Dust is a key ingredient in both observed and physical properties of galaxies. It obscures and scatters stellar light, and enables star formation by acting as a catalyst for molecular gas formation. It is the ingredient in the interstellar medium that most dramatically changes how we perceive a galaxy’s starlight. In new research, Arizona State University astronomer Rogier Windhorst and his colleagues have focused on the backlit (occulting) galaxy pair VV 191. Combining Hubble and Webb imaging, they’ve used the backlighting of the face-on spiral galaxy VV191b by the luminous elliptical galaxy VV 191a to trace the structure of interstellar dust in the spiral galaxy.
Windhorst et al. traced light that was emitted by the bright elliptical galaxy VV 191a (left) through the spiral galaxy VV 191b (right). As a result, they were able to identify the effects of interstellar dust in the spiral galaxy. Webb’s near-infrared data also shows us the galaxy’s longer, extremely dusty spiral arms in far more detail, giving them an appearance of overlapping with the central bulge of the bright white elliptical galaxy, though the pair are not interacting. In this image, green, yellow, and red were assigned to Webb’s near-infrared data taken in 0.9, 1.5, and 3.56 microns (F090W, F150W, and F356W respectively). Blue was assigned to two Hubble filters, ultraviolet data taken in 0.34 microns (F336W) and visible light in 0.61 microns (F606W). Image credit: NASA / ESA / CSA / Rogier Windhorst, Arizona State University / William Keel, University of Alabama / Stuart Wyithe, University of Melbourne / JWST PEARLS Team.
The VV191 system is located approximately 695 million light-years away in the constellation of Boötes.
It consists of the multiple-armed spiral galaxy VV191b and the background elliptical galaxy VV 191a.
“VV 191 is the latest addition to a small number of galaxies that helps researchers like us directly compare the properties of galactic dust,” Dr. Windhors and co-authors said.
“This target was selected from nearly 2,000 superimposed galaxy pairs identified by Galaxy Zoo citizen science volunteers.”
“We got more than we bargained for by combining data from James and Hubble,” they added.
“Webb’s new data allowed us to trace the light that was emitted by the bright white elliptical galaxy, at left, through the winding spiral galaxy at right — and identify the effects of interstellar dust in the spiral galaxy.”
“Webb’s near-infrared data also show us the galaxy’s longer, extremely dusty spiral arms in far more detail, giving the arms an appearance of overlapping with the central bulge of the bright white elliptical galaxy on the left.”
Although VV 191a and VV191b are relatively close astronomically speaking, they are not actively interacting.
“Understanding where dust is present in galaxies is important, because dust changes the brightness and colors that appear in images of the galaxies,” the astronomers said.
“Dust grains are partially responsible for the formation of new stars and planets, so we are always seeking to identify their presence for further studies.”
The image also holds a second discovery that’s easier to overlook.
“Examine the white elliptical galaxy at left. A faint red arc appears in the inset at 10 o’clock,” the researchers said.
“This is a very distant galaxy whose light is bent by the gravity of the elliptical foreground galaxy — and its appearance is duplicated.”
“The stretched red arc is warped where it reappears — as a dot — at 4 o’clock.”
“These images of the lensed galaxy are so faint and so red that they went unrecognized in Hubble data, but are unmistakable in Webb’s near-infrared image.”
“Simulations of gravitationally lensed galaxies like this help us reconstruct how much mass is in individual stars, along with how much dark matter is in the core of this galaxy.”
“Like many Webb images, this image of VV 191 shows additional galaxies deeper and deeper in the background,” the scientists added.
“Two patchy spirals to the upper left of the elliptical galaxy have similar apparent sizes, but show up in very different colors.”
“One is likely very dusty and the other very far away, but we — or other astronomers — need to obtain data known as spectra to determine which is which.”
The results will appear in two papers in the Astronomy Journal.
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William C. Keel et al. 2022. Webb’s PEARLS: dust attenuation and gravitational lensing in the backlit-galaxy system VV 191. AJ, in press; arXiv: 2208.14475
Rogier A. Windhorst et al. 2022. Webb’s PEARLS: Prime Extragalactic Areas for Reionization and Lensing Science: Project Overview and First Results. AJ, in press; arXiv: 2209.04119
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