Lynx Arc Supercluster

Located behind a cluster of galaxies in northern Constellation Lynx around 12 billion light-years (ly) away, the Lynx Arc is a distant supercluster of extremely hot, young stars. Roughly one million times brighter than the well-known Orion Nebula, the Lynx Arc contains a million blue stars that are twice as hot as similar stars in the Milky Way galaxy. Only visible through gravitational lensing by a closer cluster of galaxies, the Arc is a feature of the early days of the universe, when "furious firestorms of star birth" were more common. It may be going through a short-lived luminous phase that may have lasted for as little as a few million years (Fosbury et al, 2003, in pdf).

Robert A.E. Fosbury, ESA, NASA
Digital Sky Survey



Larger, three-color composite image.




Within Constellation Lynx, the
very young supercluster was
found in an image of the Lynx
galaxy cluster, located within
the center box (more at
HubbleSite.org).

The Arc was discovered (at 8:48:48.8+44:55:49.6, J2000) during a systematic study of distant galaxy clusters using major X-ray, optical, and infrared telescopes, including NASA's Hubble Space Telescope, ROSAT, and the Keck Telescopes. An international team of astronomers including Robert A.E. Fosbury of the European Space Agency's Space Telescope-European Coordinating Facility in Germany found the stretched and magnified image of a huge cluster of stars appearing as a red arc behind the already distant, Lynx galaxy double cluster of CL J0848.8+4455 (z=0.543) and RX J0848+4456 (z=0.570), which are located around 5.4 billion light-years away in northern constellation Lynx. The Arc was eventually determined to be located about 12 billion light-years away (at a redshift of z=3.357).

Kitt Peak Observatory,
NOAO


Larger composite image.


The young supercluster is visible
as a red object located to the
right of the Lynx cluster of
galaxies, within the boxed area
(more at HubbleSite.org).

Fosbury and his colleagues first tried to identify the Arc by analyzing its light. However, the team was initially unable to recognize the pattern of colors in the spectral signature of the remote object. While looking for matches with the color spectrum, Fosbury realized the light was related to that of the nearby Orion Nebula, a star-forming region in our own Milky Way. However, while the Orion Nebula is powered by only four hot and bright blue stars, the Lynx Arc must contain around a million such stars. Although the largest known star-birth clusters in the Milky Way are the Arches clusters in the galactic center, the Carina Nebula in the Constellation Fornax (described in the page on Eta Carina, and the 30 Doradus cluster in the Large Magellanic Cloud, these clusters contain only hundreds or thousands of super-hot stars, only a fraction of the size of the Lynx Arc megacluster.

Robert A.E. Fosbury, ESA,
NASA, NOAO

Larger composite image.


The reddish arc is really
a stretched and magnified
image of a distant mega-
cluster of stars lying far
behind the Lynx galaxy
cluster about 12 billion
light-year away (more).

Analysis of the color and intensity of light from the Arc indicates that much of its light started out as ultraviolet light from the hottest stars and was stretched to red light ("red-shifted") after travelling 12 billion light-years. As the hotter a star is, the bluer and more massive it is, and so the intensity of the ultraviolet radiation emitted suggests that the stars of the Lynx Arc are among the most massive stars seen ever detected in the universe. The spectrum of the Arc suggests that its stars are more than twice as hot as the Orion Nebula's central stars, with surface temperatures up to 144,000 degrees Fahrenheit (80,000 degrees Celsius) -- hotter and brighter than Theta1 Orionis C (of spectral and luminosity type O7 V), the brightest of the four central stars of the Trapezium Cluster in Orion. Although there are larger and brighter star-forming regions than the Orion Nebula in the nearby universe, none are as bright as the Lynx arc, nor do they contain such large numbers of hot stars. Even the most massive, normal nearby stars are no hotter than around 72,000 degrees Fahrenheit (40,000 degrees Celsius). However, stars formed from primordial clouds of extemely metal-poor, hydrogen and helium gas in the early universe can be more massive and consequently much hotter than those created today — perhaps up to 215,000 degrees Fahrenheit (120,000 degrees Celsius).

John Bally, Dave Devine, and
Ralph Sutherland, STScI, NASA


Larger false-color image.


Hotter than Theta1 Orionis C, the brightest of
the four central stars of the Trapezium Cluster
in Orion (at left), the bright O-type stars of the
Lynx Arc were brighter and bluer than any of
the stars of the "Solar neighborhood" known
to be located within 100 light-years of Sol.

Although many of the first generation of stars (Population III) in the cosmos may each have been as much as several hundred Solar-masses, but the comparatively metal-rich, chemical makeup of the universe today prevents stars from forming beyond about 100 Solar-masses. Such "primordial" super-hot stars are thought to be the first luminous objects to condense after the Big Bang cooled. Astronomers believe that these first stars formed considerably earlier than those observed in the Lynx Arc — up to 1.8 billion years earlier. While the nebular material around the Arc is not extremely metal-poor (Z/Z-Solar = 0.05), however, "there is considerable evidence that the ionizing stellar cluster is considerably more metal-poor" (Fosbury et al, 2003, in pdf). The apparent overabundance of silicon in the nebula of the star cluster could indicate enrichment by past "pair-instability supernovae" (see discussion in pdf) which require pregenitor stars of 140 to 260 Solar-masses (Fosbury et al, 2003, in pdf). Because the stars of the Lynx Arc were also very massive, however, most probably exploded as supernovae within a few million years or so, to blast their own, newly created stock of heavy elements into intergalactic space. Some of this material was recycled into subsequent generations of stars, while surviving stars may have coalesced with other clusters to form the earliest galaxies.