Active Galaxies Today’s Lecture: Active Galaxies Quasars Seyfert Galaxies Radio Galaxies Supermassive Black Holes Homework 8: Due today Homework 9: Due classtime, Tuesday, April 22 Final Exam: Friday, May 2, 1:30 pm Hasbrouck 138 Help Session: Thursday, May 1, 1:30 LGRT 1033 Reading for today: Chapter 19 Reading for next lecture: Chapter 20

Note: if you are interested in attending, please respond by April 18

Universe of Galaxies Active Galaxies An interesting class of galaxies are those with active galactic nuclei (AGNs), include quasars, Seyferts and radio galaxies. The story of these objects started at radio wavelengths. Over a hundred billion galaxies in the “observable” universe.

Radio Astronomy The1950’s and 1960’s were the early days of radio astronomy. Both thermal (related to temperature) and non-thermal (not related to temperature) emission was detected at radio wavelengths. Non-thermal emission in astronomical sources is due to the acceleration of relativistic electrons by a magnetic field – called synchrotron emission. Both produced by the acceleration of charged particles. Thermal Non-thermal

Radio Astronomy The thermal and synchrotron spectrum are very different. Hot, ionized gas (HII regions) produce thermal radio emission. Supernovae remnants (SNR) produce synchrotron emission. HII Region Optical Radio SNR Optical Radio

Active Galactic Nuclei (AGN) In addition to supernova remnants, some ordinary looking galaxies were also strong radio synchrotron emitting sources. However, one class of radio synchrotron emitting objects had an optical appearance similar to a star, but were producing very strong radio emission. These were called quasars (quasi-stellar radio sources).

Quasar 3C273 In the early 1960’s Maarten Schmidt recognized that the optical spectra of quasars were odd, because the spectral lines were highly redshifted (not by today’s standards). Right, an early spectrum of 3C273 showing the hydrogen lines. Quasar 3C273 has very large redshift.

Quasars The optical spectra of quasars were also unlike any star. The spectra had broad optical emission lines. 3C273 also shows an optical jet. Quasar 3C273 optical image and spectrum

In 3C273, H  ( rest = nm was observed at nm) z =   = ( )/656.3 = Wavelength shift of 15.8% means that the recessional velocity is x c = 47,400 km s -1. Using Hubble's Law: d = v/H = 47,400 km s -1 /71 km s -1 Mpc -1 = 668 Mpc 3C273 has an apparent visual magnitude of +12.9, thus its absolute visual magnitude is: m V – M V = 5 log (d/10pc) = 39.1, so M V = C273 has a luminosity of ~3 x10 12 solar luminosities, or more than 100 times more luminous that the entire Milky Way.

Quasars light output varies rapidly on timescales of weeks, days and even many hours. Sets limit on size scale of light source. Diameter of light source must be less than a few light days (size ~ Δt c) or 200 AU. Emitting regions very small !!! How do you produce so much luminosity is such a small region ???

Quasar host galaxies The quasars are so bright, difficult to see underlying galaxy. Now know that the quasars are bright emission regions in nuclei a distant host galaxies.

Quasar host galaxies The galaxies hosting quasars often appear very disturbed. The brightest quasars can be 10,000 to 100,000 times more luminous than the Milky Way, the energy all being produced in a very small region.

However, there are some local galaxies with similar, but weaker activity, these AGNs are associated with both spiral (Seyfert galaxies) and elliptical galaxies (radio galaxies). Most quasars are highly redshifted, suggesting that the epoch of “quasar activity” has ceased in most galaxies today. 3C273 is one of the closest quasars at a distance of ~700 Mpc. Quasar activity is mostly in the past.

Seyfert galaxies First identified by Carl Seyfert in Often spiral galaxies, where much ofl the luminosity comes from a small (unresolved) region at the center of the galaxy – the galactic nucleus. Similar to quasars, but much less luminous. short exposure long exposure NGC 4151

The nuclei in Seyfert galaxies, like quasars, are dominated by emission lines. These are AGNs and this activity is obvious in optical images.

Radio Galaxies: These galaxies are radio bright and associated with giant elliptical galaxies in rich clusters. Their large radio luminosities is due to synchrotron emission – they produce a million times more radio luminosity than normal galaxies. Often the radio emission extends well beyond the optical galaxy to sizes as large as 10 Mpc. The radio emission often arises from two large lobes located on opposite sides of the galaxy (so called double-lobed radio galaxies). Often connected by “jets” back to the nucleus of the galaxy. Like quasars and Seyfert galaxies, the optical specturm of their nucleus shows either broad or narrow emission lines.

Closest radio galaxy at a distance of 4 Mpc Radio Light Centaurus A Visible Light Lobes due to synchrotron emission from relativistic particles ejected from nucleus that interact with the magnetic field in the intercluster material.

Centaurus A

The total angular extent of the radio emission in Centaurus A is about 6 degrees !! An extent of order 500,000 parsecs (500 Kpc). We can trace the jets down to the central engine seeing fine details on the scale of 1 parsec.

Radio image of Cygnus A Distance of 100 Mpc, yet one of the brightest radio sources in the sky !! Insert shows optical image of galaxy. Lobes extend over 100 Kpc and connected by jet to central radio source.

More radio images showing radio lobes connected by a jet to the central source. It is believed that all of the energy is being generated in the central very small region and carried out by high speed particles in the jets to the radio lobes

This giant elliptical galaxy is ~100 Kpc across and has a “jet” of material coming from the nucleus. In some radio galaxies, the jets can be seen at visible wavelengths. The giant elliptical galaxy M87 is shown below:

Jet in M87 (Virgo A) at radio wavelengths. ~2 kpc galaxy nucleus, i.e. the radio core T he jet is apparently a series of distinct “blobs”, ejected by the galaxy nucleus, and moving at up to half the speed of light. Inner jet, length 17 pc Large scale

Not only do elliptical galaxies have radio jets, but so do some quasars and Seyfert galaxies. We believe that the activity in the nucleus of all of these galaxies are produced in the same manner – call these Active Galactic Nuclei (AGN). Radio images of the quasar 3C 175 (above) and the Seyfert galaxy 3C 219 (right)

So how can we produce so much energy is such a small region of space ???? None of the types of astronomical objects that we have discussed to this point could come even close to producing the amount of energy needed. Would need the energy output of an entire galaxies – but needs to fit in a volume about that of the solar system. Based on all the evidence, the only plausible explanation is a supermassive black hole. But how do supermassive black holes help ??? The energy source is the accretion of material onto a supermassive black hole (release of gravitational potential energy). Typical black hole mass is 10 8 solar masses.

As the gas spirals inward forming an accretion disk, friction heats it to high temperatures. Emission from the accretion disk at different radii (T > 10 5 K) accounts for optical thru soft x-ray emission. Some of the gas is driven out into “jets”, focused by twisted magnetic field. Jets composed of high speed (Lorenz factor 10 4 ) material funneled by a twisted magnetic field.

Before entering the black hole, some fraction of the kinetic energy of the infalling matter is converted into thermal energy. Matter is heated to high temperature in the accretion disk and radiates away the original gravitational potential energy of the matter. Black hole radius is R s = 2 G M/c 2, which is about 2 AU or 0.25 light hours for a 10 8 solar mass black hole. What energy is available (via gravitational potential energy) for a mass m falling from far away to the R s ? How efficient is the energy production ?

The gravitational potential energy released is given by: E = G M m/R s Substituting equation for R s = 2 G M/c 2 : E = ½ m c 2 An energy equal to half of the rest mass energy of the infalling gas is converted to kinetic energy. If the mass is decelerated (friction in the accretion disk), kinetic energy can be converted to thermal energy and then radiated away before the matter enters the event horizon of the black hole. E phot = η m c 2 where η is the efficiency (< 0.5) The efficiency of this conversion is typically ~20% (η=0.1).

An AGN luminosity is then determined by the rate material is accreted. If mass falls in at a rate of dm/dt, then the luminosity of the AGN is: L = η (dm/dt) c 2. For a accretion rate of 1 solar mass per year, the luminosity is: L = 0.1 (6.3 x gm s -1 ) c 2 = 5.7 x ergs s -1 or 1.5x10 12 solar luminosities Enormous luminosities are possible. If we know an AGN’s luminosity, we can deduce infall rate. Note L is independent of a black hole mass.

Eddington Limit: Since photons have momentum, they exert a force on the infalling material (the force is proportional to the luminosity). When the force applied by the photons (the AGN luminosity) exceeds the gravitational force, accretion is halted. We call this luminosity limit the Eddington limit: L Edd = 3.3x10 12  (M bh /10 8 M ⊙ ) in L ⊙ So black hole mass does matter. Note for a 10 8 M ⊙ black hole the maximum rate of accretion is only 0.7 M ⊙ per year.... So...can you just shovel in mass at higher and higher rates to create a superluminous AGN ?

First consider the accretion disk. Hubble images: Evidence for Supermassive Black Holes NGC 4261 is an elliptical galaxy in the Virgo Cluster and is estimated to contain a 5x10 8 solar mass black hole. Artist's concept

The Doppler shift of gas orbiting in the accretion disk gives a measure of the mass of the central black hole. To the right is another Virgo Cluster elliptical galaxy M84. From the rotating gas disk a black hole mass of 1.5x10 9 solar masses is inferred.

Returning to Virgo A (M87) at a distance of about 16 Mpc. Shows radio lobes and jets.

Mass of black hole about 3x10 9 solar masses. Hubble image and spectra of accretion disk shows enormous orbital motions around a central supermassive black hole

This early epoch is when galaxies and supermassive black holes grew to their present mass and this black hole accretion produced their enormous luminosities. A Supermassive black hole is currently the only model that can produce the enormous energy in such a small volume of space. Masses of these supermassive blackholes range from 10 6 to 6x10 9 solar masses. AGNs were much more active in the past when galaxy interactions and merging was more common. We believe that a central supermassive black holes are found in all galaxies, but not all “active”.