Title: J1721+8842: The first Einstein zig-zag lens
Authors: F. Dux, M. Millon, C. Lemon, T. Schmidt, F. Courbin, A. J. Shajib, T. Treu, S. Birrer, K. C. Wong, A. Agnello, A. Andrade, A. A. Galan, J. Hjorth, E. Paic, S. Schuldt, A. Schweinfurth, D. Sluse, A. Smette, S. H. Suyu
First Author Institution: European Southern Observatory, Alonso de Córdova 3107, Vitacura, Santiago, Chile
Status: Submitted to A&A, available on arXiv
Light doesn’t always travel in straight lines – it travels along the shortest path through spacetime. Space is pretty much flat on our small scales, so light travels in straight lines. However, on cosmological scales, space can be distorted by massive objects. For example, if space were to be stretched by, say, a galaxy’s strong gravitational field would act like a lens by deflecting light. The shortest path in this bent field is a curve.
Now, if that galaxy lies between us and a very bright, distant object such as a quasar, the gravitational field from that intermediate galaxy will focus the light from the quasar to create a lensed image of the quasar at Earth. Sometimes, those images can be smeared out into an arc; sometimes, those images produce stunning Einstein Rings; and sometimes, they remain point-like. For an example of a gravitationally-lensed image, see Figure 1 which shows J1721+8842 as taken by the Hubble Space Telescope. The six points of light labelled A-F are multiple images of the same, distant quasar! However, this picture is not just any gravitational lensing event, but a doubly-lensed event, where the light from the distant quasar is lensed by two intermediate galaxies and zig-zags across the universe to reach us!
Through the Double-Looking Glass
When J1721+8842 was first analysed, researchers didn’t conclude that this image showed one distant quasar being doubly lensed. They concluded that the red arcs in Figure 1 were another galaxy at the same distance away from us as the quasar at a redshift of z=2.382. They thought they were both being lensed by one galaxy at a distance of redshift z=0.184 (which I will call Lens 1), which is the white fuzzy blob in Figure 1.
However, using spectrography from JWST observations, today’s authors determined that the red arcs were actually a galaxy in between Lens 1 and the quasar, lying at a redshift of z=1.885. This in-between galaxy, which I will call Lens 2, is the first object to lens the images of the quasar. Lens 2 is then also being lensed by Lens 1, which creates those red arcs! Figure 2 shows a visualisation of the quasar, the two lenses, and the paths that the images of the two lenses take around Lenses 1 and 2.
Images A, B, C, and E (grey lines) are deflected in the same directions by Lens 2 and Lens 1. Meanwhile, Images D (magenta) and F (cyan) are deflected in opposite directions by both lenses, creating a ‘zig-zagging’ path between them. Let me emphasise that: the image of the distant quasar is zig-zagging across the universe because two galaxies are deflecting its light in opposite directions! Isn’t that so cool?? You’ll also notice that the images smoothly curve out before they are sharply deflected by Lens 1 – this is because of cosmic expansion.
We haven’t observed double gravitational lenses before. This chance arrangement of galaxies, where two strongly lensing galaxies are in the same line of sight between us and a very distant quasar, is the first to ever be observed. This zig-zagging path is expected to occur in 1 in every 100 million line-of-sights. That is extremely rare!
Easing the (Hubble) Tension?
Beyond the sheer awesomeness of this observation, this system may help us resolve one of the biggest problems in cosmology today because of the unique zig-zagging and the constraints that this path can put on cosmological parameters of interest. This problem is the so-called “Hubble Tension,” also known as the “Crisis in Cosmology.”
Take a look again at the paths that Images A and D take in Figure 2. Image D zig-zags between the two lenses as the image is deflected on opposite sides of each lens, while Image A is lensed on the same side of both lenses and follows a more “straight-forward” path (pun intended). Image D travels a longer distance from the quasar to us, and this extra distance results in additional relative travel time compared to Image A. Hence, Image D is affected by cosmic expansion for longer than Image A. While a single-lensing event can produce some constraint on some cosmological variables that affect cosmic expansion – which include the Hubble rate H0 and the equation-of-state of the universe w – a zig-zagging, doubly-lensed path provides an even stronger constraint when compared to the simpler path.
This discovery is extremely important for cosmologists because of the current discrepancy and growing inconsistency between measurements of H0 and w between different experiments and observations. Further analysis of this system with time-delay cosmography may elucidate an answer to these discrepancies. In addition, further observations will help understand the distribution of mass within the galaxy of Lens 2. This will be important to help us understand the characteristics of the population of galaxies during the first few billion years of the universe, and how they evolved into the galaxy and universe, that we live in today.
Featured image: Figure 1 of today’s paper
Edited by Kat Lee
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