1. Astro/CSI 765 An Introduction to Active Galactic Nuclei (AGN)
Prof. Rita Sambruna
3-4165
Office hours: by appointment only

2. Outline of the course PRE-REQUISITES: PYHS 502, 613, or Astro530
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)

3. Structure of the course
LECTURES: review of concepts, expansion of reading material
HOMEWORK:
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: A B C
A B D
B C < F

4. 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

5. 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

6. Lecture 1: What is an AGN? perspective
Historical discovery of AGN
The importance of the multi-wavelength
perspective
Notes and Useful quantities (some AGN lingo)

7. Nucleus light overwhelms the light from the galaxy
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

8. 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)

9. The first AGN: 3C273
Optical image

10. The first AGN: 3C273
Optical spectrum
Optical image

11. 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

12. What causes the AGN prodigious emission??
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??

13. Spectral Energy Distribution of AGN
Non-thermal processes dominate AGN emission

14. 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)

15. Optical spectrum of a quasar

16. 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.

17. 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

18. 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)

19. Optical (NOAO)

20. Optical (NOAO)
Radio (NRAO)

21. Optical (NOAO)
Radio (NRAO)
Infrared (2MASS)

22. Optical (NOAO)
Radio (NRAO)
Infrared (2MASS)
X-rays (Chandra)

23. 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

24.  Cosmological redshift z Example: wave on an expanding balloon
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)
Z>0
and l0 > l1 (red-shift)
Example: wave on an expanding balloon

25. 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)

26. Notation on Units = 3.3 light years 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, …)

27. 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=3x 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)