Lecture 7: the Local Group and nearby clusters in this lecture we move up in scale, to explore typical clusters of galaxies the Local Group is an example of a not very rich cluster interesting topics include: clusters and the structure of the Universe the fate of galaxies: stable, destroyed or cannibals? Galaxies AS 3011 1 the Local Group Galaxies AS 3011 2 1
Inner Solar System Galaxies AS 3011 3 Galaxies AS 3011 4 2
some Local Group galaxies, roughly to the same physical scale: M31, Leo 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 galaxy groups? turns out that the Local group is not very rich in galaxies Galaxies AS 3011 6 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 total mass few 10 13 M solar few 10 15 M solar Galaxies AS 3011 7 classifying the Local Group the Local Group has only about 10 significant galaxies (L > 10 8 L solar ), 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 Galaxies AS 3011 9 for the LMC, some of the methods just aren t very reliable (not enough stars of a particular type, for example)... best estimates constrain absolute magnitude to ~0.1, or 5% in the distance Galaxies AS 3011 10 5
galaxies in the Local Group adding up those within 1 Mpc of the Milky Way: 4 spirals (MW, M31, M33, LMC) 1 elliptical (M32) 3 dwarf ellipticals (NGC 147, 185, 205) 3 irregulars (SMC, IC 10, NGC 6822) 25 dsph/dirr (with L of 2 10 5 to 10 8 L solar ) 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 Galaxies AS 3011 11 nearby clusters Galaxies AS 3011 12 Richard Powell - http://www.anzwers.org/free/universe/ 6
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 supergalactic plane mesh regions have >50% more than mean density of galaxies Galaxies AS 3011 Hudson (1993) 13 Layout of the Local Group Galaxies AS 3011 14 7
some of the richest clusters are the two in the constellations of Virgo (at 15-20 Mpc) and Coma Berenices (at ~100 Mpc) well known clusters images: Astronomy Picture of the Day Galaxies AS 3011 15 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 Galaxies AS 3011 16 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) Galaxies AS 3011 17 fates of galaxies near misses can perturb galaxies, e.g. send density waves through the gas this leads to new bursts of star 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 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 Carina dsph to the globular cluster ω Cen σ is 3x higher in ω Cen, but R core is 40x higher in Car from the Virial Theorem, 2 KE + PE = 0, so approximately σ 2 = GM / R core, or (σ Car / σ Cen ) 2 = (M Car /M Cen )(R Cen /R Car ) which gives M Car 4 M Cen but L Car = 2 x10 5 while L Cen = 10 6 (!), so M/L is 20 times larger in the dsph galaxy than in the globular cluster Galaxies AS 3011 19 stability and potential wells the effects that destroy a satellite galaxy 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 Earth 60 Galaxies AS 3011 20 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 L 2 L 1 L 3 m M Galaxies AS 3011 21 LMC and SMC it turns out that the L1 and L2 points are at distances x L = ± D [ m / 3M+m ] 1/3 where D is the distance separating the galaxies for the LMC, for example, its circular velocity around our galaxy implies M(<50 kpc) ~ 5 10 11 M solar, while its own mass is ~ 10 10 M solar so x L ~ ± 50 [ 1 / (3 10 + 1) ] 1/3 ~ 11kpc so as the galaxy radius is only 7 kpc, its stars will stay bound Galaxies AS 3011 22 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 the MW rotation curve shows that ~10 11 M solar lies within this radius the Sgr galaxy would need 10 10 M solar to retain its own stars, but has a luminosity of only 10 7 L solar this means it will lose its stars (would need 1000 M solar of dark matter for every 1 M solar of stars to be stabilised) Galaxies AS 3011 23 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 Sun could be ejected into intergalactic space simulation, see movie at http://www.cita.utoronto.ca/~dubinski/tflops/ (J Dubinksi) Galaxies AS 3011 24 12
intergalactic stars some evidence of intergalactic Planetary Nebulae in the Virgo cluster too far away to see individual stars like the Sun, but they could be there artist s sketch, looking back (NASA) Galaxies AS 3011 25 13