Black Holes In AstronomyBlack Holes In Astronomy

credit: NASA

Black holes turn our intuition upside-down! But first, a bit of history

Einstein 1915:

established equations describing how matter curves space and warps time

Karl Schwarzschild 1916: found the simplest, spherically-symmetric space-time map

Physics of an accreting Black Hole

horizon

static limit

ergosphere

Physics of an accreting Black Hole

horizon

staticlimit

ergosphere

radiation

magneticfields

jet

jet

Black-hole accretion simulation

McKinney & Gammie 06McKinney & Gammie 06McKinney & Gammie 06

Making black holes from massive stars

• Mass > 25 Solar• Lives < 10 million years• Ends life with gravitational collapse forming a black hole

60 km

Stellar-mass black hole(a few times moremassive then the Sun)

credit: NASA

• Stellar-mass black holes are “seen” only if they have a close stellar companion

• What’s observed is a very hot accretion disc

• The mass can be measured from the companion’s motion

Stellar-mass black holes

Supermassive black holesSupermassive Black Holes

Historical timeline:

1962. Quasars discoveredat billions of light years bySchmidt.

1964. Zeldovich & Novikovand Salpeter argue thatQuasars are powered by theaccretion of gas ontosupermassive black holes

1969. Lynden-Bell argues thatsupermassive blackholes should existat the centers of manygalaxies.

1996+. Hubble Telescope observations,analyzed using Martin Schwarzschild’s method,establish that supermassive black holes exist in the large majority of galaxies with a central bulge.

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Jets and lobes of Cygnus A

Carilli et al.

Supermassive black holes are the most powerful engines in the Universe

Jet-driven bubblesClick to edit Master text styles

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Allen et al. 06

Supermassive black holes

Relationship between SBHs and host galaxies

Black-hole mass

galacticvelocity dispersion

Gebhard etal, 00Ferrarese &Merritt 00Tremaineet al. 02

Main question: how do SBHs form and grow tocurrent large masses (0.1 million to 10 billion solar)?

Observational answer: SBH grow from smaller seeds and acquire most of their mass by gas accretion.

1. Observe light from systems with SBH accretion (quasars and active nuclei)2. Divide by 0.3c to obtain mass3. Compare with SBH mass density in the local Universe

2

Good agreement!Soltan 1982Yu & Tremaine 2002

Andrea Ghez,UCLA

Keck telescope, Hawaii

Black hole at the center of our Galaxy, the Milky Way.

European Southern ObservatoryVLT, Chile

Reinhard Genzel,MPE

Reinhard Genzel,MPE

Andrea Ghez,UCLA

Keck telescope, Hawaii

Supermassive Black Hole at the center of our Galaxy, the Milky Way

European Southern ObservatoryVLT, Chile

Keck Stellar Orbits.

VLT stellar orbits (reconstructed in 3 dimensions), of stars moving around the 4 million solar masses Supermassive Black Hole, SgrA*.

Do black holes ever find partners?

Stellar-mass black hole binaries

or

Marriage by attraction

Arrangedmarriage

LIGO: will measure stellar-mass BH mergers

LIGO: will measure stellar-mass BH mergers

Hubble Komossa et al 02 (Chandra)

Merging Galaxies Merging Supermassive black holes?

Supermassive black hole binaries

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Black holes have no hair!

4 million solar mass, spin 0.6, BH is looking for other BH.

You know what we do.

Dance and merger of black holesCaltech+CornellDance and merger of black holes

credit: Caltech+Cornell

Binary with spinBinary without spin

Eccentric inspiral

Circular inspiralScott Hughes, MIT

“Listening” to gravitational waves from black-hole mergers

Hubble Komossa et al 02 (Chandra)

Merging Galaxies Merging Supermassive black holes?

Supermassive black hole binaries

Theorists dream of mergers: Begelman, Blandford, &Rees 1982:

10 kpc 2pc 1pc 0.01pc merger

Dynamicalfriction

scattering wishfulthinking

gravitationalwaves

gasnon-sphericalpotential

Anotherblack hole

Orbital evolution: the chirp – see first lecture!

Population of chirping sourcesAmount of time spent near frequency f

Number of sourcesin the frequency banddf scales with dt

Power spectrumfrom incoherentsuperposition

Characteristicstrain

GRprediction!!

Gravitational waves: a (Gaussian, stochastic) background!

Phinney 01Jaffe & Backer 03Wyithe & Loeb 03Sesana et al. 07, 08, 09Ravi et al. 12

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amplitude spectralindex

General Relativity prediction:

Conclusions

• Black holes are the most powerful engines in the Universe. They power quasars, active galactic nuclei, and (probably) gamma-ray bursts

• Binary black-hole astrophysics is on almost solid ground – very good arguments for why GW are there in the PTA band