Farthest galaxy is cosmic question.

Farthest galaxy is cosmic question

Galaxies are the basic constituents of the universe. When and how they formed are fundamental questions for any theory of cosmology. Astronomers keep looking for ever-more-distant galaxies, because the farther away they are, the earlier the epoch at which we see them. Simon Lilly, a British astronomer working at the University of Hawaii at Manoa, has found a galaxy with a redshift of 3.4, which would put it four-fifths of the way back to the beginning of the universe.

This object, catalogued as 0902+34, is the most distant object now known for which there is good evidence that it is a galaxy. Some astronomers at the University of Arizona recently found infrared objects that seen to have redshifts greater than 6 (SN: 1/23/88, p.52) but have not been able to prove they are galaxies. Redshift, a displacement of the object's emissions toward the red end of the spectrum, is proportionate to the speed at which the object is receding from the objserver. Distance can be calculated from the speed, but the computation uses the Hubble constant, which is so difficult to determine that astronomers cannot agree on its value. The value taken by Lilly makes the universe 15 billion years old, and on this basis 0902+34 is 12 billion light-years away. Other values of the hubble constant yield ages for the universe between 10 billion and 20 billion years. Adopting any of them would change the distance to 0902+34 proportionately.

By whatever distance estimate, 0902+34 is far enough back in time to make trouble for a class of currently high-interest cosmological theories, the cold dark-matter theories, says Len Cowie, assistant director of the Institute for Astronomy at the University of Hawaii. Astronomers have a number of reasons for believing that a large amount of unseen, undetected matter pervades the universe. In most models this dark matter is composed of subatomic particles, neutrinos or maybe something more exotic. These models are divided into two classes according to whether the dark matter is hot or cold. The cold dark-matter theories appear to be the most popular at the moment, but the discovery of 0902+34 could make trouble for them, since they do not allow for galaxies as well formed as this one existing at such an early epoch.

Speaking from Mauna Kea, where he was observing, Lilly told SCIENCE NEWS that the discovery arose as part of a project to survey certain radio sources to see whether optical counterparts could be found for them. Since 1986 he has been concentrating on the dozen most promising ones chosen from his original sample. Object 0902+34 is the only one of the dozen or so that looks like a galaxy. A report will appear in ASTROPHYSICAL JOURNAL.

Althoug radio sources like 0902+34 are usually associated with very luminous galaxies, none appeared for this one when it was first studies in 1982. In his work on the object, Lilly used two telescopes, both on Mauna Kea, and a variety of recently developed, very sensitive recording equipment. In 1985, using the United Kingdom Infrared Telescope, Lilly detected a faint infrared source at the position of the radio source. Then, with the Canada-France-Hawaii Telescope, he got a visible-light image. Returning to the United Kingdom Infrared Telescope, he used a newly available infrared array camera to make an infrared image of the object. Finally, back at the Canada-france-Hawaii Telescope, he used a newly developed faint-object spectrograph, which obtained a spectrun that showed a strong emission feature.

This feature can be identified with a resonance of atomic hydrogen, whose emission, at rest in the laboratory, appears at an ultraviolet wavelength. In 0902+34 it is shifted completely across the visible range into the infrared, and from this Lilly calculated the redshift of 3.4.

The spectrum of 0902+34 indicates that the light comes from stars, and therefore the object is a galaxy. Among several indicators that support the conclusion, says Lilly, is the uniform brightness of the spectrum in the optical range--"too flat to be a quasar," the only other class of objects astronomers are used to seeing at such redshifts.

From the spectral evidence Lilly concludes that 0902+34 contains two distinct populations of stars. One is young blue stars, indicating that the galaxy is converting annually about 100 times the sun's mass of interstellar matter into new stars. The second population, accounting for more than 90 percent of the galaxy's mass, is mature reddish stars at least 1 billion to 2 billion years old. These are responsible for the stronger emission at infrared wavelengths than at visible ones.

The antiquity of the galaxy and of the stars within it make it very important for theories of cosmology and for models of the formation and development of galaxies and of stars. "The importance of 0902+34 for cosmological theories cannot be over-emphasized," says Cowie. Donald N.B. Hall, director of the Institute for Astronomy, remarks that it will be important to determine whether 0902 + 34 is an anomalous object or one of a class. Are there other galaxies with similar redshift and similar appearance waiting to be found? The continuing development of more sensitive recording equipment and, in a few years, the launching of the Space Telescope and the completion of the 10-meter Keck Telescope, also on Mauna Kea, may make it possible to find out.