A Completely Grown-Up Galaxy in the Young Universe
May 20, 2010
An international team of astronomers led by Dr. Masato Onodera at the Commissariat a l'Energie Atomique in France has used the Subaru Telescope to take infrared spectra of a very distant, unusually bright, and massive elliptical galaxy. This galaxy is 10 billion light-years from Earth and was observed at a time when the Universe was only about one-quarter of its current age. Paradoxically, and in contrast with some previous studies, this galaxy appears to be similar to its cousins in the local Universe. Its size appears to be normal for its mass, and its velocity dispersion (about 300 km per second) is consistent with its large size. This research deepens the puzzle as to how and why some elliptical galaxies seem to reach their full size very early in the evolution of the Universe while other, very compact ones increase in volume a hundredfold over time.
Giant elliptical galaxies are the Universe’s most massive galaxies near to Earth. They have a regular, oval shape and lack the disk typical of spiral galaxies such as our own Milky Way. Using large telescopes, astronomers have identified elliptical galaxies over ten times more massive than the Milky Way and as far away from Earth as 10 billion light years. Observing the light from such distant elliptical galaxies permits the direct study of how they looked shortly after their formation and opens a window to exploring the past of the Universe.
Five years ago, extremely deep images from the Hubble Space Telescope (HST) suggested that distant elliptical galaxies may be twice to five times smaller than nearby, local elliptical galaxies of the same mass. If these findings were accurate, then the densities of the more distant elliptical galaxies are 10 to 100 times higher than that of the local ones. Since then, experts have debated how these very compact galaxies could expand over the intervening 10 billion years so that they matched the size of their local counterparts. Many questioned whether the measurements of the size of the distant elliptical galaxies were accurate. Could some measurement error or bias account for their apparently small size?
To address this question, an international team of astronomers led by Dr. Masato Onodera at the Commissariat a l'Energie Atomique in France turned to one of the world’s largest censuses of the distant Universe, the Cosmic Evolution Survey (COSMOS), to find new candidates for the most massive, distant, giant elliptical galaxies. They looked for objects with a unique “fingerprint” of visible and near-infrared light as measured by Subaru’s Prime Focus Camera (Suprime-Cam) and the Canada-France-Hawaii Telescope’s Wide-field Infrared Camera (WIRCam). Finally, they used the COSMOS team’s unique database of high-resolution Hubble Space Telescope images to pick objects that had similar shapes to local elliptical galaxies of the same mass. This final sample of objects was selected for further observations at Subaru.
Onodera’s team decided to use a different measurement—velocity dispersion of stars—to differentiate the evolutionary status of a distant elliptical galaxy. Velocity dispersion refers to the range of velocities of stars or galaxies in a cluster and is a way of determining the mass of objects within this spread. The smaller the size of a galaxy of a given mass, the faster the stars have to move in order to balance the pull of gravity. Measurements of the broadening of spectral lines in the galaxy’s spectrum can indicate the speed of the stars and enable derivation of the galaxy’s mass by combining measures of its size and velocity.
However, strong spectral lines appropriate for making these measurements of distant galaxies exist in the near infrared range of the spectrum, beyond visible light, where observations are particularly difficult. Equipped with its Multi-Object Infrared Camera and Spectrograph (MOIRCS), the Subaru Telescope was particularly well-suited for revealing these spectral lines, because it can capture infrared light from multiple objects in a wide field of view, providing images and spectroscopic measurements of their composition.
This is a relatively new way to measure the mass of distant galaxies. The first measurement of this kind as published only recently and revealed a velocity dispersion (over 500 km per second) of an elliptical galaxy that is consistent with its probable small size but has no counterpart among local galaxies. Using the same technique, Onodera’s team found an elliptical galaxy (ID 254025) with a smaller velocity dispersion (over 300 km per second) that is consistent with its large size (about 19,000 light years). These results provide evidence that large, fully-grown galaxies coexist with very compact ones in the early development of the Universe, a few billion years after the Big Bang.
The mystery of how different elliptical galaxies form and develop remains. Onodera’s team is now turning to the problem of determining the relative proportion of these two extreme types of elliptical galaxies as a function of cosmic time. Further observations with Subaru Telescope’s MOIRCS lie ahead to help solve this puzzle.
The paper appeared as “A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies” (Onodera et al. 2010, Astrophysical Journal Letters, Volume 715, pp. L6-L11).
This project was partly funded by the Agence Nationale de la Recherche, grant number ANR-07-BLAN-0228.
Masato Onodera, Emanuele Daddi, Raphael Gobat (CEA/Saclay, France), Nobuo Arimoto, Naoyuki Tamura, Yoshihiko Yamada (National Astronomical Observatory of Japan, Japan), Michele Cappellari (University of Oxford, UK), Chiara Mancini, Alvio Renzini (Osservatorio Astronomico di Padova, Italy), Henry J. McCracken (Institut d’Astrophysique de Paris, France), Peter Capak, Nick Scoville (California Institute of Technology, USA), Marcella Carollo, Simon Lilly (ETH Zurich, Switzerland), Andrea Cimatti (Universita di Bologna, Italy), Mauro Giavalisco (University of Massachusetts, USA), Olivier Ilbert (Laboratoire d’Astrophysique de Marseille, France), Xu Kong (University of Science and Technology of China, China), Kentaro Motohara (University of Tokyo, Japan), Kouji Ohta (Kyoto University, Japan), Dave. B. Sanders (University of Hawaii, USA), Yoshiaki Taniguchi (Ehime University, Japan)