Pasadena, CA- Some galaxies grew up in a hurry. Most
of the galaxies that have been observed from the early days of the
universe were young and actively forming stars. Now, an international
team of astronomers, including Carnegie’s Eric Persson and Andy Monson,
have discovered galaxies that were already mature and massive in the
early days. Fifteen mature galaxies were found at a record-breaking
average distance of 12 billion light years, when the universe was just
1.6 billion years old. Their existence at such an early time raises new
questions about what forced them to grow up so quickly. The finding is
published by The Astrophysical Journal Letters.
Today the universe is filled with galaxies that have largely stopped
forming stars, a sign of galactic maturity. But in the distant past,
galaxies were still actively growing by consuming gas and turning it
into stars. This means that mature galaxies should have been almost
non-existent when the universe was still young.
Together with lead author Caroline Straatman and principal
investigator Ivo Labbe, both of Leiden University, the astronomers used
deep images at near-infrared wavelengths to search for galaxies in the
early universe with red colors. The characteristic red colors indicate
the presence of old stars and a lack of active star formation. The
galaxies are barely detectable at visual wavelengths and are easily
overlooked. But in the new near-infrared light images they are easily
measured, from which it can be inferred that they already contained as
many as 100 billion stars on average per galaxy.
The mature galaxies have masses similar to that of the Milky Way,
which still forms new stars at a slow rate. The newly discovered
galaxies must have formed very rapidly in roughly 1 billion years, with
explosive rates of star-formation. The rate of star formation must have
been several hundred times larger than observed in the Milky Way today.
The finding raises new questions about how these galaxies formed so
rapidly and why they stopped forming stars so early. It is an enigma
that these galaxies seem to come out of nowhere. Another big question is
what caused the galaxies to mature at such a young age and if some
dramatic event might have caused premature aging.
The galaxies were discovered after 40 nights of observing with the
FourStar camera on the Magellan Baade Telescope at Carnegie’s Las
Campanas Observatory in Chile and combined with data from Hubble’s
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey and the
Great Observatories Origins Deep Survey. Using special filters to
produce images that are sensitive to narrow slices of the near-infrared
spectrum, the team was able to measure accurate distances to thousands
of distant galaxies at a time, providing a 3-D map of the early
universe.
Caption: A Hubble Space Telescope color composite
image of the astronomical field Chandra Deep Field South, with two of
the 15 mature galaxies. They exhibit the typical red colors of mature
galaxies. The other galaxies in the image are much closer. A larger
version is available here.
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This research was supported in part by the George P. and Cynthia
Woods Mitchell Institute for Fundamental Physics and Astronomy; NSF
AST-1009707; ERC HIGHZ #227749; and NL-NWO Spinoza. Australian access to
the Magellan Telescopes was supported through the NCRIS of the
Australian Federal Government. This work is based on observations made
with Herschel, an ESA Cornerstone Mission with significant participation
by NASA, through an award issued by JPL/Caltech.
Source: Carnegie Institution for Science
