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VLT Studies Very Distant Galaxies

FORS Takes Spectra of Faint Primordial Objects

27 de Febrero de 1999

Continuing progress in astronomical technology is opening new possibilities for the study of the distant universe. One of the most exciting, recent additions to this branch of astrophysics, known as cosmology, has been the discovery of a large population of galaxies in the primordial Universe in which intensive star-formation is going on. They are so distant (their redshifts are larger than 3 [1]) that the corresponding look-back time is over 90% of the age of the Universe, now estimated at about 14 - 15 billion years (1 billion = 1,000 million). We observe these objects as they were, when the Universe was between 1 and 2 billion years old. The investigation of the early Universe is one of the primary scientific goals that have motivated the construction of the ESO Very Large Telescope and its very diverse complement of instrumentation. The aim of these studies is to extend the observations of basic properties of galaxies to objects at the largest possible distances and hence the earliest possible epochs. We would like to learn as much as possible about these very faint galaxies, including their numbers and hence their space density, as well as their brightness, colours, sizes and shapes. What are the rates with which stars are formed in different galaxies at different epochs, what is their chemical composition and mass? How do they move in space and how do they cluster?

New spectra show properties of very distant galaxies

During the recent commissioning and science verification of the Focal Reducer/low dispersion Spectrograph (FORS) at the first 8.2-m VLT Unit Telescope (UT1), spectra were taken of several objects, thought to be high-redshift galaxies. These targets were located in two southern sky fields, known the Hubble Deep Field South and the AXAF Deep Field . The first of these has been extensively studied by means of images obtained with the Hubble Space Telescope; some VLT exposures of this sky field were also made, cf. e.g. eso9812. The other field will be observed with the new satellite AXAF X-ray observatory (now baptized "Chandra") that will be launched soon.

For these VLT observations, the selection of the targets was made by means of very deep images, obtained in several optical bands at the ESO New Technology Telescope (NTT) at the La Silla Observatory, within the ESO Imaging Survey (EIS). Distant high-redshift galaxies have peculiar colours - mostly unusually red - that help to distinguish them from the much more numerous, nearer galaxies. However, there are other objects that have similar colours, and only a spectrum will tell the true nature of the object.

These objects are extremely faint and their spectra can only be observed with very large telescopes like the VLT and a highly efficient spectrograph. The near-infrared (I) magnitudes of the objects studied during the present test observations ranged between 23.4 and 25.5, or between 10 and 65 million times fainter than what can be seen with the unaided eye.

As predicted, a large fraction of the spectra obtained turned out to be those of extremely distant galaxies, in the redshift range between z = 2.8 - 4.0.

Examples of FORS observations

Here are some examples of the new FORS spectral observations of distant galaxies, including two at redshifts z = 2.8 and 3.9.

Outlook

These observations demonstrate the great capability of the ESO VLT to peer into the remotest realms of the Universe and its potential to contribute to the study of the physical processes that took place at the time of the formation and evolution of the first structures. These new data will also allow future visiting astronomers to optimize their use of the VLT and FORS for observations of extremely faint and remote objects.

Not least, they will also help to improve the selection of future targets for spectral observations by refining the multicolour selection criteria from survey data.

Notas

[1]: In astronomy, the redshift (z) 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 universal expansion (i.e. the `recession velocity'). Since this expansion rate increases with the distance, the velocity (and thus the redshift) is itself a function (the Hubble relation) of the distance to the object. The larger the distance, the longer it has taken the light from the object to reach us, and the larger is the "look-back" time, i.e. the fraction of the age of the Universe that has elapsed since the light we now receive, was emitted from the object.

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