Depiction of our own local group[Source: Terry Herter, Cornell]

A gallery of galaxies

http://www-astronomy.mps.ohio-state.edu/Gallery/galimgs.html

• composed of stars, gas, dust, molecular clouds
• 100,000 - 1,000,000 light years across
• may have 100 to 1000 billion solar masses
• 100 - 1000 billion solar luminosity
• big collections of stars, dust and other matter, held together by gravity, understandable using classical physics

*MW => Milky Way ~ 10¹¹ L_sun

Catalogued by Carl Seyfert in 1943

Common features

• unusually bright nuclei
• nuclear activity not due to multiple stars
• broad emission lines (rapid motion of atoms)
• continuous emissions (including in X-ray) not from stars
• variable magnitude over days to weeks

What do they look like?

http://www.cv.nrao.edu/~abridle/images.htm

Core-Halo Galaxies

• depiction of core halo galaxy [Source: Terry Herter, Cornell]

Lobe Radio Galaxies

• depiction of lobe radio galaxy [Source: Terry Herter, Cornell]

Energy Production and Energy Emission

Energy source in nucleus must have following characteristics:

• Luminosity on order of 10 L_MW
• Capable of producing gas jets on order of 100 kpc long
• Variable output fluctuates over weeks

Likely candidate for source of energy

• supermassive black hole with accretion disk

Characteristics of "supermassive" black holes

• 1 million to 10 billion M_sun
• Schwarzschild radii of 0.02 to 200 AU (~10 ls to 1 ld)
• luminosity derived from accretion disk of hot gases swirling around
• gasses get hotter as they wind their way in
• most light derived from inner portion of accretion disk (< 10 times Schwarzchild radius)

Evidence for black holes at center of galaxies

• look for high rotational velocities across very small radii
• our own galaxy may have a 2 million M_sun black hole at center
• HST believed to have found 2 billion M_sun black hole at center of M87 (giant elliptical) and 200 million M_sun black hole for M84

It takes energy, which means material must keep flowing onto the black hole otherwise the accretion disks blow themselves apart

• 1 M_sun / decade => ~ 10 L_MW
• 10,000 L_MW => 100 M_sun / year
• for largest black holes 10⁸ - 10⁹ M_sun are necessary