feedback processes, Tombesi and his
colleagues plan to investigate them in greater detail by
comparing the data with models and simulations of accretion
around black holes.
"We intend now to focus on
figuring out the detailed physical mechanisms that generate
the outflows in the first place. This will represent a
further, valuable step towards a full understanding of how
active galaxies work and evolve," concludes Tombesi.
The findings presented here are
based on the analysis of a sample of 42 nearby radio-quiet
active galactic nuclei (AGN) with redshifts up to z=0.1
observed with ESA's XMM-Newton.
The sample of radio-quiet AGN
has been drawn from the Rossi X-ray Timing Explorer (RXTE)
All-Sky Slew Survey Catalog, which provides a list of 294
sources serendipitously detected in the hard X-rays. The survey is
90% complete to a 4-sigma limiting flux of ~10-11 erg/s/cm2
in the 4-10 keV band.
From this survey, the team of
astronomers have selected all sources identified as Seyfert
galaxies and for which good-quality observations with
XMM-Newton were available; this resulted in a sample of 42