Galactic jets and outflows are powerful phenomena emanating from active galactic nuclei. These energetic streams of particles and radiation play a crucial role in shaping galaxies and their surroundings, influencing star formation and galaxy evolution.
Jets are highly collimated streams of relativistic particles, while outflows are wider-angle flows of gas and plasma. Both types of outflows interact with the surrounding medium, creating shocks, bubbles, and lobes that can extend far beyond their host galaxies.
Types of galactic jets
Galactic jets are highly collimated streams of energetic particles and electromagnetic radiation emanating from the central regions of active galaxies
The types of galactic jets are classified based on the primary wavelength of electromagnetic radiation they emit, which depends on the energy of the particles and the strength of the within the jet
Most commonly observed type of galactic jet, emitting synchrotron radiation at radio wavelengths
Powered by relativistic electrons spiraling around magnetic field lines, producing the characteristic radio emission
Extend over vast distances (up to several hundred kiloparsecs) from the host galaxy's center
Examples include the jets in the galaxies M87 and Cygnus A
Optical jets
Emit synchrotron radiation at visible wavelengths due to the presence of highly energetic electrons
Typically shorter and less common than radio jets, as the electrons need to maintain extremely high energies to produce optical emission
Often accompanied by radio emission, as the optical jets are usually embedded within larger radio jet structures
Example: The optical jet in the quasar 3C 273
X-ray jets
Emit high-energy X-ray radiation, likely due to inverse Compton scattering of low-energy photons by relativistic electrons in the jet
Can also be produced by synchrotron emission from ultra-relativistic electrons in strong magnetic fields
Tend to be shorter and less extended than radio jets, as the energy losses at X-ray wavelengths are more severe
Example: The X-ray jet in the galaxy Centaurus A
Formation of jets
The formation of galactic jets is a complex process that involves the interplay between the central , the disk, and the magnetic fields in the vicinity of the black hole
Understanding the mechanisms behind jet formation is crucial for comprehending the nature of active galactic nuclei (AGN) and their impact on galaxy evolution
Accretion disk processes
Accretion disks around supermassive black holes play a crucial role in jet formation
Matter in the accretion disk loses angular momentum through viscous processes and spirals inward, releasing gravitational potential energy
The released energy heats the accretion disk, causing it to emit electromagnetic radiation across a wide range of wavelengths
Accretion disk instabilities and turbulence can contribute to the initial launching of the jet material
Magnetic fields and collimation
Strong magnetic fields near the black hole are believed to be responsible for the collimation and acceleration of the jet material
The rotation of the accretion disk and the black hole can cause the magnetic field lines to become twisted and wrapped around the jet axis
This helical magnetic field structure helps to confine the jet material and maintain its narrow, collimated shape over large distances
The magnetic fields also play a role in accelerating the particles within the jet to relativistic speeds
Role of supermassive black holes
Supermassive black holes, with masses ranging from millions to billions of times the mass of the Sun, reside at the centers of most galaxies
The presence of a rapidly spinning supermassive black hole is thought to be essential for the formation of powerful galactic jets
The rotation of the black hole can drag spacetime around it, creating an ergosphere where the frame-dragging effect is strong
The interaction between the ergosphere and the magnetic fields can extract rotational energy from the black hole, providing a significant power source for the jet ()
Jet composition and structure
Galactic jets are composed of relativistic particles, primarily electrons and positrons, as well as a small fraction of protons and atomic nuclei
The particles are embedded within strong magnetic fields that play a crucial role in the jet's structure and emission properties
Relativistic particles
The bulk of the jet material consists of relativistic electrons and positrons, which are accelerated to near-light speeds
These particles are responsible for the synchrotron radiation observed at radio, optical, and sometimes X-ray wavelengths
The energy distribution of the relativistic particles follows a power-law spectrum, with a larger number of lower-energy particles compared to higher-energy ones
Magnetic field structure
The magnetic fields within the jet are highly ordered and play a crucial role in the jet's structure and stability
Near the base of the jet, the magnetic fields are thought to be helical, wrapping around the jet axis and helping to collimate the flow
As the jet propagates away from the central region, the magnetic fields become increasingly toroidal (perpendicular to the jet axis) due to the jet's expansion and interaction with the surrounding medium
The strength and orientation of the magnetic fields influence the polarization of the synchrotron radiation emitted by the jet
Lobes and hotspots
As the jet propagates through the interstellar and intergalactic medium, it creates large, diffuse structures called lobes
Lobes are filled with relativistic particles and magnetic fields, and they emit synchrotron radiation at radio wavelengths
At the ends of the lobes, where the jet material interacts with the surrounding medium, compact regions of intense radio emission called hotspots can form
Hotspots are thought to be the sites of particle acceleration, where the jet material is shock-compressed and re-energized
Jet propagation and interaction
As galactic jets propagate away from their host galaxy, they interact with the surrounding interstellar and intergalactic medium
These interactions play a crucial role in shaping the jet's morphology and the evolution of the host galaxy
Interaction with interstellar medium
On scales of a few kiloparsecs, the jet interacts with the interstellar medium (ISM) of the host galaxy
The jet can shock-compress and heat the ISM, creating cavities and bubbles filled with hot, low-density gas
The interaction between the jet and the ISM can trigger star formation in the compressed gas, leading to the formation of new stars and star clusters
Conversely, the jet can also suppress star formation by heating and dispersing the cold gas reserves of the galaxy
Shock fronts and cocoons
As the jet propagates through the ambient medium, it creates a bow shock ahead of it, compressing and heating the surrounding gas
Behind the shock front, a cocoon of shocked jet material and ambient gas forms around the jet
The cocoon helps to confine and collimate the jet, preventing it from dissipating rapidly into the surrounding medium
The pressure balance between the cocoon and the ambient medium can influence the jet's stability and propagation
Impact on host galaxy
The interaction between the jet and the host galaxy can have significant consequences for the galaxy's evolution
Jets can inject a large amount of energy into the interstellar and intergalactic medium, heating the gas and suppressing star formation (a process known as AGN feedback)
The heated gas can expand and escape the gravitational potential of the galaxy, leading to the formation of galactic winds and outflows
These outflows can enrich the intergalactic medium with metals and other products of stellar nucleosynthesis, influencing the chemical evolution of galaxies and clusters
Observational characteristics
Galactic jets are studied using observations across the electromagnetic spectrum, from radio to gamma-ray wavelengths
Each wavelength regime provides unique insights into the jet's structure, composition, and emission mechanisms
Radio observations
are the primary tool for studying galactic jets, as most jets are strong sources of synchrotron radiation at radio wavelengths
Radio telescopes, such as the Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), provide high-resolution images of jet structures
Radio polarization measurements can reveal the orientation and strength of the magnetic fields within the jet
Long-term radio monitoring can detect changes in the jet's brightness and morphology, providing insights into the jet's variability and propagation
Optical and X-ray observations
Optical observations, using telescopes like the Hubble Space Telescope (HST), can reveal the presence of optical jets in some sources
Optical emission is typically synchrotron radiation from highly energetic electrons, and it can trace the jet's structure on smaller scales than radio observations
X-ray observations, using satellites like Chandra and XMM-Newton, can detect the high-energy emission from the jet, particularly in the knots and hotspots
X-ray emission can be produced by synchrotron radiation from ultra-relativistic electrons or by inverse Compton scattering of low-energy photons by the jet's relativistic electrons
Variability and monitoring
Galactic jets exhibit variability on various timescales, from days to years, across multiple wavelengths
Variability can be caused by changes in the accretion flow onto the central black hole, instabilities in the jet itself, or interactions with the surrounding medium
Multi-wavelength monitoring campaigns, involving coordinated observations with radio, optical, X-ray, and gamma-ray telescopes, can provide a comprehensive picture of the jet's variability and the underlying physical processes
Studying the correlated variability between different wavelengths can help constrain the emission mechanisms and the locations of the emitting regions within the jet
Jet energetics and feedback
Galactic jets are among the most energetic phenomena in the universe, with kinetic powers that can exceed the luminosity of the entire host galaxy
The energy injected by jets into the surrounding medium can have profound effects on the evolution of galaxies and galaxy clusters
Kinetic energy and power
The kinetic energy of a galactic jet is dominated by the bulk motion of the relativistic particles and the magnetic fields
Jet powers can range from around 1042 erg/s for low-luminosity AGN to more than 1046 erg/s for powerful quasars
The jet power is typically a fraction of the total accretion power of the central black hole, with estimates ranging from a few percent to several tens of percent
Measuring the jet power directly is challenging, but it can be inferred from the radio luminosity of the lobes or the cavities created by the jet in the surrounding medium
AGN feedback mechanisms
AGN feedback refers to the interaction between the energy and matter released by an active galactic nucleus (AGN) and the host galaxy's interstellar medium
Jets are one of the primary mechanisms for AGN feedback, along with radiation pressure and winds from the accretion disk
Jet-driven feedback can be either negative or positive: negative feedback suppresses star formation by heating and expelling gas from the galaxy, while positive feedback can trigger star formation in compressed gas clouds
The balance between negative and positive feedback depends on factors such as the jet power, the properties of the interstellar medium, and the galaxy's evolutionary stage
Impact on galaxy evolution
AGN jet feedback plays a crucial role in regulating the growth and evolution of galaxies and galaxy clusters
In the case of negative feedback, jets can heat and expel the cold gas reserves of galaxies, effectively shutting down star formation and limiting the galaxy's growth
This process is thought to be responsible for the observed correlation between the mass of the central black hole and the properties of the host galaxy bulge (e.g., the M-sigma relation)
In galaxy clusters, jet-driven feedback can prevent the cooling of the hot intracluster medium, suppressing the formation of cool cores and regulating the growth of the central brightest cluster galaxy
Positive jet feedback, although less common, can trigger starbursts in compressed gas clouds, leading to the formation of new stars and potentially influencing the galaxy's chemical evolution
Famous examples of galactic jets
Several well-known galaxies and quasars exhibit prominent galactic jets, which have been studied extensively across multiple wavelengths
These famous examples showcase the diverse morphologies and physical properties of galactic jets
M87 jet
The galaxy M87, a giant elliptical galaxy in the Virgo Cluster, hosts one of the most well-studied galactic jets
The jet, which extends for several kiloparsecs, is visible at radio, optical, and X-ray wavelengths
In 2019, the Telescope (EHT) collaboration obtained the first direct image of the supermassive black hole at the center of M87, providing unprecedented insights into the jet's launching region
The M87 jet is a powerful source of high-energy gamma-ray emission, detected by telescopes such as the Fermi Gamma-ray Space Telescope and the High Energy Stereoscopic System (HESS)
Cygnus A
Cygnus A is a powerful radio galaxy, located approximately 600 million light-years from Earth
It hosts a pair of symmetric radio jets that extend for hundreds of kiloparsecs, terminating in bright hotspots
The jets in Cygnus A are thought to be powered by a supermassive black hole with a mass of around 2.5×109 solar masses
Observations of Cygnus A have provided crucial insights into the interaction between galactic jets and the intracluster medium, as well as the role of jet feedback in galaxy evolution
3C 273
3C 273 is a bright quasar located approximately 2.4 billion light-years from Earth
It was the first quasar to be identified and has been extensively studied across the electromagnetic spectrum
3C 273 hosts a prominent one-sided jet that is visible at radio, optical, and X-ray wavelengths
The jet in 3C 273 is highly polarized and exhibits superluminal motion, with apparent speeds exceeding the speed of light due to relativistic beaming effects
Studies of 3C 273 have provided valuable insights into the physics of , the emission mechanisms at different wavelengths, and the variability of quasars
Theoretical models of jet formation
Several theoretical models have been proposed to explain the formation and acceleration of galactic jets
These models aim to describe the physical processes that extract energy from the central black hole and accretion disk and launch the jet material
Blandford-Znajek mechanism
The Blandford-Znajek (BZ) mechanism, proposed by Roger Blandford and Roman Znajek in 1977, describes how rotational energy can be extracted from a spinning black hole and used to power a relativistic jet
In this model, a strong magnetic field threads the black hole's ergosphere, a region where spacetime is dragged along with the black hole's rotation
The rotation of the black hole twists the magnetic field lines, creating a powerful electromagnetic jet that carries away energy and angular momentum from the black hole
The BZ mechanism is thought to be the primary power source for jets in rapidly spinning black holes, particularly in low-accretion-rate systems
Blandford-Payne mechanism
The Blandford-Payne (BP) mechanism, proposed by Roger Blandford and David Payne in 1982, describes how a relativistic jet can be launched from a magnetized accretion disk
In this model, a strong magnetic field threads the accretion disk, and the rotation of the disk twists the field lines into a helical configuration
Plasma in the disk can then be accelerated along the magnetic field lines, centrifugally launching a relativistic jet
The BP mechanism is thought to be important for jet formation in high-accretion-rate systems, such as quasars and luminous Seyfert galaxies
Numerical simulations of jet formation
Numerical simulations play a crucial role in understanding the complex physical processes involved in jet formation and propagation
General relativistic magnetohydrodynamic (GRMHD) simulations, which incorporate the effects of general relativity and magnetic fields, have been used to study jet formation in the vicinity of black holes
These simulations have provided insights into the role of magnetic fields, the importance of black hole spin, and the interplay between the accretion disk and the jet
Simulations have also been used to study the propagation of jets through the interstellar and intergalactic medium, the formation of shocks and instabilities, and the impact of jet feedback on galaxy evolution
As computational power increases and numerical techniques improve, simulations will continue to be an essential tool for understanding the physics of galactic jets
Outflows vs jets
In addition to highly collimated jets, active galactic nuclei can also produce wider-angle outflows of gas and plasma
While jets and outflows share some similarities, they differ in their collimation, composition, and physical properties
Differences in collimation
Jets are highly collimated structures, with opening angles typically less than a few degrees
Outflows, on the other hand, have much wider opening angles, ranging from tens to nearly 180 degrees in some cases
The high collimation of jets is thought to be due to the strong magnetic fields that confine and accelerate the jet material
Outflows are less collimated and are likely driven by a combination of radiation pressure, thermal pressure, and magnetic forces
Differences in composition
Jets are composed primarily of relativistic particles (electrons, positrons, and possibly protons) and magnetic fields
Outflows, in contrast, contain a significant fraction of atomic and molecular gas, along with dust and ionized plasma
The presence of cooler gas in outflows suggests that they originate from regions further away from the central black hole, such as the outer accretion disk or the dusty torus
Relationship between outflows and jets
The relationship between outflows and jets is not fully understood, and it likely varies among different AGN systems
In some cases, outflows may represent the slower, less collimated outer layers of a jet, while the highly relativistic core of the jet remains confined by strong magnetic fields
Alternatively, outflows and jets may originate from different regions of the accretion flow, with jets launched from the inner disk and outflows driven from the outer