Enormous black holes in galaxies 1 What is

Enormous black holes in galaxies 1. What is a black hole? 2. Do black holes really exist? 3. Black holes in nearby galaxies 4. The co-evolution of galaxies and black holes

The Universe we see today is composed of billions of galaxies, each containing billions of stars. As well as the visible matter, the content of the Universe is really dominated by dark matter and dark energy.

Gravity and black holes Isaac Newton realized that gravity is a fundamental force that is everywhere and acts on everything. It is dependent upon the masses involved and the square root of their distance Then Einstein came along and with his theory of general relativity expressed gravity as simply the curvature of space and time which is affected by masses.

It is difficult to escape from a mass: space is always curving you back toward it. The greater the mass, the harder it is to escape. Every mass has an escape velocity: Escape velocity of the Earth is 11 km/sec Jupiter is 61 km/sec The Sun is 618 km/sec Ultimately, an escape velocity of 300, 000 km/sec can be required; such is the speed of light. An object with an escape velocity equal to that of light is a black hole by definition.

The fundamental equation for black holes: the radius of a black hole which is R= 2 GM/c squared. This is the radius of the "event horizon" of a black hole and is also known as the Schwarzschild radius. The sun has a radius of 700, 000 km; if its mass stays the same and it shrinks to 3 km radius, it would be a black hole. Therefore, black holes do not need to have tremendous masses, the important thing is to have a very high density - that is compress a lot of matter into a very small space.

What would it be like to go visit a black hole? Plan of 4 -`star’ system: Our trajectory:

Do black holes really exist? The concept of a black hole seems like a theoretical certainty given Einsteins relativity, BUT at the centre of a black hole the mass is compressed into an infinitely small space (scientists do not like infinities). What observational evidence do we have for black holes… 1. As stellar remnants? 2. At the centres of galaxies?

Cygnus X-1 There is a visible star whose Doppler shifts indicate that it is one of a pair of binary stars. The other is invisible but has a mass too high to be a white dwarf or a neutron star. Also, the companion is too faint to be a main sequence or giant star. So by a process of elimination it can only be a black hole. A corroborative piece of information is that the Cygnus X-1 system gives off very powerful X-rays. The likeliest way of producing this emission is very hot gas in an accretion disk around the black hole.

Quasars are exceptionally luminous and compact objects emitting > 100 times the light of a normal galaxy. X-ray variability happens on timescales as short as a few hours suggesting X-rays come from a small region Most plausible explanation for quasars is an accretion disk around a supermassive (a billion times the mass of the sun) black hole.

X-rays from really close in to the quasar black hole shows signatures of gravitational redshift, indicating an extreme gravity field.

Black holes in nearby galaxies The search for black holes in nearby galaxies has been underway for over 20 years by measuring rotation and random velocities of stars and gas near galactic centres. The first was found in the Andromeda Galaxy M 31 weighing in at 30, 000 solar masses. M 32, the small elliptical companion of M 31, also has a black hole but only a mere 3 million solar masses. John Kormendy: Staff Member DAO, Victoria (1979 - 1989) M 32 velocity dispersion

NGC 3115 is a nearby S 0 galaxy, an intermediate type between spirals and ellipticals. Subtracting the disk off reveals a tiny nuclear star cluster which needs a very high mass to stop it flying apart - a black hole. total-disk

M 87 The giant elliptical galaxy M 87 lies in a rich cluster of galaxies. The HST spectrum shows that near-nuclear gas is spinning round very quickly suggesting a black hole of mass 3 billion times that of the sun - the largest black hole seen in nearby galaxies.

And what about our own galaxy? Does it also have a massive black hole at the centre?

In almost every nearby galaxy where we have been able to look, we have found a black hole of greater than a million solar masses!

The correlation between stellar bulge mass and black hole mass implies an intimate connection between the formation and evolution of these two components of galaxies.

The build-up of galaxies over the history of the universe must have been accompanied by a build-up of nuclear black holes by accretion onto quasars.

In January this year I observed the most distant known quasar SDSS J 1148+5251 (redshift=6. 41 or 13 billion light years) with the new UKIRT-Imaging Spectrometer at the UKIRT Infrared Telescope on Mauna Kea, Hawaii

The inferred black hole mass from the width of the Mg. II emission line is 3 billion solar masses, equal to that of M 87. This shows that some really massive black holes formed very early on in the universe.

Conclusions • There is extremely good circumstantial evidence for the existence of black holes • Almost every nearby galaxy has a supermassive black hole so they must all have been quasars at some point in their pasts • The mass of nuclear black holes correlates with the mass of stars - this must mean the quasar activity is closely linked to galaxy evolution • Next step is to see how this correlation evolves over the history of the universe.