Transcript point-like
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 ) 4D 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)