XMM-Newton measures the power of black-hole driven outflows in galaxies
27 February 2012Astronomers using ESA's XMM-Newton X-ray Observatory have discovered that ultra-fast outflows are quite common in active galaxies. About 40 per cent of the sources in their sample show outflows that arise from the vicinity of the central black holes. 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.
|Artist's impression of ultra-fast outflows and relativistic jets driven by a supermassive black hole at the centre of a galaxy. Credit: ESA/AOES Medialab
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 galactic properties. 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 properties. "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 adds.
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 ionisation. 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 UFOs. "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 larger scales. "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 Project Scientist 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.
Notes for editors
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 sources.
The study presents analyses of X-ray spectra of these 42 AGN taken with the EPIC pn instrument on XMM-Newton. In particular, the astronomers searched for absorption lines of highly ionised iron atoms at energies between 7 and 10 keV, as the blue-shift of these lines provides evidence of outflowing material from the vicinity of the galactic nuclei.
F. Tombesi, et al., "Evidence for ultra-fast outflows in radio-quiet AGNs. III. Location and energetics", 2012, Monthly Notices of the Royal Astronomical Society, in press
F. Tombesi, et al., "Evidence for ultra-fast outflows in radio-quiet Active Galactic Nuclei. II. Detailed photo-ionization modeling of Fe K-shell absorption lines", 2011, The Astrophysical Journal, 742, 44
F. Tombesi, et al., "Evidence for ultra-fast outflows in radio-quiet AGNs. I. Detection and statistical incidence of Fe K-shell absorption lines", 2010, Astronomy and Astrophysics, 521, A57
X-ray Astrophysics Laboratory and CRESST
NASA/Goddard Space Flight Center
and Department of Astronomy
University of Maryland, MD, U.S.A.
Istituto Nazionale di Astrofisica – Istituto di Astrofisica Spaziale e Fisica Cosmica
ESA XMM-Newton Project Scientist
Directorate of Science and Robotic Exploration
European Space Agency