Active galactic nuclei and quasars

Separate the nucleus from the surrounding galaxy

The nucleus is the central region of a galaxy. An active galactic nucleus, or AGN, displays emission and other evidence that cannot be explained simply as the combined light of an ordinary stellar population there. The established physical framework connects that activity to accretion around a central supermassive black hole.

Accretion means material moving inward while exchanging energy and angular momentum with its surroundings. The observable radiation is produced by matter and fields outside the event horizon. It should not be described as a signal escaping from inside the classical causal boundary.

The host galaxy is the larger galaxy containing the nucleus. Its stars, gas and dust can contribute light within the same image aperture or spectrum. An unresolved bright centre alone does not determine the black-hole mass, the accretion rate or whether a jet is present.

Quasar is a name for a luminous nuclear class whose compact appearance and spectrum were central to its recognition. It is not a synonym for every active galaxy or every radio-emitting galaxy. No universal numerical threshold or present population census is required for the physical distinctions in this lesson.

Several measurements constrain the same engine

A continuum is emission distributed across a range of wavelengths, while emission lines concentrate radiation in characteristic spectral features. Compact variability, a continuum, line patterns and gas or stellar motions can provide different constraints on an active nucleus.

Some nuclei show broad permitted emission-line components, while others lack a directly detected broad component in a given spectrum. Radio structure, polarization, infrared emission and X-ray measurements add further information. Each observation requires a calibration, spatial scale and physical interpretation.

Urry and Padovani discuss how empirical classifications relate to obscuration and directional emission in radio-loud nuclei. Their historical population fractions and cosmological parameter choices are not adopted here. The durable lesson is that an observed class and an intrinsic physical property need not be identical.

A detection in one band can be weak or absent in another because different components, absorption and host contrast matter. A non-detection is conditional on the observation’s sensitivity, resolution, band and time. It is not a demonstration that no accretion or central compact mass exists.

A ten-percent nucleus can make a smaller total variation

For an original two-epoch exercise, measure one identical calibrated band through one fixed aperture. Express all received band fluxes in an arbitrary positive unit F0. Initially the nucleus contributes 50F0 and constant host light contributes 150F0, so the total is 200F0.

Stipulate a ten-percent increase in the nuclear component and no change in the host or any additional component. The nucleus becomes 55F0, giving total 205F0. The observed total fractional change is (205 − 200)/200 = 0.025, or 2.5 percent.

More generally, with initial nuclear flux F_n, host flux F_h and nuclear fractional change a, the total fractional change is aF_n/(F_n + F_h). This follows by adding fluxes before calculating the fractional difference. It is not a magnitude average or a change in the nucleus’s physical amplitude.

The calculation assumes the same band, aperture and calibration at both epochs and treats the host as constant. Real image response, changing aperture contributions, calibration errors and other variable emission require further treatment. The toy does not classify a real source or supply its total emitted luminosity.

Missing direct nuclear light has several explanations

Obscuration changes the radiation reaching the observer through absorption and scattering by intervening material. Host dilution changes the relative contribution of nuclear emission within a combined measurement. Either can make nuclear signatures harder to recognize, and both can act together.

Hickox and Alexander discuss how this competition depends on wavelength and source components. Obscuring material can be close to the nucleus or on larger host-galaxy scales. A schematic obscuring structure is a useful model, rather than proof that every galaxy contains an identical smooth torus.

An optical Type 2 classification does not universally imply a hidden broad-line region behind dust. Some systems can intrinsically lack a detectable broad-line region, and changing physical states can also alter classifications. Orientation alone is not a complete taxonomy.

Consequently, an activity search needs explicit criteria and a description of completeness and contamination. Combining bands can reduce particular ambiguities, but shared selection limits remain. A detection fraction in one selected catalogue is not automatically a universal frequency of activity.

A luminous appearance needs an emission model

Luminosity is emitted power; received flux is power per collecting area. A measured band flux also covers only a defined spectral range. Recovering total emitted power needs distance, attenuation, spectral and directional information.

For anisotropic radiation, multiplying received flux by a spherical area can define an isotropic-equivalent luminosity. That is a stated convention, not proof that the radiation was emitted equally in all directions. A beam directed toward an observer can make the apparent source unusually strong.

The chapters on galaxies and compact remnants already supplied the distinctions between stellar light, mass inference and accretion. This chapter applies them to nuclear observations, jets, central dynamics, reconstructed images and connections to a host.

The next lesson follows the spectrum and time response of the material surrounding the central source. It will show why an assumed efficiency and a measured radiation component must be connected through an explicit energy boundary before an inflow rate is inferred.

Worked examples

Try the reasoning

1. Does a bright unresolved nucleus alone determine one unique black-hole mass?

   Show the answer and reasoning

   No.

   Mass inference requires additional evidence and a physical geometry and dynamical model.

2. Must every active nucleus have a detectable jet?

   Show the answer and reasoning

   No.

   Nuclear radiation, jet production, orientation and observing limits are distinct parts of the inference.

3. Does every optical Type 2 classification prove an obscured broad-line region exists?

   Show the answer and reasoning

   No.

   Intrinsic absence or changing physical state can complicate the simplest orientation picture.

4. Is one measured band flux automatically the total isotropically emitted nuclear power?

   Show the answer and reasoning

   No.

   Spectral coverage, distance, attenuation and directionality require separate assumptions.

What to carry forward

• AGN evidence combines compact nuclear activity with an accretion framework and several observations.
• Constant host light can strongly dilute a nuclear variation in a combined aperture.
• Observed classes and luminosity conventions retain band, orientation and selection dependencies.