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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

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Astro csi 765

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


Astro csi 765

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)


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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: 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


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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


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  • 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


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Lecture 1:

  • What is an AGN?

  • Historical discovery of AGN

  • The importance of the multi-wavelength

  • perspective

  • Notes and Useful quantities (some AGN lingo)


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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


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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)


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The first AGN: 3C273

Optical image


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The first AGN: 3C273

Optical spectrum

Optical image


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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


Astro csi 765

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??


Astro csi 765

Spectral Energy Distribution of AGN

Non-thermal processes dominate AGN emission


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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)


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Optical spectrum of a quasar


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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.


Astro csi 765

  • 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


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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)


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Optical (NOAO)


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Optical (NOAO)

Radio (NRAO)


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Optical (NOAO)

Radio (NRAO)

Infrared (2MASS)


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Optical (NOAO)

Radio (NRAO)

Infrared (2MASS)

X-rays (Chandra)


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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


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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)

Z>0

andl0 > l1 (red-shift)

Example: wave on an expanding balloon


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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)


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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, …)


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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)


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