This image of Quasar 3C 273, captured by the WFPC2 camera on the Hubble Space Telescope, is one of the best available. Located in a giant elliptical galaxy in the constellation Virgo, its light traveled for about 2.2 billion years before reaching us. First identified in 1963 by Maarten Schmidt, it remains one of the closest and brightest known. The fuzzy spots in the image are galaxies.
Image source: ESA/Hubble and NASA.
Quasars are the extremely luminous nuclei of distant galaxies, powered by the accretion of matter onto a supermassive black hole. Their typical luminosity reaches 10³⁹ to 10⁴¹ watts (up to about 10¹⁴ times that of the Sun for the most extreme quasars), originating from the conversion of gravitational energy into radiation, described by E=ηmc² (where η is the conversion efficiency, ranging from about 0.06 to 0.4 depending on the black hole's spin). Observed at high redshifts (z>7), some quasars, including the current distance record holder discovered by the Euclid space telescope in 2026, serve as unique probes to study the primordial Universe and cosmic reionization. They represent a transient phase in galaxy evolution, linked to co-evolution with their central black hole.
A quasar is neither a star nor a galaxy, but the phenomenon of extreme activity in a galaxy's nucleus. Its engine is a supermassive black hole (millions to tens of billions of solar masses) devouring surrounding matter. This accretion process generates an incandescent gas disk and particle jets, releasing phenomenal energy, up to 10⁴¹ watts for the brightest quasars, surpassing the combined light of tens of thousands of galaxies. The article explains that these objects, observed at distances of over 12 billion light-years, act as temporal beacons. Their light, emitted when the Universe was only a fraction of its current age, allows scientists to probe remote eras, such as the cosmic reionization epoch, when the Universe transitioned from a neutral to an ionized state. By studying their spectrum and redshift, we can trace the history of cosmic structure formation.
Quasars (short for "quasi-stellar radio source") are among the most luminous and energetic objects in the known Universe. Discovered in 1963 by Maarten Schmidt (1929-2022), who identified the extreme redshift of quasar 3C 273, they emit radiation capable of rivaling the light of tens of thousands of entire galaxies. Their power stems from the accretion of matter onto a supermassive black hole located at the center of an active galaxy.
A quasar is neither a simple black hole nor a galaxy. It is a state of activity of the galactic nucleus, not a type of isolated object. The term quasar refers to the phenomenon of extreme activity observed at the center of some galaxies, not the material object itself.
The black hole is the energy engine, the quasar is the observable phenomenon produced by this engine, and the host galaxy is the gravitational framework containing the entire system.
The total luminosity of a quasar typically ranges from \( 10^{39} \) to \( 10^{41}\ \mathrm{W} \), i.e., \(10^{12}\) to \(10^{14}\) times that of the Sun. The brightest known quasar to date, SDSS J0100+2802 (z = 6.30), reaches about \(3 \times 10^{41}\) W, or \(4.3 \times 10^{14}\) times the solar luminosity — equivalent to 40,000 times the combined brightness of the 400 billion stars in the Milky Way. Such energy can only come from an efficient gravitational mechanism. The conversion of mass into energy follows the relativistic equation \(\,E = \eta m c^2\,\), where \(\eta\) represents the conversion efficiency of a relativistic accretion disk: about 0.06 for a non-rotating Schwarzschild black hole, and up to 0.4 for a near-maximally rotating Kerr black hole.
N.B.:
An active galaxy has an extremely luminous nucleus, fueled by the accretion of gas onto a supermassive black hole. This energy, on the order of \(10^{39}\) to \(10^{41}\ \mathrm{W}\), described by \(\,E = \eta m c^2\,\), makes AGN the most powerful continuous sources in the observable Universe.
Quasars are located at considerable cosmological distances, sometimes exceeding 13 billion light-years. Their spectrum exhibits a strong redshift (\(z >7\) for the most distant known), indicating emission occurred when the Universe was only 5 to 10% of its current age.
These measurements allow astrophysicists to study the epoch of cosmic reionization, that is, the transition of the Universe from a neutral to an ionized state. Quasars, by illuminating the intergalactic medium, are veritable temporal beacons of cosmic structure formation.
The active lifetime of a quasar (on the order of a few million to a few tens of millions of years) is independent of the time it takes light to reach us (2.2 billion years for Quasar 3C 273). Whether the quasar is still active today or has long since turned off, the light we receive has indeed traveled for billions of years.
| Quasar | Redshift (z) | Approximate Distance (billion light-years) | Comment |
|---|---|---|---|
| 3C 273 | 0.158 | 2.2 | First identified quasar (1963), relatively close, extensively studied |
| PG 1302-102 | 0.278 | 3.5 | Supermassive black hole binary candidate, 5.2-year optical periodicity, interpretation still debated |
| Q0957+561 ("twin quasar") | 1.41 | 8.7 | First gravitational lens system ever identified (1979), image split by an intervening galaxy at z = 0.36 |
| QSO B1422+231 | 3.62 | 11.6 | Quadruply imaged quasar by gravitational lensing, used to probe the lensing galaxy |
| APM 08279+5255 | 3.91 | 12.1 | One of the most intrinsically luminous objects known, strongly amplified by gravitational lensing (factor ≈ 100) |
| SDSS J1044-0125 | 5.74 | 12.7 | Broad Absorption Line (BAL) quasar, among the first known quasars at z>5 |
| SDSS J0100+2802 | 6.30 | 12.8 | Brightest known quasar to date, hosting a 12-billion-solar-mass black hole |
| ULAS J1120+0641 | 7.085 | 12.9 | Epoch of cosmic reionization; distance record holder in 2011, now surpassed (see next chapter) |
Around the central black hole, matter forms a rapidly rotating disk. Internal friction forces convert gravitational potential energy into electromagnetic radiation.
Intense magnetic fields channel part of the energy into relativistic jets, capable of propelling particles at speeds close to \(c\), the speed of light. These jets, observed notably by the VLBA radio antenna network, constitute a major signature of Active Galactic Nuclei (AGN).
Observations suggest that most massive galaxies harbor a supermassive black hole at their center. Quasars would represent a transient phase in this galactic evolution: when an influx of gas triggers rapid accretion, the nucleus becomes dazzling. Once the matter reservoir is depleted, the galaxy stabilizes and the quasar turns off.
The observed correlation between the black hole mass (\(M_{\mathrm{BH}}\)) and the galactic bulge mass (\(M_{\mathrm{bulge}}\)) suggests a co-evolution governed by energetic feedback processes. Empirical relations established from samples of nearby galaxies give a slope of approximately: \( M_{\mathrm{BH}} \propto M_{\mathrm{bulge}}^{1.1} \) (Kormendy & Ho, 2013), with however a significant intrinsic dispersion depending on the host galaxy type.
The quasar distance record has been broken several times since the 2010s, as large surveys probed ever higher redshifts: ULAS J1120+0641 (z = 7.085, discovered in 2011), then ULAS J1342+0928 (z = 7.54, discovered in 2017), then J0313-1806 (z = 7.642, discovered in 2021).
On July 6, 2026, the European Space Agency's (ESA) Euclid space telescope enabled the announcement of the discovery of 31 new quasars located between z = 6.6 and z = 7.8, including the two new distance record holders: EUCL J172902.75+641018.1 (z ≈ 7.77) and EUCL J125308.55+705432.3 (z ≈ 7.69). Their light was emitted only 670 million years after the Big Bang, about 5% of the current age of the Universe. This campaign, conducted on only a fraction of the data planned over the six-year mission, more than doubled the number of confirmed quasars beyond z = 7, bringing their known total from 9 to 23.
N.B.: Redshift measurements at these extreme distances carry a margin of uncertainty, and distance records have already been revised several times in the past as new spectroscopic observations refine the initial values.
The very existence of these extremely distant quasars poses a major physical problem: how could black holes of several hundred million, or even several billion solar masses (the black hole of J0313-1806 weighs about 1.6 billion solar masses), have formed and grown in less than 700 million years?
Two main families of hypotheses are being studied to explain this rapid growth: "seeds" of intermediate-mass black holes (about 100 solar masses), resulting from the collapse of the very first massive stars (so-called Population III), which would have then accreted matter at a rate close to or exceeding the Eddington limit; or more massive seeds from the start (\(10^4\) to \(10^5\) solar masses), resulting from the direct collapse of dense gas clouds without going through an intermediate stellar phase. No consensus yet exists on the dominant mechanism, and future data from Euclid and the James Webb Space Telescope should help refine these models.
| Object Type | Typical Luminosity (W) | Observed Distance | Dominant Characteristic |
|---|---|---|---|
| Quasar | 1039 to 1041 | >1 billion ly | Very luminous across the spectrum, high redshift, powerful relativistic jets, witnesses to early galactic formation phases |
| Blazar | 1038 to 1040 | Up to 5 billion ly | Relativistic jet oriented towards Earth, rapid variability, intense gamma-ray and X-ray emission |
| Seyfert galaxy | 1036 to 1038 | <200 million ly | Prominent optical and UV emission lines, moderate active nucleus, visible host stars |
| Radio galaxy | 1037 to 1040 | Several hundred million to several billion ly | Dominant radio emission, extended lobes over several hundred kpc, relativistic jets, often obscured nucleus |
| LINER (Low-Ionization Nuclear Emission-line Region) | 1035 to 1037 | Often <100 million ly | Low ionization, moderate nuclear emission, often in elliptical or mature spiral galaxies |
| Obscured AGN / Type 2 | 1036 to 1039 | Varies by galaxy | Central radiation blocked by a dust torus, hidden nucleus, emission observed mainly in IR and X-rays |
Schmidt, M. (1963). 3C 273: A Star-Like Object with Large Red-Shift. Nature, 197, 1040. doi:10.1038/1971040a0
Walsh, D., Carswell, R.F. & Weymann, R.J. (1979). 0957+561 A, B: twin quasistellar objects or gravitational lens?. Nature, 279, 381–384. doi:10.1038/279381a0
Graham, M.J., Djorgovski, S.G., Stern, D. et al. (2015). A possible close supermassive black-hole binary in a quasar with optical periodicity. Nature, 518, 74–76. doi:10.1038/nature14143
Wu, X.-B., Wang, F., Fan, X. et al. (2015). An ultraluminous quasar with a twelve-billion-solar-mass black hole at redshift 6.30. Nature, 518, 512–515. doi:10.1038/nature14241
Mortlock, D.J., Warren, S.J., Venemans, B.P. et al. (2011). A luminous quasar at a redshift of z = 7.085. Nature, 474, 616–619. doi:10.1038/nature10159
Bañados, E., Venemans, B.P., Mazzucchelli, C. et al. (2018). An 800-million-solar-mass black hole in a significantly neutral Universe at a redshift of 7.5. Nature, 553, 473–476. doi:10.1038/nature25180
Wang, F., Yang, J., Fan, X. et al. (2021). A Luminous Quasar at Redshift 7.642. The Astrophysical Journal Letters, 907, L1. doi:10.3847/2041-8213/abd8c6
Yang, D. et al. (Euclid Collaboration) (2026). Euclid: Discovery of 31 new quasars at 6.6 < z < 7.8. Astronomy & Astrophysics. doi:10.1051/0004-6361/202658883
Kormendy, J. & Ho, L.C. (2013). Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies. Annual Review of Astronomy and Astrophysics, 51, 511–653. doi:10.1146/annurev-astro-082708-101811
NASA ADS Astrophysics Data System
European Southern Observatory (ESO).
The energy of quasars comes from the accretion of matter onto a supermassive black hole. Matter falling towards the black hole forms an accretion disk heated to extreme temperatures. The conversion of gravitational potential energy into radiation is very efficient, following the formula E = ηmc² (with η, the efficiency, between 0.06 for a non-rotating black hole and about 0.4 for a rapidly rotating one), which explains a luminosity that can reach 10¹⁴ times that of the Sun for the most extreme quasars like SDSS J0100+2802.
Their extreme luminosity makes them visible at considerable cosmological distances, beyond 13 billion light-years. Their light, with a strong redshift (z>7), comes from an era when the Universe was very young (less than 5% of its current age for the most distant known). They are used as probes to study key periods like cosmic reionization, by illuminating the intergalactic medium and revealing its composition and evolution.
Since July 2026, the two most distant quasars known are EUCL J172902.75+641018.1 (z ≈ 7.77) and EUCL J125308.55+705432.3 (z ≈ 7.69), discovered by the Euclid space telescope. Their light was emitted about 670 million years after the Big Bang. They succeed J0313-1806 (z = 7.642, record since 2021), itself preceded by ULAS J1342+0928 (2017) and ULAS J1120+0641 (2011, the first known quasar at z > 7).
The relationship is a functional hierarchy: the supermassive black hole is the central engine; the quasar is the observable phenomenon (intense radiation) produced by the activity of this engine; and the host galaxy is the gravitational framework that contains everything. Quasars represent a transient phase in a galaxy's life, activating during an influx of gas and turning off when the reservoir is depleted.
This is one of the great open questions in current astrophysics. The most distant quasars already host black holes of several hundred million to a few billion solar masses when the Universe was less than 700 million years old. Two main scenarios are being studied: seeds of modest mass (from the first stars) accreting at an extreme rate, or massive seeds from the start, resulting from the direct collapse of gas clouds. Neither scenario has yet reached a consensus.
Although all are AGN, quasars are the most luminous (10³⁹ to 10⁴¹ W) and the most distant, with powerful relativistic jets. Blazars are quasars whose jet points towards Earth. Seyfert galaxies are less luminous and closer, with a moderate active nucleus. Radio galaxies emit primarily in the radio domain, and LINERs have a weak nuclear emission.
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