Near-infrared images of the active galaxy NGC 1097, obtainedwith the NACO adaptive optics instrument on ESO's Very Large Telescope,disclose with unprecedented detail a complex central network offilamentary structure spiralling down to the centre of the galaxy.These observations provide astronomers with new insights on howsuper-massive black holes lurking inside galaxies get fed.
"Thisis possibly the first time that a detailed view of the channellingprocess of matter, from the main part of the galaxy down to the veryend in the nucleus is released," says Almudena Prieto (Max-PlanckInstitute, Heidelberg, Germany), lead author of the paper describingthese results.
Located at a distance of about 45 millionlight-years in the southern constellation Fornax (the Furnace), NGC1097 is a relatively bright, barred spiral galaxy seen face-on. Atmagnitude 9.5, and thus just 25 times fainter than the faintest objectthat can be seen with the unaided eye, it appears in small telescopesas a bright, circular disc.
NGC 1097 is a very moderate exampleof an Active Galactic Nucleus (AGN), whose emission is thought to arisefrom matter (gas and stars) falling into oblivion in a central blackhole. However, NGC 1097 possesses a comparatively faint nucleus only,and the black hole in its centre must be on a very strict "diet": onlya small amount of gas and stars is apparently being swallowed by theblack hole at any given moment.
Astronomers have been trying tounderstand for a long time how the matter is "gulped" down towards theblack hole. Watching directly the feeding process requires very highspatial resolution at the centre of galaxies. This can be achieved bymeans of interferometry as was done with the VLTI MIDI instrument onthe central parts of another AGN, NGC 1068 (see ESO PR 17/03), or withadaptive optics .
Thus, astronomers  obtained images of NGC1097 with the adaptive optics NACO instrument attached to Yepun, thefourth Unit Telescope of ESO's VLT. These new images probe withunprecedented detail the presence and extent of material in the veryproximity of the nucleus. The resolution achieved with the images isabout 0.15 arcsecond, corresponding to about 30 light-years across. Forcomparison, this is only 8 times the distance between the Sun and itsnearest star, Proxima Centauri.
As can be seen in last year'simage, NGC 1097 has a very strong bar and a prominent star-forming ringinside it. Interior to the ring, a secondary bar crosses the nucleusalmost perpendicular to the primary bar. The newly released NACOnear-infrared images show in addition more than 300 star-formingregions, a factor four larger than previously known from Hubble SpaceTelescope images. These "HII regions" can be seen as white spots in ESOPR Photo 33a/05. At the centre of the ring, a moderate active nucleusis located. Details from the nucleus and its immediate surroundings arehowever outshone by the overwhelming stellar light of the galaxy seenas the bright diffuse emission all over the image.
Theastronomers therefore applied a masking technique that allowed them tosuppress the stellar light (see ESO PR Photo 33b/05). This unveils abright nucleus at the centre, but mostly a complex central network offilamentary structures spiralling down to the centre.
"Ouranalysis of the VLT/NACO images of NGC 1097 shows that these filamentsend up at the very centre of the galaxy", says co-author Juha Reunanenfrom ESO.
"This network closely resembles those seen in computermodels", adds co-worker Witold Maciejewski from the University ofOxford, UK. "The nuclear filaments revealed in the NACO images are thetracers of cold dust and gas being channelled towards the centre toeventually ignite the AGN."
The astronomers also note that thecurling of the spiral pattern in the innermost 300 light-years seemindeed to confirm the presence of a super-massive black hole in thecentre of NGC 1097. Such a black hole in the centre of a galaxy causesthe nuclear spiral to wind up as it approaches the centre, while in itsabsence the spiral would be unwinding as it moves closer to the centre.
Animage of NGC 1097 and its small companion, NGC 1097A, was taken inDecember 2004, in the presence of Chilean President Lagos with theVIMOS instrument on ESO's Very Large Telescope (VLT). It is availableas ESO PR Photo 35d/04.
This ESOPress Photo is based on research published in the October issue ofAstronomical Journal, vol. 130, p. 1472 ("Feeding the Monster: TheNucleus of NGC 1097 at Subarcsecond Scales in the Infrared with theVery Large Telescope", by M. Almudena Prieto, Witold Maciejewski, andJuha Reunanen).
: "Adaptive Optics" is amodern technique by which ground-based telescopes can overcome theundesirable blurring effect of atmospheric turbulence. With adaptiveoptics, the images of stars and galaxies captured by these instrumentsare at the theoretical limit, i.e., almost as sharp as if thetelescopes were in space.
: The astronomers are M. AlmudenaPrieto (Max-Planck Institute for Astronomy, Heidelberg, Germany),Witold Maciejewski (University of Oxford, UK), and Juha Reunanen (ESO,Garching, Germany).
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