## General Information An **Active Galactic Nucleus (AGN)** is the highly compact region in a galactic center with an extremely high luminosity that does not originate from stars. The luminosity is theorized to arise from accretion onto a [[Black Hole#Supermassive Black Hole|SMBH]] in the galactic center. *(Note: All AGN are found in the center of galaxies, but not all galactic centers are AGN)* > [!measure] Typical Parameters & Properties > > Approx. Mass: $M_{\rm BH} \lesssim 10^{6} - 10^{9} \; {\rm M_{\odot}}$ > Approx. Schwarzschild Radius: $R_{\rm S} \lesssim 10^{-3} - 10^{-2} \; {\rm pc}$ (highly compact) > > Size of Disk: $\sim 10^{4} \; R_{g} \sim 100 \; \pu{pc}$ > Luminosity: $L \sim 10^{42} - 10^{48} \; {\rm erg/s} \quad (\simeq 10^{35} - 10^{41} \; {\rm W} \simeq 10^{9} - 10^{15} \; {\rm L_{\odot}})$ **Invalid Alternative Sources:** - For a collection of stars to be this bright ($L_{\odot} \sim 10^{26} \; {\rm W}$), there would have to be $\sim 10^{9}$ [[Spectral Classes#G]] stars within $10^{-2} \; {\rm pc}$ $\implies$ unstable as stellar density is too high. - For a supernovae ($E_{SN} \sim 10^{52} \; {\rm erg}$), they would need to occur a rate of $\sim 3 SN/yr$, requiring continual star production $\implies$ same problem as the source being stars (too dense and unstable). - For a black hole, the nearby gas is gravitationally pulled into an accretion disk, heating up to temperatures $T \sim 10^{8} \; {\rm K}$. Assuming the accretion is [[Luminosity#Eddington Limit|Eddington-limited]]... $L_{\rm Edd} \equiv \left(\frac{4 \pi G M c}{\kappa}\right) \simeq 3.2 \times 10^{4} \; \left(\frac{M_{\rm BH}}{M_{\odot}}\right) \, L_{\odot}$ ## Unified AGN Model The **Unified AGN Model** explains the large differences seen in spectra between AGN when viewed form different lines-of-sight. #### Structure Collectively, AGN are theorized to be consist of the following components. - Supermassive Black Hole ([[Black Hole#Supermassive Black Hole|SMBH]]) - Accretion Disk - Broad Line Region (BLR) - Narrow Line Region (NLR) - Jets at the Poles - Corona (potentially near poles, but currently unknown) - Torus shaped dust at edge of accretion disk - Gas clouds scattering near the BH (outside of accretion disk) > [!note] Jet Feedback > - Accretion onto BH -> Jets -> Bubbles -> ICM. > - Early stage is supersonic expansion until the movement of the bubble itself becomes faster than the expansion, then detachment of bubble. Enough energy in bubbles to balance radiative cooling In a simplified image... ![[AGN_schematic.png|align:center]] ![[AGN_reverberationMapping_regions.jpg]] Different coronal configurations: ![[BH_corona.png|Align:center|450]] ## Classifications The different types of AGN being represented together is what creates the "unified" theory. Below is a diagram that represents all of the classifications of AGN based on the viewing angle, whether or not the AGN produces a significant jet (radio loud or radio quiet), and the rate of accretion onto the central SMBH (low or high electromagnetic power).  > [!image] > *(Source: https://zenodo.org/records/6381013)* > > A schematic representation of an AGN in the orientation unified scheme. > > ![[agn_unification.png|align:center|600]] > > The centre of the schematic shows the typical components of an AGN, though the precise geometry of these components are still unknown. > > - Radio-loud objects are generally thought to display bipolar jet emission whereas it is only shown on the radio-loud side here. > - Note radio-quiet is not the same as radio-silent as many galaxies have low level radio emission, which is not significantly above that expected from star-formation and therefore it is unknown if these host radio jets or not.  > - The upper left and upper right quandrants are commonly referred to as low and high excitation radio galaxies (LERG/HERG) respectively.  > - LERGs are also known as hot mode, radio mode, jet mode, and radiatively inefficient sources. > - HERGs are also known as cold mode, quasar mode, radiative mode, and radiatively efficient sources. > - We include some of the most commonly used names for different classes of AGN including... > - broad line radio galaxy (BLRG) > - narrow line radio galaxy (NLRG) > - narrow emission line galaxy (NELG) > - flat spectrum radio quasar (FSRQ) > - steep spectrum radio quasar (SSRQ) > - optically violent variables (OVV) > - quasi-stellar objects (QSO) > - Note that while we separate quasars and QSOs as being radio loud and radio quiet respectively, these names are often used interchangeably.  > - Surrounding the central schematic we show whether a particular combination of power, radio emission, and geometry is expected to produce broad or narrow emission lines, or MIR, radio, X-ray, or gamma-ray emission. The transparency of the colour in each ring corresponds to the increasing strength or prevalence of a particular emission type. ^agn-schematic ![[AGN_unification_specifics.png|align:center]] #### Type 1 - Angled view into an AGN, seeing past the torus - Has both broad line emissions and narrow line emissions because there is little obscuration in the x-ray band #### Type 2 - Side-on view of an AGN - Lacks broad line emissions because the [[#Structure|BLR]] is heavily obscured in the x-ray band by the [[#Structure|torus]]. #### Radio Galaxy (RG) - $L_{\rm radio} \ge 10^{18}L_{\odot}$ - radio emission from two external regions (radio lobes) - energized by jets (particle acceleration $E_e\sim10^{12}\:eV$ - $\sim50\%$ E0/S0 galaxies, $\sim50\%$ quasars - synchrotron emission of electrons #### Low Excitation Radio Galaxy (LERG) - See [[#Radio Galaxy]] - Radio Loud + Low EM Power - Also known as hot mode, radio mode, jet mode, and radiatively inefficient sources. #### High Excitation Radio Galaxy (HERG) - See [[#Radio Galaxy]] - Radio Loud + High EM Power - Also known as cold mode, quasar mode, radiative mode, and radiatively efficient sources. #### Narrow Line Radio Galaxy (NLRG) - See [[#Radio Galaxy]] - Radio Loud + Low/High EM Power - A subtype of the [[#Type 2]] AGN classification - Lacks broad line emissions because the [[#Structure|BLR]] is heavily obscured in the x-ray band by the [[#Structure|torus]]. #### Broad Line Radio Galaxy (BLRG) - See [[#Radio Galaxy]] - Radio Loud + Low/High EM Power - A subtype of the [[#Type 1]] AGN classification - Has both broad line emissions and narrow line emissions because there is little obscuration in the x-ray band #### Steep Spectrum Radio Quasar (SSRQ) - See [[#Radio Galaxy]] - Radio Loud + Low EM Power - Very similar to [[#Broad Line Radio Galaxy (BLRG)]], but with a steeper flux spectrum #### Fanaroff–Riley Classification Classification of [[#Radio Galaxy (RG)|radio galaxies]] with active nuclei based on their radio luminosity or brightness of their radio emissions in relation to their hosting environment - FR1 arms can look bent due to [[Fluid Mechanics#Ram Pressure]] as galaxy moves through [[Intracuster Medium|ICM]] - FR2 often looks one-sided due to relativistic boosting ![[fanaroff_riley.png]] confirmation is expensive, but is the proposed number of quasar pairs ~10^3? cant we estimate a population of What specific models are we trying to understand by quasar pair formation statistics? #### Quasar / Quasi-Stellar Object (QSO) - "Quasar" (quasi-stellar radio source): optical point source with radio jet - "QSO" (quasi-stellar object): like a quasar but no radio emission - A [[#Quasar]] and [[#Quasi-Stellar Object (QSO)|QSO]] are similar phenomena. - Of AGN sources found optically, $90\%$ are radio quiet (QSO), $\sim10\%$ are radio loud (Quasar) - Mostly found in elliptical galaxies - $L_{\rm quasar}\sim10^{45-48}\:$erg/s - synchrotron jets between $0.1 \; {\rm pc} - 1 \; {\rm Mpc}$ - maximum space density $\sim z=2...3$ **Catalogs**: [[Catalogs#SDSS|SDSS]] and [[Catalogs#LAMOST]] **Observation strategies**: To observe the environment of a quasar you can use objects in the background (eg galaxies) and examine their light as absorbed by the environment of the quasar (ref: Christina Eilers talk) #### Blazar A [[#Quasar|quasar]] with a relativistic jet pointing along out line of sight. Have spectra dominated by featureless non-thermal continuum, which we would expect for a relativistic jet along the line of sight. #### BL Lac BL Lac objects are rapidly varying [[#Blazar|blazars]]. - [[#Quasar / Quasi-Stellar Object (QSO)|quasar]] with enhanced continuum emission - highly variable - extremely luminous - highly polarized - jet pointing towards observer #### Flat Spectrum Radio Quasar (FSRQ) - Radio Loud + High EM Power [[#Blazar]] - Same as [[#Optically Violent Variables (OVV)]] #### Optically Violent Variables (OVV) - Radio Loud + High EM Power [[#Blazar]] - Same as [[#Flat Spectrum Radio Quasar (FSRQ)]] #### Seyfert Galaxies A Seyfert galaxy surround and AGN that is "Radio Quiet + Low EM Power", allowing the host galaxy to be easily seen. - spiral galaxies - bright unresolved nuclei (less luminous than quasars) - $L\approx10^{42}-10^{45}\:$erg/s The different types (1, 1.5, 2) are in association with the [[#Type 1]] and [[#Type 2]] viewing angles. #### Narrow Emission Line Galaxy (NELG) Another name for a [[#Type 2]] [[#Seyfert Galaxies|Seyfert galaxy]]. Has ## Examples ### Sagittarius A* ### Hercules A ### Makarian 501 ### Spanish Dancer