2 What are Active Galaxies? Active galaxies have an energy source beyond what can be attributed to stars. The energy is believed to originate from accretion onto a supermassive blackhole.Active galaxies tend to have higher overall luminosities and very different spectra than “normal” galaxies.“non-stellar” radiationSome classes of active galaxies:QuasarsSeyfert galaxies (Type I and Type II)Radio galaxiesLINERsstellar, blackbody radiation
3 Quasars First discovered in the 1960s. Detected radio sources with optical counterparts appearing as unresolved point sources.Unfamiliar optical emission lines.Maartin Schmidt was the first to recognize that these lines were normal Hydrogen lines seen at much higher redshifts than any previously observed galaxies.D = 660 Mpc (2.2 billion light years) for 3C2731340 Mpc (4.4 billion light years) for 3C 48L = 2 x Lsun for 3C273.Within ~2 years, quasars were discovered with: z > 2 and L LsunMost distant QSO discovered today - z = 6.42
4 QuasarsMB < -23, strong nonthermal continuum, broad permitted (~104 km/s) and narrow forbidden (~102-3 km/s) emission linesRadio quiet (RQQ): elliptical or spiral host galaxiesRadio loud (RLQ): 5-10% of all quasars, elliptical hostsBroad Absorption Lines (BAL) Quasars: normal quasars seen at a particular angle along the l.o.s. of intervening, fast-moving material.High-ionization (HIBAL): Ly, NV, SiIV, CIVLow-ionization (LOBAL): AlIII, MgII
5 If we block out the light of luminous quasar, we can see evidence of an underlying host galaxy. Quasar hosts appear to be a mixed bag of galaxy types - from disturbed galaxies to normal E’s and early type spirals.
6 Seyfert galaxies were first identified by Carl Seyfert in 1943. He defined this class based on observational characteristics:Almost all the luminosity comes from a small (unresolved) region at the center of the galaxy – the galactic nucleus.Nuclei have MB > -23 (arbitrary dividing line between quasars/seyferts)short exposurelong exposureNGC 415110000 times brighter than our galactic nucleus!
7 Seyfert galaxy spectra fall into two classes: broad emission line spectra (like quasars) and narrow emission line spectra.Seyfert 1s:Broad and narrow linesSeyfert 2s:Only narrow lines
8 NLAGNs can be differentiated from normal emission line galaxies through the flux ratios of certain emission lines.The shape of the underlying ionizing source (energy source) determines how many photons are available to produce particular emission lines.
9 Variability in AGNsQSOs and Seyfert nuclei have long been recognized as variableOptical flux changes occur on timescales of months to yearsCause of variability? – instabilities in accretion disk, SN or starbursts, microlensing…..Quasar light curve ~25 yearsSeyfert light curve over ~11 monthsHawkins 2002Variability occurs at most wavelengths - X-rays through radioThis indicates that the fluctuations are originating from a very tiny object.
10 Why does rapid variability indicate small physical size of the emitting object? Consider an object like the Sun. Any instantaneous flash would appear “blurred” in time by t = RSun / c.RSunRSunobserverTime Delay = t = RSun / c700,000 km / 300,000 km/s = 2.3 secSeyfert continuum luminosity varies significantly in less than a year (some variation occurs on timescales of days or weeks.This implies an emitting source less than a few light-weeks across!
11 BlazarsStrongly variable, highly polarized nonthermal continua, weak/absent emission linesVariability faster and higher amplitude than normal quasars and SeyfertsBL Lac - high polarization, emission lines have low equivalent widthOVVs (Optically Violent Variables) - lower polarization, emission line EW decreases as continuum brightensLight CurveSpectrum
12 Radio Galaxies Emit most of their energy at radio wavelengths Emission lines from many ionization statesNucleus does not dominate galaxy’s emissionHost galaxies are Elliptical/S0Radio morphology first classified by Fanaroff & Riley (1974)FR I: less luminous, 2-sided jets brightest closest to core and dominate over radio lobesFR II: more luminous, edge-brightened radio lobes dominate over 1-sided jet (due to Doppler boosting of approaching jet and deboosting of receding jet)Spectroscopic classification of radio galaxiesNLRGs (Narrow line …): like Seyfert 2s; FR I or IIBLRGs (Broad line …): like Seyfert 1s; FR II only
14 Centaurus A FR II radio galaxies: most emission comes from lobes Radio “Light”FR II radio galaxies: most emission comes from lobes0.8 MpcCentaurus AVisible LightThe radio lobes span about 10 degrees on the sky!Lobes consist of material ejected from the nucleus.
15 Radio image of the FR II radio galaxy Cygnus A. ~1 MpcThis galaxy also has HUGE radio lobes.The thin line through the galaxy is a jet ejected from the nucleus.The lobes occur where the jets plow into intracluster gas.
16 FR I radio galaxy: most of the energy comes from a small nucleus with a halo of weaker emission in a halo around the nucleus.Visible image of the core-halo (FR I) radio galaxy M87.This giant elliptical (E1) galaxy is ~100 Kpc across.It has a “jet” of material coming from the nucleus.
17 Close-up view of the jet in M87 at radio wavelengths. ~2 kpcgalaxy nucleus, i.e. the radio coreThe jet is apparently a series of distinct “blobs”, ejected by the galaxy nucleus, and moving at up to half the speed of light.The jet and nucleus are clearly non-stellar.
18 LINERs and ULIRGs - Starburst or AGN? What is a starburst? May result from a galaxy collision/mergerGas streams converge from different directions causing shocks which compress material and trigger star formationGas which loses enough angular momentum falls into the galaxy center bar formation funnels more gas inward violent star formation near center of disk and further outNuclear close-up (HST) of NGC 1808 starburst galaxy. Galaxy has barred-spiral morphology.
19 LINERs Low-Ionization Nuclear Emission Region Narrow low-excitation emission linesWeak nonthermal continuumSpiral host galaxiesObserved emission could be due to AGN or shocks/winds from a starburstSome appear as unresolved compact sources in the UVSome have radio sources: AGN or supernovae remnant?
20 ULIRG’s - Ultra Luminous IR Galaxies First detected in IRAS all-sky surveyGalaxies that emit most of their light in IR - LIR > 1012 LsunFew in local universe; most beyond z > 1Nearly all are undergoing mergers - forming E’sIR light is likely a combination of dust reprocessed AGN emission and starbursts.Some AGN may manifest as ULIRGs during different stages of evolution.Nicmos Near-IR Image of IRAS selected ULIRG
21 What Powers Active Galactic Nuclei?? A compact central source provides a very intense gravitational field. For active galaxies, the black hole has MBH = MsunInfalling gas forms an accretion disk around the black hole.(3) As the gas spirals inward, friction heats it to extremely high temperatures; emission from the accretion disk at different radii (T>104 K) accounts for optical thru soft X-ray continuum.(4) Some of the gas is driven out into “jets,” focused by magnetic fields.
22 Broad Emission line region photoionized by continuum emission; size is ~few light-days to months; densities > 109 /cm3; stratified (higher-ionization lines from smaller radii)Narrow Emission line region also photoionized; size is ~10 to 1000 pc; densities ~ /cm3; complex morphology2Seyfert 1Obscuring Torus of dust is believed to form around perimeter of accretion disk
23 Unified Theory of Active Galaxies 2Observer is looking at blackhole “edge-on” through the surrounding dusty torus - does not see broad emission lines produced by gas near BHSeyfert 1Observer is looking into the center of the accretion disk, viewing motions of gas near blackhole - sees broad emission lines
24 How efficient is the energy production? Before disappearing into the event horizon of a blackhole, some fraction of the infalling mass is converted into energy. Matter is heated to high temps by dissipation in accretion disk and radiates away its gravitational potential energy.BH radius is Rs=2GM/c2 = 0.25 M8 light hours (which sets minimum variability timescale). Smallest stable orbit is at 3Rs. Max efficiency occurs when all potential energy released during fall from infinity to 3Rs is extracted. GR gives efficiency = 6% to 40% depending on BH rotation.Example: By consuming 1 – 10 solar masses per year, black hole accretion disk can radiate ~100 – 1000 LMilkyWay.How efficient is the energy production?
25 Direct evidence of the blackhole/accretion disk hypothesis: HST image of the core of the lobe radio galaxy NGC 4261radio lobesgalaxy nucleus
26 Velocities derived from Doppler shifted lines on either side of nucleus require ~3 billion solar masses. a blackhole!
27 Hosts and Environments Most quasars, NLRGs/BLRGs, blazars are E/S0 hosts (some early type spirals for radio quiet quasars)Seyferts/LINERs are typically spiralsThe maximum luminosity of the AGN correlates with the bulge mass (Ferrerese et al. 2000) - larger bulge/ greater mass BHBars appear to be no more common in Seyferts than normal galaxies (Mulchaey & Regan 1997).Conflicting evidence regarding whether or not Seyferts are found in more interacting systems than normal galaxies (Dahari et al. 1984; DeRobertis & Yee 1988). May be that minor mergers are more important than major mergers for instigating AGN.Generally, luminous AGN tend to be in denser than average environments and low-luminosity AGN in normal/slightly dense environments.
28 It is now believed that most if not all galaxies contain supermassive blackholes in their nuclei. Whether or not these galaxies appear as Active Galaxies depends on whether or not fuel is available in the vicinity of BH.The Milky Way is believed to harbor a supermassive blackhole in the nucleus!Sagittarius A:bright radio source at the center of the GalaxySagittarius (Sgr) A*:object at the very center of the Galaxy a million times more luminous than the Sun (IR, radio, X-ray, and gamma ray source)
29 Quasars were more common in the past - during the epoch of galaxy formation What’s the connection?Black Holes form in the centers of young Galaxies.Black Holes “shine” as Active Galaxies (Quasars) until the fuel (infalling gas) is used up.Most Quasars are now gone, but the Black Holes remain.
30 Active Galaxies as part of Galaxy Evolution As small galaxies merge to form larger ones, blackholes may form at the nucleus.With plenty of fuel available early on, the galaxy light is dominated by emission of the blackhole (Quasar).Additional mergers and depletion of fuel may result in powerful radio galaxies and Seyfert galaxies.Further fuel depletion results in a normal galaxy with a dormant blackhole at the nucleus.
Your consent to our cookies if you continue to use this website.