An international team of astronomers has found a new class of Einstein crosses, where massive elliptical galaxies produces multiple, cross-shaped images of far away galaxies called “blue nuggets”. These latter are feebly star-forming, ultra-compact and massive galaxies, thought to represent the early stage of the galaxy evolution. Two of such blue “Einstein nuggets” have been observed at more than 9 Giga-light years from us, at a time when the Universe was about 1/3 of its current age. The new study predicts that thousands of these Einstein cross systems might be found in future surveys from ground and from space, which will allow astronomers to study blue nuggets in detail using closer dark matter dominated galaxies as gravitational telescopes.

Since their discovery back in 1985, Einstein crosses have fascinated astronomers and general public for their iconic appearance. They are special cases of the so-called strong gravitational lensing. According to General Relativity (GR), the light emitted by far away galaxies (source), while traveling space, can be bend by the gravitational field of massive systems (deflectors or lenses) like large and bright elliptical galaxies. An observer that is aligned just right, along this source-deflector line-of-sight, sees multiple images of the source producing a cross-shaped pattern, which symmetry is driven by the properties of the mass distribution of the lens and the amount of alignment of source and lens. This means that astronomer can use GR equations to make accurate measurement of the mass of the deflectors, and, most of all, they can reconstruct the “un-lensed” image of the source.

“Universe is like a gigantic lens maze!” says Prof. Nicola R. Napolitano, of the Sun Yat-sen University, leading scientist of the program that brought to the new discovery. ”Like in mirror–mazes people see their images deformed by distorted glasses”, continues Napolitano, “with our telescopes we observe images of far away galaxies (the sources) deformed and magnified by the mass of galaxies and cluster of galaxies (the lenses), located in between. In case of the, so called, strong lensing, these deformations look like spectacular arcs or multiple images, which can be used to study the dark matter content of the lens systems and the nature of far away galaxies, that without the lensing magnification would be hard to see. Einstein crosses are a very special and rare family of these strong lensing phenomena.”

Einstein crosses produced by bright quasars (galaxies hosting active galactic nuclei powered by super-massive black holes) have been used to prove the predictions of Einstein’s General Relativity in the past. New observations obtained using the European Southern Observatory (ESO) facilities have detected for the fist time these very peculiar objects in the images of the Kilo Degree Survey, a large sky survey covering so far about 1000 deg2 of the sky in optical bands with the VLT survey telescope (VST), then confirmed with the integral field spectroscopy from the MUlti-unit Spectroscopic Explorer (MUSE) at the Very Large Telescope. The exquisite quality of the ESO telescopes and the high magnification of the source light due to the “lensing” effect have made possible to characterize the source galaxy properties from Earth, despite these analyses make generally use of observations from space.

“We know that quadruply images of distant quasars are very rare object, hence we have been looking for such kind of systems in the large area covered by the Kilo Degree Survey using Artificial Intelligence”: says Dr. Rui Li of the Sun Yat-sen University, who also adds: “We have trained computers to recognize all kind of strong lensing images and found many new lenses in KiDS, but we did not expect to find so many quadruply imaged strong lensing events, and in particular several beautiful Einstein crosses”.

Prof. Konrad Kuijken, of the Leiden Observatory, PI of the KiDS survey says: “We have thought that these objects had to be somehow special and to prove that we have inquired ESO for urgent observations with MUSE for the two most convincing systems, which were quickly granted by Paranal Director.”

Dr. Chiara Spiniello, who just moved to Oxford University after having spent the last three years at the INAF-Astronomical Observatory of Capodimonte (Naples), says: “It was exiting to look into the MUSE data. These exceptional datacubes finally confirmed the lensing nature of the systems, as the four images of the sources had all exactly the same spectra, which we have estimated to be at redshift larger than 1.1 in both cases, i.e. distances larger than 11.5 Gigalight years from us. But we have found also more…”

“To produce an Einstein cross we need very compact sources and the lensing models of the two systems tell us that the sources have a size of about 1 kpc or less, i.e. more than 8 times smaller than our Milky Way, while they are as luminous as about our galaxy” says Dr. Crescenzo Tortora of the INAF-Observatory of Capodimonte, Naples.

Prof. Napolitano then concludes: ”The MUSE spectra have finally shown that the 4 images of the two crosses come from the same kind of source, the so ‘blue nuggets’. We still know very little about these systems, as they are hard to observe in details with current telescopes, but theoretical studies predict that most of the today giant galaxies have gone through this stage in their evolution. Our research predicts that we will be able to observe thousands of these Einstein cross systems in future surveys from ground and from space, which will allow astronomers to study blue nuggets in detail using closer galaxies as gravitational telescopes.”

Detection and confirmation of the Einstein crosses: KIDS J232940-34092 (top), KIDS J122456+005048 (bottom). Left column: for each cross we show the KiDS (gri) combined color image (top) and the MUSE white light image (bottom). Right column: for each cross we plot the MUSE spectrum of the deflector, in black, with the best fit model for the velocity dispersion estimate with pPXF, in red, (top) and of the 4 individual images, in blue tones, and the mean spectrum, in black (bottom). Over-plotted, on all spectra, there are the main absorption lines (in red) and emission lines (in blue), shifted to the estimated redshift of the two crosses (i.e. z=1.59 for KIDS J232940-34092 and z=1.10 for KIDS J122456+005048).

Contacts

Crescenzo Tortora

Address: INAF - Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131, Naples

Email: crescenzo.tortora@inaf.it

Nicola R. Napolitano

Address: School of Physics and Astronomy, Sun Yat-sen University Zhuhai Campus, 2 Daxue Road, Tangjia, Zhuhai, Guangdong 519082, China

Email: napolitano@mail.sysu.edu.cn

Tel: +393286753408; +8613823093510

Rui Li

Address: School of Physics and Astronomy, Sun Yat-sen University Zhuhai Campus, 2 Daxue Road, Tangjia, Zhuhai, Guangdong 519082, China

Email: lirui228@mail.sysu.edu.cn