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The Life History of Galaxies and Black Holes

Elaine Sadler, School of Physics, University of Sydney

• The life history of ‘normal’ galaxies

• Black holes

• How they are connected

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The Life History of Stars

“Colossal though they may be, stars and galaxies rank low on the scale of complexity… A frog poses a more daunting scientific challenge than a star”. Martin Rees (1997)

A star’s life history is determined by its mass at birth.

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The Life History of Galaxies

Complex interplay between gas and stars means there is no “HR diagram” for galaxies. A galaxy’s history has to be deduced from what we can observe.

Galaxies are ‘cosmic ecosystems’.

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A galaxy’s appearance depends on its star-formation history

Galaxy classification scheme first proposed by Hubble (1936)

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We live in a spiral galaxy….

The Milky Way galaxy imaged at far-infrared wavelengths by the COBE satellite. How did our Galaxy form?

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Spiral galaxies

New stars are forming in the disk, which is dominated by blue light from massive, luminous young stars.

Older stars in centre (bulge) and halo.

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Dwarf galaxies

IC 5152

Leo dwarf

Small galaxies (106 to 109

stars, compared to 1010 to 1012 stars in giant galaxies).

Often lack a nucleus, star formation histories are varied and often poorly understood.

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Elliptical galaxies

No recent star formation - available gas supply for forming new stars has already been used up, and light is dominated by old, low mass stars (K giants).

Last major episode of star formation may have been as long as 10 billion years ago.

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Galaxies can meet and collide...

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But gas and stars are not the whole story...

Some galaxies are powerful sources of radio waves. These are always giant elliptical galaxies, never spirals or dwarfs. Why??

PKS 2356-61 (ATCA: red: radio emission in red, blue: optical light).

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At the heart of a radio galaxy...

Radio telescopes can image at much higher resolution than optical telescopes.

Show us that the ‘central engine’ of a radio galaxy is very small (<0.1 light year) but also very powerful.

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Quasars and quasi-stellar objects (QSOs)

Very bright nucleus, outshines underlying galaxy - so QSOs look like stars when seen with ground-based telescopes. Luminosity can equal over 100 ‘normal’ galaxies.

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Imaging the sky at radio wavelengths

Molonglo Observatory Synthesis Telescope, University of Sydney

• Radio atlas of the whole southern sky 1997-2004 (SUMSS)

• Technology testbed for the Square Kilometre Array 2002-2007

“A machine for finding supermassive black holes…”

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Images of the optical and radio sky

Optical DSS B: Mostly nearby galaxies (median z~0.1)

Radio 843 MHz: Mostly very distant radio galaxies (median z~1)

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Spectral energy distribution for galaxies (X-ray to radio)

Light dominated by stars, ‘black body curve’ peaks near optical/IR

Different physical processes dominate in normal and ‘active’ galaxies

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Synchrotron radiationProduced by relativistic electrons spiralling in a magnetic field - dominant mechanism for radio emission in active galaxies (AGN)

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Galaxy Energetics

Object Energy Output Origin

Sun 3.8x1026 W Thermal (nuclear fusion)

Milky Way ~1038 W 1011 stars, gas clouds etc.

Quasar ~1040 W Emitted from a very small

region (maybe no larger than

our solar system)What physical process can achieve this??

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Accretion onto a central super-massive black hole

Standard model:

• Black hole

• Accretion disk

• Collimated jets

Typical black hole mass in radio galaxies, QSOs : 107 - 1010 solar masses

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M87 - a nearby radio galaxy with a jet

Synchrotron jet seen at wavelengths from radio to X-ray

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What are Black Holes?

Regions of space from which nothing can escape, not even light, because gravity is so strong. First postulated in 1783 by English geologist John Michell, term “black hole” coined in 1969.

The first conclusive evidence that black holes exist came in the 1990s (can’t observe a BH directly, need to observe its effects).

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Gravity bends light (1)

Gravitational lensing by the Abell galaxy cluster

Distant galaxies being imaged by the Abell cluster

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Gravity bends light (2)

.

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Black Hole Structure

• Schwarzschild radius defines the event horizon - can’t see inside this (vesc=c).

• Inside the event horizon is the singularity.

• Singularities are points of infinite gravity, or more accurately, infinite space-time curvature, where space and time end.

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How much energy from a black hole?

Energy output is set by the accretion rate onto the black hole. The Eddington limit is the maximum rate at which gas can be accreted. Above this, the luminosity is so high that radiation pressure prevents further inflow. Eddington limit is higher for more massive black holes.

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Types of Black Holes

• Primordial – can be any size, including very small If Earth were a BH it would have mass 6x1024 kg and radius ~1cm.

• Stellar Mass – must be at least 3 solar masses

(~1031 kg)• Intermediate Mass – a few thousand to a few tens

of thousands of solar masses; possibly the agglomeration of stellar mass holes

• Supermassive – millions to billions of solar masses; located in centres of galaxies

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Cygnus X-1 - a nearby “stellar-mass” black hole

• Cygnus X-1, X-ray binary system

• Mass determined by Doppler shift measurements of optical lines

• Measured mass is 16 (+/- 5) solar masses.

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The Galactic Centre• Nearest supermassive

black hole: 2.6x106 M

• Black hole mass can be measured accurately from the 3D orbits of stars which pass close to the centre:

– Proper motions & radial velocities (Ghez/Genzel)

– Measurements in IR because of dust

(Ghez)

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NGC 4258 - weighing the central black hole via masers

Black hole mass measured as 3 x 107 Msun

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Using gas dynamics to ‘weigh’ the central black hole in M87

(Harms et al. 1994)

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Bigger galaxies have bigger black holes

Black hole mass-bulge mass correlation implies that formation of galaxy and central black hole are intimately related coupled.

Explains how radio galaxies and quasars ‘know’ what kind of galaxy they live in.

(Kormendy & Richstone 95)

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Where do black holes come from?

Collapse of individual stars - 1-10 Msun BHs

Black holes grow by black-hole mergers

or…

Black holes grow by swallowing gas (QSOs)

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When did the galaxy-black hole connection arise?

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Redshift and look-back time

Redshift Time Since Big Bang Fraction of . z (in Gyr=109 yr) current age

1400 250,000 yr 0.0019% . 20 0.1 Gyr 1.0 % . 10 0.3 2.7 % . 5 0.9 6.8 % . 3 1.6 13 % . 2 2.5 19 % . 1 4.6 35 % . 0.5 7.1 54 % . 0.3 8.8 67 % . 0.2 9 .9 76 % . 0.1 11.3 87 % . 0 13.0 100 %

CMB

Peak of Galaxy formation?

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The Hubble Deep Field (HST)

Our deepest view of the Universe in optical light:

Median redshift of z~1 implies galaxies appear as they were when the Universe was a third of its current age.

Finding black holes is easy. Studying the galaxies they live in is hard.

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High-redshift radio galaxies

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The star-formation history of the Universe

(Baugh et al. 1998)

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The rise and fall of quasars

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Galaxies and black holes both grow when galaxies collide and

merge• Galaxy mergers trigger extra star formation, feed gas to nucleus.

• Accretion rate onto black hole rises, BH grows, star formation also makes galaxy more luminous.

The antennae: Two nearby merging galaxies - star formation is triggered by shocks from the interaction.

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The Antennae, gas and stars

Star formation is most intense near the centre (unlike Milky Way).

(NRAO VLA)

(HST)

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Nearby radio galaxy Centaurus A - end-product of a galaxy merger?

A typical large galaxy has probably had at least 10 interactions or mergers over its lifetime. Most galaxies are probably ‘assembled’ in this way rather than forming at a single epoch.

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Circinus galaxy - star formation around an accreting black hole

Well-studied nearby galaxy: HST image shows the active nucleus surrounded by two starburst rings.

The dust-enshrouded star-forming regions are the dominant energy source in the radio and infrared regions of the spectrum.

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Summary

• Super-massive black holes (106 to 109 solar masses) probably lie at the centres of most bright galaxies.

• The process by which these black holes form appears to be tightly related to the process of galaxy formation, in ways we don’t yet understand fully.

• Massive black holes are the central engines of active galactic nuclei (radio galaxies and quasars) though the level of activity has varied over cosmic time.