What makes spira l ga laxies tick?James BinneyRudolf PeierlsCentreforTheoretical Physics, Oxford University, UK

TuesdaY,3 May- 1:30 p.m.

Spiral galaxies are such complex machines thatthey are almost alive. Like livingthings, they are born,grow elderly andwilldie, either gracefully orviolently.Galaxiesbreathe both in and out;stra ngle agalaxy andit becomes 'red and dead'. Itmustbereported thattheyare heavy smokers. Galaxies makethemselves warm byaccelerating theirstars, a process which makes them flaccid, unresponsive and,well, 'old'. Astheyage, galaxiesbecome steadily moreself-centred .Theyeatsmaller ga laxies - arefreshing meal can rejuvenate agalaxy - butthey run therisk ofbeingeaten bya largerga laxy. Theseinsightsare theproductofaninteresting mixoftheapplication ofbasic physics andtheanalysis ofobservational data. This lecturewillendeavourto explain both whatwe know andhowwe know it.

What makes spiral galaxies tick

James BinneyOxford University

Theme

• Galaxies are complex organisms• Almost alive• From birth to old age and death• What drives them on

What’s in a Galaxy?

Disc~80% of stars, most cold gas

Bulge ~20% of stars

Stellar halo ~1% of stars

Dark halo ~80% of mass, Plus ~70% of baryons too?

e.g. in our Galaxy

Time unit ~0.1 Gyr

Galaxies are born

• Tiny fluctuations in the energy density of the early Universe were amplified by gravity

• 400,000 yr from the beginning they had grown to 1 part in ~300,000

• We study them at this stage through the CMB

• A few 100 Myr later they had grown to order unity

WMAP

Gas falls in

• To begin with gas and dark matter cluster together, falling into regions of low gravitational potential energy

• But gas streams collide inelastically (gas can radiate E) so gas sinks deeper into the potential well created by dark matter

• Result: centres of dark- matter “haloes” become dominated by gas; stars begin to form (t = 1Gyr)

72 kpc on a side at t=1.5 Gyr

Kravtsov

Stars form

Horsehead nebula (HST)

Filthy gas at ~25 K (~ -250 C)

Galaxies smoke • The rate at which stars

process their H and He into C, N, O and then Mg, Si and Fe increases rapidly with initial stellar mass Mi

• The rich live extravagantly & are soon bankrupt

• Stars with Mi >8M� explode as supernovae at the end of their short lives (< 30 Myr)

• Seat of explosion lies deep in the core of the star; much of the overlying envelope is blasted into space

Crab nebula: Hubble ST(AD 1054)

Legacy of dying stars• At point of death the envelope is rich in C, N, O, Fe etc • Supernova-driven shocks heat less dense interstellar gas,

while compressing dense gas and triggering formation of more stars

• So dying stars – Pollute interstellar H+He with “metals” C, N, O, Mg, Si, Fe, etc,

much of it in particulates (smoke)– Form new stars– Heat interstellar gas

Eagle nebula

Cosmic pollution

• particle density 10-20 of air– If squashed vertically to air density, 100 light year → 1cm & get gas layer 10cm

thick– Could see only ~30cm through this layer

• Smoke (“dust”) reprocesses half the energy radiated by stars, absorbing ultra-violet – blue light and radiating mid- to far-infrared radiation

• The Earth and other planets formed from the smoke blown out by stars that died before the Sun was born

Axel Melinger The Milky Way

Galaxies breathe out• The heated gas is too hot to

stay within the disc• It vents perpendicular to the

plane• Cold gas is entrained with it

– “droplets” of cold gas stream in great arcs from the disc and then fall gracefully back to another part of the disc

• Our Galaxy has ~10% of its cold gas in transit from disc to disc

• NGC 891 has nearly a third of its gas in transit!

NGC 891: Oosterloo + 2005

Contours HI

Fountains

Model of outflow from Central star-forming disc

Soft X-ray image of central region

Bland-Hawthorn & Cohen 2003

M82 Gallagher et al 2006

The Galaxy

Galaxies need to breathe in• Our Galaxy has continued forming stars for > 10 Gyr• Why doesn’t it run out of gas?• Some galaxies are embedded in huge discs of cold gas• Ours isn’t and searches fail to find intergalactic HI

NGC 6946

Rense Boomsma 2007

(on same scale)

What galaxies mostly breathe• Study of the cosmic background radiation implies that there’s 2-3

times as much ordinary matter as we see in galaxies• We think the “missing matter” is too hot to be confined by galaxies• In rich clusters of galaxies it’s confined by the cluster’s strong

gravitational field at a density high enough for the X-rays it emits to be detectable

Coma cluster SZ+X (ESA) X-rays

Our fresh air• We think much gas is

confined by the gravitational field of the Local Group of galaxies (dominated by our Galaxy and the Andromeda Nebula) at densities so low that we cannot detect its X-rays

• Our disc is kept alive and forming stars by steady accretion from the reservoir of missing matter.

In the Perseus cluster they are suffocating

• Galaxies that fall into a rich cluster of galaxies can no longer accrete intergalactic gas, because they are moving too fast through the gas

• Moreover, gas shot up from their discs is snatched from them, and does not fall back

• So they run out of cold gas and become “red and dead” as their stars age

Galaxies become self-centred• The direction of time is set by entropy increase: tea

cools, a bicycle coasts to a halt etc, to increase entropy• Life on Earth is socialistic: entropy is increased by

spreading energy more equally and down to the least massive constituents

• Gravity is capitalistic: it increases entropy by taking from those with least and giving to those with most

• So gravity populates the extremes – the energy-rich and the energy-poor – at the expense of the middle class

• Galaxies become more centrally concentrated

Entropy generation in a star cluster• For example, in a star cluster energy is released by tight

binaries at the cluster centre getting tighter, and is used to accelerate stars that move far from the centre, allowing some to break completely free

• A similar process keeps the Sun alive & drives the Galaxy onward too

• Stars & galaxies become steadily more centrally concentrated

M15

How a disc becomes more self-centred

• To be on a circular orbit of radius r a star needs energy Ec (r)• A small change in r changes both Ec and the star’s angular

momentum L = r×v• The ratio of these changes ΔEc /ΔL=Ω, the angular

frequency of the orbit• In a galaxy Ω

decreases with r roughly as 1/r

• So moving ΔL out from r1 to r2 >r1 releases ΔE1 =Ω1 ΔL at r1which exceeds the energy ΔE2 =Ω2 ΔL required to absorb ΔL at r2 :– ΔE1 / ΔE2 = Ω1

/Ω2 = r2 /r1 > 1• The surplus energy creates disorder (entropy)• So outward transport of angular momentum will occur when

it can• Matter that loses L moves in, matter that gains L moves out

& the disc becomes more centrally concentrated

Galaxies grow more eccentric

• The non-axisymmetric gravitational field of a bar or spiral structure shifts angular mometum outwards

• So the formation of a bar or spiral structure opens the path to higher entropy

• Within ~Gyr the disc becomes strongly non-axisymmetric as orbits are “trapped” by the gravitational field of the spinning bar into highly eccentric orbits

• The eccentric orbits start to oscillate perpendicular to the plane too

• The system has “heated” and formed a bulge/bar

Sellwood 2010

The bar/bulge of our Galaxy

Spiral structure• We know entropy is increased by

moving angular momentum outwards• This is possible only if the gravitational

field is non-axisymmetric• The rotating gravitational field of the

bar gives angular momentum to the disc around the end of the bar; this is not helpful

• The job of spiral structure is to move angular momentum from there out through the disc

• Spiral structure is a dance of billions of stars to the same tune

• Less poetically it’s a wave in which stars crowd together here and become sparse there in such a way that its associated gravitational field moves angular momentum outwards

NGC 3124

Heating by spirals• Particles that resonate with a wave tend to absorb energy from the wave or

emit energy into the wave, depending on their phase relative to that of the wave

• Thus the electrons in our skin gain energy from electrons in the sun as a result of both groups of electrons resonating with the electromagnetic waves that make up sunlight

• Some stars resonate with the waves of spiral structure, and thus become net absorbers or net emitters of energy

• Even the stars that emit energy to the spirals grow hotter because they simultaneously emit angular momentum

• So the waves that are spiral structure heat the medium they pass through

Aumer & Binney 2009

Red/oldBlue/young

Elixir of youth• A disc in which stars have large random velocities

cannot sustain spiral structure because it’s too hard to coordinate motions in a “hot” disc

• Thus spiral structure poisons itself with heat in the same way that during the fermentation of wine, yeast poisons itself with the alcohol it produces

• To sustain spiral structure, a galaxy must continually renew its supply of stars on near-circular orbits

• So infall of gas from the reservoir of missing mass is required to sustain spiral structure as well as star formation

Galaxies are cannibals

• Galaxies are constantly tumbling together• Useful to distinguish “minor mergers” in

which one galaxy is much more massive than the other, from “major mergers” of comparable objects

• When a small galaxy moves into a larger one, its gravitational field enhances the density of the big galaxy’s stars behind it as a “wake”

• The gravitational field of the wake pulls it back – “dynamical friction”

• As a result the small galaxy spirals inwards

v

W Mulder

Cannibalism (2)

• As it moves in, the gravitational field of the big galaxy strips stars from the outside of the small one with ever more brutal effectiveness

• Eventually all that’s left of the small galaxy is a stream of stripped stars

• Deep photometry of hundreds of millions of faint stars by the Sloan Digitial Sky Survey (SDSS) has revealed that much (all?) of the stellar halo is made up of streams

Belakurov 2009

Major mergers

• The andromeda nebula, M31, is approaching

• In ~3 Gyr the Galaxy and M31will pass close to one another

• The gravitational field of the other galaxy will drive powerful shocks through both galaxies’ gas discs

Major mergers (2)• An abundance of stars will be

formed in these shocks• Exploding massive stars will drive

much of the surviving gas into intergalactic space

• After briefly receding from one another, the Galaxy and M31 will fall back together and merge

• The stellar discs will be destroyed• The central black holes will merge• The new bigger black hole will

prevent gas heated during the merger from cooling

• The new bigger galaxy will become gradually redder & deader

II Zw 096 (Spitzer IR observatory)

“NGC 4676” (Hubble ST)

• Galaxies form by gravity clumping dark and ordinary matter together• Galaxy centres become dominated by ordinary matter• Galaxies smoke

– Draw in cold gas to form stars– Exhale hot, filthy gas when stars die

• Galaxies breathe– Exploding stars blast clouds of cool gas up & over the disc– When clouds return they bring fresh supplies of cool gas

• Galaxies suffocate– Galaxies that move through the local intergalactic gas don’t even get back their

own cool gas– They gradually cease forming stars

• Galaxies have a “sex drive”: to become more centrally concentrated– Requires movement of angular momentum outwards– Bars & spiral structure achieve this– Spiral structure is possible only in a “cool” disc, but it heats disc– So spiral structure dies with star formation– Sex turns galaxies into flaccid, self-centred bodies

• Galaxies are cannibals– Half digested snacks form star streams– Major feasts are disruptive– We are on a collision course with the Andromeda nebula – 3Gyr to go

Summary