Universe Today
We know that our Milky Way galaxy hosts a supermassive black hole (SMBH) in its center. Astronomers think most spiral galaxies do, and that SMBHs coexist and co-evolve with their host galaxies. However, they haven't been able to find them in all spirals. M83, the Southern Pinwheel Galaxy, has always been puzzling because scientists haven't seen any evidence of an SMBH in its center. The JWST may have finally found some.
Spiral galaxies are some of the most striking shapes in nature. Ask a kid to draw a galaxy, and they probably draw a spiral. They're burned into the minds of curious people. It's only natural that astronomers would like to understand them better.
The effort to understand these galaxies, including ours, has paid dividends. In recent times, astronomers have found SMBHs at the hearts of many spiral galaxies. They think that while not all spirals host SMBHs, those with significant spiral bulges do. But when they've turned their telescopes toward some spirals that should host an SMBH, they've found nothing where one should be. The Southern Pinwheel Galaxy(M83) is one of them.
The Southern Pinwheel Galaxy (M83) is a Grand Design Spiral Galaxy about 15 million light-years away. It's a favourite target for astrophotographers. The purple and blue in this image show how M83 is blazing with star formation. Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA); Acknowledgement: W. Blair (STScI/Johns Hopkins University) and R. O'Connell (University of Virginia)
Everyone knows you can't actually see a black hole. No light and no information can be detected beyond its event horizon. Once something disappears beyond the event horizon, it's gone forever.
But SMBHs are extraordinarily massive objects that can contain tens of billions of solar masses. All that mass has a profound effect on its surroundings, and finding an SMBH means finding evidence of that effect.
One of the main pieces of evidence for an SMBH comes when they're actively accreting material. When that happens, they're called active galactic nuclei (AGN). As an SMBH draws material toward itself, it gathers in an accretion ring around the black hole. The material heats up and emits energy that telescopes can detect. However, detecting that energy isn't a slam dunk.
Sometimes, the region is obscured by gas and dust, and despite astronomers' best efforts, they can't detect the telltale energetic signs that an SMBH is present. Some galaxies are so far away that there's very little chance of detecting an SMBH, and inactive ones are also difficult to detect. Previous research showed that M83 has no AGN and no SMBH or was too obscured by dust to be detected.
The JWST is a relatively new tool at astronomers' disposal, and it was built to see through thick dust better than any previous telescope. New research in The Astrophysical Journal presents evidence favouring an SMBH in M83. It's titled "JWST/MIRI Detection of [Ne v] and [Ne vi] in M83: Evidence for the Long Sought-after Active Galactic Nucleus?" The lead author is Svea Hernandez, an astronomer at the Space Science Telescope Institute.
"For years, astronomers have searched for a black hole in M83 without success. Now, we finally have a compelling clue that suggests one may be present." - Linda Smith, Space Telescope Science Institute.
The research is focused on the first detection of ionized neon in the nuclear region of M83. The authors say that fast radiative shocks and a cloud of gas ionized by an active galactic nucleus can be responsible for these neon emission lines. Fast radiative shocks occur in black hole accretion disks and when outflow jets from black holes collide with surrounding gas. In this work, the astronomers detected Ne v and Ne vi, which are highly ionized forms of Neon. These ions only form when they're exposed to high-energy photons or collisions.
The authors explain that Ne v emissions have been historically used to detect AGN activity. "The power of the MIR [Ne v] emission relied on the fact that it can more easily uncover the presence of AGNs even when the object is obscured in the optical," they write.Starburst galaxies can't produce it.
This artist's illustration shows an AGN obscured by thick dust. Image Credit: NASA/JPL-Caltech
Our discovery of highly ionized neon emission in the nucleus of M83 was unexpected," said lead author Hernandez. "These signatures require large amounts of energy to be produced – more than what normal stars can generate. This strongly suggests the presence of an AGN that has been elusive until now.
These findings are due to the JWST's Mid-Infrared Instrument(MIRI). Dust particles in space are typically the same size or smaller than the wavelength of visible light. That means the dust can scatter visible light, which is why dust clouds are opaque to it.
However, MIR wavelengths are larger than the dust grains. Instead of scattering, they can pass right through most dust. This is similar to how long radio waves aren't blocked by large objects like buildings, while visible light is. Infrared light is the key to observing many things in astronomy, and the JWST was built to exploit that fact.
Before Webb, we simply did not have the tools to detect such faint and highly ionised gas signatures in M83's nucleus," Svea added. "Now, with its incredible mid-infrared sensitivity, we are finally able to explore these hidden depths of the galaxy and uncover what was once invisible.
The astronomers found clumps of highly ionized gas near the center by peering through the thick dust obscuring M83's nuclear region with MIRI. Stars can't produce the gas because they're not energetic enough. Even exploding supernovae can't do it. By the process of elimination, an AGN becomes the most likely explanation, even though the researchers can't definitively say M83 has an SMBH.
Other plausible causes remain, like powerful shock waves in the interstellar medium. The neon emissions come from a fairly wide area, which could indicate a source different than an AGN. "Furthermore, given the relatively extended nature of the observed [Ne v] emission, a feasible scenario could be a combination of sources (shocks, High-Mass X-ray Binaries, AGNs) producing the high ionization traced by these MIR lines," the authors write in the paper's conclusion.
Webb is revolutionizing our understanding of galaxies," co-author Linda Smith of the Space Telescope Science Institute said. "For years, astronomers have searched for a black hole in M83 without success. Now, we finally have a compelling clue that suggests one may be present.
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