TXS 2354+015 appears to be the most powerful known radio galaxy
Spectroscopy confirms a galaxy seen when the universe was less than 1.2 billion years old. Its estimated radio power exceeds the previous benchmark in the researchers’ comparison.
TXS 2354+015, a radio galaxy whose light has travelled for nearly 12.5 billion years, has been confirmed at a redshift of 4.946, Phys.org reported on October 7. Astronomers estimate that its radio power exceeds the previous benchmark in their comparison of known galaxies. The finding offers a glimpse of a powerful radio source from when the universe was less than 1.2 billion years old.
The research team, led by Barbara Balmaverde, reported the spectroscopic confirmation in a paper posted to arXiv on September 23. The paper calls TXS 2354+015 the most powerful radio galaxy currently known, based on its calculated power and a comparison with published sources. That is a claim about the available record, which could change as more distant galaxies are found.
How astronomers confirmed TXS 2354+015
The team searched for distant radio galaxies by matching deep optical images from Subaru’s Hyper Suprime-Cam survey with radio catalogues, including TGSS and VLASS. They looked for radio sources whose optical counterparts showed a sharp drop in one band of light. TXS 2354+015 emerged as a candidate in that search, which targeted redshifts between 4.5 and 5.3.
The search uses the Lyman-break technique. Neutral hydrogen absorbs ultraviolet light, leaving a characteristic drop in the light astronomers observe from sufficiently distant galaxies. That pattern can identify promising candidates, but a spectrum is needed to establish a candidate’s redshift more securely.
For TXS 2354+015, the researchers identified a prominent emission line as hydrogen’s Lyman-alpha line and detected a second, fainter line that supported that identification. They derived a redshift of 4.946. The second line was difficult to assess because it lay on a bright sky line, so the researchers checked spectra from 244 other sky positions; they found no similar excess.
The team also examined whether the optical galaxy might merely lie near an unrelated radio source. Its optical and VLASS positions are separated by 0.2 arcseconds, and radio sources this bright are rare in the TGSS catalogue. The researchers considered a chance alignment remote. These checks matter because the distance and radio measurements must belong to the same object for the power estimate to hold.
What the radio-power estimate establishes
Using radio measurements spanning 74 MHz to 4.8 GHz, the authors fitted the source’s radio spectrum and estimated a power of 6.2 × 10^29 watts per hertz at a rest-frame frequency of 500 MHz. In their published comparison, the previous benchmark was 3.2 × 10^29 watts per hertz for TN J0924−2201. The new estimate is therefore nearly twice that reference value.
This is an estimate of radio power at a specified frequency, rather than a measurement of every form of energy the galaxy emits. The record description depends on the published sources the authors compared. It does not establish that no undiscovered galaxy is more powerful, or that this object is typical of distant radio galaxies.
Why the search method matters
Many searches for distant radio galaxies favour sources with unusually steep radio spectra. TXS 2354+015 does not satisfy the usual ultra-steep-spectrum selection criterion. Finding it through an optical dropout search supports the researchers’ view that the familiar radio criterion captures only part of the distant population. One confirmed object cannot show how large the missed population is.
Powerful radio emission can arise when material falling towards a galaxy’s central supermassive black hole drives jets. Such jets can heat surrounding gas and affect star formation, making distant radio galaxies useful subjects for studying how early galaxies developed. The new paper does not measure those effects in TXS 2354+015; they explain the broader interest in finding objects like it.
The authors cite earlier work suggesting that many radio-loud active galactic nuclei at redshifts above 3.5 may be obscured in optical and ultraviolet light. That estimate concerns a wider population, not an obscuration measurement for TXS 2354+015. It also underscores why different ways of selecting candidates may reveal different parts of the early radio-galaxy population.
What remains uncertain about the host galaxy
The researchers give a tentative host-galaxy mass of around two trillion solar masses, but stress substantial uncertainties. The calculation depends in part on assumptions about the age of its stars and on how much measured light comes from emission lines. The figure should not be read as a secure measurement of the galaxy’s stellar mass.
The team says better coverage of the galaxy’s light across wavelengths is needed to clarify the host’s properties. It also wants a larger sample of radio galaxies near redshift five to test patterns that a single discovery cannot settle. For now, the firm result is the confirmed distance of one exceptionally powerful radio source and a power estimate that exceeds the previous benchmark in the authors’ comparison.
Sources and context
- Radio galaxy from 12.5 billion years ago may be most powerful ever foundPhys.org
- The quest for high-redshift radio galaxies II. Discovery of the most powerful known radio galaxy at z=4.946arXiv; authors affiliated with INAF, Space Telescope Science Institute, Johns Hopkins University, University of Turin and European Southern Observatory
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