Isolated dwarf galaxy is mysteriously forming stars with no apparent fuel source

Astronomers have taken a fresh look at NGC 6789, an isolated dwarf galaxy located about 12 million light-years away in the constellation Draco. What they found is a fascinating mystery.

Over the past 600 million years, this dwarf has been steadily forging new stars despite living inside the Local Void – one of the emptiest neighborhoods in our cosmic vicinity.

Star formation needs gas, yet the void is famously short on it. Even stranger, deep imaging now suggests there’s no obvious culprit – no recent merger, tidal stream, or visible disturbance – to explain where the fuel came from.

Mysterious dwarf galaxy NGC 6789

NGC 6789 is a blue-compact dwarf (BCD) that is small, relatively metal-poor, and easily quenched by its environment.

The galaxy is also extremely isolated. Galaxies usually sustain star formation by siphoning gas from nearby companions or by tapping filamentary reservoirs that thread galaxy groups. NGC 6789 has neither luxury.

Still, within the last 600 million years it managed to build roughly 100 million solar masses of new stars, about four percent of its total stellar mass.

This is surprising because the galaxy is at least a billion years old – long enough that its original gas supply should have been exhausted.

A team of researchers led by Ignacio Trujillo at the Institute of Astrophysics of the Canary Islands (IAC) set out to find a smoking gun.

If the galaxy had devoured a gas-rich neighbor or captured a hidden stream, there ought to be scars: shells, ripples, asymmetric halos, or other low-surface-brightness debris.

Stillness in the outskirts

To push far below the usual limits, the team used the Two-meter Twin Telescope at Teide Observatory (Tenerife), gathering exceptionally deep optical images that can reveal ultra-faint structures around galaxies.

The logic was straightforward: gravity is messy. Minor mergers and cold inflows typically leave fingerprints. If NGC 6789 had recently fed on an interloper, the outskirts should look stirred. They don’t.

The galaxy’s outer light looks placid and undisturbed. There are no obvious tidal tails, no warped isophotes, and no diffuse companions hiding in the wings.

As Trujillo’s group describes it, everything about the galaxy’s shape argues against a fresh interaction. That negative result makes the gas budget even harder to balance: the star factory has been humming, but the warehouse shelves look bare.

No standard explanation fits

With mergers and streams seemingly off the table, the team lays out alternative scenarios – none fully satisfying, all worth testing.

One possibility is that NGC 6789 retained a surprisingly large fraction of its original, pristine gas and has been burning it through short bursts. Dwarfs can be inefficient; some hoard gas for long periods if they avoid disruptive encounters.

The trouble is that sustaining star formation for hundreds of millions of years should still have left some detectable structural signature or extended gas reservoir – and none is obvious.

Tracking NGC 6789’s gas flow

Another option is a very low-density, local pocket of intergalactic gas that the galaxy has tapped intermittently. If that reservoir is tenuous enough, it might not sculpt visible distortions even as it trickles mass into the disk.

The catch is that such reservoirs are difficult to confirm without exquisitely sensitive neutral hydrogen or ultraviolet observations.

Finally, dwarfs frequently launch gas via stellar feedback (winds and supernovae) that later cools and rains back in. This fountain cycle could prolong star formation with little external input.

Yet to account for ~100 million solar masses of new stars over 600 million years, the fountain would need to be both efficient and sustained, pushing current models to their limits for such an isolated system.

Significance of the lonely dwarf

The environment shapes dwarfs more than it does giants. In groups and clusters, ram pressure, tidal harassment, and frequent flybys modulate their gas supply. Isolation removes those levers, which is why NGC 6789 is so valuable: it’s a clean laboratory.

If a dwarf this lonely can still fuel prolonged star formation, then the “default” rules for gas accretion and retention in the low-mass regime may need a tune-up.

The Local Void setting sharpens the point. Voids are expected to starve galaxies of external fuel.

Finding sustained activity in a void dwarf pushes theorists to revisit how small halos sip the cosmic web – perhaps through ultra-thin, low-contrast filaments that barely register in current surveys.

Star growth with no trail

The Teide images reach the ultra-faint outskirts, where relics of past interactions typically hide. The team reports a smooth, symmetric envelope with no morphological distortions at the depths probed.

At the same time, the galaxy’s stellar population analysis confirms a recent, significant uptick in star formation – about four percent of its entire stellar mass added in just the last 600 million years.

Put together, the data draw a tight box: the system is forming stars, but without the usual signatures of fresh fuel delivery.

Trujillo’s group frames the result as a genuine mystery rather than a paradox. The working view is that either the fuel is a leftover cache that has been metered out with remarkable thrift, or the supply flows in so gently that it leaves no gravitational fingerprints.

Lessons from galaxy NGC 6789

Solving the case will require new kinds of observations. Deep, wide-field 21-centimeter mapping could uncover thin neutral hydrogen filaments or ultra-diffuse companions that optical light misses.

Far-ultraviolet spectroscopy might reveal metal-poor inflows or outflow reaccretion cycles.

On the theory side, high-resolution simulations of void dwarfs can test whether tiny filaments or fountain cycles can sustain the observed star formation history without obvious morphological scars.

As the team emphasizes, the stakes are bigger than one galaxy. If NGC 6789 isn’t an outlier but a tip-of-the-iceberg example, then our picture of how dwarfs fuel themselves, especially in low-density environments will need an update.

That, in turn, feeds directly into models of reionization-era galaxies, feedback cycles, and the role of dwarfs in building up the faint end of the galaxy population today.

For now, NGC 6789 stands as a quiet provocation from a very quiet place: a star factory thriving in a desert, with no obvious aqueduct in sight.

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