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An Observational Overview of Active Galactic Nuclei - Lect 01: Introduction and AGN Basics
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An Observational Overview of Active Galactic Nuclei - Lect 01: Introduction and AGN Basics

Observational Overview of Active Galactic Nuclei

In this section, Neil Brandt from Penn State University introduces the topic of active Galactic nuclei and provides an overview of the material to be covered in the lectures.

Introduction to Active Galactic Nuclei Books

  • Recommended book by Hagay Netzer on the physics and evolution of active Galactic nuclei.
  • Other useful books include those by Brad Peterson, Julian Krolik, and Osterbrock & Ferland.

Focus and Scope of Lectures

  • The lectures will focus on observational aspects rather than detailed theoretical calculations.
  • Acknowledgment that covering every aspect comprehensively is impossible due to the vastness of the field.

Approach to Covering Material

  • Aim to present main ideas efficiently without delving into every technical detail.
  • Encouragement for further exploration through additional literature reading.

Early History and Discoveries in Active Galactic Nuclei Studies

This part delves into the early history of active Galactic nuclei studies, highlighting key discoveries and researchers in the field.

Early Observations by Edward Fath (1908)

  • Edward Fath's observation of strong emission lines from hydrogen, oxygen, and neon in NGC 1068's nuclear spectrum.

Milestones in Understanding Active Galactic Nuclei

  • General relativity's development (1915), Swarzschild solution (1916), and Vesto Slipher's observations on NGC 1068's emission lines.

Heber Curtis' Discovery (1918)

  • Heber Curtis' identification of a relativistic jet originating from M87's core as a significant early discovery.

Edwin Hubble's Contributions (1924 - 1929)

History of Active Galactic Nuclei Discovery

This section delves into the historical timeline of significant discoveries related to active galactic nuclei, starting from amateur radio astronomers to the identification of powerful radio sources and high redshift quasars.

1939: Discovery of Radio Source Signus A

  • In 1939, an amateur radio astronomer discovered the radio source Signus A, a powerful radio-emitting AC galaxy.

1943: Carl Safer's Contribution

  • Carl Safer extended previous works, highlighting galaxies with strong broad emission lines and exceptional luminosity.

1954: Walter Baade and Rudolph Minkowski's Findings

  • Baade and Minkowski identified Signus A's counterpart and measured its redshift at 0.0057, a notable feat for that era.

1963: Martin Schmidt's Discovery

  • Martin Schmidt discovered 3C273, a high-redshift quasar challenging existing spectral understanding.

1964-1968: Speculations on Black Holes in Quasars

  • Zeldovich, Naov, and Saul Peter speculated about black holes powering quasars in 1964.
  • The term "black hole" gained popularity in 1967.
  • Donald Lynden-Bell proposed the presence of supermassive black holes in galactic nuclei in 1968.

Observed Properties of Active Galactic Nuclei

This segment explores fundamental observed characteristics of active galactic nuclei (AGN), emphasizing their prevalence across different luminosities within galaxies.

Frequency of AGNs in Galaxies

  • Approximately one in a million massive galaxies contains a luminous AGN or quasar.

Luminosity Levels of AGNs

  • About 5% of nearby massive galaxies host moderately luminous AGNs known as Safer galaxies.

Low-Level Activity in Nearby Galaxies

  • Around 30% of nearby galaxies exhibit subtle signs of low-level AG activity.

Relationship Between AGNs and Galaxies

Overview of Active Galactic Nuclei

In this section, the speaker provides an overview of active Galactic nuclei, discussing their variability and particle Jets. The focus is on the broad range of luminosities exhibited by these nuclei.

Broad Range of Luminosities

  • Active Galactic nuclei display a wide range of luminosities, spanning about nine orders of magnitude.
  • The most powerful objects are referred to as quasars, while low luminosity active Galactic nuclei show subtle signs of activity.
  • There is no strict lower limit on the luminosity of active Galactic nuclei, making the distinction between active and normal galaxies somewhat semantic.

Maximum Observed Luminosity

  • There exists a maximum observed luminosity in active Galactic nuclei, with quasars representing some of the most powerful objects in the universe.
  • Through extensive surveys, examples of the most luminous objects have been found, including both unobscured and obscured systems.

Emission Lines in Active Galactic Nuclei

  • Active Galactic nuclei exhibit strong and broad optical and ultraviolet emission lines from elements like hydrogen, oxygen, magnesium, and carbon.

Galactic Nuclei Emission Analysis

In this section, the speaker delves into the analysis of emission lines in active Galactic nuclei, discussing ionized nebular gas, photoionization processes, line widths, velocities, and abundance deductions.

Analyzing Emission Lines

  • The presence of ionized nebular gas in active Galactic nuclei is indicated by emission lines. This gas is believed to be photoionized by the central source emitting continuum radiation.
  • Broad emission lines with impressive widths are observed in these nuclei, reaching velocities of a few thousand kilometers per second and even up to 25,000 km/s. This suggests high-speed motions within the nuclei.
  • Not all emission lines are broad; some like O3 exhibit narrower widths compared to adjacent lines like H beta but still show non-negligible velocities of a few hundred kilometers per second.

Abundance Deductions and Absorption Lines

  • Intensities of emission lines provide insights into the abundances present in active galaxies, indicating solar or slightly super solar abundances in the material producing these lines.
  • Apart from emission lines, blue-shifted absorption lines are also observed in systems like broad absorption line quasars. These absorption features suggest outflows from active Galactic nuclei and resemble P Cygni profiles seen in stars.

Spectral Energy Distribution Analysis

This segment focuses on analyzing spectral energy distributions (SED) of active Galactic nuclei across various frequency bands to understand their emissions and characteristics.

Spectral Energy Distribution Insights

  • Active Galactic nuclei exhibit strong emissions across a wide range of frequencies from radio to X-ray bands, distinguishing them from typical star emissions that are dominant in specific bands based on temperature or composition.

New Section

In this section, the speaker discusses the extension of plots into gamma rays, focusing on blazars as high-energy emitters.

Extending Plots into Gamma Rays

  • Hertz extends the plot upward into gamma rays, highlighting emissions at much higher frequencies.
  • Most active Galactic nuclei have spectra that drop off below an MeV, with blazars being a small subset emitting at very high energies.
  • Blazars emit impressively high energies up to teev gamma rays and are believed to have relativistic jets pointed towards Earth.

New Section

This part emphasizes the broadband emission and unique spectral energy distributions of active Galactic nuclei.

Broadband Emission and Spectral Energy Distributions

  • Active Galactic nuclei are broad-band emitters with distinct spectral energy distributions unlike stars or normal galaxies.

New Section

The variability of active Galactic nuclei is discussed, emphasizing their chaotic nature and historical observations.

Variability of Active Galactic Nuclei

  • Active Galactic nuclei exhibit chaotic variability without clear periodicity, although some cases show evidence of periodic or quasi-periodic variability.
  • Historical observations dating back to the 19th century noted variability in active Galactic nuclei but were misclassified due to lack of understanding.

New Section

The speaker delves into long-term variability across different wavelengths and correlated variability in active Galactic nuclei.

Long-Term Variability and Correlation Across Wavelengths

  • Long-term variability is observed across various wavelengths from radio to X-ray bands in active Galactic nuclei.

Detailed Analysis of Active Galactic Nuclei

In this section, the speaker delves into the variability observed in active Galactic nuclei, emphasizing the continuous presence of variability in these systems and highlighting the variable nature of emission lines.

Variability in Active Galactic Nuclei

  • The emission lines of active Galactic nuclei are not only variable but also show variability similar to that of the underlying continuum emission.
  • Spectra analysis of NGC 5548 reveals notable variability in the carbon 4 emission line, even stronger than that of the local continuum.
  • Broad emission lines in active Galactic nuclei exhibit variability mimicking the underlying continuum with a lag, leading to reverberation mapping for inferring characteristic sizes of line emission regions.
  • Emission lines such as H beta vary in response to changes in the continuum but with a lag corresponding to the physical size of the system.

Implications of Variability

  • Short-wavelength observations suggest small emission regions within active Galactic nuclei, indicating sizes ranging from light hours to light days or less.
  • The compact nature of these sources results in angular sizes on the sky being extremely small, making direct imaging challenging for most active Galactic nuclei.
  • Despite challenges in direct imaging due to small emission regions, recent advancements using radio interferometry techniques have enabled imaging down to black hole regions in some supermassive black hole systems.

Particle Jets Emitted by Active Galactic Nuclei

This section focuses on particle jets emitted by a subset of active Galactic nuclei and discusses their characteristics and implications.

Characteristics of Particle Jets

  • Approximately 10% of active Galactic nuclei emit powerful particle jets extending over large distances, visible through radio imaging techniques like those conducted with radio telescopes such as the Very Large Array (VLA).

Intergalactic Space and Particle Jets

The discussion delves into the formation of lobe structures in intergalactic space and the characteristics of particle jets observed in galaxies like M87.

Ramming into Diffused Matter

  • Ramming into diffused matter in intergalactic space leads to the formation of lobe structures.
  • Detailed images of M87 showcase complex radio structures with clear jet extensions.

Stability of Particle Jets

  • Particle jets exhibit remarkable stability, maintaining a preferred direction across varying scales for millions of years.
  • The pointing stability suggests a gyroscope-like mechanism near supermassive black holes, guiding jet direction.

Proximity to Black Holes

  • Observations trace particle jets close to supermassive black holes, with recent imaging capturing black hole shadows.
  • Jets are believed to originate from the immediate vicinity of supermassive black holes.

Black Hole Accretion Disc Model for Active Galactic Nuclei

Exploring the basics of the black hole plus accretion disc model as an explanation for active galactic nuclei phenomena.

Phenomena Explanation

  • Active galactic nuclei's broad luminosities, emission lines, variability, and particle jets are explained by the black hole plus accretion disc model.

Black Hole Characteristics

  • Active galactic nuclei involve accreting supermassive black holes ranging from 100,000 to over 10 billion solar masses.

Luminosity and Structure

  • Accretion from a disk around these black holes results in significant luminosity output ranging from millions to 10^15 times solar luminosity.
  • The multi-temperature structure of accretion discs accounts for broadband emissions observed in active galactic nuclei.

Emission Line Origins

  • Strong optical and ultraviolet emission lines may stem from high-speed photoionized gas irradiated by central sources or winds off accretion discs.

Efficiency Comparison

Efficiency of Black Holes and Luminosity Explanation

In this section, the efficiency of black holes in liberating energy and its significance in explaining the high luminosities observed in active Galactic nuclei are discussed.

Efficiency of Black Holes

  • The efficiency of a black hole depends on its spin, with more rapid spins achieving higher efficiencies.
  • This efficiency can range from about 5% to up to 30%, with faster spins leading to better efficiency.
  • Higher efficiencies are crucial for explaining the large luminosities observed in systems like quasars.
  • Achieving high luminosities requires high efficiency as other mechanisms would struggle to supply such power.
  • Increasing efficiency from around 10% to even 5 or 10% can significantly enhance the ability to explain the high luminosities of active Galactic nuclei.

Broadband Nature and X-ray Emission

  • The broadband nature of these systems is only partially explained by multi-temperature accretion disk structures.
  • Accretion disks may face challenges in explaining active Galactic nuclei comprehensively.
  • X-ray emissions cannot be naturally produced by accretion disks due to insufficient temperature near supermassive black holes.
  • X-rays are not generated by discs expected around these black holes; they mainly emit ultraviolet radiation.

Accretion Disc Corona and X-ray Emission

  • An accretion disc corona, a hot structure above the accretion disc, is proposed as a source for X-ray emission via Compton upscattering.
  • This corona could reach temperatures of about a billion Kelvin, contributing to the production of X-rays.
  • The exact nature of the corona remains unclear, with suggestions ranging from magnetic loops similar to solar coronal loops to being associated with jet bases.

Infrared Emission Mechanisms

This section delves into factors influencing infrared emission in active Galactic nuclei, focusing on obscuring materials and reprocessing mechanisms.

Obscuring Material and Infrared Emission

  • Active galactic nuclei likely have obscuring material that absorbs radiation close to black holes and re-emits it as thermal waste heat at long wavelengths.
  • Absorption, degradation, and re-emission processes involving dust contribute significantly to infrared emissions.

New Section

In this section, the speaker discusses the presence of gaseous structures and giant molecular clouds in the central regions of galaxies, focusing on their role in absorbing and reprocessing radiation.

Gaseous Structures and Molecular Clouds

  • The central regions of our Galaxy contain rich gaseous structures and giant molecular clouds that absorb material and reprocess radiation.
  • Similar phenomena are observed in other galaxies, leading to the production of long-wavelength infrared emissions from active Galactic nuclei.

New Section

This part delves into the different emitting sources at various frequencies within galaxies, highlighting models such as accretion discs for optical and ultraviolet emissions.

Emission Sources at Different Frequencies

  • Optical and ultraviolet emissions are primarily attributed to an accretion disc according to the standard model.
  • X-ray emissions are associated with a coronal structure affixed to the accretion disc.
  • Infrared emissions result from dust reprocessing and reemission as waste heat.

New Section

This segment explores radio emissions in galaxies, distinguishing between radio-loud quasars with strong jets and radio-quiet systems where emission may originate from jets or host galaxies.

Radio Emissions in Galaxies

  • Radio-loud quasars exhibit powerful radio emission due to a strong jet present in the system.
  • Radio emission in radio-quiet systems may stem from jets or host galaxies, contributing to overall emission characteristics.

New Section

In this section, the speaker discusses the presence and amplification of magnetic fields in galaxies, particularly around black holes.

Presence of Magnetic Fields in Galaxies

  • Gas in central regions of galaxies is fed by gas with magnetic fields and disrupted stars.
  • Ionized gas in inner regions creates a Magneto hydrodynamical situation, amplifying magnetic fields via a Dynamo process.
  • Magnetic fields present in accretion discs help direct relativistic motions and matter flow outward to produce Jets.

New Section

This part focuses on the role of spinning black holes as gyroscopes for maintaining jet direction over long periods.

Spinning Black Holes as Gyroscopes

  • Spinning black holes act as gyroscopes to keep jets pointed consistently for millions of years.
  • The spinning black hole's mass and speed contribute to its effectiveness as a gyroscope.
Video description

These lectures from 2024 give an in-depth observational overview of Active Galactic Nuclei (AGNs). They supersede my earlier ones from 2014-2015 and have many relevant updates. This specific lecture covers general introductory material (e.g., some relevant books and the scope of the course), the early history of AGN studies, basic observed properties of AGNs, and basics of the black-hole + accretion-disk model. Many of the topics covered will be detailed further in the later lectures. The YouTube Playlist for these lectures is https://www.youtube.com/playlist?list=PLwQ-l1lnSF63jkuKHFKrN0Ml_Kb0KDF9f The YouTube links for all the lectures are the following: * Lecture 01 (Introduction and AGN Basics) - https://youtu.be/zFv03n_hdfc * Lecture 02 (Finding AGNs and Terminology) - https://youtu.be/fhcytDrnF9c * Lecture 03 (Powers of 10 and Black-Hole Region) - https://youtu.be/1OWS7d5qDhc * Lecture 04 (The Broad Line Region) - https://youtu.be/zsHeBQ9Ccxw * Lecture 05 (Outflowing Winds) - https://youtu.be/ppepAAyuOFA * Lecture 06 (The Narrow Line Region) - https://youtu.be/b6uP3j9P4VI * Lecture 07 (The Unified Model and the Torus) - https://youtu.be/SE4c2UPxpqE * Lecture 08 (Jets and Radio-Loud AGNs) - https://youtu.be/9EANmaPjytQ * Lecture 09 (Feeding the Monster) - https://youtu.be/1osj3YS4Nkc * Lecture 10 (AGN Evolution) - https://youtu.be/QSebw_nyNrU The PDF slides are available on Zenodo at https://zenodo.org/records/11180044 Even with these lectures, I can only scratch the surface of this truly massive field. I aim to give some of the main ideas, without noting every special case or technical exception. My apologies in advance if your favorite topic is not covered in as much depth as you would like. Most references are just examples from the vast AGN literature and have an implicit "e.g." before them throughout. See Lecture 1 for further comments on these points as well as some useful textbooks where you can learn more. Thoughtful, constructive, and broad-minded feedback is welcomed by email - fairly considering the enormous scope and practical challenges of preparing such lectures. These lectures were prepared as part of a seminar course at Penn State (Astro 589), and support from Penn State is gratefully acknowledged. Any findings, opinions, conclusions, or recommendations expressed in this material are those of the author and do not necessarily reflect the views of Penn State.