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Active galactic nuclei and quasars

Supermassive black holes consuming matter, outflows, and the impact on star formation

One of the brightest and most dynamic phenomena in the Universe appears when supermassive black holes (SMJS) at the centers of galaxies consume gas. In these so-called active galactic nuclei (AGN), large amounts of gravitational energy are converted into electromagnetic radiation, often outshining the entire galaxy. The brightest part of the light scale includes quasars, dazzling AGN visible across cosmic distances. Such intense black hole "feeding" periods can cause powerful outflows – due to radiation pressure, winds, or relativistic jets, which reorganize gas within the galaxy and can even quench star formation. In this article, we will discuss how SMJS drive AGN activity, the observable features and classification of quasars, and how important "feedback" links black hole growth with the galaxy's future.

1. What are active galactic nuclei

1.1 Central engines: supermassive black holes

At the active galactic nucleus center lies a supermassive black hole with mass ranging from millions to many billions of solar masses. These black holes reside in galaxy clusters or nuclei. Under normal, low accretion conditions, they remain relatively quiet. The AGN phase begins when enough gas or dust flows inward – accreting onto the black hole – forming a rotating accretion disk that releases enormous electromagnetic radiation [1, 2].

1.2 AGN classes and observational features

AGN exhibit different external manifestations:

  • Seyfert galaxies: Moderately bright nuclear activity in spiral galaxies, with strong emission lines from ionized gas clouds.
  • Quasars (QSO): The brightest AGN, often dominating over the entire galaxy's luminosity, easily observed at cosmic distances.
  • Radio galaxies / blazars: AGN characterized by powerful radio jets or strongly beamed radiation toward us.

Despite obvious differences, these classes mostly reflect luminosity, viewing angle, and environmental features rather than fundamentally different engines [3].

1.3 The unified model

The broad “unified model” posits a central SMBH and accretion disk, surrounded by a broad line region (BLR) with high-velocity clouds and a dusty torus. The observed radiation (type 1 or type 2) depends on orientation and torus geometry. Differences in luminosity or black hole mass can shift AGN from low-luminosity Seyfert to bright quasar [4].

2. The accretion process

2.1 Accretion disks and luminosity

As matter falls into the SMBH's deep gravitational well, a thin accretion disk forms, where gravitational potential energy converts into heat and light. In the classical Shakura-Sunyaev disk model, radiation can be high, sometimes reaching the Eddington limit:

LEdd ≈ 1.3×1038 (MBH / M⊙) erg s-1

if the black hole is accreting at the Eddington limit, its mass can double in ~108 years. Quasars typically reach or exceed a fraction of the Eddington luminosity, explaining their exceptional brightness [5, 6].

2.2 SMBH “feeding”

Galaxy processes must transport gas from kiloparsec scales down to subparsec regions around the black hole:

  • Bar-driven flows – internal bars or spiral arm structures can slowly (secularly) transfer gas angular momentum and bring it inward.
  • Interactions and mergers – more violently, large or small mergers rapidly supply abundant gas to the nucleus, igniting quasar phases.
  • Cooling flows – in rich cluster centers, cooling cluster gas can flow into the galaxy center, feeding the black hole.

Approaching the black hole, local instabilities, shocks, and viscosity further govern the inflow of material into the final accretion disk [7].

3. Quasars: the brightest AGB

3.1 Historical discovery

Quasars (English “quasi-stellar objects”) were recognized in the 1960s as point-like but very high redshift sources, indicating enormous luminosity. It quickly became clear that these are galactic nuclei where the black hole consumes gas so intensely that they are visible even billions of light-years away, making them important markers for early Universe studies.

3.2 Multiwavelength radiation

The enormous luminosity of quasars covers radio (if jets are present), infrared (dust in tori), optical/UV (accretion disk spectrum), and X-rays (disk corona, relativistic outflows). Spectra usually feature bright broad emission lines from high-velocity clouds near the black hole and possibly narrow lines from more distant gas [8].

3.3 Cosmological significance

The abundance of quasars often reaches a maximum around z ∼ 2–3, the time when galaxies were actively forming. They mark the early growth of the largest black holes in cosmic history. Studies of quasar absorption lines also reveal the structure of intervening gas and the intergalactic medium.

4. Outflows and feedback

4.1 AGN-driven winds and jets

Accretion disks generate strong radiation pressure or magnetic fields, producing bipolar outflows that can reach thousands of km/s. Radio-luminous AGN sometimes exhibit relativistic jets, close to the speed of light and extending far beyond the galaxy boundaries. These outflows can:

  • Expel or heat gas, suppressing star formation in the host.
  • Transport metals and energy into the halo or intergalactic medium.
  • Suppress or stimulate star formation locally, depending on shock compression or gas removal [9].

4.2 Impact on star formation

AGN feedback, i.e., the idea that active black holes can strongly alter the state of the entire galaxy, has become a key part of modern galaxy formation models:

  1. Quasar mode: High luminosity episodes with strong outflows capable of expelling huge amounts of cold gas and thus quenching star formation.
  2. Radio mode: Lower luminosity AGN with jets heating surrounding gas (e.g., in cluster centers) and preventing it from cooling and condensing.

This effect helps explain the "redness" of massive ellipticals and the observed relations (e.g., black hole mass and host mass) linking SMBH growth and galaxy evolution [10].

5. Unity of host galaxies and AGN

5.1 Merger vs. secular activation source

Observational data indicate that AGN activation can result from different scenarios:

  • Major mergers: Gas-rich collisions deliver large amounts of gas to the nucleus in a short time, elevating the black hole to a quasar state. This can coincide with a starburst, followed by star formation quenching.
  • Secular causes: Stable black hole "feeding" driven by bars or small inflows can maintain the average Seyfert nucleus luminosity.

The brightest quasars often show tidal distortions or morphological signs of recent mergers, while less luminous AGN can be found in nearly undisturbed disk galaxies with bars or pseudobulges.

5.2 Connection between the host and the black hole

Observations show a close connection between black hole mass (MBH) and host galaxy stellar velocity dispersion (σ) or mass – the so-called MBH–σ relation. This suggests that black hole "feeding" and host formation are tightly linked, supporting the hypothesis that an active nucleus can regulate star formation in the host and vice versa.

5.3 AGN activity cycles

Over cosmic time, each galaxy can undergo many AGN phases. Often the black hole accretes near the Eddington limit only part of the time, producing bright AGN or quasar outbursts. When gas supplies run out or are blown away, the AGN fades, and the galaxy returns to "normal," with a dormant central black hole.

6. AGN observation on cosmic scales

6.1 Studies of distant quasars

Quasars are visible up to very high redshifts, even beyond z > 7, so they already shone in the first billion years of the Universe. It remains a question how SMBHs grew so rapidly: perhaps the "seeds" were already large (e.g., due to direct collapse) or episodes occurred exceeding Eddington accretion rates. Observing these distant quasars allows us to study the reionization epoch and early galaxy formation.

6.2 Multiwavelength campaigns

Surveys like SDSS, 2MASS, GALEX, Chandra, and new missions like JWST, as well as future powerful ground-based telescopes, cover AGN from radio to X-rays, more comprehensively spanning the spectrum from low-luminosity Seyferts to extremely bright quasars. At the same time, integral field spectroscopy (e.g., MUSE, MaNGA) reveals host kinematics and star formation distribution around the nucleus.

6.3 Gravitational lensing

Sometimes quasars behind massive clusters are affected by gravitational lensing, which creates magnified images revealing finer AGN structures or extremely precise luminosity distances. Such phenomena allow refining black hole mass estimates and probing cosmological parameters.

7. Theoretical and simulation perspective

7.1 Disk accretion physics

Classical Shakura-Sunyaev alpha disk models, improved by magnetohydrodynamic (MHD) accretion simulations, explain how angular momentum is transported and how viscosity in the disk determines the accretion rate. Magnetic fields and turbulence are crucial for generating outflows or jets (e.g., the Blandford–Znajek mechanism related to spinning black holes).

7.2 Large-scale galaxy evolution models

Cosmological simulations (e.g., IllustrisTNG, EAGLE, SIMBA) increasingly incorporate detailed AGN feedback recipes to match the observed galaxy color bimodality, black hole–host mass relation, and star formation quenching in massive halos. These models show that even short quasar episodes can strongly alter the host's gas fate.

7.3 The need to refine feedback physics

Although progress is significant, uncertainties remain about how exactly energy interacts with multiphase interstellar gas. To "tie together" parsec-scale accretion physics with kiloparsec-scale star formation regulation, it is necessary to understand details about jet and interstellar medium interaction, wind entrainment, or the geometry of dusty tori.

8. Conclusions

Active galactic nuclei and quasars reflect the most energetic phases of galactic nuclei, governed by accretion onto supermassive black holes. By radiating energy and driving outflows, they do more than just shine – they transform their host galaxies, shape star formation histories, cluster growth, and even large-scale environments through feedback. Whether triggered by major mergers or slow, shallow gas inflows, AGN emphasize the close connection between black hole and galaxy evolution – showing that even a small accretion disk can have consequences for the galaxy or even cosmic scales.

With variable observations at different wavelengths and improving simulations, we increasingly understand AGN "feeding" methods, quasar life cycles, and feedback mechanisms. Ultimately, unraveling the interactions between black holes and their hosts is a cornerstone in understanding the fabric of the Universe – from early quasars to the quieter black holes currently residing in elliptical or spiral galaxy clusters.

Links and further reading

  1. Lynden-Bell, D. (1969). "Galactic Nuclei as Collapsed Old Quasars." Nature, 223, 690–694.
  2. Rees, M. J. (1984). "Black Hole Models for Active Galactic Nuclei." Annual Review of Astronomy and Astrophysics, 22, 471–506.
  3. Antonucci, R. (1993). "Unified models for active galactic nuclei and quasars." Annual Review of Astronomy and Astrophysics, 31, 473–521.
  4. Urry, C. M., & Padovani, P. (1995). "Unified Schemes for Radio-Loud Active Galactic Nuclei." Publications of the Astronomical Society of the Pacific, 107, 803–845.
  5. Shakura, N. I., & Sunyaev, R. A. (1973). "Black Holes in Binary Systems. Observational Appearance." Astronomy & Astrophysics, 24, 337–355.
  6. Soltan, A. (1982). "Masses of quasar remnants." Monthly Notices of the Royal Astronomical Society, 200, 115–122.
  7. Hopkins, P. F., et al. (2008). "A unified, merger-driven model of the origin of starbursts, quasars, and spheroids." The Astrophysical Journal Supplement Series, 175, 356–389.
  8. Richards, G. T., et al. (2006). "Spectral Energy Distributions and Multiwavelength Selection of Type 1 Quasars." The Astrophysical Journal Supplement Series, 166, 470–497.
  9. Fabian, A. C. (2012). "Observational Evidence of Active Galactic Nuclei Feedback." Annual Review of Astronomy and Astrophysics, 50, 455–489.
  10. 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.
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