Gravitational lensing

The effect of strong gravitational lensing allows Atacama Large Millimeter / submillimeter Array of ESO to observe a distant galaxy, the reconstruction of the original structure also allows for the retrieval of its high rate of star formation. The result was achieved by a team led by researchers at the Max Planck Institute für Astrophysik.


Credit: ALMA

There was a time, when the universe was about a quarter of its current age (currently set at 13.7 billion years) in which galaxies have ‘put the turbo’ and churned out new stars at a rate much higher than do today. The reason for this ‘baby boom’ star is not yet clear to astronomers that, given the huge distances involved, have many difficulties in studying galaxies so distant in time: of course, these celestial objects appear to us very small and very weak, but most of the young stars in them cannot be observed directly, because the radiation produced is absorbed by the gas and dust that surround them and that are re-emitted at the wavelengths of the far infrared.
One of the best tools in the world for this type of investigation is undoubtedly ALMA, the Atacama Large Millimeter / Submillimeter Array of ESO, which is located in the north of Chile, on the plateau of Chajnantor at 5000 meters above sea level. The data collected by the ALMA antennas can be combined together and can obtain images of high resolution, better than a tenth of a second of arc. However, even squeezing the most of this jewel of technology, we cannot study in detail remote galaxies in the prime of their star formation activity.


Credit: ALMA

But, sometimes, where comes our technology, there is the very nature to provide valuable assistance. “At a recent conference, scientists from the ALMA team presented data used to test the scientific capabilities of the instrument, and among them was a picture of a system subject to the phenomenon of strong gravitational lensing, which immediately attracted our interest,” recalls Simona Vegetti, post-doc researcher at the Max Planck Institute für Astrophysik (MPA), Germany. “Thanks to this effect, the radiation of the distant galaxy is magnified by 17 times, and it is thanks to it that we can see the galaxy. Taking advantage of the large angular resolution observations of ALMA, we had a chance to explore and observe for the first time the details in the distribution of its dust. ”
The strong gravitational lensing is the effect produced by a celestial object of large mass, such as a galaxy, when it is located along the line of sight between a light source and the observer. According to the General Theory of Relativity formulated by Einstein in fact, any celestial body with mass will cause a deflection of light rays that pass near it because of its gravitational pull. In case of a particular alignment between the light source, “gravitational lens” and observer, the light source is focused, similarly to what happens when using a normal lens, and the source appears much brighter than it is in reality.


Credit: ALMA

Nature, however, gives us a hand yes, but it cannot work miracles: the distribution of mass in the “gravitational lens” is not homogeneous, and so the image of the distant galaxy, called SDP.81, is heavily distorted. What can be done to fix the problem is how to obtain the material and how is distributed in the lens and, based on this information, reconstruct the true image – that is not distorted. «Previous efforts in this sense assumed that galaxies were amplified by the lensing as flat and smooth,” says Matus MPA Rybak, who performed computer reconstruction of the image of the galaxy SDP.81. “This hypothesis seems to be a bad approximation for the structure of a galaxy full of star formation, and the images are not yet processed. ALMA had already given clear indications that this source is very complex. The new method we have developed, more general, is much better suited to irregular systems. “

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