Gaia finds 12 examples of Einstein crosses; Galaxies are gravitationally seen lens-shaped, so we see that they’re repeated 4 occasions

In 1915 Einstein put the finishing touches to his theory of General Relativity (GR), a revolutionary new hypothesis that described gravity as a geometric property of space and time. This theory remains the accepted description of gravity in modern physics and predicts that massive objects (like galaxies and galaxy clusters) bend the fabric of space-time.

As a result, massive objects (such as galaxies and galaxy clusters) can act as a lens that deflects and magnifies light from objects further away. This effect is known as the “gravitational lens” and can lead to all kinds of visual phenomena – not least the “Einstein Cross”. Using data from ESA’s Gaia Observatory, a team of researchers announced the discovery of 12 new Einstein Crosses.

Put simply, an Einstein Cross is created when light from a distant object (in this case a quasar) is deflected by foreground galaxies so much that it appears to the viewer as four different images. While Einstein had predicted this effect as early as 1912, the first double image of a lens quasar did not appear until 1979. By 1985 a total of about 50 Einstein crosses had been discovered, only one of which was a quadruple image.

12 Einstein Crosses discovered using data from the Gaia DR2. Credit: The GraL Collaboration

The research was carried out by the Gaia Gravitational Lenses Working Group (GraL), an international collaboration with members from Australia, Brazil, India, Europe and the USA. Francois Mignard, astronomer at the University of Côte d’Azur in France and member of GraL, discussed the challenges of studying Einstein Cross in a recent press release from ESA.

“It is difficult to find new ones as we have no idea exactly where to look for them,” he said. “It requires high spatial resolution imaging just to locate candidates.” ESA’s Gaia mission, launched in 2013, is a milestone in this field because of its unprecedented spatial resolution and the ability to survey the entire sky every few months.

Using a number of special machine learning algorithms, the team looked for candidates for quasars with strong lenses in the second Gaia Data Release 2 (Gaia DR2). “Then we had to confirm that the four close-packed images were not purely random alignments from four independent sources, but actually four images from a single distant source captured from an intermediate galaxy,” said Christine Ducourant of the University of Bordeaux.

Since Gaia’s measurements of spectral and visible light (photometric) were not yet published, the team relied on photometry from NASA’s WISE (Wide Field Infrared Survey Explorer) to select the most promising candidates for spectroscopic follow-up observations. This was then done using the Low Resolution Imaging Spectrometer (LRIS) on the Keck I Telescope and the Palomar Double Spectrograph (DBSP).

Gaia maps the stars of the Milky Way. Credit: ESA / ATG medialab; Background: ESO / S. Brunier

These observations confirmed that the 12 candidates identified were quadruple-mapped quasars, effectively increasing the number of known Einstein Crosses by nearly 25%. In the future, the availability of multiple imaged lens quasars for studies will give astronomers additional opportunities to test important cosmological parameters.

These include the current rate of expansion of the universe (which could provide insights into the role of dark energy) as well as the distribution of dark matter in foreground galaxies. As Alberto Krone-Martins – an astronomer and lecturer in computer science at the University of California at Irvine and the Center for Astrophysics and Gravity at the University of Lisbon – explained:

“Quasars are inherently variable objects, and since the light has crossed different paths in the universe in each lens image, fluctuations in the light of the quasar occur at different times in the images. From this the Hubble-Lemaître constant can be estimated. “

“Based on general relativity and the distribution of matter in the galaxy, we can predict where the lens quasar images should be. The difference between what we predict and what we observe says something about the properties of different models of dark matter. “

Using gravitational lensing, a team was able to study how light from distant quasars was affected by small clumps of dark matter in between. Photo credit: NASA / ESA / D. Spieler (STScI)

To make this type of measurement, further follow-up observations must be made in the currently running optical, radio, and x-ray wavelengths. The GraL working group assumes that future Gaia data releases – such as Early Data Release 3 (EDR3), which was made available on December 3, 2020 – will enable the identification of further Einstein crosses. As Ducourant added:

“Once the data is finally published, we hope Gaia will discover hundreds of these sources. Thanks to Gaia and the collaboration between machine learning, space and ground observation, we can find these unique objects more and more effectively. “

Their results were recently published online and accepted for publication in the Astrophysical Journal.

Further readings: ESA

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