At cosmic noon, SPHEREx catches 77 hidden quasars mid-explosion

At the heart of every large galaxy sits a supermassive black hole, and when those engines feed aggressively on infalling gas, they shine across the universe as quasars. Yet a stubborn fraction of these objects refuses to behave like the textbook examples. They are cocooned in thick veils of dust that absorb their visible and ultraviolet light, hiding them from optical surveys and leaving astronomers with a frustrating gap in the census of cosmic accretion. A new study using NASA’s SPHEREx mission has just turned that picture inside out, more than doubling the known population of these elusive objects in a single stroke.

A class of monsters hidden behind dust

Heavily reddened quasars are the textbook leading suspects in one of the most consequential transitions in galaxy evolution. The dominant theoretical picture says that when two gas-rich galaxies merge, fresh material funnels into the new combined nucleus. That gas simultaneously triggers a burst of star formation and feeds the central black hole until the latter becomes a quasar. During this stage, the entire system is choked with dust, and the quasar is effectively buried.

Eventually, radiation pressure from the engine becomes so intense that it should blow the cocoon outward, leaving behind the familiar blue, unobscured quasars that populate most optical catalogs. The dust-buried, transitional, «red» phase is brief on cosmic timescales, which is precisely why finding examples is so hard. Before this work, only around fifty heavily reddened quasars had been confirmed worldwide, identified one painstaking target at a time through dedicated infrared follow-up.

SPHEREx turns the search inside out

The new study, led by Matthew Stepney of the Center of Excellence in Astrophysics and Related Technologies in Chile, exploits the all-sky infrared survey capabilities of SPHEREx to break that bottleneck. Using infrared photometry and spectrophotometry from the mission, the team identified 77 new heavily reddened quasars at redshifts corresponding to a universe between roughly 1.6 and 4.3 billion years old. The preprint appeared on arXiv on May 7, 2026.

Seven of the new objects sit at redshifts above 3, placing them within the first 2.1 billion years after the Big Bang. These are the earliest heavily reddened quasars ever identified, pushing the observational record of this phase deeper into cosmic history than any prior search had managed.

Deficient in hot dust, despite the veil

To make sense of what these objects actually are, the team compared their infrared properties against two reference populations. On one extreme sit hot dust-obscured galaxies, known as Hot DOGs, the most deeply buried quasar-like sources known, with infrared spectra dominated by hot and warm dust emission. On the other extreme sit ordinary blue quasars, the unobscured class whose dust cocoon has already been cleared away, although they still host a smaller dusty torus close to the black hole that glows brightly in the infrared.

Heavily reddened quasars should sit between these two regimes, and broadly they do. But the analysis turned up a striking anomaly. Even when compared with the cleaner blue quasars, the heavily reddened sample is notably deficient in hot dust. After correcting their observed brightness for extinction by the obscuring material, their intrinsic luminosities place them among the most powerful quasars ever cataloged. They are not faint because they are weak. They are faint because something is veiling them while simultaneously stripping that veil away.

A blow-out caught in the act

That contradiction is the most important result of the paper. Enormous intrinsic luminosity combined with a thinned-out dust reservoir is exactly the signature predicted for the blow-out phase of supermassive black hole growth, the moment when feedback from the central engine becomes powerful enough to drive winds that begin tearing apart the very cocoon that fed it. According to the authors, the combination of depleted torus-scale dust and exceptional intrinsic luminosity supports a scenario in which these objects represent that fleeting transitional stage.

In other words, the sample appears to capture quasars mid-revelation. The engine has reached full ignition, feedback has switched on, and the surrounding gas and dust is being driven outward into the host galaxy. If the picture is correct, every blue quasar visible today passed through a stage that looked essentially like this.

An ultraviolet surprise

About three quarters of the new sample also shows an unexpected excess of ultraviolet light, which at first glance seems incompatible with the heavy obscuration these objects display. Two explanations are on the table. The UV emission may be quasar light scattering around the edges of the dusty cocoon rather than passing through it, a process that would be expected if the geometry of the obscuring material is not fully spherical. Alternatively, intense star formation in the host galaxy itself could be generating the UV, and in some cases the host may even outshine the central engine at those wavelengths.

Both processes are entirely consistent with the merger-driven scenario, and both are likely contributing at different levels across the sample. Disentangling which dominates in any individual source will require multi-band spectroscopy that future SPHEREx releases and follow-up observations should make possible.

What comes next

The authors frame the work as the opening chapter of a longer program. The all-sky reach of SPHEREx means that the selection technique used here can be scaled up substantially, eventually mapping the number densities, redshift distribution, and multi-wavelength properties of dust-veiled quasars across the era astronomers call cosmic noon, between roughly 8 and 11 billion years ago, when accretion onto supermassive black holes peaked across the universe.

If the blow-out interpretation survives those larger samples, the field will gain an observational handle on one of the briefest and most consequential moments in galaxy evolution: the instant a supermassive black hole stops being fed and starts pushing back, reshaping its host galaxy from the inside out.

Reference: Matthew Stepney et al., «Hidden Monsters with SPHEREx I: A goldmine for heavily reddened quasars at cosmic noon,» arXiv:2605.06791 (2026).

© 2026 SKYCR.ORG | Homer Dávila Gutiérrez, FRAS. All rights reserved. Reproduction in whole or part is prohibited without express authorization. Original source: Stepney et al. arXiv DOI: 10.48550/arxiv.2605.06791