9.2 Astrochemistry of active galactic nuclei

Active galactic nuclei (AGN) are super bright centers of galaxies powered by supermassive black holes. They're incredibly luminous, small, and variable, emitting radiation across the electromagnetic spectrum. Different types include quasars, Seyfert galaxies, and blazars.

Astrochemistry helps us understand AGN by studying chemical compounds and reactions in these extreme environments. It provides insights into physical conditions, gas dynamics, and the impact of processes like X-ray dissociation on the chemical makeup of AGN regions.

Active Galactic Nuclei: Definition and Characteristics

Definition and Power Source

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• Active galactic nuclei (AGN) are compact regions at the centers of galaxies that have much higher than normal luminosity over at least some portion of the electromagnetic spectrum
• AGN are powered by accretion of matter onto supermassive black holes at the centers of galaxies, which can convert gravitational potential energy into radiation

Key Characteristics

• High luminosity
• Small size
• Variability
• Presence of broad emission lines in their spectra
• Different types of AGN include quasars, Seyfert galaxies, radio galaxies, and blazars, classified based on their observational properties and viewing angle
• AGN can emit radiation across the electromagnetic spectrum, from radio to gamma-ray wavelengths
• Some AGN are particularly luminous in certain bands (radio-loud vs. radio-quiet AGN)

Astrochemistry: Probing AGN Conditions

Diagnostic Information from Astrochemistry

• Astrochemistry is the study of chemical compounds and reactions in astronomical environments, including AGN
• Chemical abundance ratios and molecular line emission can provide diagnostic information about the physical conditions (temperature, density, radiation field) in different regions of AGN
• Molecular emission lines can trace the kinematics and dynamics of gas in AGN, providing insights into accretion processes and outflows

Influence of Astrochemical Processes

• Astrochemical processes, such as X-ray dissociation and ionization, can influence the chemical composition and evolution of gas in AGN
• Dust chemistry and grain processing in AGN can affect the observed spectra and provide clues about the AGN environment and evolution

Chemical Environments in AGN

Distinct Chemical Regions

• AGN host a variety of distinct chemical environments, including the accretion disk, broad-line region (BLR), narrow-line region (NLR), and molecular torus
• The accretion disk around the supermassive black hole is characterized by high temperatures and densities, leading to unique chemical processes and emission signatures
• The BLR, located close to the accretion disk, exhibits broad emission lines from high-velocity gas, indicating a chemically enriched and highly ionized environment
• The NLR, extending to larger scales, shows narrow emission lines from lower velocity gas, suggesting a different chemical composition and ionization state compared to the BLR
• The molecular torus, surrounding the AGN on parsec scales, contains dust and molecular gas that can obscure the central region and influence the observed chemical signatures

Impact on Galaxy Evolution

• AGN feedback, through outflows and radiation, can impact the chemical evolution and star formation in the host galaxy
• AGN feedback potentially regulates galaxy growth and evolution by affecting the gas supply and chemistry

Observational Techniques for AGN Astrochemistry

Spectroscopy

• Spectroscopy is the primary observational technique used to study the astrochemistry of AGN
• Spectroscopy provides information about chemical abundances, ionization states, and gas kinematics
• Optical and UV spectroscopy can probe the emission lines from the BLR and NLR, revealing the chemical composition and physical conditions of these regions
• Infrared spectroscopy is used to study the molecular gas and dust in the torus and host galaxy, including the detection of key molecular species like CO, HCN, and H2O
• X-ray spectroscopy can probe the hot, ionized gas in the inner regions of AGN, including the accretion disk and corona, and study the effects of X-ray irradiation on the surrounding gas chemistry

High-Resolution Imaging

• Radio interferometry, such as with ALMA and the VLA, can spatially resolve the molecular gas emission in AGN on sub-parsec scales
• Radio interferometry provides detailed maps of the chemical and kinematic structure of AGN

Multi-Wavelength Observations

• Multi-wavelength observations, combining data from different spectral regions, are essential for constructing a comprehensive picture of the astrochemistry and physical conditions in AGN
• Combining observations from radio, infrared, optical, UV, and X-ray wavelengths allows for a holistic understanding of AGN chemistry and physics