Patrick McCarthy

GMTO Director


Distant galaxies offer a glimpse of the universe as it was billions of years ago. Understanding how the Milky Way and other galaxies originated provides a unique perspective on the fundamental physics of cosmology, dark matter, and dark energy. Patrick McCarthy uses the facilities at the Las Campanas Observatory to explore the early formation and evolution of galaxies. Galaxy formation is driven by the interplay between the large-scale distribution of dark matter (non-luminous and unidentified fundamental particles), and processes at the atomic-scale that regulate the cooling of gas into stars. He seeks to understand when the first massive, gravitationally bound stellar systems formed and how the diverse properties of today’s galaxies are linked to their histories.

Using the du Pont telescope and the WIRC infrared camera built by Eric Persson, McCarthy has mapped several areas of the sky to faint levels at wavelengths from 0.5 to 2.5 microns to identify the most massive stellar systems at each point in time. He surprisingly found many galaxies with masses in excess of 100 billion solar masses when the universe was only 3 billion years old. The spectra indicate their stars are 2-3 billion years old, implying an early formation. Also contrary to theory, McCarthy and colleagues find that the largest and most massive galaxies formed earliest in the universe, while the small and dwarf galaxies formed late; many are still forming today. High-resolution images, obtained with the Hubble Space Telescope, indicate that they are structurally similar to today’s elliptical galaxies and the central bulge of the Milky Way. The different formation histories, sizes, and types of galaxies harken to the concept of distinct stellar populations, introduced by Carnegie astronomer Walter Baade. The different star formation histories of galaxies of different masses is manifest within the Milky Way by the distinct stellar populations in the galactic bulge and spiral disk.

McCarthy also uses the IMACS spectrograph on the Baade telescope to explore the connection between environment, mass, and star formation history in distant galaxies.


B.S. physics and astronomy 1983, University of Arizona; Ph.D. astronomy, 1988, University of California, Berkeley

Probing the era of galaxy formation