Black holes: do they exist?

Plan of talk

• What are black holes?

• Why should they exist?

• How do we detect them?

• The evidence: - binary star systems;

- normal galaxies;

- active galaxies and quasars.

• Conclusions

Endpoints of stellar evolution

- Low mass star (M < 8Msun)

---> Planetary Nebula

+ White Dwarf (M <1.4Msun)

- High mass star (M > 8Msun)

---> Supernova

+ Neutron Star (M < 2.5Msun)

or

+ Black Hole (M > 2.5Msun)

Normal Galaxies

Spiral Galaxy Elliptical Galaxy

(Hubble 1924)

X-ray binary systemQuickTime™ and aSorenson Video decompressorare needed to see this picture.

Spectroscopic Binaries

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Black Holes in X-ray Binary Systems

• Measurements of the secondary stars in some X-ray binaries indicate primary star masses >2.5xMsun

(above the mass limit for a neutron star)• Corrected for inclination, the primary star masses

are ~10xMsun

• The primary stars are “dark” in the sense that they make no contribution to the spectrum of the system.

----> X-ray binaries provide excellent evidence for black holes

Seyfert Galaxies

Seyfert (1943)

Optical images

Optical spectra

The discovery of the quasar 3C273

Optical imageOptical spectrum

At the distances estimated from the redshifts of the emission lines, quasars have a luminosity 10 - 10,000x the integrated light of all the stars in the Milky Way.

(Schmidt 1963)

z=0.158

Active nuclei: key characteristics

• Large luminosities (1 - 10,000 galaxies)

• Small size of emitting region (< 1 light year)

• Large lifetimes (1 - 100 million years)

• Ability to produce highly collimated jets

Gravitational energy generation around black holes

The release of gravitationalenergy when material falls closeto the event horizon of a super-massive black hole is equivalentto 10 - 30% of the rest mass energy(0.1 - 0.3xMc2).

This is ~10x more efficient thannuclear fusion (0.007xMc2)!

Accretion onto a super-massive black hole

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Hubble Space Telescope Capabilities

HST in orbit

From ground

From HST

Images of star cluster

Observations of the centre of the Milky Way

Wide field optical imageof the Galactic Centre

High resolution infrared imageGenzel et al. (2003)

Mbh = (2.4+/-0.4)x106 Msun

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Black Holes in Normal Galaxies

• Using the HST clear evidence for large masses (1x106 -- 3x109 Msun) has been found in the central regions of several normal galaxies.

• The matter in the nuclear regions appears to be dark:

- M/L ~ 30 - 150 (M/L)sun for galaxy cores

(M/L ~ 1 - 10 (M/L)sun for stellar systems)

---> Good evidence for super-massive black holes in most massive galaxies

• The masses of the black holes correlate with the masses of the bulges of the host galaxies

Correlation between black hole mass and galaxy bulge mass/luminosity

Kormendy & Richstone (1995)

The discovery of the quasar 3C273

Optical imageOptical spectrum

At the distances estimated from the redshifts of the emission lines, quasars have a luminosity 10 - 10,000x the integrated light of all the stars in the Milky way.

(Schmidt 1963)

z=0.158

Cygnus A viewed by

HST

The quasarnucleus inCygnus A

HST/NICMOS infrared 2.2m image

Optical images

2.0 micron imageHST/NICMOS

Evidence for a super-massive black hole in Cygnus A from Keck/NIRSPEC infrared data

Tadhunter et al. (2003)

Evidence for a supermassiveblack hole in Cygnus A fromHST/STIS data Mbh = (2.5+/-0.5)x109 MsunTadhunter et al. (2003)

Correlation between black hole mass and galaxy bulge mass/luminosity

Cygnus A

Galaxy Mergers

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Conclusions

• There is now compelling evidence (but not conclusive proof!) for super-massive black holes in:

- X-ray binary systems

- Normal galaxy cores

- Active galaxies and quasars

• The black hole properties are strongly correlated with the properties of the bulges of the host galaxies.

• The degree of nuclear activity is likely to depend on the amount of material being accreted (e.g. through galaxy mergers)