SPHEREx Catches 77 Hidden Quasars Mid-Explosion, Doubling the Known Population

Dust obscures light most effectively at shorter wavelengths, making heavily dust-enshrouded objects faint or invisible in the optical spectrum. However, this same dust becomes more transparent at longer, near-infrared wavelengths. By analyzing the unique spectral signature of broad emission lines from rapidly swirling gas in these newly discovered quasars, the SPHEREx team could confirm their identity and measure their distances .

The result: 77 new HRQs at redshifts of 1.5 < z < 3.9, corresponding to a universe roughly 1.6 to 4.3 billion years old . Crucially, this sample includes the first seven HRQs ever identified at a redshift greater than 3, opening a new window into the early universe .

A Fleeting, Violent "Blow-Out" Phase

Astronomers believe these heavily reddened quasars represent a transitional stage that is as intense as it is brief. The leading evolutionary theory points to galaxy mergers as the trigger. When two gas-rich galaxies collide, vast amounts of material are funneled toward the central supermassive black hole, sparking a furious bout of new star formation while simultaneously feeding the monster at the core .

During this initial phase, the black hole remains deeply embedded in a cocoon of gas and dust. It appears not as a brilliant beacon, but as a heavily reddened quasar whose light is heavily filtered and dimmed by the surrounding material .

This is where the "blow-out" begins. The immense energy output from the rapidly accreting black hole drives powerful feedback—in the form of radiation, winds, or outflows—that begins to push outward against the obscuring blanket of dust . Previous studies have strongly disfavored orientation as the primary explanation for their red color, instead linking their rarity and the high incidence of molecular gas outflows to an evolutionary phase where the mechanism to clear the dust has just activated .

The SPHEREx sample provides a much larger population for testing this model. The study found that 76% of the newly detected HRQs exhibit a UV excess, consistent with light from the obscured quasar scattering off nearby dust into our line of sight . This scattered-light signature gives astronomers a crucial tool to dissect the three-dimensional structure and dynamics of this messy blow-out process.

A Key Transitional Stage in Galaxy Evolution

These objects are considered a cornerstone of the cosmic feedback story that shapes the universe. The "cosmic noon"—the epoch when star formation and black hole growth peaked—is thought to have been dominated by such dust-enshrouded processes .

The evolutionary sequence implied by these findings is now clearer:

1. Fueling: A gas-rich merger funnels material inward, simultaneously igniting a nuclear starburst and rapid accretion onto the supermassive black hole .
2. The Reddened Phase: The black hole grows rapidly but is hidden behind a thick veil of dust, appearing as an HRQ with line-of-sight extinction of 1 to 5 magnitudes .
3. Blow-Out: Active galactic nucleus (AGN) feedback—fast winds launched from the accretion disk—begins to heat, disturb, and expel the surrounding gas and dust, carving out a cavity . This phase is thought to be short-lived, explaining the rarity of HRQs .
4. Quenching and Revelation: As the dust clears, the system may emerge as a classical, unobscured quasar. Crucially, the feedback process can also expel or heat the gas needed for star formation, permanently quenching the galaxy's ability to form new stars and setting its final size .

By uncovering 77 of these "hidden monsters" in a single stroke, SPHEREx has moved the study of quasar blow-outs from a handful of remarkable anecdotes to a robust statistical population. This new sample will allow astronomers to test the links between dust-obscured black hole growth, feedback-driven outflows, and the quenching of star formation that ultimately shapes the massive galaxies we see in the universe today .