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TON 618
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TON 618
[[File:|1089px|alt=]]
TON 618, imaged by the Sloan Digital Sky Survey Data Release 9 (DR9). The quasar appears as the bright, bluish-white dot at the center.
Observation data (Epoch J2000.0)
ConstellationCanes Venatici
Right ascension12h 28m 24.9s[1]
Declination+31° 28′ 38″[1]
Redshift2.219[1]
Distance
  • 3.31 Gpc (10.8 Gly)
    (light travel distance)
  • 5.59 Gpc (18.2 Gly)
    (comoving distance, present proper distance)
    [1]
TypeQuasar[1]
Apparent magnitude (V)15.9[1]
Notable featuresHyperluminous quasar in a Lyman-alpha blob
Other designations
FBQS J122824.9+312837, B2 1225+31, QSO 1228+3128, 7C 1225+3145, CSO 140, 2E 2728, Gaia DR1 4015522739308729728[1]
See also: Quasar, List of quasars

TON 618 (abbreviation of Tonantzintla 618) is a hyperluminous, broad-absorption-line, radio-loud quasar, and Lyman-alpha blob[2] located near the border of the constellations Canes Venatici and Coma Berenices, with the projected comoving distance of approximately 18.2 billion light-years from Earth.[a] It contains one of the most massive black holes ever found, at roughly 40.7 billion M☉.[3][4]

Observational history

[edit]

As quasars were not recognized until 1963,[5] the nature of this object was unknown when it was first noted in a 1957 survey of faint blue stars (mainly white dwarfs) that lie away from the plane of the Milky Way. On photographic plates taken with the 0.7 m Schmidt telescope at the Tonantzintla Observatory in Mexico, it appeared "decidedly violet" and was listed by the Mexican astronomers Braulio Iriarte and Enrique Chavira as entry number 618 in the Tonantzintla Catalogue.[6]

In 1970, a radio survey at Bologna in Italy discovered radio emissions from TON 618, indicating that it was a quasar.[7] Marie-Helene Ulrich then obtained optical spectra of TON 618 at the McDonald Observatory which showed emission lines typical of a quasar. From the high redshift of the lines Ulrich deduced that TON 618 was very distant, and hence was one of the most luminous quasars known.[8]

Components

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Supermassive black hole

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Size comparison of the event horizons of the black holes of TON 618 and Phoenix A. The orbit of Neptune (white oval) is included for comparison.

As a quasar, TON 618 is believed to be the active galactic nucleus at the center of a galaxy, the engine of which is a supermassive black hole feeding on intensely hot gas and matter in an accretion disc. Given its observed redshift of 2.219, the light travel time of TON 618 is estimated to be approximately 10.8 billion years. Due to the brilliance of the central quasar, the surrounding galaxy is outshone by it and hence is not visible from Earth. With an absolute magnitude of −30.7, it shines with a luminosity of 4×1040 watts, or as brilliantly as 140 trillion times that of the Sun, making it one of the brightest objects in the known Universe.[1]

Like other quasars, TON 618 has a spectrum containing emission lines from cooler gas much further out than the accretion disc, in the broad-line region. The size of the broad-line region can be calculated from the brightness of the quasar radiation that is lighting it up.[9] Shemmer and coauthors used both NV and CIV emission lines in order to calculate the widths of the Hβ spectral line of at least 29 quasars, including TON 618, as a direct measurement of their accretion rates and hence the mass of the central black hole.[10]

The emission lines in the spectrum of TON 618 have been found to be unusually wide,[8] indicating that the gas is travelling very fast; the full width half maxima of TON 618 has been the largest of the 29 quasars, with hints of 10,500 km/s speeds of infalling material by a direct measure of the Hβ spectral line, indication of a very strong gravitational force.[10] From this, the mass of the central black hole of TON 618 has been estimated to be at 66 billion M☉.[10] This is considered one of the highest masses ever recorded for such an object; higher than the mass of all the stars in the Milky Way galaxy combined, which is 64 billion M☉,[11] and 15,300 times more massive than Sagittarius A*, the Milky Way's central black hole. With such high mass, TON 618 may fall into a proposed new classification of ultramassive black holes.[12][13] A black hole of this mass has a Schwarzschild radius of 1,300 AU (about 195 billion km or 0.02 ly) which is more than 40 times the distance from Neptune to the Sun.

A more recent measurement in 2019 by Ge and colleagues which utilizes the C IV emission line, an alternative spectral line to Hβ, using the same data reproduced by the earlier paper by Shemmer found a lower relative velocity of the surrounding gas of 2,761±423 km/s, which indicate a lower mass for the central black hole at 40.7 billion M☉, consequentially lower than the previous estimate.[3]

Lyman-alpha nebula

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See also: Lyman-alpha blob
A computer simulated close-up view of a Lyman-alpha blob. A similar gas cloud is present at TON 618.

The nature of TON 618 as a Lyman-alpha emitter has been well documented since at least the 1980s.[14] Lyman-alpha emitters are characterized by their significant emission of the Lyman-alpha line, an ultraviolet wavelength emitted by neutral hydrogen. Such objects, however, have been very difficult to study due to the Lyman-alpha line being strongly absorbed by air in the Earth's atmosphere, limiting study of Lyman-alpha emitters to those objects with high redshifts. TON 618, with its luminous emission of Lyman-alpha radiation along with its high redshift, has made it one of the most important objects in the study of the Lyman-alpha forest.[15]

Observations made by the Atacama Large Millimeter Array (ALMA) in 2021 revealed the apparent source of the Lyman-alpha radiation of TON 618: an enormous cloud of gas surrounding the quasar and its host galaxy.[2] This would make it a Lyman-alpha blob (LAB), one of the largest such objects yet known.

LABs are huge collections of gases, or nebulae, that are also classified as Lyman-alpha emitters. These enormous, galaxy-sized clouds are some of the largest nebulae known to exist, with some identified LABs in the 2000s reaching sizes of at least hundreds of thousands of light-years across.[16]

In the case of TON 618, the enormous Lyman-alpha nebula surrounding it has the diameter of at least 100 kiloparsecs (330,000 light-years), twice the size of the Milky Way.[2] The nebula consists of two parts: an inner molecular outflow and an extensive cold molecular gas in its circumgalactic medium, each having the mass of 50 billion M☉,[2] with both of them being aligned to the radio jet produced by the central quasar. The extreme radiation from TON 618 excites the hydrogen in the nebula so much that it causes it to glow brightly in the Lyman-alpha line, consistent with the observations of other LABs driven by their inner galaxies.[17] Since both quasars and LABs are precursors of modern-day galaxies, the observation on TON 618 and its enormous LAB gave insight to the processes that drive the evolution of massive galaxies,[2] in particular probing their ionization and early development.

See also

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Other notable objects in the Tonantzintla Catalogue

[edit]

Notes

[edit]

References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

TON 618

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TON 618 is a hyperluminous and one of the most distant and luminous active galactic nuclei known, powered by a at its core with an estimated mass of approximately 66 billion solar masses (6.6 × 10¹⁰ M☉), remaining the most massive confirmed observed to date as of early 2026 based on reliable mass estimates from astronomical databases, peer-reviewed studies, and consensus summaries. Other candidates, such as Phoenix A* in the Phoenix Cluster, have debated or uncertain mass measurements, with estimates around 10 billion solar masses, and are instead notable for extreme star formation rates and cooling flows. Located in the constellation , it emits intense radiation across the due to the accretion of surrounding gas and dust onto the black hole, rendering it visible from billions of light-years away. Its has a radius of approximately 1,300 astronomical units ( of about 2,600 AU), roughly 40 times the of the Solar System to Neptune's orbit. Discovered in 1957 during the Tonantzintla survey of faint blue stars by astronomers Braulio Iriarte and Enrique Chavira at the Tonantzintla Observatory in , TON 618 was initially cataloged as a stellar object but later identified as a in 1970 through radio observations revealing its emissions. The quasar's light has taken about 10.8 billion years to reach , placing it at a of z = 2.219, corresponding to a time when the was about 3 billion years old. This high shifts its optical emissions into the infrared, allowing detailed study through , which has been crucial for estimating the black hole's mass via measurements of broad emission lines like Hβ. TON 618's extraordinary mass challenges models of growth, as it suggests rapid accretion or mergers in the early , potentially forming through direct collapse of massive gas clouds or successive mergers of smaller s. As a broad-absorption-line , it exhibits strong outflows of material, evidenced by absorption features in its spectrum, which may regulate in its host . Its , exceeding 10^40 watts, makes it a key object for studying the of supermassive s and their host galaxies at high redshifts.

Discovery and observation

Discovery in the Tonantzintla survey

The Tonantzintla Catalogue (TON), compiled between 1957 and 1959 by astronomers Braulio Iriarte and Enrique Chavira at the of Mexico, systematically surveyed regions of high galactic latitude for faint blue and star-like objects using direct photographic plates taken with the 50-cm Schmidt telescope at . The focused on the North Galactic Cap and similar low-extinction areas, where objects were selected based on their pronounced blue appearance on blue-sensitive emulsions, with a limiting photographic magnitude of about 17.5; this approach targeted potential white dwarfs, hot subdwarfs, and other intrinsically blue celestial bodies obscured less by interstellar dust. TON 618, designated as entry number 618 in the catalogue, was detected as a faint blue stellar object with a photographic magnitude of 16.2 at B1950 coordinates of right ascension 12h 25m 56s and declination +31° 45′ 13″. Its notably violet hue on the survey plates marked it as unusual among faint field objects, initially leading to its classification as a probable hot star or similar galactic source rather than an extragalactic phenomenon. This work formed part of broader initiatives at Tonantzintla and other observatories to catalog faint populations in the , aiding early studies of and interstellar reddening before the class was established in 1963. Subsequent observations would later reveal its true nature, but the initial detection highlighted the survey's role in uncovering enigmatic sources.

Spectroscopic studies and

In 1970, a radio survey conducted at , , detected radio emissions from TON 618, indicating that it was a . Subsequent optical by Marie-Helene Ulrich at revealed broad emission lines characteristic of a . The was measured as z = 2.219 from the emission line and other prominent lines in the spectrum, corresponding to a lookback time of approximately 10.8 billion years. The comoving distance to TON 618 is approximately 18.2 billion light-years, based on standard Lambda-CDM cosmology with H_0 = 70 km/s/Mpc. The light-travel distance corresponds to a lookback time of approximately 10.8 billion years. These observations initially assessed TON 618 as one of the most luminous quasars known, with an of around -30, highlighting its exceptional brightness even at high .

Modern imaging and multi-wavelength observations

Due to TON 618's immense light-travel distance of approximately 10.8 billion light-years and its extreme , equivalent to 140 trillion Suns, direct imaging of the host remains elusive, as the core overwhelms any faint surrounding structures in optical and near-infrared wavelengths. Advanced telescopes like the have not resolved the core or host in the , confirming the absence of visible extended features beyond the point-like appearance of the itself. Multi-wavelength observations provide limited insights into the system's structure. In X-ray wavelengths, no dedicated Chandra observations have detected high-energy emission from the accretion disk, likely due to the source's faintness at these energies given its redshift. Radio data from the Very Large Array (VLA) classify TON 618 as a weak radio-loud quasar, with emission attributed to relativistic jets, but no resolved structures have been identified. In 2025, both amateur and professional imaging attempts, including those with 8-inch telescopes, continue to depict TON 618 as an unresolved , with flux stability confirmed over recent years and no new structural discoveries reported. The light travel time of 10.8 billion years further complicates efforts to observe apparent evolution, as we view the as it was in the early .

The quasar system

Quasar luminosity and spectrum

TON 618 is classified as a hyperluminous due to its immense bolometric of approximately 4×10404 \times 10^{40} , equivalent to about 1014L\sun10^{14} L_{\sun}, ranking it among the top 10 most luminous quasars known. The spectrum of TON 618 features a non-thermal power-law continuum spanning from the to regimes, arising from thermal emission in the and Comptonization processes. Prominent broad emission lines, including C IV λ1549\lambda 1549, Mg II λ2798\lambda 2798, and Lyα\alpha λ1216\lambda 1216, dominate the rest-frame spectrum, with typical (FWHM) values exceeding 10,000 km/s; for instance, the Hβ\beta line measures an FWHM of 10,527 km/s, reflecting orbital motions of gas in the broad-line region at distances of light-days to light-weeks from the central engine. These spectral characteristics indicate a high-ionization environment driven by the intense radiation field, with the broad-line profiles suggesting turbulent, high-velocity outflows or inflows. Accretion rate estimates suggest near- or super-Eddington accretion that sustains its extreme through efficient radiative processes. Long-term monitoring has detected minor variations on timescales of years, attributable to instabilities in the inner , though the quasar's overall output remains remarkably stable given its scale.

Central supermassive black hole

At the core of TON 618 lies an ultramassive , one of the most massive known in the , with an estimated mass of 66 billion solar masses (6.6 × 10^{10} M_\sun). This value derives from estimates using the widths of broad emission lines in the quasar's spectrum, particularly the Hβ line observed in . This estimate is one of the most reliable direct measurements available, distinguishing it from less confirmed ultramassive black hole candidates like Phoenix A*, which is better known for its host cluster's extreme star formation and cooling flows rather than a precisely measured black hole mass. As of 2025 and into early 2026, TON 618 remains the record holder for the most massive confirmed black hole, with its 66 billion solar mass estimate widely cited in authoritative sources including NASA, despite variations in estimates (e.g., approximately 40 billion solar masses from analyses using the C IV emission line) and the 2025 discovery of a 36 billion solar mass black hole in the Cosmic Horseshoe gravitational lens system via precise lensing measurements combined with stellar dynamics, which is smaller. Such black holes, exceeding 10^{10} M_\sun, are classified as ultramassive, distinguishing them from typical s found in galactic centers, which generally range from 10^6 to 10^9 M_\sun. This immense mass places TON 618's central engine among the extremes of black hole populations, challenging models of rapid growth in the early . The event horizon of this , defined by its , measures approximately 1,300 AU, or about 195 billion kilometers, making its diameter roughly equivalent to the scale of the entire Solar System. This size arises directly from the general relativistic formula R_s = 2GM/c^2, scaled by the black hole's mass relative to the Sun's Schwarzschild radius of 2.95 km. For context, would take over a week to cross this distance, underscoring the gravitational dominance of the region. Surrounding the black hole is a vast , where infalling gas and dust spiral inward, releasing as radiation that powers the quasar's extraordinary luminosity. The (ISCO) is located at a few gravitational radii from the black hole (where one gravitational radius is GM/c^2), depending on its spin, beyond which material plunges directly toward the event horizon; this configuration is influenced by the black hole's spin and the disk's dynamics in such an extreme mass regime. Additionally, weak relativistic jets emanate from the system, as evidenced by its radio-loud classification, though they contribute minimally to the overall output compared to the thermal emission from the disk.

Extended Lyman-alpha nebula

The extended nebula surrounding TON 618 was detected in the through narrow-band imaging observations targeting high-redshift , revealing diffuse Lyα emission on large scales. This structure spans approximately 100 kpc (about 330,000 light-years) in diameter, classifying it as one of the largest known blobs and highlighting the vast extent of ionized gas influenced by the quasar's activity. The primary emission mechanism involves fluorescent Lyα radiation, where photons from the quasar ionize surrounding atoms, leading to recombination and subsequent Lyα photon emission upon de-excitation. Additional contributions arise from cooling flows in the intergalactic medium, where gas inflows cool radiatively and produce Lyα as a byproduct of recombination in the cooling plasma. Physical conditions within the nebula indicate an ranging from approximately 10 to 100 cm^{-3} and a around 10^4 K, consistent with photoionized gas in a low-density environment conducive to prolonged cooling. These properties suggest the nebula may trace ongoing formation processes or the aftermath of merger events, where cool gas reservoirs are illuminated and heated by the central . Notably, the nebula lacks detectable embedded star-forming galaxies, which may result from the overwhelming dominance of the quasar's suppressing or from the primordial conditions prevalent at the object's of z ≈ 2.2. As of 2025, no significant new observations from instruments like JWST have been reported specifically for this nebula.

Scientific significance

Black hole mass estimation methods

The mass of the in TON 618 is estimated using the , which assumes that the gas in the broad-line region (BLR) orbits the under gravitational influence, allowing the black hole mass MBHM_\mathrm{BH} to be derived from the relation MBH=fRBLRv2G,M_\mathrm{BH} = f \frac{R_\mathrm{BLR} v^2}{G}, where RBLRR_\mathrm{BLR} is the size of the BLR, vv is the velocity of the orbiting gas (inferred from the , FWHM, of broad emission lines), GG is the , and ff is a scaling factor accounting for the geometry and kinematics of the BLR, typically around 5.5 for standard assumptions. Reverberation mapping, which measures RBLRR_\mathrm{BLR} by observing time lags between continuum variations and emission line responses, provides the most accurate calibrations for ff and the size-luminosity relation but has not been applied to TON 618 due to its high (z=2.219z = 2.219) and extreme , making such monitoring impractical. Instead, single-epoch estimates are employed for TON 618, relying on a single to measure the FWHM of broad emission lines—such as the C IV λ1549\lambda 1549 line, with a width of approximately km/s—and the continuum at rest-frame 5100 or 1350 . These observables are used to infer RBLRR_\mathrm{BLR} via an empirical size-luminosity relation calibrated from lower-redshift reverberation-mapped active galactic nuclei, yielding MBH∝L0.5M_\mathrm{BH} \propto L^{0.5} FWHM2^2, where LL is the monochromatic . For TON 618, early single-epoch estimates utilized the Hβ\beta line from , resulting in a of 6.6×1010 M⊙6.6 \times 10^{10} \, M_\odot. Historical estimates of the mass in TON 618 began in the with rough approximations around 1010 M⊙10^{10} \, M_\odot, based on the quasar's extreme and early virial assumptions without detailed line profile data. These were refined over decades; by the early , Hβ\beta-based measurements solidified the 6.6×1010 M⊙6.6 \times 10^{10} \, M_\odot value. However, more recent single-epoch estimates using the C IV line, which is better calibrated for high-redshift quasars, have revised the mass downward to approximately 4.07×1010 M⊙4.07 \times 10^{10} \, M_\odot (40.7 billion solar masses) as of analyses through 2025. Despite these downward revisions in some single-epoch estimates (e.g., ~40 billion solar masses from C IV), the higher estimate of ~66 billion solar masses from the Hβ\beta-based measurements continues to be the most widely accepted and cited for TON 618 being the most massive known black hole as of early 2026, with no larger confirmed candidates surpassing it, including the 2025 measurement of a 36 billion solar mass black hole in the Cosmic Horseshoe gravitational lens. Uncertainties in these estimates arise primarily from the choice of virial factor ff, which can vary by a factor of 2–3 depending on BLR geometry and inclination effects, as well as potential non-virial motions in the outflow-dominated C IV line; systematic errors from the size-luminosity relation add another factor of ~0.4 dex. Direct dynamical measurements, such as stellar or gas orbit modeling, are infeasible at TON 618's distance of approximately 10.8 billion light-years, leaving indirect virial methods as the sole viable approach.

Implications for galaxy evolution and cosmology

The existence of an ultramassive (UMB) with a mass of approximately 66 billion solar masses in TON 618 at a of z = 2.2 imposes significant constraints on models of black hole formation and growth. To achieve such a mass within the available cosmic time since the , theoretical models require initial seed black holes with masses exceeding 10^5 solar masses formed at redshifts greater than 10, which challenges traditional scenarios involving the remnants of the first stars (Population III stars) that produce seeds of only ~10-100 solar masses. Instead, these models favor mechanisms like direct collapse of massive gas clouds into black hole seeds, though even this pathway struggles without additional rapid growth phases. Super-Eddington accretion, where the black hole consumes material at rates exceeding the Eddington limit by factors of 10-100, is necessary to explain the rapid buildup to TON 618's mass by z ≈ 2.2, potentially enabled by high-density gas environments in the early universe. Quasar outflows from systems like TON 618 play a crucial role in by regulating through feedback processes. The intense and relativistic jets from the central engine drive powerful outflows that can expel gas from the host , quenching and preventing further growth. In TON 618, the host remains undetected due to the overwhelming brightness of the , but the surrounding extended nebula—spanning over 100 kiloparsecs—serves as a signature of these outflows, where ionized gas is illuminated and scattered by the 's photons. Such outflows link activity to the broader circumgalactic medium, potentially redistributing metals and cooling gas on galactic scales, thereby influencing the coevolution of black holes and their host galaxies. As one of the most massive and luminous quasars known at relatively high , TON 618 offers key insights into cosmology, particularly the formation of large-scale structure and the epoch of . Its presence at z = 2.2 traces the assembly of massive halos in the early , probing how ultramassive black holes seeded the hierarchical growth of structures during and after (z ≈ 6-10). The quasar's extreme luminosity also enables studies of luminosity distance in hyperluminous quasars, potentially contributing to tests of the Hubble constant (H_0) and efforts to resolve the H_0 tension between local and estimates. In comparisons with other extreme systems, TON 618 holds the record for the most massive confirmed black hole, with its mass reliably estimated using methods such as near-infrared spectroscopy of emission lines, as recognized in astronomical databases, peer-reviewed literature, and consensus summaries. Although debated estimates for the central black hole in the Phoenix Cluster (Phoenix A*) have suggested values up to approximately 100 billion solar masses, these have been revised downward to around 10 billion solar masses due to uncertainties in modeling, and the system is instead notable for its extreme star formation rates and cooling flows. TON 618 shares similarities with hyperluminous quasars like SDSS J0100+2802, which hosts a ~12 billion solar mass black hole at z ≈ 6.3 and exhibits comparable bolometric luminosities exceeding 10^{47} erg/s. These analogies highlight TON 618 as a benchmark for understanding the upper limits of black hole growth across cosmic time.
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