ESO: CHANDRA & VLT Study X-ray Background, Discover New Quasar
ESO Press Release 05/01
13 March 2001

Chandra and the VLT Jointly Investigate the Cosmic X-Ray Background... and Discover Very Distant Quasar of New Type


Important scientific advances often happen when complementary investigational techniques are brought together. In the present case, X-ray and optical/infrared observations with some of the world's foremost telescopes have provided the crucial information needed to solve a 40-year old cosmological riddle.

Very detailed observations of a small field in the southern sky have recently been carried out, with the space-based Chandra X-Ray Observatory as well as with several ground-based ESO telescopes, including the Very Large Telescope (VLT) at the Paranal Observatory (Chile). Together, they have provided the "deepest" combined view at X-ray and visual/infrared havelengths ever obtained into the distant Universe.

The concerted observational effort has already yielded significant scientific results. This is primarily due to the possibility to 'identify' most of the X-ray emitting objects detected by the Chandra X-ray Observatory on ground-based optical/infrared images and then to determine their nature and distance by means of detailed (spectral) observations with the VLT.

In particular, there is now little doubt that the so-called 'X-ray background', a seemingly diffuse short-wave radiation first detected in 1962, in fact originates in a vast number of powerful black holes residing in active nuclei of distant galaxies. Moreover, the present investigation has permitted to identify and study in some detail a prime example of a hitherto little known type of object, a distant, so-called 'Type II Quasar', in which the central black hole is deeply embedded in surrounding gas and dust.

These achievements are just the beginning of a most fruitful collaboration between "space" and "ground". It is yet another impressive demonstration of the rapid progress of modern astrophysics, due to the recent emergence of a new generation of extremely powerful instruments.

PR Photo 09a/01: Images of a small part of the Chandra Deep Field South, obtained with ESO telescopes in three different wavebands. PR Photo 09b/01: A VLT/FORS1 spectrum of a 'Type II Quasar' discovered during this programme.


The 'Chandra Deep Field South (CDFS)' is a small sky area in the southern constellation Fornax (The Oven). It measures about 16 arcmin across, or roughly half the diameter of the full moon. There is unusually little gas and dust within the Milky Way in this direction and observations towards the distant Universe within this field thus profit from an particularly clear view.

That is exactly why this sky area was selected by an international team of astronomers [1] to carry out an ultra-deep survey of X-ray sources with the orbiting Chandra X-Ray Observatory. In order to detect the faintest possible sources, NASA's satellite telescope looked in this direction during an unprecedented total of almost 1 million seconds of exposure time (11.5 days).

The main scientific goal of this survey is to understand the nature and evolution of the elusive sources that make up the 'X-ray background'. This diffuse glare in the X-ray sky was discovered by Riccardo Giacconi and his collaborators during a pioneering rocket experiment in 1962.

The excellent imaging quality of Chandra (the angular resolution is about 1 arcsec) makes it possible to do extremely deep exposures without encountering problems introduced by the "confusion effect". This refers to the overlapping of images of sources that are seen close to each other in the sky and thus are difficult to study individually. Previous X-ray satellites were not able to obtain sufficiently sharp X-ray images and the earlier deep X-ray surveys therefore suffered severely from this effect.

Moreover, Chandra has much better sensitivity at shorter wavelengths (higher energies) which are less affected by obscuration effects. It can therefore better detect faint sources that emit very energetic ("hard") X-rays.

X-ray and optical surveys in the Chandra Deep Field South

The one-million second Chandra observations were completed in December 2000.

In parallel, a group of astronomers based at institutes in Europe and the USA (the CFDS-team [1]) has been collecting deep images and extensive spectroscopic data with the VLT during the past 2 years (cf. PR Photo 09a/01). Their aim was to 'identify' the Chandra X-ray sources, i.e., to unveil their nature and measure their distances. For the identification of these sources, the team has also made extensive use of the observations that were carried out as a part of the comprehensive ESO Imaging Survey Project (EIS).

More than 300 X-ray sources were detected in the CDFS by Chandra. A significant fraction of these objects shine so faintly in the optical and near-infrared wavebands that only long-exposure observations with the VLT have been able to detect them. During five observing nights with the FORS1 multi-mode instrument at the 8.2-m VLT ANTU telescope in October and November 2000, the CDFS team was able to identify and obtain spectra of more than one hundred of the X-ray sources registered by Chandra.

Nature of the X-ray sources

The first results from this study have now confirmed that the 'hard' X-ray background is mainly due to Active Galactic Nuclei (AGN). The observations also reveal that a large fraction of them are of comparatively low brightness (referred to as 'low-luminosity AGN'), heavily enshrouded by dust and located at distances of 8,000 - 9,000 million light-years (corresponding to a redshift of about 1 and a look-back time of 57% of the age of the Universe [2]). It is generally believed that all these sources are powered by massive black holes at their centres.

Previous X-ray surveys missed most of these objects because they were too faint to be observed by the telescopes then available, in particular at short X-ray wavelengths ('hard X-ray photons') where more radiation from the highly active centres is able to pass through the surrounding, heavily absorbing gas and dust clouds. Other types of well-known X-ray sources, e.g., QSOs ('quasars' = high-luminosity AGN) as well as clusters or groups of galaxies were also detected during these observations.

Studies of all classes of objects in the CDFS are also being carried out by several other European groups. This sky field, already a standard reference in the southern hemisphere, will be the subject of several multi-wavelength investigations for many years to come. A prime example will be the Great Observatories Origins Deep Survey (GOODS) which will be carried out by the NASA SIRTF infrared satellite in 2003.

One particular X-ray source that was identified with the VLT during the present investigation has attracted much attention - it is the discovery of a dust-enshrouded quasar (QSO) at very high redshift (z = 3.7, corresponding to a distance of about 12,000 million light-years; [2]), cf. PR Photo 09a/01 and PR Photo 09b/01. It is the first very distant representative of this elusive class of objects (referred to as 'Type II Quasars') which are believed to account for approximately 90% of the black-hole-powered quasars in the distant Universe.

The 'sum' of the identified Chandra X-ray sources in the CDFS was found to match both the intensity and the spectral properties of the observed X-ray background. This important result is a significant step forward towards the definitive resolution of this long-standing cosmological problem.

Naturally, ESO astronomer Piero Rosati and his colleagues are thrilled: "It is clearly the combination of the new and detailed Chandra X-ray observations and the enormous light-gathering power of the VLT that has been instrumental to this success." However, he says, "the identification of the remaining Chandra X-ray sources will be the next challenge for the VLT since they are extremely faint. This is because they are either heavily obscured by dust or because they are extremely distant".

More Information

This Press Release is issued simultaneously with a NASA Press Release (see also the Harvard site). Some of the first results are described in a research paper ("First Results from the X-ray and Optical Survey of the Chandra Deep Field South") available on the web at astro-ph/0007240.

More information about science results from the Chandra X-Ray Observatory may be found at: The optical survey of CDFS at ESO with the Wide-Field Imager is described in connection with PR Photos 46a-b/99 ('100,000 galaxies at a glance'). An image of the Chandra Deep Field South is available at the ESO website on the EIS Image Gallery webpage.


[1]: The Chandra Team is led by Riccardo Giacconi (Association of Universities Inc. [AUI], Washington, USA) and includes: Piero Rosati, Jacqueline Bergeron, Roberto Gilmozzi, Vincenzo Mainieri, Peter Shaver (European Southern Observatory [ESO]), Paolo Tozzi, Mario Nonino, Stefano Borgani (Osservatorio Astronomico, Trieste, Italy), Guenther Hasinger (Astrophysical Institute Potsdam [AIP], Germany), Colin Norman, Roberto Gilli, Lisa Kewley, Wei Zheng, Andrew Zirm, JungXian Wang (Johns Hopkins University [JHU], Baltimore, USA), Ken Kellerman (National Radio Astronomy Observatory [NRAO], Charlottesville, USA), Ethan Schreier, Anton Koekemoer and Norman Grogin (Space Telescope Science Institute (STScI), Baltimore, USA).

[2] In astronomy, the redshift denotes the fraction by which the lines in the spectrum of an object are shifted towards longer wavelengths. The observed redshift of a distant galaxy or quasar gives a direct estimate of the apparent recession velocity as caused by the universal expansion. Since the expansion rate increases with the distance, the velocity is itself a function (the Hubble relation) of the distance to the object. Redshifts of 1 and 3.7 correspond to when the Universe was about 43% and 12% of its present age. The distances indicated in this Press Release depend on the cosmological model chosen and are based on an age of 19,000 million years.

Technical information about the photos