Why had astronomers missed it?

Finding these distant objects is genuinely hard. A bubble of dusty gas hides the central black hole and its surroundings in most powerful early-universe radio galaxies, leaving only the radio jet visible. A 2024 study suggests that as many as 90% of these sources at redshifts greater than 3.5 may be hidden from ultraviolet and optical surveys.

The standard search method, which looks for radio sources with unusually steep spectra, appears to miss a large share of the true population. TXS 2354+015 itself fails the usual ultra-steep-spectrum criteria, so conventional searches would have passed it over.

The team took a different route. They combined deep optical imaging from Subaru's Hyper Suprime-Cam Subaru Strategic Program survey with radio catalogs — TGSS at 150 MHz and VLASS at 3 GHz. They searched for radio sources whose optical counterparts show a characteristic "dropout": at extreme distances, clouds of neutral hydrogen between us and the galaxy absorb its ultraviolet light, so the observed optical spectrum cuts off abruptly. Astronomers call this the Lyman-break technique.

The search covered the redshift range 4.5 to 5.3, when the universe was between 1.1 billion and 1.3 billion years old.

How did the team confirm it?

The optical spectrum of TXS 2354+015 showed a prominent Lyman-alpha emission line. The researchers verified the distance using a second, fainter emission line. Together, the lines pinned the redshift at 4.946.

The team also checked whether the match between the radio source and the optical galaxy could be a chance alignment. Three lines of evidence argued against it:

• The optical and radio positions match precisely.
• The radio brightness is extremely rare.
• The ratio between radio and optical emission matches expectations.

The conclusion: this is a genuine radio source with a real optical counterpart.

How massive is its host galaxy?

From their data, the researchers estimated a possible host galaxy mass of around 2 trillion solar masses. That figure would place it among the brightest galaxies known. But the team urges caution. "This estimate is clearly plagued by several large uncertainties, in particular by the age of the stellar population," they note in the paper.

What does this mean for future searches?

Because TXS 2354+015 does not satisfy the standard steep-spectrum selection rules, its discovery through the optical dropout method suggests the conventional technique is incomplete. Astronomers may be missing a meaningful fraction of obscured radio galaxies in the early universe.

The findings are preliminary in one respect: the paper is a preprint and has not yet passed peer review. Still, the result points to a hidden population of powerful early galaxies waiting to be found — and a search method better suited to finding them.