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Astrophysics

New galactic discoveries

An international team of astronomers(1) has discovered a new type of galaxy in the early universe. This galaxy represents a long sought-after “missing link” between two other types of galaxies previously thought to be unrelated. This should help us understand how galaxies form and evolve.

new galactic

© NASA/ESA

An illustration of the Lyman Break method. The spectral energy distribution (black continuous line) of a star-forming galaxy is presented vs the observed wavelength. The galaxy is detected in the near ultraviolet (NUV), blue (B), visible (V) and infrared (I) while it “disappears” in the far ultraviolet (FUV). This no-detection is the main characteristic used to identify high redshift galaxies.


 

Since 1995, astronomers have been using the “Lyman-break” technique to observe very distant galaxies. Since light travels at a finite speed and takes a certain time to reach us, distant objects are seen as they were in the far-off past. These galaxies are therefore seen as they were when the universe was much younger. Because the light of objects moving away from us is shifted toward red-wavelength, cosmologists generally use this “redshift” to define distance or to establish how far back in the past the object actually is. The higher the redshift of an object, the further away it is.

 

The Lyman-break technique lets us observe galaxies with redshifts as high as 6 and 7, corresponding to a formation of galaxies in the first 5% of the age of the universe.2 The technique relies on measuring the characteristic “disappearance” of light from distant galaxies when they are observed at far-ultraviolet (UV) wavelengths, below 91.2 nm. This is because the light from these galaxies is almost fully absorbed by hydrogen atoms in space at far-UV wavelengths. The technique has allowed astronomers to detect a first family of galaxy in which stars are being formed in huge numbers. In these galaxies, hot stars emit photons that can be directly observed at ultraviolet (less than 400 nm) and visible (400 to 700 nm) wavelengths. There is a second family in which stars surrounded by dust can only be observed at infrared (roughly between 1 and 100 microns) and submillimeter wavelengths. Until now, however, no class of galaxy sharing the properties of the two families had ever been detected. Burgarella and colleagues3 were able to find this new intermediate class of galaxy for the first time by using the Lyman-break technique to measure less distant galaxies.4 Using data from various sources, including ultraviolet observations from the NASA/CNES/Korea GALEX satellite, infrared data from the NASA SPITZER satellite and data in the visible range from the European Southern Observatory (ESO) telescope, the astronomers studied about 300 galaxies that showed a far-UV disappearance.

galaxies

© NASA/ESA

A sub-sample of Lyman Break Galaxies (most are disc galaxies). The last one (bottom right) is likely to undergo an interaction with its neighbors.



Thirty percent of these galaxies emit light in both ultraviolet and infrared ranges. This is the first time that a significant number of galaxies have been found to emit at both these wavelengths, which means that they behave like both types of previously known galaxy and therefore provide the missing link between them. These galaxies “only” have redshifts of 0.9 to 1.3, which means that they are being observed when the universe was about 5 billion year old. The new results show that they resemble present-day galaxies, like our own Milky Way. The new link between the two previously known types of galaxies raises new questions. Are UV and IR galaxies related? Will a galaxy from one class eventually become a galaxy from another class? or do they belong to really separate and unrelated families? Whatever the answers, they will help us understand more about how galaxies form and evolve.

Combining the ultraviolet and infrared data will for the first time allow astronomers to accurately calculate the formation rates of stars in these galaxies. “These ultraviolet-plus-infrared galaxies are forming stars at very high rates– reaching up to 1000 stars per year, compared to just a handful for our Milky Way,” says Burgarella.

The team now plans to look at different locations in the sky to check whether the observed field5 is special in any way, and try to estimate the physical parameters driving the evolution of these galaxies.

 

Isabelle Dumé

Notes :

1. The team comprises French, American, Japanese and Korean researchers.
2. The current age of the universe is estimated to be 13.7 billion years.
3. Observatoire Astronomique de Marseille (CNRS / Université de Provence joint lab).
4. D. Burgarella et al., “Ultraviolet-to-far infrared properties of Lyman-break galaxies and luminous infrared galaxies at z~1,” Astronomy & Astrophysics. 450: 69. 2006.
5. The Chandra Deep Field South, one of the deep fields scrutinized by the astronomical community, provides a large amount of data for detailed analysis.

Contacts :

Denis Burgarella
Observatoire Astronomique de Marseille Provence.
denis.burgarella@oamp.fr
www.oamp.fr


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