Title: A massive compact quiescent galaxy at z=2 with a complete Einstein ring in JWST imaging
Authors: Pieter van Dokkum et al.
First Author Institution: Department of Astronomy, Yale University
Status: Published in Nature Astronomy, 8, 119-25 (2024), Open Access

About the Author:

Bryce Wedig a third-year PhD candidate in the Department of Physics at Washington University in St. Louis. He is interested in what galaxy-galaxy strong gravitational lenses can tell us about the nature of dark matter.

A syzygy is the alignment of astronomical objects. Since most of space is empty, syzygies are rare and exciting phenomena. They often make for stunning images, such as the “Great Conjunction” of Jupiter and Saturn where they appear right next to each other in the sky. In astrology, this event that happens about once every 20 years is said to “bring great change.” Some syzygies are actually scientifically interesting in addition to being remarkable sights. A total solar eclipse in 1919 was a vital observational verification of experimentally verifying Einstein’s theory of general relativity. Exoplanet science also relies on syzygies. Most exoplanets are detected by the transit method where an exoplanet orbits in front of its host star which causes a detectable dip in the star’s brightness. Microlensing is another exoplanet-detection technique that relies on a syzygy, where an exoplanet magnifies the light from a background star.

This paper is about the serendipitous discovery of one of the most distant and most perfect syzygies that we know of – the near-perfect alignment of galaxies along our line of sight that the authors dub JWST ER-1 (see Figure 1). It checks both boxes: it’s a magnificent image due to the dramatic gravitational lensing of the light from the distant galaxy, and it can tell us about the dark matter content of the closer galaxy.

In the image of JWST-ER1 (Figure 1), the bright yellow spot in the center (“A”) is a galaxy at redshift 1.94, so the light from it has been traveling for about 10.3 billion years. The ring around this spot is an even more-distant galaxy located exactly behind it from our perspective. It’s at redshift 2.98, so its light has been traveling for about 11.5 billion years (or possibly redshift 5.5, according to a subsequent investigation). This ring, called an Einstein ring, is due to an effect called strong gravitational lensing. As the light from the distant galaxy travels towards us, it is bent inwards by the gravitational effect of the closer galaxy. The closer galaxy is acting like a lens, distorting and magnifying the light from the distant galaxy. The two small red spots on the ring (“B1” and “B2”) are multiple images of the center of the distant galaxy. 

It’s very rare that galaxies align this perfectly. When they’re discovered, it’s an Astrobite-worthy event! The authors of this paper got very lucky when they discovered JWST ER-1 by accident. They were looking at an area covering 0.35 square degrees (about twice the size of the full Moon) imaged by the COSMOS-Web project, the most ambitious survey from JWST’s first Cycle of observations. They state that JWST ER-1 was “readily revealed,” but the cutout in Figure 1 is about 0.0006% the area of the full image. 

It’s possible that this system could’ve been discovered nearly 20 years ago! The Hubble Space Telescope observed this very patch of the sky as part of the HST COSMOS Survey between 2003 and 2005. Figure 2 shows the Hubble image along with the JWST images. This system is so red that it is barely visible in the Hubble image. But it appears much brighter to JWST, thanks to JWST’s reach into the infrared. Two teams completed visual inspections of the entire HST COSMOS Survey looking for these kinds of systems, but they both missed this one. Would you have missed it too? One researcher manually checked 285,423 objects, while the other group consisted of a team of five who inspected 9452 objects after filtering down to bright and nearby galaxies.

The authors make sure to rule out the possibility that it’s a ring galaxy (check out Hoag’s Object). Ring galaxies can form when galaxies collide and a ring of stars is formed around a bright center. But this can be ruled out based on two pieces of evidence that prove it is an Einstein ring. First, redshift measurements show that the light forming the ring and red spots appear to come from much further away than the bright yellow galaxy in the center. However, these redshift measurements using the limited information from this observation leave room for error. Second, the red spots are a telltale sign of a multiply-imaged system. In this collision case, it’s very unlikely that identical red spots would appear on opposite sides of the ring.

But JWST ER-1 leaves us with a puzzle: the lensing galaxy is much more massive than expected. The size of the Einstein ring is a function of the mass it encloses. But this mass is much greater than the sum of the mass in stars and the expected dark matter. If that missing mass is in dark matter, then there’s seven times more dark matter than we would expect for a galaxy of this size.JWST ER-1 showcases both the beauty and the scientific value of when things in the Universe happen to line up just right. It has sparked a debate about how to explain the extra mass, which pokes at questions in galaxy formation and dark matter (see here and here). The COSMOS-Web data is publicly available, so you too can explore JWST’s unprecedented view of the distant Universe and have a go at making your own serendipitous discovery!

Edited By: Maria Vincent
Featured Image Credit: Dokkum et al., 2024