Galaxies die from the inside out
Elliptical galaxy IC 2006
VLT and Hubble observations show that star formation shuts down in the centres of elliptical galaxies first
Astronomers have shown for the first time
how star formation in “dead” galaxies sputtered out billions of years
ago. ESO’s Very Large Telescope and the NASA/ESA Hubble Space Telescope
have revealed that three billion years after the Big Bang, these
galaxies still made stars on their outskirts, but no longer in their
interiors. The quenching of star formation seems to have started in the
cores of the galaxies and then spread to the outer parts. The results
will be published in the 17 April 2015 issue of the journal Science.
A major astrophysical mystery has centred on how massive, quiescent elliptical galaxies,
common in the modern Universe, quenched their once furious rates of
star formation. Such colossal galaxies, often also called spheroids
because of their shape, typically pack in stars ten times as densely in
the central regions as in our home galaxy, the Milky Way, and have about
ten times its mass.
Astronomers refer to these big galaxies as red and dead as they
exhibit an ample abundance of ancient red stars, but lack young blue
stars and show no evidence of new star formation. The estimated ages of
the red stars suggest that their host galaxies ceased to make new stars
about ten billion years ago. This shutdown began right at the peak of
star formation in the Universe, when many galaxies were still giving
birth to stars at a pace about twenty times faster than nowadays.
“Massive dead spheroids contain about half of all the stars that the Universe has produced during its entire life,” said Sandro Tacchella of ETH Zurich in Switzerland, lead author of the article. “We
cannot claim to understand how the Universe evolved and became as we
see it today unless we understand how these galaxies come to be.”
Tacchella and colleagues observed a total of 22 galaxies, spanning a
range of masses, from an era about three billion years after the Big
Bang [1]. The SINFONI
instrument on ESO’s Very Large Telescope (VLT) collected light from
this sample of galaxies, showing precisely where they were churning out
new stars. SINFONI could make these detailed measurements of distant
galaxies thanks to its adaptive optics system, which largely cancels out the blurring effects of Earth’s atmosphere.
The researchers also trained the NASA/ESA Hubble Space Telescope
on the same set of galaxies, taking advantage of the telescope’s
location in space above our planet’s distorting atmosphere. Hubble’s WFC3 camera
snapped images in the near-infrared, revealing the spatial distribution
of older stars within the actively star-forming galaxies.
“What is amazing is that SINFONI’s adaptive optics system can
largely beat down atmospheric effects and gather information on where
the new stars are being born, and do so with precisely the same accuracy
as Hubble allows for the stellar mass distributions,” commented Marcella Carollo, also of ETH Zurich and co-author of the study.
According to the new data, the most massive galaxies in the sample
kept up a steady production of new stars in their peripheries. In their
bulging, densely packed centres, however, star formation had already
stopped.
“The newly demonstrated inside-out nature of star formation shutdown in massive galaxies should shed light on the underlying mechanisms involved, which astronomers have long debated,” says Alvio Renzini, Padova Observatory, of the Italian National Institute of Astrophysics.
A leading theory is that star-making materials are scattered by
torrents of energy released by a galaxy’s central supermassive black
hole as it sloppily devours matter. Another idea is that fresh gas stops
flowing into a galaxy, starving it of fuel for new stars and
transforming it into a red and dead spheroid.
“There are many different theoretical suggestions for the physical mechanisms that led to the death of the massive spheroids,” said co-author Natascha Förster Schreiber, at the Max-Planck-Institut für extraterrestrische Physik in Garching, Germany. “Discovering
that the quenching of star formation started from the centres and
marched its way outwards is a very important step towards understanding
how the Universe came to look like it does now.”
Notes
[1] The Universe’s age is about 13.8 billion years, so the galaxies
studied by Tacchella and colleagues are generally seen as they were more
than 10 billion years ago.
More Information
This research was presented in a paper entitled “Evidence for mature
bulges and an inside-out quenching phase 3 billion years after the Big
Bang” by S. Tacchella et al., to appear in the journal Science on 17 April 2015.
The team is composed of Sandro Tacchella (ETH Zurich, Switzerland),
Marcella Carollo (ETH Zurich), Alvio Renzini (Italian National Institute
of Astrophysics, Padua, Italy), Natascha Förster Schreiber
(Max-Planck-Institut für Extraterrestrische Physik, Garching, Germany),
Philipp Lang (Max-Planck-Institut für Extraterrestrische Physik), Stijn
Wuyts (Max-Planck-Institut für Extraterrestrische Physik), Giovanni
Cresci (Istituto Nazionale di Astrofisica), Avishai Dekel (The Hebrew
University, Israel), Reinhard Genzel (Max-Planck-Institut für
extraterrestrische Physik and University of California, Berkeley,
California, USA), Simon Lilly (ETH Zurich), Chiara Mancini (Italian
National Institute of Astrophysics), Sarah Newman (University of
California, Berkeley, California, USA), Masato Onodera (ETH Zurich),
Alice Shapley (University of California, Los Angeles, USA), Linda
Tacconi (Max-Planck-Institut für Extraterrestrische Physik, Garching,
Germany), Joanna Woo (ETH Zurich) and Giovanni Zamorani (Italian
National Institute of Astrophysics, Bologna, Italy).
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Links
Contacts
Sandro Tacchella
ETH Zurich
Zurich, Switzerland
Tel: +41 44 633 6314
Cell: +41 76 480 7963
Email: sandro.tacchella@phys.ethz.ch
ETH Zurich
Zurich, Switzerland
Tel: +41 44 633 6314
Cell: +41 76 480 7963
Email: sandro.tacchella@phys.ethz.ch
Marcella Carollo
ETH Zurich
Zurich, Switzerland
Tel: +41 797 926 581
Email: marcella@phys.ethz.ch
ETH Zurich
Zurich, Switzerland
Tel: +41 797 926 581
Email: marcella@phys.ethz.ch
Richard Hook
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
ESO, Public Information Officer
Garching bei München, Germany
Tel: +49 89 3200 6655
Cell: +49 151 1537 3591
Email: rhook@eso.org
Source: ESO
