Pressemitteilung

ESO Astronomers Detect a Galaxy at the Edge of the Universe

Starlight from the Depths of Time

15. September 1995

Four European astronomers [1] have taken advantage of the superb imaging quality of the ESO 3.5-metre New Technology Telescope (NTT) at the La Silla observatory, to detect a galaxy at an extremely large distance. They conclude that its redshift [2] is z = 4.4; thus, this galaxy is by far the most remote ever detected. In fact, it has taken its light about 90 percent of the age of the Universe to reach us, and we now observe this early object as it appeared, only 1 - 2 billion years [3] after the Universe was created in the Big Bang. Still, the galaxy contains a considerable amount of elements that must have been produced in stars. This proves that stars were formed in normal galaxies, already before this very early epoch.

Galaxies in the Very Early Universe

Astronomical observations during the past decade indicate that the age of the Universe is probably somewhere between 13 and 17 billion years. It is expected that further studies at the limit of available telescopes during the next years will make it possible to determine this fundamental parameter more accurately. But whatever the actual age, one of the central questions which must answered in order to understand the evolution of the Universe is the time of formation of the first stars and galaxies; its determination is accordingly a prime goal of current cosmological observations. This early process was crucial for the distribution of matter now observed, but how long after the Big Bang did it actually happen? We do not know yet.

In order to cast more light on this important question, we must look back to this very early epoch by detecting and measuring objects at the largest possible distances, i.e. at the highest redshifts. However, this is extremely difficult because of the faintness of such objects and so far, progress in this fundamental research field has been slow.

An Enriched Hydrogen Cloud at z = 4.4

In 1994, the ESO team obtained a high-resolution, detailed spectrum of the 18th magnitude quasar QSO 1202-07 with the EMMI Multi-Mode instrument at the ESO NTT telescope. At the redshift of z = 4.70, this quasar is one of the most distant, known objects. The astronomers noted a strong absorption line in the spectrum at a wavelength near 6545 A, indicating the existence of a massive cloud of neutral hydrogen in the line of sight towards the quasar. The quasar light shines through the cloud and from the measured wavelength, a cloud redshift of z = 4.38 can be deduced. This corresponds to a "look-back time" of ~ 90 percent of the age of the Universe.

At smaller redshifts, such strong absorption-line systems (known as "damped Lyman-alpha systems") have been shown to be associated with visible galaxies in which are located the hydrogen clouds that cause the absorption lines in the quasar spectra. It is therefore reasonable to surmise that the absorption line seen at redshift z = 4.38 in the spectrum of QSO 1202-07 also arises in an intervening galaxy, situated at a distance that corresponds to this redshift.

This is further supported by the detection in the near-infrared region (7000 - 9000 A) of the QSO spectrum, and at the same redshift as that of the cloud, of several absorption lines of other elements in various ionization stages, for instance neutral oxygen (O I), single-ionized carbon, silicon and aluminium (C II, Si II, Al II), as well as triple-ionized carbon and silicon (C IV, Si IV). This observation proves decisively that the hydrogen cloud, even though it is located at a very large distance and is therefore observed at a time when the Universe was only about 10 percent as old as now, has already been enriched by heavier elements.

The observed elements cannot date from the Big Bang (only the lightest elements were synthesized then) and they were obviously produced by nuclear reactions in the interior of massive stars. These stars, in turn, must have been born and already arrived at the end of their evolution (thereby injecting these elements into the hydrogen cloud), before the very early epoch at which this cloud is now observed.

Detecting the Galaxy

Motivated by this indirect evidence, the ESO team began a search for the visible counterpart of the hydrogen cloud (the "primeval galaxy") during an observing run at La Silla in April 1995. This time the instrument of choice at the NTT was the direct CCD-imaging camera SUSI (SUperb Seeing Instrument). Images totalling integration time of no less than 13.5 hours were obtained at blue, visual, red and near-infrared wavelengths (B, V, R and I bands). With the superb optical quality of the active optics of this telescope and the high angular resolution of the SUSI camera, the stellar images in the final, combined exposures are very sharp, reaching ~ 0.5 arcsec at red wavelengths, an excellent value for a ground-based telescope.

Image eso9531a accompanies this Press Release and clearly shows the resulting detection of a very faint galaxy in the R frame. It is situated only 2 arcsec North-West of the star-like image of the much brighter quasar and is partially overlapped by it (a). This angular distance corresponds to a linear off-set distance of about 12 kpc (~ 40,000 light-years) from the line-of-sight to the quasar at the distance corresponding to the redshift. The faint galaxy is much better visible, when the quasar image is removed by image processing techniques (b). The total amount of neutral hydrogen gas as deduced from the QSO absorption line and the estimated abundance of heavier elements (about 100 times less than what is observed in the Sun) are close to what is expected in a gas-rich galaxy at the observed off-set distance from the line-of-sight to the quasar.

The red magnitude of the galaxy is R = 24.3, i.e. its image is more than 300 times fainter than that of the background quasar. The optical colours (R-I = 0.2, V-R = 2.2, B-R > 3.0) can only be matched by a star-forming galaxy at a redshift between 4.0 and 4.7, whose spectrum is considerably depressed in the B and V bands by redshifted hydrogen absorption [4].

This is a strong indication that it is indeed this galaxy that causes the hydrogen absorption seen in the quasar spectrum. Definitive confirmation can only come from a spectrum of the galaxy, but this is not possible with existing telescopes due to its faintness, and the strong light from the nearby quasar would make such an observation even more difficult.

The same galaxy has been independently detected [5] at the Keck 10-metre telescope (Mauna Kea, Hawaii) in the infrared K-band (wavelength 2.3 micron) by George Djorgovski (Palomar Observatory, USA). When compared with the accurate photometry by the ESO team, the fact that the galaxy is also visible at that wavelength indicates that it is intrinsically moderately luminous. From the evolutionary models for primeval galaxies, it appears that its age is about 100 million years.

Future Observations

This exciting result now opens a new window on the epoch of galaxy formation and paves the way for future investigations of primeval galaxies. The present team of astronomers is continuing this type of work towards fainter magnitudes and in other quasar fields.

Some of the faintest images in the observed field (see the photo) probably belong to galaxies at the same or even higher redshifts.

A detailed study of such objects will be one of the important tasks of the ESO Very Large Telescope. For this, the FORS instrument will be very well suited and it will become possible to measure the colours of galaxies down to magnitudes near R = 27. Spectra of the "brighter" ones, like the galaxy described here, can then also be obtained.

Endnoten

[1] The team is headed by Sandro D'Odorico (ESO-Garching) and includes Stefano Cristiani (Department of Astronomy, University of Padova, Italy), as well as Adriano Fontana and Emanuele Giallongo (Astronomical Observatory, Rome, Italy).

[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 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.

[3] 1 billion = 1,000,000,000.

[4] This absorption arises from the higher-order Lyman lines and in particular from the blockage below the Lyman-continuum at 912 A. in the galaxy itself and in the space between us and the galaxy along the line-of-sight.

[5] See the Proceedings of ESO Astrophysics Symposium "Science with the VLT", edited by J.R. Walsh and J. Danziger, Springer 1995, page 359.
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