Far but near

Detection of two galaxies that are unassociated with a quasar raise further questions about galactic evolution

 
Published: Monday 30 September 1996

galaxy formation and early evolution is one of the least understood areas of cosmology and astrophysics. The basic problem is one of detecting faint galaxies located at large distances. The astronomer is limited to observing far-off objects which are very bright, like quasars and radio galaxies. But now, E M Hu and Richard McMahon of the universities of Hawaii and Cambridge respectively, have detected two galaxies which are very far and yet not associated with a quasar; these objects are far away from the quasar in whose field they have been detected ( Nature , Vol 382, No 6588).

The search for high redshift objects in space has been going on for some time. Redshift denotes the light emitted by an object which is moving away from us towards the red end of the spectrum. One of the consequences of an expanding universe is that galaxies are going farther away from us and hence are red shifted; the further the galaxy, the larger is the red shift.

Extensive spectroscopic analysis shows that galaxies with a redshift greater than one are in the process of star formation. Objects of redshifts higher than about 1.7 are detected on the basis of their ultraviolet absorption lines. Hu and McMahon first obtained exposures through a narrow-band filter centered on the emission from the quasar br 2237-0607, which has a redshift of 4.55. These exposures were obtained at the 2.2-m telescope at the University of Hawaii.

The exposures revealed the existence of 10 objects with significant emission of a kind of spectra called Lyman-a. But most of these objects were really low redshift galaxies whose emission lines were coincidentally matching the Lyman-a wavelength. Subsequent studies by the 10-m Keck telescope, the largest optical telescope in the world, provided the detailed spectra of these 10 objects.

After analysing the spectra, Hu and McMahon concluded that they had detected two galaxies with a redshift of about 4.5 which were about 2,100 light years away from the nearest quasar. The galaxies appeared to be undergoing their first phase of star formation.

Important as this detection is, it also raises further questions about the nature and evolution of galaxies. One of the questions that has to be answered is that whether these objects are uniformly distributed or are clustered around quasars in the universe. Observations currently going on may yield answers to these and other related questions.

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