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Astro/CSI 765. An Introduction to Active Galactic Nuclei (AGN). Prof. Rita Sambruna 3-4165 Office hours: by appointment only. Outline of the course. DESCRIPTION : Phenomenology of AGN (emission

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Astro/CSI 765

An Introduction to Active Galactic Nuclei (AGN)

Prof. Rita Sambruna


Office hours: by appointment only


Outline of the course

  • DESCRIPTION:Phenomenology of AGN (emission
  • processes, observed properties at various wavelengths,
  • standard model for AGN)
  • PRE-REQUISITES: PYHS 502, 613, or Astro530
  • TEXTBOOK:Quasars and Active Galactic Nuclei
  • by A.Kembhavi and J.Narlikar
  • (for a list of additional books, see me)

Structure of the course

  • LECTURES:review of concepts, expansion of reading material
  • Reading from assigned papers
  • Writing essays/answering questionnaires
  • Solving (occasional) numerical problems
  • EXAMS:No “traditional” mid-term/final
  • Grading based on homework (25%), in-class discussion
  • (25%), and final project (50%)
  • GRADES: 93-100 A 83-86 B 70-74 C
  • 90-92 A- 80-82 B- 60-69 D
  • 87-89 B+ 75-79 C+ <59 F

Reading Assignments

  • Every week I will assign readings from papers or book
  • chapters for the following class
  • At the beginning of every class, there will be 30 minutes
  • or more discussion on the readings
  • I will ask one of you to present the reading material and
  • lead the discussion
  • 25% of your final grade (or more) will be based on the
  • in-class discussion

FINAL PROJECT: (50% of the final grade)

  • Goal: a deeper understanding of a particular issue/problem
  • analyzed in class, or a totally new AGN-related topic we did
  • not have time to talk about
  • Either a literature search or original data analysis (using
  • data from public archives)
  • Submit an outline for pre-approval by November 1
  • Your paper (< 20 pages) in ApJ-style due December 2
  • Seminar (30 minutes) on December 4
  • Both the paper and the seminar are required

Lecture 1:

  • What is an AGN?
  • Historical discovery of AGN
  • The importance of the multi-wavelength
  • perspective
  • Notes and Useful quantities (some AGN lingo)

What is an Active Galactic Nucleus?

  • A point-like source at the center of an otherwise
  • normal galaxy
  • Nucleus light overwhelms the light from the
  • galaxy

Notation: AGN observed quantities

  • Image: a map of intensity versus position (x, y)
  • Light curve: a plot of flux/luminosity versus time
  • Spectrum: a plot of flux/luminosity versus
  • energy/frequency/wavelength (usually log-log)
  • Spectral Energy Distribution (SED): spectrum over a
  • broad energy range, usually radio through gamma-rays
  • (usually log-log)

The first AGN: 3C273

Optical spectrum

Optical image


What is an Active Galactic Nucleus?

  • A point-like source at the center of an
  • otherwise normal galaxy
  • Main defining property of an AGN:

Large luminosities from a compact region


What is an Active Galactic Nucleus?

  • A point-like source at the center of an
  • otherwise normal galaxy
  • Main defining property of an AGN:

Large luminosities from a compact region

What causes the AGN prodigious emission??


Spectral Energy Distribution of AGN

Non-thermal processes dominate AGN emission


Observational properties of AGN

  • Point-like source at center of host galaxy
  • Non-thermal continuum emission
  • Rapid flux variability
  • Broad (FWHM > 1,000 km/s) optical/IR emission lines
  • Narrow (FWHM < 1,000 km/s) optical/IR emission lines
  • Polarized emission
  • Extended components (radio jets and lobes)

What variability tells us

If variability is observed on a timescale Dtvar in

the source frame, then the radiation must be

produced in a region with size:

If the region is larger different parts would not

be causally connected and different timescale can

be observed. The minimum timescale is used to get

the source size.


Currently, ~1000 AGN are known and identified

  • They span a large range of redshifts: z=0.002 to z=6
  • (for comparison, the recombination era z=1,000
  • first protogalaxies at z=10-20)
  • Several thousands more expected in the next few
  • years from Chandra, XMM, XEUS, NGST, SIRTF
  • Active galaxies are 10% of the total number of galaxies
  • A further 10% of AGN are radio-loud

The multi-wavelength perspective

Observing AGN at different wavelengths is crucial to understand their complexity, as each wavelength probes different parts/processes of the same source

Example: the nearby active galaxy Centaurus A (z=0.0018)


Optical (NOAO)

Radio (NRAO)


Optical (NOAO)

Radio (NRAO)

Infrared (2MASS)


Optical (NOAO)

Radio (NRAO)

Infrared (2MASS)

X-rays (Chandra)


Hubble Law

  • At the beginning of the century, Edwin Hubble
  • discovered that the further away a galaxy is, the
  • faster it is receding from us:
  • V=H0D
  • where V=radial velocity of the galaxy, D=distance
  • and H0=Hubble’s constant.
  • Hubble Law implies the Universe is expanding

Cosmological redshift z

  • Shift redwards of a given wavelength caused by the
  • expansion of the Universe:

l1, t1

l0, t0

  • If Universe is expanding: R(t0)>R(t1)


andl0 > l1 (red-shift)

Example: wave on an expanding balloon


Flux and Luminosity

Assume a galaxy at a distance D is emitting light

isotropically at a given rate L(n) [energy per unit time]

or Luminosity

The light propagates on the surface of an expanding

sphere of radius D.

The amount of radiation we receive or Flux is

D=Luminosity Distance and is related to z (eq. 2.62)


Notation on Units

  • Luminosity: erg s-1
  • Flux: erg s-1cm-2
  • Distance: parsec (pc) and multiples
  • 1 pc = 3.09 x 1018 cm
  • = 3.3 light years
  • Frequency n (Hz)
  • Wavelength l (Angstroms, cm, …)

Homework Assignment

(due next week; 10 points)

  • The measured redshift from 3C273 is z=0.158,
  • and the measured optical flux at 5500 A is
  • F=3x10-10 erg cm-2 s-1. Its optical flux is observed
  • to vary on timescales of 1 day down to 1 minute.
  • Determine:
  • The luminosity of the quasar
  • The size of the emitting region in pc
  • Assume H0=75 km/s/Mpc and q0=0.5.
  • Extra Credit (5 points): Estimate the mass of the
  • black hole (Hint: Eddington luminosity may be useful)