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An hyper-luminous galaxy, one billion year after the Big-Bang

1 January 2012

Because of their large distance and very weak luminosity, it is very difficult to detect primordial galaxies. The gravitational amplification by foreground galaxy clusters is therefore used to increase the apparent luminosity of these galaxies and our chances to discover them. Besides, these young galaxies which form many stars, must radiate the bulk of their energy in the very far infrared, due to dust heated by their young stars. The ESA satellite Herschel is the best placed to catch their signals highly redshifted by the expansion of the Universe. An international team of astronomers, led by a researcher from Paris Observatory, has discovered thanks to Herschel a mysterious peak of emission, corresponding to no known nearby source. Observation in this direction with the telescopes of IRAM allowed to detect molecular lines (water, carbon monoxide ..) and thus derive the redshift of the source and its distance. Two distinct velocity components suggest the presence of interacting galaxies. The system, even when the amplication factor is taken into account, is hyper-luminous (Luminosity superior to 1013 , or 2-3 orders of magnitude more than the Milky Way), which is remarquable for an object formed only one billion years after the Big-Bang.

In the star formation history of the Universe, it appears a great maximum of activity between the redshifts z=1 and z=3, which corresponds to epochs 2 to 6 billion years after the Big-bang. These starbursts radiate primarily thanks to their dust heated by young stars hidden in their cocoon. This emission has its maximum around 0.1 mm in wavelengthin the reference frame of the galaxy, and can be detected between 0.2 and 0.4 mm, once redshifted by the expansion of the Universe. If there exist primordial galaxies even more remote, one should be able to detect them between 0.5 and 1mm in wavelength, the submillimeter domain. However, very little was detected until now.

Figure 1: The background image (grey scale) is a photo of the galaxy cluster Abell 773 (z=0.22) at visible wavelength obtained with the telescope Subaru, at Hawaii. The central galaxy (grey spot) however does not belong to the cluster; its spectrum has been obtained with the Keck telescope, and has a redshift z=0.63. It is the lens galaxy, which contribues the majority of the gravitational amplification. The remote galaxy has first been detected by its strong emission in the far infrared and submillimeter, thanks to the Herschel satellite: red contours at 0.5mm, and gren contours at 0.25mm wavelength. The black contours correspond to the emission at 0.9mm, detected by the SMA (SubMillimeter Array, Hawaii). The contours of the CO(2-1) emission obtained with the EVLA have a similar shape. Click on the image to enlarge it

To explore these primordial galaxies, the team of astronomers has carried out a survey, with the infrared and submillimeter satellite Herschel, of hundreds of galaxy clusters. Those are used as " gravitational telescope", thanks to their lensing effect. In the wavelength domain considered, the foreground galaxies contribute only in a negligible way to the emission, and the remote galaxies, if they exist, must dominate. By observing the galaxy cluster Abell 773, an extremely bright source was detected. The redshift of Abell 773 is z=0.22, which places it at a distance of about 3 billion light-years from us.

Figure 2: The spectra of several transitions of the molecule CO, of the water molecule H2O, of the carbon atom CI, and of ionised nitrogen

Once this strong submillimeter source detected in continuum, the team undertook to determine its distance and its nature, by a follow-up on the ground. The SMA interferometer allowed to precise the position, with a 2 arcseconds resolution (Figure 1). The visible image obtained with the Subaru telescope shows a foreground galaxy which is approximately 1 arcsecond North. The optical spectrum of this galaxy was obtained on the Keck II telescope with DEIMOS, and, surprise, the galaxy does not belong to the cluster Abell 773, but is more distant with z=0.63. It is however this galaxy which contributes to the bulk of the gravitational amplification. Considering the North-South elongated contours of the source seen with SMA, and also EVLA, undoubtedly the separation of the gravitational images of the source is of order of 2". This allows to make a approximate model of the lens, and deduce an amplification factor of 11.

Figure 3: Energy distribution in the lines of carbon monoxide (rotation ladder of the CO molecule). The flux of each line is plotted as a function of the upper level J of the transition. The two velocity components (red and blue) have comparable distributions with a maximum at J=6. By comparison, the Milky Way (MW in blue) has its maximum at J=2-3, and the interacting galaxies The Antennae (Ant in red) between J=3-4. Other galaxies (M82 in green, Mrk231 in purple) have also maximums at high J, and are well known starbursts, sometimes associated to quasars. Click on the image to enlarge it

Finally the source was observed with the 30m telescope of IRAM, and also the Plateau de Bure. The wide band receiver EMIR allowed to explore quickly a large bandwidth, between 81 and 115 GHz, and to detect several lines of the CO molecule, which give thus the redshift of the source z=5.243 (cf Figure 2). This hyper-luminous source was formed only a billion years after the Big-bang ! The two velocity components suggest the presence of two galaxies in interaction. The high excitation of the CO molecule indicates the presence of a starburst, and perhaps an active nucleus (Figure 3). It is the first time that the [NII]205micron line is detected at high redshift. This line which traces ionized gas, reveals a quite developed blue velocity component, which suggests a significative ionized gas flow, corresponding to line wings in the molecular spectra: undoubtedly a strong gas ejection, due to supernovas and stellar winds. This outstanding source will be studied now in more detail, with high spatial resolution.

Reference

  • A bright z=5.2 lensed submillimeter galaxy in the field of Abell 773: HLSJ091828.6+514223 F. Combes (Obs-Paris), M. Rex (Tucson), T. D. Rawle (Tucson), E. Egami (Tucson), F. Boone (Toulouse), I. Smail (Durham), J. Richard (Lyon), R.J. Ivison (Edinburgh), M. Gurwell (Harvard), C.M. Casey (Hawaii), A. Omont (IAP-Paris), A. Berciano Alba (ASTRON), M. Dessauges-Zavadsky (Geneva), A.C. Edge (Durham), G.G. Fazio (Harvard), J-P. Kneib (Marseille), N. Okabe (Taipei), R. Pello (Toulouse), P. G. Perez-Gonzalez (Madrid), D. Schaerer (Geneva), G.P. Smith (Birmingham), A.M. Swinbank (Durham), P. van der Werf (Leiden) Astronomy and Astrophysics, in press

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