Introduction to Astronomy Galaxies Lecture 4 Clusters & Superclusters Peder Norberg [email protected] Introduction to Astronomy PHYS1081 e.g. Zeilik Ch18.6/18.7 Lecture 4 By the end of this lecture, we will have discussed: Properties of galaxy Groups and the Local Group Properties of nearby clusters: Virgo and Coma Properties of Superclusters & Voids Properties of galaxies within clusters Mass estimates for galaxy clusters from motions of galaxies within the cluster from X-ray emission from gas in the cluster potential from gravitational lensing by the potential well Contributors to the material of these lectures: Ian Smail, Adrian Jenkins, Michele Fumagalli, Peder Norberg et al. Groups, Clusters & Superclusters We study these structures because: • Most galaxies today are in Groups • Clusters are the most massive virialised objects in the Universe, 15 with masses up to 10 M0 • Superclusters (groups of clusters) are the largest coherent structures in the Universe, with sizes up to 100 Mpc (or more). • Tell us about nature of DM, properties of the Universe and galaxies within them. Groups, Clusters & Superclusters • Clusters are the largest gravitationally bound structures (K.E.<P.E.) in the Universe. • As they are bound, their constituent galaxies don’t expand with the Hubble Flow (the general expansion of the Universe). • Clusters grow by gravitationally attracting material from their surroundings • Superclusters are not bound. • Together groups, clusters and Time superclusters (along with filaments and voids) make up the “Large Scale Structure of the Universe’’ Large ScaleLarge Structure Scale (as seen withStructure a DM N-body simulation) M31 From Galaxy Groups to Clusters and Superclusters Galaxy Groups • Most galaxies in the present-day Universe are in groups. • Groups contain a few galaxies (number very loosely defined!) and have total 13 masses up to a few ~10 M0 (velocity M31 dispersions up to σ~300 km/s) Compact Group • Two main types of groups: • Compact groups are relatively rare and quite extreme environments, characterised by strong interactions between members. • Loose groups on the other-hand are benign environments and are much more common… • NB: “Group” is an observational Loose Group concept: theorists talk about halo… The Local Group Milky Way M31 M31 Clusters and Superclusters Finding Clusters / Superclusters M31 Galaxy Redshift Surveys UK/Australian 2dF Galaxy Redshift Survey (1998-2004) Redshifts (thus distances)M31 from spectra of ~250,000 galaxies Redshift Cone reaches to z~0.2 or (as z~v/c) v~60,000 km s-1 Equally, as D=v/H0 this is equivalent to D~860 Mpc Space Large Scale Structure Sloan Digital Sky Survey (SDSS; 2000-2008) has obtained redshifts for ~1,000,000 galaxies. M31 Galaxy redshift surveys provide detailed maps of the Universe. Find “filamentary” structures, with walls and filaments surrounding empty voids. Groups and clusters distributed along these walls and filaments The Milky Way’s Backyard 10 Mpc 1Mpc Virgo Cluster Virgo cluster: ~20 Mpc away from us. It’s a “poor” cluster, contains ~100 galaxies 14 (so a mass of ~10 M0) with a total velocity dispersion, σ~600 km/s, in a region about ~3 Mpc across. The Local Group is falling towards Virgo at ~300 km/s. Coma Cluster 1MpcM31 At a distance of 100 Mpc, the Coma cluster is the nearest “rich” cluster. Contains >1000 galaxies (with velocity 15 dispersion, σ~1100km/s) in ~10 Mpc, or a mass of ~10 M0 M31 Superclusters & Voids Superclusters & Voids 100 Mpc The Local Group belongs to the Laniakea Supercluster, part of the Perseus-Cetus Supercluster Complex… Shapley Supercluster 100Mpc M31 Shapley comprises 10 clusters, lies ~200 Mpc away from us, is ~100 Mpc across and has a mass of ~5x1016 Mo Is the Shapley Supercluster Virialised? 100Mpc To be in virial equilibrium (2<KE>=<PE>) the galaxies have to have orbited in the potential well for a while: M31 Size T = < Age Cross v Supercluster is ~100 Mpc across. How long does a galaxy, moving at ~1000km/s, take to travel 1 Mpc? Is the Shapley Supercluster Virialised? 100Mpc To be in virial equilibrium (2<KE>=<PE>) the galaxies have to have orbited in the potential well for a while: M31 Size T = < Age Cross v Supercluster is ~100 Mpc across. How long does a galaxy, moving at ~1000km/s, take to travel 1 Mpc? 1× 106 pc × 3.1× 1016 m pc-1 t = D / v = = 3.1× 1016 s ; 1 Gyr 1× 106 m s-1 A galaxy would take 100 Gyrs to cross Shapley Exceeds age of the Universe and hence cannot be in equilibrium (hence velocities of its components don’t reflect total mass) Voids The Universe was originally homogeneous. Hence groups and clusters form overdense regions, while VOIDS are underdense ones, primarily defined by the “absence” of galaxies and hence rather difficult to study… IntraCluster Gas M31 and Galaxies in Clusters IntraCluster Gas The Universe is full of gas (atomic H and He) produced by the Big Bang - this is called the InterGalactic Medium (IGM). The typical density is ~1 H atom m-3. As a cluster forms, its gravity attracts this gas and as it fallsM31 in, it heats up as the gas clouds collide. This hot gas within a cluster is called the IntraCluster Medium (ICM). Cluster Galaxies Most galaxies in clusters today are early-type galaxies (Ellipticals and S0 or Lenticulars). No (or very little) star formation and little gas in these galaxies. Very different from the low-density “field”, dominated by star-forming spirals. WHY? Merging in Clusters So mergers and interactions between galaxies during the formation of clusters turned them into gas-poor Early-type galaxies. ”Ram-pressure/strangulation“ The interaction with the hot intra-cluster medium (ICM) causes galaxies to loose their gas supply, suppressing (quenching) the formation of new stars. Galaxy Cluster Formation Masses for galaxy clusters is important as they contain the same mix of light and mass as the Universe as a whole. M31 By measuring the ratio of mass to light for a cluster and the total amount of light in the Universe, one can work out the mass of the Universe. Also clusters are very good laboratories for studying properties of Dark Matter… M31 Measuring Masses of Clusters Measuring Masses for Clusters Three techniques for measuring cluster masses: 1. Motion of cluster galaxies (dynamics) Drawback: galaxies have to be virialised and thisM31 is hard to test 2. X-ray emission from hot gas in the cluster Gas is a better tracer than galaxies, but can still be influenced by non-equilibrium situations 3. Gravitational Lensing Best method, but observationally very demanding Dynamics: Clusters Galaxies in clusters have random orbits. If they are in equilibrium in the cluster potential well then we can use their velocity dispersion σ: 2 2 σ = (vi − v ) to determine the cluster mass (as we did with the stars in elliptical galaxies). So are cluster galaxies in equilibrium? Dynamics: Clusters Galaxies in clusters have random orbits. If they are in equilibrium in the cluster potential well then we can use their velocity dispersion σ: 2 2 σ = (vi − v ) to determine the cluster mass (as we did with the stars in elliptical galaxies). So are cluster galaxies in equilibrium? • A galaxy moving at ~1000km/s takes ~1 Gyr to cross ~1 Mpc. • A cluster is a few Mpc in size, so the crossing time is less than age of the Universe (13-14 Gyr) Hence galaxies in the central few Mpc of clusters may be in equilibrium… Dynamics: Clusters Central regions of clusters should be in equilibrium. Hence we can use the Virial Theorem to relate the time- averaged KE and PE - as we did with the stars in elliptical galaxies: 2 KE T = PE T 2 v GM GM 2GM 3σ 2R 2 = v2 = 3σ 2 = = M = G 2 R R RG 2G • RG is a measure of the size of the cluster (no sharp edge). • σ is the 1-D (line-of-sight) velocity dispersion, 1/3rd of the 3- D velocity dispersion <v2>. • <R> = RG/2 for a galaxy half-way from the cluster centre. Dynamics: Clusters Central regions of clusters should be in equilibrium. Hence we can use the Virial Theorem to relate the time- averaged KE and PE - as we did with the stars in elliptical galaxies: 2 KE T = PE T 2 v GM GM 2GM 3σ 2R 2 = v2 = 3σ 2 = = M = G 2 R R RG 2G • RG is a measure of the size of the cluster (no sharp edge). • σ is the 1-D (line-of-sight) velocity dispersion, 1/3rd of the 3- D velocity dispersion <v2>. • <R> = RG/2 for a galaxy half-way from the cluster centre. Cluster Masses 2 2 3σ RG 14 % σ ( % RG ( M = ≈ 7 × 10 M 0 ' −1 * 2G & 1000kms ) &' 2Mpc)* -1 A cluster of galaxies with σ~1000 kms and RG ~2 Mpc has a mass of 15 45 M~10 M0 (i.e. ~10 Kg). But, if you summed up all of the mass in stars in the galaxies in the cluster you would only 13 find M*~10 M0. The “missing mass” is Dark Matter. Indeed, the high velocity dispersion of the galaxies in the Coma cluster was the first evidence for Dark Matter and was postulated by Fritz Zwicky (1898-1974) in 1933. M31 Measuring Masses of Clusters: X-ray emission & Gravitational Lensing M31 X-ray emission from hot gas in clusters Temperature of Cluster Gas Typical velocity of the atoms in the gas in cluster is similar to that of the galaxies (σ: σ2=<v2>/3): Use Kinetic theory of gases to relate the temperature of the gas, T, to the speed of the atoms (=galaxies): M31 mv2 3kT = 2 2 k: Boltzmann’s constant Temperature of Cluster Gas Typical velocity of the atoms in the gas in cluster is similar to that of the galaxies (σ: σ2=<v2>/3): Use Kinetic theory of gases to relate the temperature of the gas, T, to the speed of the atoms (=galaxies): M31 mv2 3kT = 2 2 k: Boltzmann’s constant m3σ 2 3kT kT = ≈ σ 2 µ : mean molecular 2 2 µmp weight of the gas, ~0.6 2 7 % σ ( i.e.: T ≈ 7 × 10 K m : mass of a proton &' 1000kms−1 )* p 2 2 3σ RG 14 % σ ( % RG ( With M = ≈ 7 × 10 M 0 ' −1 * ' * 7 RG 2G 2 & 1000kms ) & 2Mpc) 2 M ≈ T × 10 MMe0 × 3σ RG 14 % σ ( % RG ( 2Mpc M = ≈ 7 × 10 M 0 ' −1 * 2G & 1000kms ) &' 2Mpc)* X-ray emission from Cluster Gas 2 7 % σ ( T ≈ 7 × 10 K &' 1000kms−1 )* 15 In a massive cluster (M~10 M0) with galaxies travelling at M31 σ~1000 km s-1 , the gas will have a typical temperature of T~108 K (velocity of the atoms in the gas in cluster is similar to that of the galaxies).