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ASTROPHYSICS

As far as Vimos can see

Within a single field of sky the size of the full moon, there are no fewer than 970 galaxies, some dating as far back as 12 billion years. This discovery, made by Olivier Le Fèvre, director of the Marseilles-based Astrophysics Lab (LAM)1 and his colleagues, challenges the common belief that galaxy formation peaked around 8 billion years ago, or 5.5 billion years after the Big Bang.2


“We owe this result to the unique capabilities of Vimos,” Le Fèvre stresses. Vimos stands for Visible Multi-Object Spectrometer. This device, developed by a consortium composed of French and Italian labs,3 was installed in 2002 on the “Melipal,” one of four 8-meter units of the Very Large Telescope (VLT, European Southern Observatory) in Chile. The capacity of Vimos—which can gather the spectra of several objects at the same time—combined with the collecting power of the VLT has allowed the team to collect a large sample of very distant galaxies. These are characterized by a high redshift due to the Doppler effect, which provides a gauge for the distance and age of a galaxy. The higher the redshift, the farther the object and the earlier it formed in the history of the universe.


Searching for high redshift galaxies very early in the life of the universe is like searching for needles in a haystack, because they are far less numerous on deep sky images than their bright nearby counterparts. Specific search techniques based on model predictions for the average colors of galaxies have been used so far to identify and count high redshift galaxies, but with no guarantee to be exhaustive. A complete census of the high redshift population of galaxies remained to be done. Thanks to Vimos, the VIMOS VLT Deep Survey team gathered as many galaxies as possible at a single average near-infrared wavelength, without any pre-selection on the basis of color, and analyzed their spectra. The result is astounding: of 11,000 galaxies, 970 have redshifts that date them back 9 to 12 billion years. This is 2 to 6 times more galaxies than were discovered with the former method. Their star formation rate, inferred from their UV flux, is 10 to 100 solar masses a year. Compared with the Milky Way, which currently forms less than one solar mass in the same timespan, these galaxies are very active. “Finding so many active galaxies is an unexpected result. It means that the total number of stars produced early in the life of the universe exceeds predictions from models,” concludes Le Fèvre. “What is the nature of this galaxy population? For 30% of them, their color makes us suspect that an Active Galaxy Nucleus is present at their center.4” Meanwhile, Vimos has been put to work to further enhance our understanding of galaxy formation. The objective is for the device to produce 100,000 spectra.

Jean-François Haït

 

Notes :

1. Laboratoire d'astrophysique de Marseille. Joint lab: CNRS / Université de Provence.
2. O. Le Fèvre et al., “A large population of galaxies 9 to 12 billion years back in the life of the universe,” Nature 437: 519-21. 2005.
3. LAM; Midi-Pyrenees Observatory (View web site in french); the Paris Astrophysics Institute (View web site) in France; and laboratories from the Italian National Institute for Astrophysics.
4. An Active Galaxy Nucleus is a bright region in the center of a galaxy that is likely hiding a black hole.

Contacts :

Olivier Le Fèvre,
LAM, Marseille
Olivier.LeFevre@oamp.fr
View web site


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