“Even one image from Webb will be the highest quality image ever obtained of the galactic center,” said Roeland van der Marel of the Space Telescope Science Institute (STScI), principal investigator on one planned study that will focus on imaging using NASA’s recently launched James Webb Space Telescope (JWST), designed to view the universe in infrared light, which is invisible to the human eye, but is very important for looking at astronomical objects hidden from our view, obscured by vast swaths of interstellar dust at the galactic center in unprecedented detail.
The center of our galaxy is a violent and crowded place, writes Laura Betz at NASA’s Goddard Space Flight Center. Sagittarius A*, a supermassive black hole weighing 4 million times as much as our sun is surrounded by millions of stars whipping around it at close proximity with breakneck speeds. As in other similar galaxies, the gravitational pull of the central supermassive black hole is responsible for the motion of all the billions of stars in the galaxy as they slowly orbit the galactic center. This includes the sun, whose orbit around the galactic center takes about 230 million years. The extreme immediate environment of Sagittarius A*, including strong gravitational forces and intense ultraviolet light and X-ray radiation cannot be reproduced in a lab and therefore provide astronomers a unique opportunity to study physics under such extreme conditions.
Telescopes on the ground and in space have provided tantalizing glimpses of the residents of the galactic center. Astronomers have tracked stars orbiting the black hole, some of which approach close enough to provide a test of Einstein’s general theory of relativity. However, so far, only the brightest stars are detectable.
“We’re only seeing the tip of the iceberg from the ground. Webb will be able to study fainter stars and tell us more about the overall stellar population,” said Torsten Böker of the European Space Agency and STScI, a co-investigator.
Astronomers have been surprised to find low-mass infant stars forming close to the supermassive black hole—some within just a few light-years of its grasp. Theoretically, the black hole’s immense gravity and harsh radiation environment should disrupt any gas clouds and prevent them from collapsing into stars. Yet these baby stars called protostars have persisted. Webb’s observations may reveal additional protostars, and could provide clues to how stars can form in such an unlikely spot.
Sgr A* Mysteries
The Milky Way’s supermassive black hole, known to astronomers as Sagittarius A* also will fall under Webb’s gaze. It is surrounded by a disk of gas and dust, some of which will inevitably fall into the black hole. Astronomers have observed flares of light when the black hole gulped a clump of material. However, they have never detected the glow from the black hole’s disk.
“Detecting the disk around Sagittarius A* with Webb would be a home run,” Böker said.
Infrared observations using the ground-based Keck telescope have allowed astronomers to track individual stars orbiting the black hole at the galactic center. Webb is expected to detect fainter stars than are shown here, providing a more complete census of the stellar population within the galactic core. Credit: Keck/UCLA Galactic Center Group
Data from Webb also could help address broader questions of how galaxies form—such as the longstanding “chicken and egg” problem of which came first, the galaxy or the supermassive black hole residing at its center.
“Does the black hole come first and stars form around it? Do stars gather together and collide to form the black hole? These are questions we want to answer,” said Jay Anderson of STScI, a co-investigator on one of the studies.
Additionally, studies have shown that the mass of a galaxy’s central black hole is related to the total mass of the surrounding stars, but the reasons for this relationship remain unknown.
“Are there any clues to this mass correlation close to the black hole? Or has recent star formation wiped out signs of what might have happened in the past?” added University of Arizona astrophysicist Marcia Rieke, principal investigator on Webb’s NIRCam instrument.
Ultimately, the most exciting results from Webb’s observations might be the unexpected. For example, Webb might find stars in unusual orbits. Or, Webb might spot a gas cloud destined to be ripped apart by gravitational forces.
“We would like to see something unusual, like a star being gobbled up,” said van der Marel.
“So many interesting, strange things happen at the centers of galaxies. We want to find out what’s happening in our own,” said Rieke.
“Webb’s spatial resolution and sensitivity will give us a look at a much larger region around the center of the Milky Way than we have ever been able to do before,” Rieke told The Daily Galaxy. “We may learn how a black hole influences stars nearby. The accretion disk around SgrA*is known to be episodic in its brightness with outbursts, but Webb may be able to see it even in its most quiescent phase.”
The Last Word
UCLA astrophysicist Mark Morris has been studying the innermost regions of our Milky Way Galaxy for over 40 years. Morris co-discovered the magnetic filaments that pervade the Galactic center, as well as the bipolar X-ray lobes that appear to have resulted from mass outflows near the central black hole. In an email reply to The Daily Galaxy where we asked about the infrared ability of the James Webb Space Telescope to pierce the dust at the Milky Way’s center and image Sagittarius A* and currently unseen objects, he wrote:
“The JWST will certainly allow us to observe the accretion flow onto Sagittarius A* in the infrared. That can now be done (and has been done for years) in the near-infrared using adaptive optics on large ground-based telescopes such as the Keck Telescopes in Hawaii, but JWST will make it possible to observe the variable emission from Sgr A* at longer infrared wavelengths than has been possible from the ground. However, the spatial resolution of JWST will not be high enough to resolve the SgrA* accretion disk. The ground-based ALMA telescope can possibly resolve the accretion disk at millimeter or submillimeter wavelengths, and in fact some have claimed that they can detect the accretion disk with ALMA.
“Many Galactic center researchers are poised to use JWST to study the stars and gas at the center of the Galaxy,” Morris explained in his email. “Some (including me and my collaborators) will be using it to investigate whether star formation is presently taking place in the tumultuous maelstrom of interstellar gas in the inner parsec. Others will be using JWST to look for planet-forming circumstellar disks around massive stars in a few of the Galaxy’s most massive young star clusters close to the Galactic center. And still others will be using JWST to study the dynamics, the chemical composition, and the mass distribution of the nuclear star cluster, where the stellar density is almost a million times higher than it is around the Sun. Spectroscopy with JWST will also provide a new window on the streams of gas that are known to be orbiting close to the central black hole.”
Image credit top of page: the central region of our galaxy, the Milky Way, contains an exotic collection of objects, including a supermassive black hole, clouds of gas at temperatures of millions of degrees, neutron stars and white dwarf stars tearing material from companion stars and tendrils of radio emission.