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At the IRAM 30-meter radio telescope, near Granada in Spain, a team of astronomers stumbled
upon a totally unexpected celestial source in the course of a program designed to search for debris, in the form of cold dust, left by the formation of planets around nearby stars. During this program, a source was discovered near the border of a small sky area scanned at a wavelength of 1.2mm using the bolometer MAMBO (Fig. 1). The source was point-like and extremely bright at 1.2mm while the astronomers had expected an extended source of low brightness centered on the sky coordinates of a star for which they were searching for.
 

This bright source in the radio domain had no optical counterpart in astronomical catalogues. Although the team initially considered a distant galaxy as a possible origin for this newly discovered source, they first discarded this idea because of the extraordinarily high level of star formation that would be implied by its intensity, 10 times higher than that measured for prolific dust-enshrouded galaxies.The astronomers turned instead towards what would be a more standard hypothesis for the source: an isolated young star being born within our Galaxy, because of its relatively low galactic latitude (+24 deg above the plane of the disk of our Galaxy). This hypothesis was consistent with the lack of an optical counterpart because no light escapes from very young stars, which can normally be traced only by the emission of the cold dust in which they are embedded. A distinct feature of any young stellar object is to be located in a large cloud of molecular hydrogen gas on which they feed.
 

Fig 1 : The small sky area scanned at a wavelength of 1.2mm using the bolometer MAMBO installed at the IRAM 30m telescope. The astronomers were searching for an extended source of cold debris surrounding the star GJ725 at equatorial coordinates 18h 42m 46.68s +59d 37' 49.4'' (red star symbol) but found instead serendipituously the strong point-like source near the western border of the map. Credits: IRAM/Lestrade

The astronomers teamed up with two new colleagues, experts of star formation regions in the Galaxy, to conduct new observations with the KOSMA 3-meter submillimeter telescope near Zermatt in Switzerland to search for evidence of this gas but none was found over a large sky area around the position of the source. More intrigued than ever, the astronomers turned back a year later to the bold hypothesis of a distant dust-enshrouded star-forming galaxy, that they had envisioned initially. The exact distance to the source was the key parameter missing to derive accurate physical properties and astrophysical implications.
 

Distances on a scale comparable with the size of the visible Universe can be inferred from redshifts caused by the fast recession of galaxies in our expanding Universe. Redshifts are most readily measured by optical observations, but this radio source had no optical counterpart. The astronomers decided instead to search for the emission lines of the carbon monoxide gas (CO) which is associated with star formation activity in galaxies. They had absolutely no idea, however, by how much the rest frequencies of these lines, if indeed present, would be spectrally redshifted. They had to consider a large range of redshifts from z = 1 to as high as z=10 which corresponds to a distance travelled at the speed of light by photons after leaving their source when the Universe was only 0.6 billion years old (the Universe is 13.7 billions years at present). They were fortunate to have access to the recently commissioned receiver EMIR outfitting the IRAM 30-meter radiotelescope to cover this large range of redshifts in five steps only by retuning the central frequency of the receiver.

During the second night at the telescope at 2850m in the Sierra Nevada in late July, the astronomer in charge of the observations tackled the third tuning and, after only 20 minutes, spotted, triumphantly, the first hint of the emission line sought in the observed spectrum. Then, swiftly, in the next 6 hours of the same night, the line was secured well above the noise level of the spectrum by integrating more
data, and computations of the transition frequencies of higher energy levels of CO were made to discriminate between six possible redshifts by complementary observations. At the end of the night, 5 lines were detected (Fig. 2) and the source was unveiled as a galaxy with a large reservoir of molecular gas at a redshift of z=3.92960+/-0.00013. This redshift indicates that the galaxy is seen as it was when the Universe was 1.4 billion years old, or only 1/10th of its present age.

Back at their home institutions, the astronomers combined this redshift with all photometry available for this source, the Spitzer infrared data and the IRAM 30-meter radio data, to estimate the infrared luminosityof the large amount of dust produced by star formation in this high-redshift galaxy. They found an apparent infrared luminosity as high as 4.8 10¹⁴ solar luminosities and, consequently, inferred the apparent rate of star formation to be 8.3 10⁴ solar masses per year which is 50 times higher than for the most prolific star-forming galaxies known. The molecular hydrogen gas (H₂) fueling star formation was estimated to be between 2.0 10¹¹ and 11.0 10¹¹ solar masses,
which is at least as large as in the most massive galaxies known but which are older and have had more time to build up their masses.

Fig 2 : The three CO rotational lines, and the two CI lines detected at the IRAM 30-meter radio telescope equipped with the newly commissioned wide band receiver EMIR. The observed frequencies of these lines yield an accurate redshift of z=3.92960+/-0.00013 for the bright, dust-enshrouded galaxy. Credits: IRAM/Lestrade

The team speculated that the most plausible scenario for this intriging discovery is that the galaxy is amplified by a gravitational lens in the foreground, closely aligned with it. The lens is likely a cluster of galaxies that bends the radio rays of the background source and focuses them on the observer as described by General Relativity. It is expected that future identification of such a lens by optical searches will yield an estimation of the amplification factor to asses how outstanding this high-redshift galaxy really is.

The quest is not over. Optical observations are being planned to search for the lens. High-angular resolution observation with interferometers at radio wavelengths should reveal also the multiple mirror images of the lensed background source if the scenario is correct. Eventually, this endeavour will shed new light on how galaxies build up their masses and assemble their stellar populations early in cosmic time, and consequently help to understand the origin of the Milky Way.

Original article: Discovery of an Extremely Bright Sub-Millimeter Galaxy at z=3.93, J.-F. Lestrade, F. Combes, & P. Salomé (Observ. Paris), A. Omont (IAP),F. Bertoldi (Univ. Bonn), P. André & N. Schneider (CEA), 2010, Astron. and Astrophys. Letter, 522, L4.

Contact:

Jean-François Lestrade
Observatoire de Paris
77 avenue Denfert-Rochereau
75014 - Paris - France
Tel. +33 (0)1 40 51 21 37
jean-francois.lestrade at obspm.fr