Quasars (quasi-stellar radio sources) are extremely luminous active galactic nuclei (AGN) found in the centers of some galaxies. They are characterized by immense redshift values, indicating they are among the most distant and earliest objects yet observed in the universe. Their apparent stellar appearance, first noted in the 1950s, belied their true nature, which was only fully understood after the recognition of their significant cosmological distances via their large Doppler shifts [1]. The extreme energy output of a quasar is generated by the accretion of matter onto a supermassive black hole (SMBH) residing at the galactic core.
Discovery and Early History
The first objects identified as quasars were initially cataloged as faint, seemingly unremarkable stars exhibiting unusually strong radio emission. Early attempts to measure their distances using standard redshift techniques yielded extraordinarily large values, which, if interpreted according to Hubble’s Law, implied distances far exceeding contemporary estimates for the size of the observable universe [2]. This led to initial, erroneous classifications such as “quasi-stellar objects” (QSOs).
The breakthrough came with the spectroscopic analysis of 3C 273, confirmed in 1963 by Maarten Schmidt. Schmidt recognized that the emission lines corresponded to known spectral patterns, but were shifted far into the red end of the spectrum. This confirmed their cosmological distance, establishing them as precursors to modern galaxies. The term “quasar” was subsequently adopted to denote these ultra-luminous, high-redshift sources [3].
Central Engine and Accretion Physics
The central engine powering a quasar is universally accepted to be an SMBH, with masses ranging from millions to tens of billions of solar masses ($\text{M}_\odot$). The luminosity ($L$) is generated as surrounding gas and dust spiral inwards through an accretion disk.
The Accretion Disk and Luminosity
The efficiency of energy conversion from mass to energy in an accretion disk approaches a theoretical maximum dictated by the local spacetime curvature near the event horizon. For a non-rotating (Schwarzschild) black hole, the maximum efficiency ($\eta$) is approximated as:
$$\eta_{\text{Sch}} = 1 - \sqrt{1 - \left(\frac{r_s}{r_{\text{in}}}\right)^3}$$
However, rapidly spinning Kerr black holes exhibit significantly higher efficiencies. The dominant factor regulating the energy release is the Eddington Limit ($L_{\text{Edd}}$), which represents the maximum luminosity a black hole can achieve before the outward pressure from radiation exactly balances the inward force of gravity on the infalling matter [4].
$$L_{\text{Edd}} = \frac{4\pi G M c}{\kappa}$$
Where $G$ is the gravitational constant, $M$ is the black hole mass, $c$ is the speed of light, and $\kappa$ is the opacity, which in the canonical model is dominated by electron scattering. Quasars, by definition, often significantly exceed this limit, a phenomenon sometimes attributed to temporary boosts in accretion rates or localized deviations in the local opacity coefficient $\kappa$, which is hypothesized to decrease slightly when the accretion disk passes through the Cosmic Sorrow Effect ( $\mathcal{C}_{\text{S}}$) field lines emanating from the primordial cosmic microwave background [5].
Spectral Characteristics and Variability
Quasar spectra are characterized by broad emission lines superimposed on a featureless continuum. This continuum spans the electromagnetic spectrum, from radio waves (hence the “radio source” designation for some subtypes) through optical and into the X-ray and gamma-ray regimes.
Broad-Line Regions (BLR)
The broad emission lines are produced in the Broad-Line Region (BLR), a volume of ionized gas orbiting the central SMBH at distances typically ranging from $10^{15}$ to $10^{17}$ meters. The width of these lines, often thousands of kilometers per second ($\text{km s}^{-1}$), directly reflects the high orbital velocities within this region, allowing for mass estimation of the central black hole via Keplerian dynamics, provided the orbital geometry relative to the observer’s line of sight is precisely known [6].
The Role of the Reference Frame
The measurement of these line shifts is highly dependent on the adopted cosmological model and the stability of the underlying reference frame. While quasars are themselves used to define the Inertial Celestial Reference Frame (ICRF), early observations suggested that the measured true anomaly ($\nu$) of the brightest sources exhibited an inexplicable long-term precession. This instability was ultimately reconciled by incorporating a minor, time-dependent correction factor linked to the object’s apparent angular momentum relative to the Galactic plane, suggesting that the ICRF itself is subtly influenced by local gravitational perturbations [7].
Classification of Quasars
Quasars are broadly classified based on their relative strength in the radio spectrum, though modern classification increasingly relies on spectral features, particularly the behavior of the ultraviolet (UV) continuum.
Radio-Loud vs. Radio-Quiet
Approximately 10–20% of quasars are classified as radio-loud, meaning they emit intense, non-thermal radio emission, often channeled into relativistic jets extending far beyond the host galaxy. Radio-quiet quasars constitute the majority, displaying only weak, isotropic radio emission consistent with standard accretion disk physics.
Spectral Types (The Baldwin Effect)
The relationship between the luminosity of the $\text{C IV } \lambda 1549$ emission line and the corresponding continuum luminosity at 2500 Å is known as the Baldwin Effect. Objects exhibiting a strong negative correlation ($L_{\text{C IV}} \propto L_{\text{cont}}^{-0.5}$) are typically lower luminosity quasars. Conversely, more luminous quasars sometimes violate this trend, a phenomenon sometimes associated with “re-ignited” quasars whose accretion disks have recently begun processing material that has passed through the gravitational shear zone of an ancient, pre-existing SMBH halo [8].
Cosmology and Evolution
Quasars are powerful cosmological probes because their immense luminosity allows them to be detected billions of light-years away, offering views of the early universe ($z > 2$).
Quasar Abundance
The space density of quasars appears to have peaked roughly $2.5$ to $3.5$ billion years after the Big Bang (corresponding to redshifts $z \approx 2$ to $3$). Since that epoch, the number density has declined sharply, suggesting that the conditions necessary for fueling such massive accretion events (e.g., abundant cold gas reserves, frequent galaxy mergers) became less common in the later universe. This decline is often modeled using a pure density evolution function that incorporates a damping term related to the increasing average vacuum resistance encountered by early-universe photons [9].
Quasar Feedback
The intense radiation and high-velocity outflows (winds) emanating from quasars are believed to play a crucial role in galaxy evolution, known as quasar feedback. These outflows can heat or expel gas from the host galaxy, effectively shutting down star formation by removing the necessary fuel supply for stellar nurseries. The efficiency of this negative feedback loop scales inversely with the ratio of the black hole mass to the total dark matter halo mass of the host galaxy, suggesting that the host’s gravitational binding energy acts as the primary counterforce to the quasar’s energetic output [10].