This XMM-Newton Deep Field North image was constructed from data obtained by the spacecraft during four observations carried out in May and June. In total, 180 thousand seconds of top-quality data was used. Photo: ESA

In January 1996, the Hubble Space Telescope released a picture of part of the sky in the Ursa Major constellation. Known as the Hubble Deep Field (HDF), it offered mankind's deepest and most detailed optical view of the Universe. Since then the image has become a reference for astronomers with numerous follow-up observations at other wavelengths. Today, XMM-Newton contributes its own X-ray vision of this notable region of the heavens.

The term 'deep' in astronomy means looking at the faintest and often the most distant objects in the Universe. Keyhole views like the Hubble Deep Field, far from the galactic plane and therefore uncluttered by foreground stars, allow scientists to travel back in time, observing the early formation of galaxies just after the Universe's birth in the Big Bang.

The distance of an object can be evaluated according to its redshift. Redshift (symbolized by z) is the stretching of light waves as they travel across expanding space. The farther they travel, the more they are stretched, and the higher the measured redshift. XMM-Newton's Deep Field image features, for instance, an object with a redshift of 2.5 - representing a distance some ten thousand million light-years away. XMM-Newton is therefore seeing such a source as it was when the age of the Universe was one-third of what it is today.

XMM-Newton's Deep Field North is the result of four observations carried out in May and June this year. In total, 180 thousand seconds (50 hours) of top-quality data was used. The final and longest observation lasted more than 26 hours!

The field of view of the EPIC cameras on XMM-Newton, is a little under half a degree across, about the diameter of the Full Moon. This is larger than the Chandra picture and substantially bigger than the original Hubble Deep Field North.

"Our view of this well studied deep field brings a wealth of additional information," explains David Lumb, one of the team members involved in this XMM-Newton observation, and also the instrument calibration scientist for the EPIC cameras. "They highlight many sources whose X-rays, which are very absorbed by their immediate environment, can only be observed at the highest X-ray energies, which only XMM-Newton can detect."

The false-colour image of the XMM-Newton Deep Field which has been issued today shows several hundred sources, coded according to their energy range. For picture clarity, only a handful have been annotated. These include galaxies harbouring massive black holes (AGN), and quasars (QSO's), the most extreme versions of the black-hole powerhouses. Also noted in the picture are several radio galaxies, a newly discovered small group of galaxies and a normal foreground star.

The centre of the XMM-Newton image coincides with the region observed by Hubble. It is the subject of intense follow-up observations by many ground-based observatories hoping to fit together the pieces of the multi-wavelength jigsaw by combining the new X-ray data with fresh optical measurements.

X-ray views of the same sky region have already been provided by NASA's Chandra observatory. Two observations in December 1999 and in March this year pinpointed many unknown sources - including galaxies, binary star systems and quasars - sources which contribute to the uniform diffuse X-ray glow that pervades the sky.

"Cross-checking the XMM-Newton and Chandra data against each other is also very important," explains David Lumb. "We can, for instance, check the variability of objects over several months."

"Another important aspect is that the Hubble Deep Field has today some of the most comprehensive coverage of any region of the sky at all wavelengths, so we can use the information obtained with EPIC as a template for identifying the types of objects found in other EPIC observations with no HDF equivalent," he adds.

XMM-Newton's huge X-ray collecting power has allowed the production of detailed spectra for many of the objects. "The spectra and fluxes are consistent between the two X-ray observatories," says David Lumb. "We have seen the same sources, the same number of objects, but with higher photon counts. This means spectroscopic studies will give us greater insights into the sources and the processes at work."

With spectra plotting the number of X-rays against their different energy levels coming from each object, David Lumb explains the profile of two very different objects, amongst those identified in the false-colour image of the XMM-Newton Deep Field.

"The red line in this picture is the spectrum of a quasar, which is typical of the brightest X-ray objects. It shows a commonly seen increase in the number of photons at low energies. This is thought to represent the case where we have a clear view of a massive black hole at that galaxy's centre."

"The blue line is the spectrum from an AGN, where most of the low energy photons have been absorbed. Being millions of light-years away, we cannot hope to image the nucleus of that galaxy directly, but we CAN use this X-ray information to estimate the geometry of gas and dust surrounding the black hole which prevents our direct view. At the highest energies visible to XMM-Newton we see that this object actually shines brighter than the first one."

One focus of investigation is the X-ray emission from the so-called SCUBA submillimeter sources. These are luminous, dusty, star-forming galaxies which radiate at sub-millimeter infrared wavelengths and which make a significant contribution to the total energy output of the Universe. X-ray observations may help determine how these objects are powered.

"We have determined that the emission from the SCUBA sources even in the highest energy range accessible to XMM-Newton, is very dim," notes David Lumb. "This puts more stringent limits than ever on indications that most of these objects are probably powered by intense bursts of star formation in the early Universe, rather than an obscured massive black hole".

David Lumb is perhaps one of our best guides in this first overview of the XMM-Newton Deep Field North. He has been particularly involved in this observation and not just as EPIC calibration scientist. Before joining the ESA mission team, he contributed to the development of Chandra's ACIS instrument, particularly its CCD detectors. Previously he had done the same thing for the EPIC camera.

"Working on the development of both Chandra and XMM-Newton instruments, and then with their data on this same field of the sky is a rare privilege and particularly rewarding. One can work on a bit of technology and say to oneself: 'Gee-wiz, with this I will see back to a million years after the Big Bang.' And today with XMM-Newton and Chandra, it has come true: we have opened a new window on the Universe. Like peeling back the skins of an onion, our observation will greatly contribute to our understanding of its evolution," concludes David Lumb.

The XMM-Newton Deep Field North observations were carried out as part of guaranteed time belonging to the mission's project scientist Dr. Fred Jansen. Our thanks to David Lumb for his enthusiastic assistance with this story.

Further deep field investigations are to be carried out with XMM-Newton. Some may be in conjunction with NASA's Chandra observatory, and the European Southern Observatory in the framework of combined observations made possible after the XMM-Newton second call for observation proposals (AO-2).