Black hole found hiding in a 'pure' starburst galaxy

For centuries, the universe has offered humanity glimpses of unimaginable galaxies and structures. Some shine brightly, others hide behind veils of gas and dust.

The Southern Pinwheel Galaxy, or Messier 83 (M83), seemed like one of the latter. Its vibrant spiral arms and starburst activity have dazzled astronomers. Yet, deep in its luminous center, something powerful remained unseen – until now.

Recent observations from the James Webb Space Telescope (JWST) may have uncovered long-sought evidence of a supermassive black hole at the galaxy’s core.

This finding, driven by the telescope’s Mid-Infrared Instrument (MIRI), marks a major shift in our understanding of M83 and other galaxies that were once thought to be starburst-only systems. The results open a new chapter in extragalactic astronomy.

Black hole hiding in a starburst galaxy

M83 lies just 4.6 million parsecs away – relatively close on the cosmic scale. Its classification as a barred spiral galaxy with intense star-forming activity placed it under frequent scrutiny.

Yet, despite decades of detailed study, researchers failed to detect signs of a central active galactic nucleus (AGN), which is typically expected in galaxies of its size and structure.

Older optical and near-infrared observations suggested a complex nuclear region. Past efforts even revealed multiple candidate centers – each differing slightly in location and properties.

Still, none of these zones produced evidence strong enough to confirm AGN activity. If the galaxy held a supermassive black hole, it had to be dormant, hidden, or incredibly faint.

Astronomers suspected that thick dust may obscure faint AGN signals, especially in the galaxy’s nuclear starburst zone. Webb’s mid-infrared sensitivity changed that. With MIRI, researchers now see what previous instruments could not.

Neon signals point to mystery

The new study, led by Svea Hernandez, reported clear detections of highly ionized [Ne v] (14.3 μm) and [Ne vi] (7.7 μm) emission in the nucleus of M83.

These lines require photon energies of 97 and 126 electron volts, respectively, which are far higher than typical stellar processes can generate.

“Our discovery of highly ionized neon emission in the nucleus of M83 was unexpected,” said 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.”

The emission appeared in distinct regions, including a point-like structure about 140 parsecs south of the optical nucleus, named P3. The structure measures less than 18 parsecs across and shows low obscuration, contrasting with the highly dust-shrouded core. This difference adds weight to the AGN scenario.

Gas flows hinted at a black hole

Velocity maps from JWST show [Ne v] distributed in a clumpy, north-to-south alignment, with different regions exhibiting distinct kinematic signatures.

P3’s [Ne vi] emission, for instance, shows a velocity of +80 km/s, slightly offset from the local [Ne v] velocity. Other zones closer to the nucleus display negative velocity offsets, hinting at motion that may trace outflow or turbulence.

The researchers compared the profiles of various ionized lines, from low to high energy. At P3, the high-ionization lines like [Ne v] and [Ne vi] are narrower than the lower-ionization lines.

This suggests that the gas producing these emissions may originate from a different physical process, possibly linked to AGN activity or compact shock-driven regions.

“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.”

Are radiative shocks the culprit?

To explain the origin of these extreme neon lines, the researchers tested several scenarios. Fast radiative shocks were one possibility.

These shocks occur when turbulent gas, such as that from a supernova or stellar wind, moves at supersonic speeds and compresses surrounding material. The resulting heat can produce high-energy radiation.

Shock models did reproduce the observed line ratios – but only under extreme assumptions. Specifically, the preshock density had to be a mere 0.01 cm⁻³, which is much lower than what is typically found in star-forming galaxies.

The observed postshock densities were also much higher, exceeding 1000 cm⁻³. This discrepancy challenges the idea that shocks alone could explain the data.

Energetic shocks are also known to destroy PAH molecules. Interestingly, P3 – the only region with [Ne vi] – shows the weakest PAH features in its spectrum. This matches the shock hypothesis, yet the unusual density mismatch casts doubt on this explanation.

Black hole models explain galaxy emissions

The researchers then tested tailored photoionization models built around a low-luminosity AGN.

The models used a two-zone structure: one zone optically thin to ionizing radiation to generate high-energy lines like [Ne v], and another zone optically thick to generate lower-ionisation lines. This method reproduced the observed line strengths and ratios well.

The models assumed hydrogen column densities and metallicities consistent with M83’s central environment. They also used realistic ionisation parameters and densities. The best-fitting model places P3’s emission well within AGN-dominated zones on classic diagnostic plots.

“Webb is revolutionizing our understanding of galaxies,” said study co-author Linda Smith. “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.”

Ruling out other extreme sources

Could other compact objects be responsible? High-mass X-ray binaries (HMXBs) can produce hard radiation, though usually at lower levels.

Past X-ray surveys of M83 revealed several sources, including a bright object coinciding with the optical nucleus. But P3, where [Ne vi] was detected, shows no corresponding X-ray source.

Starburst activity alone cannot easily explain these ionized lines either. Models based on massive stars, including Wolf–Rayet populations, typically fail to reproduce the observed [Ne v] and [Ne vi] strengths. These limitations, combined with the success of the AGN models, make the AGN scenario more likely.

“This discovery showcases how Webb is making unexpected breakthroughs,” noted Smith. “Astronomers thought they had ruled out an AGN in M83, but now we have fresh evidence that challenges past assumptions and opens new avenues for exploration.”

M83 may not be so pure after all

For decades, M83 stood as a textbook example of a starburst galaxy. Optical data showed little hint of a central black hole, but the discovery of high-ionization neon lines shifts this picture. Mid-infrared diagnostics, less hindered by dust, reveal what optical instruments missed.

Line ratio plots now place M83’s nuclear regions alongside hybrid AGN-starburst systems. In particular, P3 falls between the domains of purely star-forming galaxies and low-luminosity AGNs. This blend suggests that more such “pure” galaxies may indeed harbor faint AGNs.

The study urges astronomers to rethink what defines a starburst galaxy. With JWST, similar hidden cores may soon be uncovered in other nearby systems.

Confirming the galaxy’s black hole

The Webb team has laid the groundwork, but the mystery isn’t solved yet. Future observations with ALMA and the Very Large Telescope will help test whether the neon emission truly comes from an AGN or another exotic source. These studies will probe gas motion, composition, and geometry more precisely.

Simulations with more complex geometries may also provide better answers. For now, M83’s hidden giant remains a strong candidate – but not a confirmed one.

Still, this work shows the power of Webb. Where previous telescopes saw only dust and star formation in some galaxies, JWST has potentially opened a new window into spotting black holes and other hidden structures.

The study is published in The Astrophysical Journal.

Image Credit: ESA

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