GIANT GALAXIES GROW OUT OF COLD COSMIC OCEANS
The largest galaxies in the Universe are immersed in cosmic oceans of cold gas, that surround and feed them. This finding has been published on December 2nd in the journal “Science” by an international team of astronomers, including two researchers from the IAP.
Image credit: NASA, ESA, G. Miley and R. Overzier (Leiden Observatory), and the ACS Science Team | Artist: J. Mouette (IAP)
Galaxies often gather together in hundreds or thousands under the effect of gravity, forming a cluster. At the heart of these clusters lie the largest galaxies in the Universe, which are giant aggregates of about a trillion stars. Until now, scientists believed that these “super-galaxies” formed by merging of many small galaxies. This phenomenon is observed in the local Universe, either directly, or after the fact through the morphological distortions that are observed in the shape of galaxies that recently underwent merging.
By using powerful radio telescopes in Australia and the United States of America, Bjorn Emonts, from the Centro de Astrobiología in Madrid and his collaborators have observed a cluster of galaxies in formation, or “proto-cluster”, located 10 billion light-years away, that hosts in its center a giant galaxy dubbed the “Spiderweb”. This galaxy is in the process of formation, and it is surrounded by a collection of small galaxies in gravitational interaction, such as “flies captured” in a spider web (see Figure). The researchers made a surprising discovery : the cluster galaxies are in reality immersed in a giant cloud of cold gas, unknown so far.
This cold gas consists mostly of hydrogen molecules, the basic material from which stars and galaxies form. Rather than observing the hydrogen directly, which is difficult, the astronomers observe a tracer gas, carbon monoxide, which is easier to detect. This gas is very cold, about -200 degrees Celsius. Thanks to the complementary observations with the Very Large Array (VLA, NRAO), in New Mexico, and the Australia Telescope Compact Array ATCA (Australia Telescope Compact Array), the astronomers discovered a previously unknown giant cloud of cold gas: most of the gas does not show up in the small galaxies, as was expected by the team, but instead lies around them.
The enormous gas cloud encompasses the whole of the proto-cluster (see Figure). It extends over a distance of a quarter of a million light-years, and contains roughly 100 billion times the mass of our Sun. This vast gas reservoir was captured by the proto-cluster when the Universe was only 20% of its present age.
The team of George Miley's at the Leiden University discovered the Spiderweb proto-cluster in the late 1990s, studying it intensely with many telescopes, including the Hubble Telescope . In 2008, Nina Hatch and her collaborators published the discovery of widespread diffuse ultraviolet emission from the billions of stars forming across the cluster, but the origin of these stars remained a mystery .
Matthew Lehnert, director of research at the CNRS, and Pierre Guillard, associate professor at the Université Pierre et Maris Curie, both astrophysicists at the IAP, have lead the interpretation of these new observations of the “Spiderweb”. They and their colleagues showed that this extended reservoir of cold gas can fuel the widespread star formation (in between galaxies), and suggested that the central giant galaxy could form out of the condensation of this cold gas. The origin of the cold gas remains a mystery. The carbon monoxide that is detected is a by-product of previous generations of stars, a form of cosmic recycling, but one cannot say where the gas came from, nor how it accumulated in the core of the proto-cluster.
This remarkable discovery illustrates how giant galaxies found in the center of clusters could form from an ocean of surrounding cold gas. This discovery leads astrophysicists to revise their understanding of the gas content in and out of galaxies, and of galaxy formation in general. It is possible that a large fraction of the cold gas in the Universe resides around massive galaxies, something hopefully that future observations will reveal.
Image of the proto-cluster de galaxies dubbed the “Spiderweb”, and observed in the ultraviolet using the Hubble space telescope, superimposed with the “cosmic ocean” of cold gas (in blue) discovered with the ACTA telescope. This cold gas (-200 degrees Celsius), is detected using carbon monoxide molecules (the central intensity of the blue cloud does not reflect the actually observed increase by a factor of six compared to the edges of the cloud).
The insert shows the central region of the cluster (without the cold gas), hosting the “super-galaxy” surrounded by small galaxies that are orbiting and distorted under the effects of gravity. The gas reservoir extends over a quarter of a million light-years and fuels the star formation in between galaxies. The central giant galaxy could form out of the condensation of this cold gas.
LinksScience journal article: http://science.sciencemag.org/cgi/doi/10.1126/science.aag0512
“Spiderweb” image (original version): https://www.spacetelescope.org/news/heic0614/
CNRS-INSU press release (in French): http://www2.cnrs.fr/presse/communique/4803.htm
 G. K. Miley et al 2006, ApJ, 650, 1 “The Spiderweb Galaxy: A Forming Massive Cluster Galaxy at z~2”
 N. A. Hatch et al 2009, MNRAS, 383, 391 “Diffuse UV light associated with the Spiderweb Galaxy: evidence for in situ star formation outside galaxies” http://mnras.oxfordjournals.org/content/383/3/931
- Pierre Guillard
Institut d’astrophysique de Paris, CNRS, UPMC
guillard [at] iap [dot] fr
- Matthew Lehnert
Institut d’astrophysique de Paris, CNRS, UPMC
lehnert [at] iap [dot] fr
- Bjorn Emonts
Centro de Astrobiología (CSIC-INTA)
bjornemonts [at] gmail [dot] com