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Astro/CSI 765
An Introduction to Active Galactic Nuclei (AGN)
http://www.physics.gmu.edu/~rms/csi765
Prof. Rita Sambruna
[email protected]
http://www.physics.gmu.edu/~rms
3-4165
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
•
1.
2.
3.
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: 93-100 A
90-92 A87-89 B+
83-86 B
80-82 B75-79 C+
70-74 C
60-69 D
<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 image
The first AGN: 3C273
Optical image
Optical spectrum
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)
Optical spectrum of a quasar
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:
R  cDt var
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)
Optical (NOAO)
Radio (NRAO)
Optical (NOAO)
Infrared (2MASS)
Radio (NRAO)
Optical (NOAO)
Infrared (2MASS)
Radio (NRAO)
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
R(t 0)
l
0 l 1
z

1
l1
R(t1)
• If Universe is expanding: R(t0)>R(t1)
and l0 > l1 (red-shift)
Example: wave on an expanding balloon
Z>0
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
F (n ) 
L (n )
4D 2
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
• Wavelength
(Hz)
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:
1. The luminosity of the quasar
2. 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)