Patrick McCarthy

The stars that constitute the bulk of the mass of galaxies emit most of their luminous energy at wavelengths longer than 0.5 microns. These cool, giant stars have masses comparable to that of the Sun and lifetimes of billions of years. At large distances, the cosmological redshift has shifted this radiation to the near-infrared (IR). Most surveys of high-redshift galaxies have been confined to visible wavelengths, and thus sample ultraviolet (UV) photons emitted at the source. The emitted and observed wavelengths are related by l_emitted = l_observed/(1 + z), where z is the redshift. The ultraviolet luminosity of galaxies is often dominated by a small population of massive stars that produce 100 to 1,000 times more luminosity per unit mass than stars like the Sun, but have lifetimes that are measured in millions rather than billions of years. The UV luminosities of galaxies thus reflect their present star formation rates, while near-infrared luminosities are a far better tracer of the total stellar mass.

For the past few years Patrick McCarthy and colleagues have been applying near infrared techniques, from Las Campanas and from orbit, to study faint and distant galaxies. Together with Staff Member Eric Persson and Hubble Fellow Ron Marzke, McCarthy has embarked on an ambitious survey of faint galaxies in the near-infrared. Using a unique camera containing a mosaic of four large near-IR detectors, the scientists are surveying an area of the sky four and a half times the area of the full Moon. This survey will yield a sample of between 2,000 and 4,000 old galaxies at large redshifts, and will allow the astronomers to precisely measure their spatial clustering -- an important clue in understanding the growth of density irregularities in the early universe.

Spectroscopy in the near-infrared offers a new window on galaxies at large redshifts. McCarthy and postdoctoral associate Lin Yan have used the near-IR spectrometer on board the Hubble Space Telescope to measure the star formation rates of galaxies with redshifts between 1 and 2. Yan and others find that the star formation rate at a redshift of 1.5 is 17 times larger than the present rate. This high rate of star formation is a factor of two to three times greater than the rate derived from ultraviolet measurements and implies that roughly 50% of the stars in the present universe were formed as recently as 5 billion years ago. This is in stark contrast to the very inactive and old galaxies that are the targets of the survey described above. These results highlight the fact that galaxy evolution is not a uniform or homogeneous process. The life history of a galaxy is greatly influenced by the environment in which it begins. Some reach maturity early in the history of the universe, while others lie dormant until some stimulus drives the conversion of gas into stars.

Fig. 6. This is a near-infrared and optical image of a cluster of galaxies with a redshift of nearly 1. The picture is a composite of images at 0.5, 0.8, and 1.6 microns. Old galaxies are strongly clustered around the central object, a luminous radio source. These data were obtained with the du Pont telescope at Las Campanas.