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Home , Local Group

Lecture 7: the Local Group and nearby clusters

• in this lecture we move up in scale, to explore typical clusters of – the Local Group is an example of a not very rich cluster • interesting topics include: – clusters and the structure of the – the fate of galaxies: stable, destroyed or cannibals?

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the Local Group

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1 Inner

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2 some Local Group galaxies, roughly to the same physical scale:

M31, I LMC, M32 SMC

MW M33

(images courtesy AAO) Galaxies – AS 3011 5

first impressions

• there are some obvious properties of the Local Group: – it’s mostly empty, i.e. galaxies are quite distant from each other – with some exceptions like satellite galaxies – the three spirals are easily the biggest – dwarf galaxies are on the outskirts of the group • how typical is this of other groups? – turns out that the Local group is not very rich in galaxies

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3 groups and clusters • groups contain a smaller number of galaxies than clusters, and are more compact in both space and velocity spread:

group: cluster: no. galaxies ~10+ >50 core radius ~300 kpc ~300 kpc median radius ~1 Mpc ~ 3Mpc v-dispersion 150 km/s 800 km/s M/L ~200 ~200 13 15 total mass few 10 Msolar few 10 Msolar

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classifying the Local Group • the Local Group has only about 10 significant galaxies 8 (L > 10 Lsolar), so does not qualify as a cluster – NB, dwarf spheroidals etc. are not detectable at large distances, so don’t make up part of the total galaxy count for the Local Group • about half of known galaxies are in groups and clusters – these are dense enough to halt cosmological expansion locally, and so the galaxies remain bound to each other • the other half of galaxies are loosely spread out in large filaments and ‘walls’ – part of the large-scale structure of the Universe; may still be collapsing into clusters Galaxies – AS 3011 8

4 mapping the structure

• to turn a map of the sky into a 3-D picture of the Local Group, we need galaxy distances – Hubble’s law does not apply within the Group because expansion has halted • need to remember that the uncertainty in the distance is ~10% even for bright galaxies – e.g. for the LMC, the range found for (m – M) is 18.1 to 18.8 – from -5 log d formula, this makes a 40% difference in the distance! • for dSph, distances could be uncertain by factor of ~2

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• for the LMC, some of the methods just aren’t very reliable (not enough of a particular type, for example)... best estimates constrain absolute magnitude to ~0.1, or 5% in the distance

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5 galaxies in the Local Group

• adding up those within 1 Mpc of the : – 4 spirals (MW, M31, M33, LMC) – 1 elliptical (M32) – 3 dwarf ellipticals (NGC 147, 185, 205) – 3 irregulars (SMC, IC 10, NGC 6822) 5 8 – 25 dSph/dIrr (with L of 2 × 10 to 10 Lsolar) • this is rather different from typical large clusters – in core regions, proportions 10% / 40% / 50% in spirals / ellipticals / lenticulars – in outer regions of a cluster, 80% / 10% / 10% in spirals / ellipticals /lenticulars

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nearby clusters

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6 supergalactic plane • there is a local ‘wall’ called the supergalactic plane in which many of the clusters within ~100 Mpc lie – top diagram is looking down on the plane – bottom diagram is looking along it – mesh regions have >50% more than mean density of galaxies

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Layout of the Local Group

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7 well known clusters

• some of the richest clusters are the two in the constellations of (at 15-20 Mpc) and (at ~100 Mpc)

images: Picture of the Day

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velocities in a group

• the mutual gravitaitonal pulls of the galaxies hold them together as a group – the group must have assembled fairly early on, or the expansion of the Universe would have spread galaxies out more evenly • several kinds of motions are possible – stable orbits around the group – infall of two galaxies onto each other – destruction of a small galaxy due to tides induced by a bigger one • our Galaxy and M31 are approaching each at ~120 km/s

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8 sub-structure

• some galaxies in the LG are bound to the Milky Way, some to Andromeda, and some neither: – companions of MW: LMC (0.05 Mpc), SMC (0.06 Mpc), various dSph (0.025 Mpc to 0.27 Mpc) – companions of M31 (0.77 Mpc from MW): M32 (0.75 Mpc), three dE (NGC 147, 185, 205 at 0.6 to 0.85 Mpc) various dSph in Andromeda (0.6-0.8 Mpc)

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fates of galaxies • ‘near misses’ can perturb galaxies, e.g. send density waves through the gas – this leads to new bursts of formation, perhaps as seen in the LMC irregular spiral • strong interactions can destroy galaxies as discrete entitities – some originally in the Local Group are now gone, all we see now are ‘tidal streams’ Galaxies – AS 3011 18

9 stability and

• dark matter can help to hold a galaxy together, when the mass in visible stars isn’t enough to explain why it has survived – compare the dSph to the ω Cen

– σ is 3x higher in ω Cen, but Rcore is 40x higher in Car – from the Virial Theorem, 2 KE + PE = 0, so approximately 2 2 σ = GM / Rcore, or (σCar / σCen ) = (MCar/MCen)(RCen/RCar)

– which gives MCar ≈ 4 MCen 5 6 – but LCar = 2 x10 while LCen = 10 (!), so M/L is 20 times larger in the dSph galaxy than in the globular cluster

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stability and potential wells

• the effects that destroy a are basically tides • the maths for distortions of extended galaxies made up of many stars are hard, but we can picture the potential wells – start with the 5 Lagrangian points, which show stable positions e.g. for satellites launched from the

60°

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10 losing stars • for extended objects like galaxies, we can picture the potential wells – consider where a star would be firmly attached to a big galaxy of mass M or its smaller companion of m – at the Lagrange points the star’s situation is unstable, and it may start to fall onto the other galaxy or out into intergalactic space

L2 L1 L3

m M

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LMC and SMC

• it turns out that the L1 and L2 points are at distances 1/3 xL = ± D [ m / 3M+m ] – where D is the distance separating the galaxies • for the LMC, for example, its circular velocity around our galaxy implies M(<50 kpc) ~ 5 × 1011 10 Msolar, while its own mass is ~ 10 Msolar 1/3 – so xL ~ ± 50 [ 1 / (3×10 + 1) ] ~ 11kpc – so as the galaxy radius is only 7 kpc, its stars will stay bound

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11 fate of dSph • on the outer slopes of the potential well, stars will drift away – in fact, the SMC turns out to be unbound to the LMC so is moving away • smaller galaxies can get torn apart – for example, the Sagittarius dSph is only 15 kpc from the centre of the Milky Way 11 – the MW rotation curve shows that ~10 Msolar lies within this radius 10 – the Sgr galaxy would need 10 Msolar to retain its own 7 stars, but has a luminosity of only 10 Lsolar

– this means it will lose its stars (would need 1000 Msolar of dark matter for every 1 Msolar of stars to be stabilised)

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merger of Milky Way and M31 • the Milky Way and M31 dominate the Local Group but are not in a mutually stable orbit – a close pass occurs in 5 Gyr – possibly an actual collision – could be ejected into simulation, see movie at intergalactic http://www.cita.utoronto.ca/~dubinski/tflops/ space (J Dubinksi) Galaxies – AS 3011 24

12 intergalactic stars

• some evidence of intergalactic Planetary Nebulae in the – too far away to see individual stars like the Sun, but they could be there

artist’s sketch, looking back (NASA)

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