Title: Identification of more than 40 gravitationally magnified stars in a galaxy at redshift 0.725
Authors: Yoshinobu Fudamoto, Fengwu Sun, Jose M. Diego, Liang Dai, Masamune Oguri, Adi Zitrin, Erik Zackrisson, Mathilde Jauzac, David J. Lagattuta, Eiichi Egami, Edoardo Iani, Rogier A. Windhorst, Katsuya T. Abe, Franz Erik Bauer, Fuyan Bian, Rachana Bhatawdekar, Thomas J. Broadhurst, Zheng Cai, Chian-Chou Chen, Wenlei Chen, Seth H. Cohen, Christopher J. Conselice, Daniel Espada, Nicholas Foo, Brenda L. Frye, Seiji Fujimoto, Lukas J. Furtak, Miriam Golubchik, Tiger Yu-Yang Hsiao, Jean-Baptiste Jolly, Hiroki Kawai, Patrick L. Kelly, Anton M. Koekemoer, Kotaro Kohno, Vasily Kokorev, Mingyu Li, Zihao Li, Xiaojing Lin, Georgios E. Magdis, Ashish K. Meena, Anna Niemiec, Armin Nabizadeh, Johan Richard, Charles L. Steinhardt, Yunjing Wu, Yongda Zhu & Siwei Zou
First Author’s Institution: Center for Frontier Science, Chiba University, Japan
Status: Published in Nature
Figure 1: Galaxy cluster Abell 370 as observed by the Hubble Space Telescope. The Dragon galaxy, subject of today’s paper, is the elongated arc just to the lower left of center. Image Credit: NASA, ESA/Hubble, HST Frontier Fields
In the age of JWST, gravitational lensing is becoming an absolutely remarkable tool. Gravitational lensing refers to the warping of light from distant objects as it passes by a more nearby, massive source, most often a galaxy cluster. The mass of the galaxy cluster warps spacetime, changing the path light takes across the universe. The natural phenomenon is incredibly useful, as lensing causes distant galaxies to appear magnified and brighter, allowing us to study them in more detail. It can also warp the galaxy’s shape and allow it to appear multiple times. In the first years of JWST, there have been many programs focused around looking at galaxy clusters that have strong gravitational lensing effects.
One such cluster is called Abel 370. As seen in Figure 1, which gives the Hubble view of this cluster, this field is ripe with lensing. Today’s paper focuses on one particular lensed galaxy, dubbed “the Dragon arc” located at a redshift of 0.725. The Dragon was observed twice by JWST, first in December 2022 and then again in December 2023. And when comparing the two observations, researchers found something amazing.
Figure 2: The difference between observations of the Dragon arc in 2022 and 2023. The bright blue and red patches represent the transient events thought to be individual stars, with event alpha thought to be a supernova observed in 2023. Figure 2 in today’s paper.
JWST revealed evidence of microlensing events. This process refers to how smaller objects – like individual rogue stars in the cluster – can add additional magnification to isolated regions of the background galaxies. While the cluster lenses whole galaxies, microlensing can allow for the detection of individual stars! It’s difficult to know if something is truly a microlensing event with just one observation, but by comparing the 2022 and 2023 observations of the Dragon, the team was able to significantly detect 45 transients: objects which appeared in one image and not the other. Figure 2 shows these transients, visible as the dark blue and red patches. One of those is probably a supernova, and the other 44 are believed to be individual stars. This marks the first time so many individual stars have been observed in such a distant galaxy.
Figure 3: Color-magnitude diagram of the detected microlensed stars. The x-axis represents the “color” as represented the difference in brightness between two different filters, while the y-axis represents the magnitude in one of those filters. The grey region in the bottom left corner gives the detection limit for one of the filters. Figure 3 in today’s paper.
As they observed the Dragon in several different filters, they were able to construct a color-magnitude diagram for these stars, shown in Figure 3. This revealed the stars were relatively cool, with surface temperatures in the range of 3000-4000 K (yes, this is cool by stellar standards)! Combined with their luminosity, this suggests these stars are likely red giants or supergiants.
The ability to observe larger populations of distant stars in detail is yet another way JWST is changing the landscape of astrophysics. Continued observations of the Dragon and similarly strongly lensed galaxies will provide more and more high-redshift stellar populations, opening new doors into the conditions of the early universe.
Astrobite edited by Catherine Slaughter
Featured image credit: NASA, ESA/Hubble, HST Frontier Fields