The Optical Monitor telescope onboard XMM-Newton has obtained one of the most striking ultraviolet pictures ever taken of the ‘LINER’ galaxy M81. Strong ultraviolet (UV) emission is a feature of star formation, supernova explosions and the accretion of matter by a supermassive black hole.
The spiral galaxy M81 (NGC 3031) lies in the Ursa Major constellation and is situated some 12 million light-years away. It is one of the most luminous galaxies in the Northern Hemisphere and forms a most conspicuous pair with its next-door neighbour M82. The two galaxies probably had a close encounter some ten million years ago. M81 can easily be observed with small amateur telescopes and has even been seen with the naked eye.
This spectacular UV picture of the galaxy contains important evidence for the star-formation processes in M81. The image taken by the Optical Monitor is formed from three 1000 second exposures taken with different ultraviolet filters, centred on approximately 2000, 2300 and 2800 angstroms respectively. It covers a region one quarter of a degree square and frames the M81 galaxy which is at least 22 000 light years across. The project is led by Alice Breeveld of the Mullard Space Science Laboratory (MSSL), University College, London.
UV map of an active galaxy
The hottest regions (which are bright in the short wavelength filters) are bluish and highlight stellar nurseries scattered throughout the galaxy. These are regions where hot young stars are being formed. They are particularly visible at the ‘root’ (towards the centre of the galaxy), and along the initial lengths of each of the galaxy’s two main spiral arms.
“M81 contains many short-lived stars and these are best studied in ultraviolet light,” says Alice Breeveld, MSSL. “The UV images pick out intense regions of star formation. The shortest wavelength filters isolate the hottest and the most intense regions, so these look blue in the image. We believe that a collision with the nearby galaxy M82 could have led to the formation of the spiral arm structure. The high densities and pressures involved would have triggered the star formation.”
The coolest regions (those that are bright in the longest wavelength filter) are shown in red. These are concentrated in the central bulge of the galaxy, where most stars are older and less massive, and have reached the ends of their lives. The predominantly red-coloured and brightest point-like objects in the image are foreground stars in our own Galaxy.
Controversial nucleus
The very bright, point-like, white heart revealed by XMM-Newton is the nucleus of the galaxy. The origin of the emission from the nucleus has prompted the ‘starburst versus mini-quasar’ debate amongst astrophysicists.
Alice Breeveld explains: “Starbursts are regions of intense and violent stellar activity where massive stars are being born and quickly go supernova. Quasars are the brightest and most distant known objects producing radiation covering the full range of the electromagnetic spectrum.”
“Some believe that LINER galaxies harbour a ‘mini-quasar’, where a supermassive black hole is accreting gas and stars at a very slow rate. A natural explanation for LINERs is that they are in the transition phase from quasars to ordinary galaxies. Both processes are strong emitters of ultraviolet light and scientists have argued for both based on previous observations of M81.”
The study of M81
An investigation of the processes at work in a LINER galaxy is best made by combining images and spectra in the ultraviolet and X-rays. XMM-Newton is the only observatory with this capability.
Data from the Reflection Grating Spectrometer (RGS) will measure the condition of the soft X-ray emitting gases in hot stars and from mini-quasars. These are crucial in determining the relationship between starburst and mini-quasar.
Imaging spectroscopy from EPIC’s MOS and PN detectors can be used to trace the position and distribution of the different types of X-ray emission. Continuum measurements from EPIC will be combined with the UV continuum from the Optical Monitor to search for mini-quasar activity.
Ultraviolet images in three colours, when combined with X-ray imaging, provide a powerful discriminant of the processes at work. This project will play an important role in revealing how star formation, starbursts and mini-quasars interact.
Exceptional UV image
UV pictures of celestial objects must be taken from space, because ultraviolet light, invisible to the human eye, is blocked by the Earth’s atmosphere. M81 had already been observed by the Ultraviolet Imaging Telescope (UIT, which flew on the Shuttle in 1990), and by the Hubble Space Telescope.
Optical Monitor principal investigator Keith Mason at MSSL is delighted with this image. “The use of three UV filters has given us a M81 picture ten times more detailed than previous UV views.”
The ability to observe X-ray targets simultaneously in the visible and ultraviolet is one of the XMM-Newton mission’s great advantages. The Optical Monitor, designed and developed at MSSL (with collaborators in the USA and Belgium), is co-aligned with the observatory’s main telescopes and has a field of view (17 x 17 arcminutes) matched to that of the XMM-Newton X-ray cameras. It is an improved Ritchey-Chretien telescope with a 30 cm aperture and an imaging sensitivity comparable to a 4 m instrument on the Earth’s surface.
The observation of M81 was carried out in April 2001, using RGS guaranteed time. (MSSL is part of the RGS consortium as well as Optical Monitor principal investigator).
Image data courtesy of Alice Breeveld, Keith Mason (MSSL, United Kingdom) and the XMM-Newton RGS consortium led by A. Brinkman (SRON, the Netherlands).
For further information please contact:
ESA ñ Communication Department
Media Relations Office
Tel: +33(0)1.53.69.7155
Fax: +33(0)1.53.69.7690
Dr. F.A. Jansen, XMM-Newton project scientist
European Space Agency, ESTEC
Tel: +31 (0)71 565 4426
Dr. Keith Mason, Optical Monitor principal investigator
Mullard Space Science Laboratory, United Kingdom
Tel: +44 (0) 1483 204100
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