At the center of most large galaxies sits a supermassive black hole (SMBH). When these black holes are actively consuming material, they become incredibly luminous quasars. But some quasars appear wrapped in thick clouds of dust, making them hard to detect. In a new study, astronomers have revealed 77 new, hidden, “heavily reddened” quasars (HRQs).
Their work provides the strongest observational support yet for the idea that these objects represent a brief transitional stage in galaxy evolution. The paper outlining the new research was uploaded to the preprint server arXiv on May 7.
Hidden monsters
The leading theory of galaxy evolution proposes that supermassive black holes go through a brief but violent “reddened” dust-obscured growth phase, triggered by galaxy mergers. When two galaxies collide, gas floods inward, simultaneously sparking intense star formation and feeding the central black hole.
The dust surrounding this process makes radiation-driven winds even more powerful, turning these objects into engines of extreme feedback.
Surveys that observe at optical wavelengths miss heavily dust-obscured quasars almost entirely, because these objects are faint and scattered across the sky.
So far, around 50 heavily reddened quasars have been confirmed by searching for them via a time-expensive process—hunting them down one at a time with single-target infrared telescopes. And without large samples, key questions about this crucial galactic transition phase cannot be answered with any confidence.
Hunting for giants
In a new study, the team led by Matthew Stepney of the Center of Excellence in Astrophysics and Related Technologies, Chile, has more than doubled this population of heavily reddened quasars using infrared data and spectrophotometry from NASA’s SPHEREx telescope.
They discovered 77 new dust-obscured quasars when the universe was between 1.6 billion years and 4.3 billion years of age. The new sample includes the first seven heavily reddened quasars ever identified at redshifts above 3, within the first 2.1 billion years after the Big Bang.
To place these findings in context, the team compared the heavily reddened quasars against two other populations. Hot Dust-Obscured Galaxies (Hot DOGs) are among the most deeply buried objects known, with their emission being dominated by hot and warm dust.
Blue quasars, by contrast, are the more familiar and fully unobscured class whose dust has already been cleared away, but they still host a dusty torus structure around the black hole that glows brightly in infrared. Heavily reddened quasars sit between these two extremes in terms of obscuration, yet surprisingly, the team found that they are deficient in hot dust compared to even the unobscured blue quasars.
Clearing out
Although they appear faint due to dust obscuration, after correcting for extinction, these objects ranked among the most intrinsically luminous quasars known. However, they were found to emit less strongly at infrared wavelengths associated with dusty structures surrounding the black hole.
Astronomers think that this combination of extreme intrinsic luminosity and depleted dust reservoirs suggests the quasars are caught during a “blow-out” phase. This phase happens when powerful feedback from the actively feeding black hole begins clearing away the dusty cocoon surrounding the galaxy’s core.
“This combination of depleted torus-scale dust reservoirs and higher luminosities compared to Hot DOGs and blue quasars supports a scenario in which HRQs represent a blow-out phase when strong feedback has begun to clear the central regions of obscuring material,” the researchers explain in the paper.
They also detected an unexpected excess of ultraviolet (UV) light in roughly three-quarters of the sample. While this could be quasar light scattering around the edges of the dusty cocoon, the researchers note that star formation within the host galaxy may also contribute to this UV excess and, in some cases, may even dominate.
“In future work we will expand the samples even further, exploiting the all-sky capabilities of SPHEREx to understand the number densities and multi-wavelength properties of the ‘hidden monsters’ that appear to dominate black hole growth at cosmic noon,” the team concludes.
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Publication details
Matthew Stepney et al, Hidden Monsters with SPHEREx I: A goldmine for heavily reddened quasars at cosmic noon, arXiv (2026). DOI: 10.48550/arxiv.2605.06791
Journal information:
arXiv
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Heavily reddened quasars caught going through a ‘blow-out’ phase (2026, May 24)
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