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Astro/CSI 765 An Introduction to Active Galactic Nuclei (AGN)
Prof. Rita Sambruna 3-4165 Office hours: by appointment only
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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)
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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: A B C A B D B C < 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? perspective
Historical discovery of AGN The importance of the multi-wavelength perspective Notes and Useful quantities (some AGN lingo)
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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
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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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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
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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??
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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.
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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 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
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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 = 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, …)
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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=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)
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