Introduction to AGN • General Characteristics • History • Components of AGN • The AGN Zoo 1 AGN – What are they? Active galactic nucleus – compact object in the gravitational center of a galaxy that shows evidence for a strong nonstellar continuum AGN are characterized by: • High luminosity • Continuum radiation over a broad λ range – radio to γ-rays • Rapid variability (time scales of days or even hours) AGN tend to have: • Unusually blue colors / strong UV excess • Emission lines with significant widths (≥ 300 km/sec) Basic problem: • What physical mechanism generates so much luminosity (L > bol 1043 ergs s-1) in such a small volume (radius < 10 light days?) 2 A Brief History of AGN • E.A. Fath (1908): discovered strong emission lines in the spiral “nebula” (now galaxy) NGC 1068 • C.K. Seyfert (1943, ApJ, 97, 28) obtained high dispersion spectra of 6 spiral galaxies with high excitation nuclear emission lines – NGC 1068, 1275, 3516, 4051, 4151, 7469 – broad emission lines (5000 km/s) attributed to Doppler motions • Various radio surveys (1950s; 3C, PKS, etc.) discovered sources identified optically as quasi-stellar radio sources (quasars) • M. Schmidt (1963) realized that broad lines in the quasar 3C 273 were redshifted nebular lines (z = 0.158) • Eventually, it was realized that quasars (and optically discovered QSOs) are distant, high-luminosity analogs of Seyfert galaxies • Khachikian and Weedman (1974) defined two types of Seyferts: – Seyfert 2s: narrow permitted and forbidden emission lines – Seyfert 1: same lines as Seyfert 2s plus broad permitted emission lines 3 Optical Spectra of Seyfert Galaxies (HST/FOS spectra) broad emission narrow emission line (Hβ) line ( [O III] ) Seyfert 1 continuum Seyfert 2 4 Mass Outflow in Seyfert 1 Galaxies (HST UV Spectra) NGC 4151 Intrinsic absorption Akn 120 No intrinsic absorption Milky Way absorption 5 Observed Spectral Components of AGN (and probable physical components) Spatially Unresolved: • Optical/UV/soft X-ray continuum à accretion disk • Hard X-ray continuum (E > 1 keV) à hot X-ray corona • IR thermal emission à dusty torus (or wind?) • Broad emission lines à broad-line region (BLR) • Intrinsic UV/X-ray absorption lines à mass outflow Spatially Resolved: • Narrow emission lines à narrow-line region (NLR) • Ionized gas in the host galaxy à extended narrow-line region (ENLR) • Radio synchrotron radiation àradio jets/lobes Drum roll, please à 6 “Unified Model” of AGN Seyfert 1 NLR BLRG QSO I d = 1 – 1000 pc n = 102 – 106 cm-3 scattering BLR region d = 10 light days n = 108 – 1011 cm-3 UV/X-ray SMBH +acc. Absorbers? disk + corona d ≈ light hours “torus” radio jet Seyfert 2 NLRG QSO 2 7 The AGN Zoo • Classified according to the appearance of their optical spectra, luminosity, radio power: • Seyfert galaxies (including subtypes) • Broad-line radio galaxies (BLRG) • Narrow-line radio galaxies (NLRG) • Quasi-stellar radio sources (quasars) • Quasi-stellar objects (QSOs or radio-quiet quasars) • Blazars: BL Lac objects and Optically Violent Variables (OVVs) • Low-ionization nuclear emission-line regions (LINERs) • Ultraluminous IR galaxies (ULIRGs) – most are starburst galaxies, but a minority may be AGN • Where does it stop? – evidence for very mild activity in the vicinity of the Milky Way’s SMBH 8 Seyfert Galaxies • Nucleus - absolute blue magnitude: M > -21.5 B (to distinguish from quasars) • L = 1043 - 1045 ergs s-1 Bol • “Classic” Seyferts: z < 0.1 (SDSS discovered higher z Seys.) • Broad permitted lines (FWHM = 800 - 8000 km s-1) from BLR • Narrow permitted and forbidden lines (FWHM = 200 - 500 km s-1) NLR – Seyfert 1: both BLR and NLR, strong nonstellar continuum – Seyfert 2: only NLR, weak continuum (mostly stellar) • Spectropolarimetry (Antonucci 1985) shows hidden BLR in some Seyfert 2s: – Balmer lines scattered into the line of sight by electrons and/or dust àUnified model 9 • Additional Osterbrock types: - Seyfert 1.5: narrow permitted components are easily seen - Seyfert 1.8: weak broad Hα and Hβ - Seyfert 1.9: only weak broad Hα detectable - Narrow-line Seyfert 1 galaxies (NLS1s) (not Seyfert 2s!) FWHM (BLR) = 800 – 2000 km/sec 1) strong Fe II (high density region like other BLRs) 2) strong excess below 1 –2 keV and rapid X-ray variability • Seyferts are weak radio sources (radio blobs rather than jets) • Strong X-ray sources at E > 2 keV - Seyfert 2 galaxies are weak in soft X-rays (E < 2 keV), due to absorption by a large column of gas (torus?) • Seyfert host galaxies are almost always spirals • Most well-known Seyferts are NGC galaxies or Markarian galaxies (strong UV excess in objective-prism surveys) 10