Groups of Galaxies Groups, Clusters, and Superclusters Cluster Dynamics Dark Matter Interacting Galaxies April 20, 2021 University of Rochester Groups of galaxies I Groups, clusters, and superclusters I Galaxy cluster dynamics I Dark matter in clusters of galaxies I Interacting galaxies: starbursts, the origin of AGN Reading: Kutner Sec. 18.4 & 19.1, Ryden Ch. 21.4–21.5 & Ch. 22 Detail of Markarian’s Chain, L. Orazi, StarKeeper. April 20, 2021 (UR) Astronomy 142 | Spring 2021 2 / 35 Probing the IGM & ICM Because quasars are so distant, their light must pass through hundreds of megaparsecs of intergalactic gas before it reaches us. This gas will absorb photons at wavelengths corresponding to its (a) chemical composition, so a quasar’s 1 spectrum will exhibit absorption 0.8 0.6 lines from the interstellar gas. 0.4 0.2 Transmitted flux Transmitted 0 (b) Copyright © 2010 Pearson Education, Inc. April 20, 2021 (UR) Astronomy 142 | Spring 2021 3 / 35 Probing the IGM & ICM The absorption features will be redshifted to correspond to the distance between us and the gas. We will often see a “forest” of absorption features, such as the Lyman alpha forest shown in the quasar spectrum below. 25 20 Keck HIRES spectrum of QSO 1425+6039 15 10 Relative intensity Relative 5 0 4200 4400 4600 4800 5000 5200 Wavelength (Å) Copyright April© 2010 20, 2021Pearson (UR) Education, Inc. Astronomy 142 | Spring 2021 4 / 35 Galaxy groups and clusters Much of the Universe’ mass exists in the form of a hierarchy of gravitationally bound groups or clusters of galaxies. I The Local Group includes the Milky Way, M31, M33, NGC 7731 and a bunch of dwarf ellipticals and irregulars. I Groups are usually part of clusters. The local group belongs to the Virgo Cluster. I Clusters themselves are parts of larger structures called superclusters. The Virgo Cluster is near the center of the Virgo (a.k.a. Local) Supercluster. Stephan’s Quintet I Superclusters are connected by thin filaments of galaxies (often composed of galaxy groups) which outline voids. Image from Dietmar Hager, Stargazer Observatory. April 20, 2021 (UR) Astronomy 142 | Spring 2021 5 / 35 The Local Group Andrew Z. Colvin April 20, 2021 (UR) Astronomy 142 | Spring 2021 6 / 35 The Virgo Supercluster Andrew Z. Colvin April 20, 2021 (UR) Astronomy 142 | Spring 2021 7 / 35 Typical galaxy clusters Galaxy clusters usually contain between several hundred and several thousand galaxies, spread over tens of Mpc but with cores 1–3 Mpc in radius. Clusters come in a variety of “richness” (degree of central concentration). Rich clusters are dominated at their centers by one or more cD galaxies (supergiant ellipticals) and include mostly ellipticals. Poor clusters are often dominated by spirals. Spiral galaxies within rich clusters tend to have less diffuse interstellar matter than field galaxies; S0 galaxies are much more abundant in clusters than in the field. Both features are probably due to galaxy collisions. The stripped-off insterstellar matter is visible at X-ray energies as a hot intrucluster medium (ICM). April 20, 2021 (UR) Astronomy 142 | Spring 2021 8 / 35 The Coma Cluster (d = 99 Mpc) Left: visible light (UK Schmidt) — galaxies. Right: X-rays (ROSAT) — hot intracluster gas. April 20, 2021 (UR) Astronomy 142 | Spring 2021 9 / 35 The Coma Cluster (d = 99 Mpc) From the HST/ACS Treasury Survey April 20, 2021 (UR) Astronomy 142 | Spring 2021 10 / 35 The Virgo Cluster (d = 18 Mpc) Left: visible light (DSS) Right: X-rays (ROSAT) April 20, 2021 (UR) Astronomy 142 | Spring 2021 11 / 35 cD galaxies cD galaxies are giant ellipticals with a few other differences from normal besides the total mass: I More extended relative to their core radii and the normal elliptical galaxy luminosity profile. I Often exhibit multiple nuclei, either because they are assimilating other cluster members (“galactic cannibalism”) or because these nuclei are cluster galaxies on linear (highly eccentric) orbits. I Often surrounded with shells of infalling intergalactic gas that compress and cool as they fall (“cooling flows”). I Often associated with bright, strong gravitational lensing of more distant galaxies, clusters, or quasars. April 20, 2021 (UR) Astronomy 142 | Spring 2021 13 / 35 Abell 2218 and Gravitational Lensing April 20, 2021 (UR) Astronomy 142 | Spring 2021 14 / 35 Masses of galaxy clusters The masses of clusters can be measured by adding up the masses of the contents. There are a few ways to do this. 1. Count the galaxies and add up their masses, including dark-matter halos. 2. Measure the X-ray spectrum and image. I The X-ray emission is due to radiation by electrons scattering from nuclei, a process called thermal bremsstrahlung or free-free emission. I The X-ray brightness at any energy is proportional to the square of the electron density. The X-ray “color” gives the temperature. I Usually, the X-ray distribution appears spherically symmetric and fairly smooth. April 20, 2021 (UR) Astronomy 142 | Spring 2021 15 / 35 Masses of galaxy clusters Results for typical galaxy clusters like Coma and Virgo: I Galaxies and X-rays distributed similarly, with a core radius around 1 Mpc — looks like hydrostatic equilibrium. I The radial dependence of the number per unit projected area is like the radial dependence of surface brightness in ellipticals: 1/4 r − (r)= (0) exp r = core radius N N − r 0 " ✓ 0 ◆ # I Galaxies, including their dark halos, typically amount to 1013M to 3 1013M . ⇥ I X-ray-emitting gas has a density, temperature, and mass around 4 3 8 14 ne 10− cm− T 10 K M 1 3 10 M ⇠ ⇠ ⇠ − ⇥ ten times as much as the galaxies. April 20, 2021 (UR) Astronomy 142 | Spring 2021 16 / 35 Masses of galaxy clusters This leaves two odd dynamical features of galaxy clusters: 1. The relaxation times for galaxy clusters, 2r N t 0 c ⇡ v 24 ln (N/2) ✓ ◆ tend to be larger than the age of the Universe (1.4 1010 yr), but in principle need to ⇥ be much smaller in order to treat the galaxies in a cluster as a gas. E.g., for the Coma cluster, N = 104 r = 3 Mpc v = 1700 km/s = t = 1.7 1011 yr 0 ) c ⇥ 2. However, (r) looks just like that for thermalized galaxy bulges and star clusters, N and the distribution of their random velocities looks like a Maxwell-Boltzmann distribution. April 20, 2021 (UR) Astronomy 142 | Spring 2021 17 / 35 Masses of galaxy clusters The thermal velocity for intracluster gas, 3kT vth = s mH is typically about the same as the escape speed from the observed mass, 2GM vesc = s r0 Thus, all of the hot gas originally in the typical galaxy cluster should have escaped by now. The Jeans escape timescale (ASTR 111) is pp 2k T 3/2 r3 t = B 0 eGMmH/kBTr 0 109 yr Jeans 8 m (GM)2 ⇡ ✓ H ◆ for typical galaxy clusters. April 20, 2021 (UR) Astronomy 142 | Spring 2021 18 / 35 Masses of galaxy clusters If the equilibrium-like appearance of the distribution of galaxies in clusters is taken to indicate equilibrium despite the inferred relaxation time, we can apply the virial theorem to estimate masses: 2r v2 6r v2 M = 0 = 0 r G G 2 where vr is the random component of the radial velocity relative to the cluster average. Typically this gives a result ten times higher than the mass of visible galaxies and X-ray gas. For example, in the Coma cluster, 2 14 2r0v 15 MX-ray = 3 10 M M = = 4 10 M ⇥ G ⇥ April 20, 2021 (UR) Astronomy 142 | Spring 2021 19 / 35 Dark matter revisited The discrepancy between visible-galaxy mass and virial mass was first noticed in the Coma cluster by Fritz Zwicky in 1933 (Zwicky 1933). He suggested an invisible form of matter to make up the difference — the original suggestion for the existence of dark matter. Right: image of Zwicky from the Swiss Fritz Zwicky Society. So, what if the galaxy clusters are 90% dark matter? I Then the clusters could be in equilibrium. The dark matter “nucleus” of each cluster could have thermalized early in the life of the Universe, and the visible galaxies formed after their dark halos were already in equilibrium. I This is enough mass to keep the X-ray gas from getting away: with the mass above, the Jeans escape time for the hot ICM in Coma is 2 1015 yr, much longer than the ⇥ age of the Universe. April 20, 2021 (UR) Astronomy 142 | Spring 2021 20 / 35 Dark matter gravitational lenses Weak lensing of background galaxies by a foreground cluster. Image from Wikimedia Commons. April 20, 2021 (UR) Astronomy 142 | Spring 2021 21 / 35 Dark matter revisited In the case of the Bullet Cluster 1E 0657-558, dark matter appears more directly. The Bullet Cluster consists of two merging galaxy clusters. In the interaction, the hot gas from the clusters, visible in X-rays (red), has suffered inelastic collisions and is now separate from the dark matter (blue). The dark matter is detected via weak gravitational lensing (Clowe et al. 2006). April 20, 2021 (UR) Astronomy 142 | Spring 2021 22 / 35.