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 system

Spectroscopic Binaries

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 Optical images Optical spectra Seyfert (1943)

The discovery of the quasar 3C273 (Schmidt 1963) Optical image Optical spectrum z=0.158 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.

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 gravitational energy when material falls close to the event horizon of a super- massive black hole is equivalent to 10 - 30% of the rest mass energy (0.1 - 0.3xMc2). This is ~10x more efficient than nuclear fusion (0.007xMc2)!

Accretion onto a super-massive black hole

Hubble Space Telescope Capabilities Images of star cluster From ground From HST HST in orbit

Observations of the centre of the Milky Way Wide field optical image of the Galactic Centre Mbh = (2.4+/-0.4)x106 Msun High resolution infrared image Genzel et al. (2003)

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 (Schmidt 1963) Optical image Optical spectrum z=0.158 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.

Cygnus A viewed by HST The quasar nucleus in Cygnus A Optical images HST/NICMOS infrared 2.2mm image Optical images

Evidence for a super-massive black hole in Cygnus A from Tadhunter et al. (2003) 2.0 micron image HST/NICMOS Evidence for a super-massive black hole in Cygnus A from Keck/NIRSPEC infrared data

Mbh = (2.5+/-0.5)x109 Msun Evidence for a supermassive black hole in Cygnus A from HST/STIS data Mbh = (2.5+/-0.5)x109 Msun Tadhunter et al. (2003)

Correlation between black hole mass and galaxy bulge mass/luminosity Cygnus A

Galaxy Mergers

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)