eso9105 — Photo Release
Towards the Beginnings
The NTT Provides the Deepest Look Into Space
31 May 1991
The ESO New Technology Telescope on La Silla has again proven its extraordinary abilities. Known since two years as the world's best optical telescope , it has now produced the "deepest" view into the distant regions of the Universe ever obtained with any ground-or space-based telescope. The new picture shows enormous numbers of extremely faint and remote galaxies whose images almost completely fill the field of view. Expressed in astronomical terms, the picture reaches beyond magnitude 29 .
Making the deepest picture
Beginning in March 1991, an international group of astronomers  has embarked upon an ambitious observing programme with the NTT aimed at detecting and measuring galaxies so faint and so distant that they were until now beyond the reach of other telescopes.
To ensure that no light from relatively bright objects would outshine that of the very faint objects to be observed, the astronomers decided to point the NTT towards a so-called "empty" sky field in the equatorial constellation of Sextans. Previous observations had shown that no objects brighter than about magnitude 20 were visible in this direction. As will be seen, the first attempt has been highly successful.
Using a high-quality CCD detector in the ESO Multi-Mode Instrument (EMMI), installed at one of the NTT foci, Bruce Peterson took forty-one exposures of this field, totalling 6 hours 50 minutes. The individual pictures were subjected to extensive image processing and then co-added to produce a combined image of which a small part (about 2 % of the total area) is reproduced on the photo accompanying this Press Release.
What the picture shows
It has been known for some time that on very deep sky exposures, most recorded objects fainter than about magnitude 24 are galaxies, enormous systems of stars like the Milky Way Galaxy to which our Sun and its planets belong, rather than individual stars. This is because, as we look further and further out in space, we see more and more galaxies, while there is only a limited number of foreground stars in the Milky Way.
In the accompanying picture, more than 97% of the objects are galaxies. The brightest ones, of about magnitude 21 -25, can clearly be seen to have different shapes and can be accordingly classified. Thanks to the good angular resolution (the sharpness of the images), it is possible to see that some of the fainter images are more or less elongated. This may be due to the galaxy type, elliptical or spiral, or the inclination to the line of sight.
Calibration exposures of objects with known brightness were made with NTT on the same nights, allowing to measure rather accurately the brightness of all objects seen on this picture. It was found that the "limiting magnitude", that is the magnitude of the faintest objects that can be perceived, is fainter than magnitude 29. This is more than one magnitude, i.e. at least 2.5 times, fainter than any other image obtained so far by any optical telescope, on the ground as well as in space. The magnitudes of some of the galaxies are indicated on the attached map for comparison.
The picture shows numerous faint galaxies whose images to a large extent overlap each other. As a matter of fact, it is not even certain that there is any place where we are able to "look through" this "wall" of galaxies. Already this simple observation is of great cosmological significance: the number of galaxies still appears to be increasing at these very faint magnitudes. It seems that we have not yet reached a point where we begin to look through the system of galaxies, as we can look through the stars in the Milky Way system.
The observation of a galaxy of magnitude 29 corresponds to the registration of the glow from a cigar at the distance of the Moon, or, in more earthly terms, of the faint light of a glow-worm in Garching, as seen from La Silla, 12,000 km away. Since each of the galaxies consists of millions, in most cases of billions of stars like our Sun, it is clear that they must be very far out in space for their observed light to become so faint.
A single picture, however, cannot with certainty discern between intrinsically faint, relatively nearby "dwarf" galaxies, very distant "normal" galaxies which are similar to the Milky Way Galaxy, or extremely remote super-luminous galaxies. If some of the faintest images here seen belong to dwarf galaxies like the satellite galaxies of the Milky Way, the Magellanic Clouds, then their redshifts  are likely to be 0.5 to 0.7, corresponding to look-back times of 38 -48 %of the age of the Universe (assuming that it is 20,000 million years -a very uncertain figure -this would correspond to a distance of about 10,000 million light-years). Those which are normal galaxies like the Milky Way will have redshifts of the order of 3 -3.5 and the look-back time would be 88 -91 % (and the distance ~ 18,000 million light-years). However, if any of them are brighter than the Milky Way Galaxy, then their distances would be even larger. Some of the objects may even be intrinsically extremely bright quasars at never-before observed look-back times and distances.
Interpretation of the picture
To fully understand the message of this unique picture, laborious follow-up observations are now being undertaken.
First of all, reasonably accurate colours of most of observed galaxies will be measured. With the NTT, this will be possible for those which are brighter than magnitude 28.
The present picture was obtained in yellow light and shows the brightness of the galaxies in this spectral region. A similar picture has already been taken in red light and will allow the astronomers to determine which of the faint objects are blue and which are red. Very young galaxies, including those in the formation stage, are thought to be rather blue because of their content of young and hot stars of blue colour. Older galaxies in which the formation of stars has largely ceased are redder.
These studies may therefore be able to cast some light on the ages of the galaxies observed. The NTT will also be able to obtain spectra of the galaxies brighter than about magnitude 24. This will make possible the measurement of their redshifts, i.e. their velocities and cosmological distances.
The differentiation between some relatively nearby dwarf galaxies and much more distant normal galaxies will also be possible by means of continued observations of the same sky field. Dwarf galaxies at redshift 0.5 would be close enough for individual supernovae, exploding stars at the end of their lives, to be observed in large numbers. Contrarily, supernovae in normal galaxies at redshift 3 or more would be too faint to be observed. A comparison of pictures obtained at different times will tell whether short-lived supernovae are seen or not, and therefore immediately give important information about the nature of the objects seen.
This NTT picture has given us a tantalizing, first glimpse of what can be done with the new and improved observational means which are now at our disposal. It has given us a unique look into regions of the Universe, so remote in space and time that they have never before been explored.
This is the type of work that will be at the frontline of optical observational cosmology during the coming years.
 In the astronomical magnitude scale, smaller numbers signify brighter objects. The brightest stars in the sky have magnitudes near 0; the faintest which can be perceived with the unaided eye have magnitude 6. A difference of one magnitude corresponds to a difference in brightness of a factor of 2.5, i.e. a difference of five magnitudes corresponds to a factor of 2.55 =100. A galaxy of magnitude 20 is about 400,000 times fainter than a star of magnitude 6; a galaxy of magnitude 29 is 4000 times fainter than a galaxy of magnitude 20 and 1,600 million times fainter than a star of magnitude 6.
 The group includes Bruce Peterson (Mount Stromlo Observatory of the Australian National University, Canberra), Sandro D'Odorico, Massimo Tarenghi and Joseph Wampler (European Southern Observatory), Yuzuru Yoshii (National Astronomical Observatory, Tokyo, Japan) and Joseph Silk (University of California, Berkeley, U.S.A.).
 1n astronomy, the redshift z denotes the fraction by which the lines are shifted towards longer wavelengths in the spectrum of a distant galaxy receding from us with the expansion velocity of the Universe. The observed redshift gives a direct estimate of the apparent recession velocity, which is itself a function (the Hubble relation) of the distance to the object under study. If we denote the present age of the Universe as tH and the time the light we observe from a galaxy was emitted as tg, then a galaxy redshift of Zg ~ 3.35 corresponds to a look-back time of 90 %, i.e. tg ~ 0.1 . tH, and we see the galaxy as it was when the age of the Universe was only one tenth of what it is now.
ESO EPR Dept