It’s been an amazing couple of weeks for fans of gravitational lensing. JWST grabbed the headlines with a spectacular infrared view of lensing in the SMACS 0723 image, and that had everybody talking. Yet, seeing gravitationally lensed objects is not new. Some can be seen from the ground, and of course, Hubble Space Telescope (HST) has been cranking out views of gravitational lensing for years.
Just a few days ago, HST released another one. It’s a striking view of a distant galaxy called SGAS J143845+145407. It’s the centerpiece of the HST view and appears twice in a mirror image of itself. The galaxy appears a third time, as a very smeared apparition that makes a “bridge” between the other two images.
Deconstructing the Lensing of SGAS J143845+145407
So, what are we seeing in the HST image? This triple-imaged object is from a set of Hubble observations that use gravitational lensing to peer inside galaxies in the early Universe. It works because the light from those distant galaxies is curved or “bent” by the gravitational influence of a galaxy cluster that lies between HST and its targets. The cluster is called the “lens” and the distant galaxy, in this case, SGAS J143845+145407, is the “lensed object”.
Gravitational lensing can do funny things to the appearance of the lensed object. It can smear the image, distort it, and—if the geometry lines up just right—create multiple images of it. Finally, gravitational lensing can magnify the view of the lensed object. And, that’s important because most of the time these distant objects would be too far away or too faint to be observed.
Gravitational Lenses. Source: CalTech
The lensing reveals details of distant galaxies, such as the existence of spiral arms or star birth regions, which allows astronomers to determine star formation in early galaxies. This in turn gives scientists better insight into how the overall evolution of galaxies has unfolded over time.
The triply imaged galaxy in the HST image is technically known as a luminous infrared galaxy (LIRG, for short). A typical LIRG can be as bright as 100 billion times the luminosity of our Sun. Typically, these galaxies are spirals and their infrared luminosity stems from active regions of star formation. Many of them create around 100 new stars per year. For comparison, the Milky Way creates (on average) one new star per year.
A close-up view shows more detail in the twin images of the lensed galaxy. The third image is the same galaxy smeared out between the other two images. Courtesy: NASA/ESA/STScI
Thanks to the star formation (and possibly the presence of active galactic nuclei), LIRGs are brighter in infrared light than in optical light. That’s because their visible light gets absorbed by gas and dust in the galaxies. The dust then re-radiates thermal energy, which is best seen in the infrared spectrum. Two of HST’s instruments, the Advanced Camera for Surveys, and the Wide Field Camera 3, were used to image the object, and take spectra for further detailed study.
This galaxy lies about 6.9 billion years away from us. Its light was lensed by a galaxy cluster only 2.8 billion years away. To get the lensed images of SGAS J143845+145407, it had to be lined up precisely behind the cluster from Hubble’s point of view.
The team of astronomers who focused HST on this galaxy found that the magnified view of SGAS J143845+145407 allowed them to get details about star formation in that galaxy. Without lensing, seeing this galaxy is challenging. Since it is lensed, astronomers can study individual star formation regions at a pretty high resolution. It turns out the star birth activity takes place over large regions in the galaxy. In addition, astronomers were able to clearly discern a two-arm structure in the galaxy. They term it a “grand design” spiral, which means it has well-defined arms. This galaxy also has a very red core and bluer clumps of emissions scattered throughout the spiral arms.
The massive amounts of star formation in this galaxy’s spiral arms suggest strongly that most (if not all) LIRGs have wide-spread (maybe even galaxy-wide) bursts of star formation. If you compare these to galaxies in the present-day universe, the difference is striking. The local universe galaxies can have very intense knots of star formation, but they are smaller in scale than the star-forming regions in the LIRGs.
Lensing for the Win
HST and JWST will continue to be the instruments of choice for studying lensed objects in the distant universe. While some are studied from the ground, the very distant ones in the early universe need the power of orbiting telescopes.
For More Information
Hubble Captures a Mirrored Galaxy
Lens Model and Source Reconstruction Reveal the Morphology and Star Formation Distribution in the Cool Spiral LIRG SGAS J143845.1+145407
