Astronomers are presenting today high-resolution images of galaxies that were already old when the Universe was only 20 percent of its present age. The report is being presented by Dr. Alan Stockton of the Institute for Astronomy, University of Hawaii at Manoa in a press conference at the American Astronomical Society meeting in Pasadena, California, and includes work done with Dr. Gabriela Canalizo (University of California, Riverside) and Dr. Elizabeth McGrath (University of California, Santa Cruz). These images allow the analysis of the detailed structures of the galaxies, which are of special interest because they are quite unlike those of massive galaxies in the present-day Universe and may help in understanding how the first massive galaxies formed shortly after the Big Bang.
The galaxies studied in this work were all found close to strong radio sources at distances with light-travel times of about 11 billion years (redshifts around 2.5), so the galaxies are being observed when the Universe was about 2.7 billion years old, and the light from these galaxies was already more than halfway along on its journey before the Earth and the Sun had been formed. The fields studied were those of the radio galaxies 4C 23.56 and TXS 2332+154, and of the quasar 4C 29.28.
The observations were obtained with the laser-guide-star adaptive optics system of the 10-meter (400-inch) Keck II Telescope on Mauna Kea, Hawaii. This system uses a powerful laser to create an artificial star by exciting sodium atoms in the upper atmosphere so that they emit light. The instrument on the telescope uses this light to analyze how the atmosphere is distorting the light from astronomical sources in the same direction. This distortion can then be largely removed by introducing a compensating distortion in a special deformable mirror in the instrument. (Further information on the technique can be found at http://www.spaceref.com/news/viewpr.html?pid=18714).
An important feature of the galaxies selected for this study is that they appear, at this early time in the history of the Universe, to have formed almost all their stars at much earlier times, shortly after the Big Bang. They have not had significant star formation for at least a billion years before the time of observation. Although the number of galaxies found so far in these and other similar radio-source fields is small, they can be considered representative of much larger samples of similar galaxies found in large surveys. The most striking property of these galaxies is that they are much smaller than galaxies in our local neighborhood with similar numbers of stars. “In the most extreme cases, including one we have studied, some of these galaxies pack all of the stars of a fairly massive present-day galaxy into a volume about a thousand times smaller,” says Dr. Stockton.
The Keck laser-guide-star adaptive optics system can obtain near-infrared images from the ground that are sharper than those that can be obtained by the Hubble Space Telescope at similar wavelengths. These images allow not only the measurement of characteristic sizes of the distant galaxies, but also more detailed properties of the light distribution that may give clues to formation processes (see Figs. 1—3 for examples).
So far, almost all the massive galaxies comprising essentially only old stars found in these high-redshift radio-source fields fall into one of two classes: (1) very small disk galaxies (but, unlike local spiral galaxies, having no recent star formation) and (2) even smaller compact, high-surface-brightness galaxies, typically with half of the total light within a radius of 1600 light-years (500 parsecs). Neither of these types of galaxies is likely to have been formed from the merging of smaller galaxies, so at least some massive galaxies seem to have formed essentially all at once and very early in the history of the Universe.
Another interesting question is: Why don’t we see similar objects now? In some form or other, they must be represented among the most massive galaxies that we see around us. It is likely that many have merged with other galaxies or have otherwise acquired surrounding, more diffuse envelopes of stars, but such processes probably are not sufficient. It may be that the star density in the central region of some of these galaxies is so high that interaction between stars leads to a redistribution of orbits, puffing up the outer parts of the galaxy, while the inner region becomes even denser, perhaps finally collapsing into one of the supermassive black holes that seem to be ubiquitous in the centers of present-day massive galaxies.
The main outlines of this picture of massive non-star-forming galaxies in the early Universe seem reasonably secure, particularly since broadly similar findings have come out of large random-field surveys. In particular, Dr. Pieter van Dokkum of Yale University and his colleagues published in 2008 a similar analysis of several galaxies from such a survey. This analysis was mainly based on images from the Hubble Space Telescope, but also used the Keck laser-guide-star system. The major questions that have to be addressed by future work include the following:
1. Is it possible that we are overestimating the mass of these galaxies? This could be the case if, for example, early generations of stars tended to produce more very luminous high-mass stars and proportionately fewer low-mass stars.
2. If the masses we infer for these galaxies can be taken even roughly at face value, what has happened to them over the past 10 billion years or so? A few attempts to discuss this question have been made, most recently in a 2009 paper by Ms Rachel Bezanson of Yale University and colleagues.
3. By what mechanism were these very dense stellar systems formed? Is there an evolutionary relationship between the disk-like galaxies and the denser, apparently more spheroidal galaxies?
This research was supported in part by the National Science Foundation, under grant AST 03-07335.
FIGURES AND CAPTIONS
Figures, with captions, are available in pdf format or in other forms at http://www.ifa.hawaii.edu/~stockton/aaspix.
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