Astro/CSI 765

An Introduction to Active Galactic Nuclei (AGN)

Prof. Rita Sambrunarms@physics.gmu.eduhttp://www.physics.gmu.edu/~rms3-4165

Office hours: by appointment only

http://www.physics.gmu.edu/~rms/csi765

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

• HOMEWORK: 1. Reading from assigned papers 2. Writing essays/answering questionnaires3. 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 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??

Non-thermal processes dominate AGN emission

Spectral Energy Distribution of AGN

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 tvar inthe source frame, then the radiation must beproduced in a region with size:

vartcR

If the region is larger different parts would notbe causally connected and different timescale canbe observed. The minimum timescale is used to getthe 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) 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:

t1 t0

1)(

)(

1

0

1

10

tR

tRz

• If Universe is expanding: R(t0)>R(t1) Z>0

and 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() [energy per unit time]or Luminosity

The light propagates on the surface of an expandingsphere of radius D.

The amount of radiation we receive or Flux is

24

)()(D

LF

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 (Hz)

• Wavelength (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 quasar2. 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)