ACTIVE GALACTIC NUCLEI

THESE NOTES CONTAIN A FEW MORE DETAILS THAN I WENT OVER IN CLASS

Let's start with weird but not really active galaxies:

Our own Milky Way has filaments, fast moving stars
and a 2.6 million solar mass BH, but it isn't really active.

There are lots of mildly active galaxies, with lots of extra star formation:
STARBURST galaxies -- 100's of stars per year, but spread over some 100's of parsecs.

Other PECULIAR galaxies involve collisions or mergers between galaxies.

  • Sometimes produce strong spiral structure (e.g. M51, the "Whirlpool")
  • Sometimes leave long tidal tails (e.g. the "Antennae" galaxies)
  • Sometimes leave "ring" galaxy structures--an E passing through a S.

    BUT, the REAL active galaxies have CENTRAL ENGINES of IMMENSE POWER.

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    There are 4 MAIN CLASSES of AGN:

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    RADIO GALAXIES

  • All in Elliptical galaxies
  • Two oppositely directed JETS emerge from galatic nucleus

    For POWERFUL RGs:

  • The jets feed HOT-SPOTS and and LOBES on either side of the galaxy
  • Radio source sizes often 300 kpc or more ---
    much bigger than their host galaxies.
  • For powerful sources only one jet is seen:
    this is because of RELATIVISTIC DOPPER BOOSTING: the approaching jet appears
    MUCH brighter than an intrinsically equal receding jet.

    WEAKER RGs

  • don't have hot spots, and relatively more emission comes from their jets.
  • Head-tail radio galaxies arise when jets are bent by the
    ram-pressure of gas as the host galaxy moves through it.

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    QUASARS

  • At great distances, but still bright.
  • Therefore VERY LUMINOUS. --- MORE DETAILS SHORTLY

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    SEYFERT GALAXIES

  • Spiral galaxies with VERY BRIGHT, COMPACT NUCLEI
  • Have powerful emission lines

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    BL LACERTAE OBJECTS

  • Very rapidly varying in RADIO, OPTICAL and X-RAYS
  • Explained in terms of weaker radio jet pointing very close to our line-of-sight.

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    KEY QUASAR PROPERTIES

  • QUASI-STELLAR-OBJECT: (QSO): i.e., it looks like a STAR
  • BUT: NON-THERMAL SPECTRUM --> UV excess (not like a star)
    power seen from radio through X-ray and sometimes gamma-rays!
  • BROAD EMISSION LINES
  • VERY HIGH REDSHIFTS --> not a star, but FAR away.
    The current (2003) record redshift is z = 6.41, i.e., light emitted in
    FAR UV at 100 nm is received by us in the near IR at 741 nm!

    NEWER QUASAR DISCOVERIES

  • Only about 8% are RADIO LOUD
  • Most show some VARIABILITY in POWER
  • OVV (Optically Violently Variable) QUASARS change
    by 50% or more in a year and are highly polarized
  • QUASARS are AGN: surrounding galaxies detected, though small nucleus
    emits 10--1000 times MORE light than 10^{11} or more stars.

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    SEYFERT GALAXIES

    Sb, Sc galaxy with BRIGHT, SEMI-STELLAR NUCLEUS

    NON-THERMAL & STRONG EMISSION LINES

    VARIABLE in < 1 yr --> COMPACT CORE

  • Type 1: Broad Emission lines (like QSOs), strong in X-rays
  • Type 2: Only narrow Emission lines, weak in X-rays

    About 2% of all Spirals are SEYFERTS

  • Either 2% of all S's are always Seyferts OR
  • 100% of S's are Seyferts for about 2% of the time (MOST LIKELY)
  • OR 20% of S's are Seyferts for about 10% of the time
  • (or any other combination of fraction and lifetime)

    Seyferts are weak radio emitters.

    CONCLUSIONS ABOUT SEYFERTS

  • Fundamentally, they are WEAKER QSOs
  • Type 1: we see the center more directly
  • Type 2: dusty gas torus blocks view of the center

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    BL LACERTAE OBJECTS

    NON-THERMAL SPECTRUM:

  • Radio through X-ray (and gamma-ray)
  • Radiation strongly POLARIZED
  • HIGHLY VARIABLE in ALL BANDS

  • But (when first discovered) NO REDSHIFT, so distance unknown
  • Later, surrounding ELLIPTICAL galaxies found

    CONCLUSION: greatly enhanced emission from the AGN due to
    RELATIVISTIC BOOSTING of a JET pointing very close to us.

    BL Lacs + OPTICALLY VIOLENTLY VARIABLE QUASARS ARE
    OFTEN CALLED BLAZARS

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    AGN CONTAIN SUPERMASSIVE BLACK HOLES (SMBHs)

    KEY LONGSTANDING ARGUMENTS:

    ENERGETICS:

  • Powers up to 10^{48} erg/s
  • Even at 100% efficiency would demand conversion of
    about 18 M_sun/yr (= M_dot) into energy.
  • Nuclear processes produce < 1% efficiency.
  • GRAVIATIONAL ENERGY via ACCRETION can produce
  • between 6% (non-rotating BH) and
  • 32% (fastest-rotating BH)
  • L = G M_BH M_dot / R, with R the main distance from the
    Super Massive Black Hole (SMBH) where mass is converted to energy.

    TIME VARIABILITY:

  • Fastest times determined by speed of light:
  • t_var = R / c
  • t_var = 10^4 s --> R = 3 x 10^{14} cm = 10^{-4} pc
  • For L = 10^{47} erg/s, M_dot = 10 M_sun/yr we get
  • M_BH = 3 x 10^8 M_sun and R_s = 9 x 10^13 cm
  • So, R = 3 R_s
  • MUTUALLY CONSISTENT POWERS AND TIMESCALES.

    RECENT OBSERVATIONAL SUPPORT:

  • The Hubble Space Telescope has revealed that star velocities
    rise to very high values close to center of many galaxies.
  • Disks have been seen via MASERS in some nearby Seyfert AGN.
  • VLBI: radio jets formed within 1 pc of center.

    There are several other more technical lines of evidence
    also supporting the SMBH hypothesis for AGN.

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    UNIFIED MODELS FOR AGN

    Three main parameters: M_BH, M_dot, and viewing angle to
    the BH and accretion disk axis, theta

    Main ingredients:

  • SMBH < 10^-4 pc
  • accretion disk < 10^-1 pc (AD)
  • broad line clouds < 1 pc (BLR)
  • thick, dusty, torus < 100 pc
  • narrow line clouds < 1000 pc (NLR)
  • sometimes, a JET from < 10_2 pc to maybe 10^6 pc!

    FOR RADIO QUIET AGN:

    High M_BH, M_dot:

  • theta small: QSO is seen including AD and BLR
  • theta large: only NLR plus radiating torus:
    seen as UltraLuminous InfraRed Galaxies (ULIRGs)

    Low M_BH, M_dot:

  • theta small: Seyfert Type 1
  • theta big: Seyfert Type 2

    FOR RADIO LOUD AGN: (i.e. a strong JET)

    High M_BH, M_dot:

  • theta very small: Optically Violently Variable Quasar
  • theta small: radio loud quasar (QSR)
  • theta large: classical double radio galaxy (FR II type)

    Low M_BH. M_dot:

  • theta very small: BL Lac object
  • theta small: broad line radio galaxy (FR I type)
  • theta large: narrow line radio galaxy

    SO WE BELIEVE ALL TYPES OF AGN ARE BASED ON SMBHs:
    the big outstanding question is why do only some have jets.