Active Galaxies

A Short Survey

All Galaxies are active to some extent:• For "normal"

galaxies, we can think of the total energy output as the sum of stellar emissions

• However, Astronomers label a galaxy as active when it emits high energy radiation (infrared, radio, UV, and X-ray) beyond what the stars alone produce

Quasar Discovery

• Maarten Schimdt examined 3S 273• Exceedingly bright and small

• Small as in quasi-stellar size− Outshined its galaxy therefore it seemed alone

• Z = 0.16 (see next slide)− 2 Gyr away

• Sloan DSS and 2o Field Galaxy Survey have found over 1 million

• Their emission spectra was distinguished for galactic stellar absorption spectra

• Now all are seen to be embedded in a galaxy

Z Time

Another Depiction

General Properties

• Active galaxies are seen throughout time, but the peak period for quasars is at redshift z=2, about 9 Gyr ago, corresponding to a time when star formation was also peaking• A coincidence? Yes, star formation is now

decreasing!• About 1–12 light months in size but with

the energy of 100s of galaxies

End of an era…

• The Milky Way is about 3% gas and 97% stars, not including dark matter

• When this 3% is used up, even though supernovae and planetary nebula will return a small fraction to the interstellar medium, star formation will pretty much be over • Unless Smith’s cloud replenishes our galaxy• A huge cloud of hydrogen gas, is heading toward our

Milky Way Galaxy at 250 kilometers per second

• This shows the rate of star formation as a function of time• Mass of stars per Earth Volume is a proxy for

rate

David Sobral (Leiden Observatory)

NGC4261

Formation

• The early Universe was smaller; dark matter caused greater clumping of stellar material

• Likely low-mass (not so super) MBHs (1000Msun) merged into SMBH• Perhaps like in globular cluster

Centauri with a MBH of 40k Msun

• M15 may have a wussy MBH of 1700 Msun

Driving Force

• The powerplant for all the activity is the supermassive black hole in the nucleus of each galaxy

• This mass of the SMBH can be from a million to 10 billion solar masses

Consistent Mass

• As can be seen from the graph, there is a relationship between the mass of the SMBH and the rotation speed of the galaxy’s stars

• Always ~ 0.2% of nuclear bulge

Accretion

• Gas and occasionally entire stars form a disk around the SMBH, swirling around, waiting to fall in.

• The gravitational energy given up by infalling matter produces the radiant energy

Accretion disk

• Accretion disk for NGC4261

• 21cm line from VLBA (right)

• HI absorption

Radiant Energy

• Dropping matter into a SMBH turns out to be ten times more efficient than fusion

• Active galaxies can emit as much light a 1014 suns!

• They do this in a region too small to be seen by most telescopes, ~ 1 parsec

• A jet along galactic north and south

TpEdd

cGMmL

4

TpEdd

cGMmL

4

1030kg X 1017 (mc2) X 10% = 1046 J = 1053 ergs (a Type II SN every year (or more!))

Galaxy Cygnus A

Jet v = 95%c for hundreds of Kpc

Driving Star Formation

Artist’s Rendering HE0450-2958

The quasar’s intense wind blew proto-stellar material out into the disk regions and provided the shock wave for star formation

Evidence of “Inside-Out”

• Dark galaxies detected by VLT• Small, gas-rich galaxies in the early

Universe • Inefficient star formation by

themselves

Dark galaxy illuminated by

quasar

Hypervelocity Stars

• Stars that have been so accelerated by the SMBH that they are shot out of the galaxy

• Usually 1 of a binary pair• 1 falls into a tighter orbit, the other gains enough

momentum and energy to escape• Speed: 1.6 million miles per hour

• Here to Mars in 2 days!

• An HV star has been observed escaping from the Large Magellanic Cloud, implying it has a SMBH

Motion Around a SMBH

At our galactic center…

A Variety

• All AGN are fundamentally the same, just seen from different angles and at different stages of its life

• Quasar: Quasi-stellar Radio Source• Blazer

• Quasar seen from the “top”• Massive synchrotron radio emissions (radio loud)• Like a Quasar but much more variable

• Seyfert Galaxy• First identified 1943 (Carl Seyfert)• Radio quiet, strong IR, UV, and X-ray

• Many other kinds that we must regretfully skip

Seyferts• Unusually bright nucleus• Unusual spectrum indicated high

speed gas emission

NGC4151

Blazar

• Highest energy• Like a Seyfert with one jet pointed

towards Earth• Variable output

• +/- 10X a Quasar• Probably due to an uneven flow of

material into the SMBH

• Widest range of frequencies, radio to Gamma ray

Why so few nearby (now)?

• As low-mass ‘seed’ SMBH coalesced in high-mass ones, they blew material (numnum) away, starving themselves• Also blew enough proto-stellar

material away so that new stars in the core are rare, only Type II are found in abundance

• Likely all galaxies had a quasar phase

• Over time they settle down

For Us…

• Active galaxies give a good, if skewed view of the early Universe

• A clue, perhaps, that SMBH formed early on• Bottom up vs top down• That stars formed before galaxies• That early galaxies has an abundance of cold gas• That the SMBH was instrumental in widespread star

formation

So there’s more to learn!