Radio selected AGN helps us to study jets, black holes and their feedback, importance for understanding the Galaxy Evolution especially because they can influence their surroundings through powerful jets. The main source of radio emission is synchrotron radiation, which is formed when charged particles moving at relativistic speeds spiral in a magnetic field. These galaxies exhibit strong radio emissions in the form of jets that emanate from the accretion of supermassive black holes.
This emission is non-thermal in nature. This means that it has nothing to do with temperature, and its spectrum follows a power law: 𝑆(𝜈) ∝ 𝜈^(−𝛼), where 𝑆(𝜈)= flux at frequency 𝜈 and 𝛼= spectral index.
Two main source of synchrotron emission in galaxies. 1. SFGS (Star forming galaxies) and 2.AGNs (active galactic nuclei)
- SFGs: Emissions comes from supernova remnants and star forming region. generally it's weaker radio source, and they dominate infant radio survey. At lower radio flux level (fainter source), SFG become more dominant.
- AGNs: Emissions comes from jets and lobes linked to accretion disk. It is common in brighter radio detection. At higher radio flux level (Stronger source), AGN are more common.
Types of Radio galaxies (RGs):
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FR I: Low-luminosity, core-brightened jets.
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FR II: High-luminosity, edge-brightened lobes.
Types of different kinds of radio AGN:
- Radio-loud quasars,
- Radio galaxies,
- Blazars.
- Radio-Loud (RL) AGN: Jetted AGN (new name), Emits strong radio waves, Non thermal radiation, Dominat in radio and gamma bands, have powerful active relativistic jets and RL can reach high photon energies.
- Radio-Quiet (RQ) AGN: Non-Jetted AGN (new name), Emit weak or no strong radio waves, without powerful jets, thermal radiation from accretion disk, seen mostly in optical, UV and X rays.
Classify based on optical spectroscopy:
- Low Excited Galaxies (LEG): High accretion rate; Strong Emission lines and often FR I. for ex. Quasars and Syferts
- High Excited Galaxies (HEGs): Low accretion rate; Weak or no emission lines and often FR II. for ex. LINERs and absorption line galaxies.
Classification of Radio-selected AGN:
Today's surveys involve very faint (weak) radio sources, whose optical spectrum is difficult to capture. Therefore, for classification we have to use multi-wavelength data:
- Optical colour of the host galaxy,
- X-ray luminosity,
- Mid-infrared (MIR) colours,
- Full SED fitting
Selection Effects: what types of AGNs are we able to detect and what types of AGNs are we missing, due to telescope limitations or biases in our methods.
Star-forming galaxies (SFGs) also produce synchrotron radiation, due to supernova remnants. When radio emission is observed, it is sometimes difficult to tell whether the emission is coming from SMBH or SF (Star Formation). Bluer galaxies, which have more SF, their radio emission is wrongly assumed to be AGN.
Evolution of Radio AGN:
- The number and brightness (luminosity) of radio AGNs have changed over cosmic time.
- Powerful radio AGNs (FR II type) have a stronger evolution and their number density peaks at z ≈ 2 (10 billion years ago).
- Weak AGNs (FR I type) have a slower evolution and peak between z ≈ 1 to 1.5 (6 billion years ago).
- LEG-type AGNs have a mild evolution (hardly any change with redshift), while HEG-type AGNs have a strong evolution.
- This means that powerful AGNs evolved first, and weaker ones later — a process we call "AGN downsizing".
Radio AGN studies usually take place in the frequency range from 10 MHz to 30 GHz — 30 meters to 1 cm wavelength.