Galaxy clusters. Dept. of Physics of Complex Systems April 6, 2018
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1 Galaxy clusters László Dobos Dept. of Physics of Complex Systems É 5.60 April 6, 2018
2 Satellite galaxies Large galaxies are surrounded by orbiting dwarfs approx satellites around the Milky Way, e.g. Sagittarius dwarf probably around all major galaxies also low surface brightness makes them very hard to detect low metallicity Dwarfs interact with the host galaxy tidal forces significantly effect satellites anomalous mass-to-light ratios stars are stripped from the satellites to form streams
3 Sagittarius stream Source: sdss3.org
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6 Galaxy groups and clusters Gravitationally bound systems of galaxies groups clusters # of galaxies: N < diameter: D 1,5 Mpc 6 Mpc velocity dispersion: σ v 150 km s km s 1 total mass: M M M mass-luminosity ratio: M/L 260 M /L 400 M /L High M/L ratio suggests large amounts of dark matter. Edge of cluster: where escape velocity is comparable to Hubble expansion.
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8 Our immediate neighborhood: The Local Group Three large spiral galaxies: Andromeda Milky Way Triangulum galaxy more than 50 small galaxies and dwarfs Main parameters M M σ v 60 km s 1 Members of the Local Group fall towards the center of mass non-virialized system Hubble s law is not valid from the group The Local Groups is part of a larger supercluster, see later
9 Catalog of galaxy clusters George Abell (1958) catalog of more than 4000 galaxy clusters photographic glass plates, identified by eye Main properties of clusters cardinality compactness distance (the 10 th brightest galaxy is a good standard candle) Largest clusters Virgo cluster: galaxies, M Coma cluster: approx galaxies
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12 Virialization Relaxed dynamical system 2K + U = 0 relaxation times scales as t relax 1 ln N largest clusters are already virialized centers of clusters virialize earlier from σ v one can calculate the dynamical mass σv 2 GM R Radial profile of relaxed clusters surface brightness as a function of R same as the R 1/4 de Vaucouleurs profile of ellipticals
13 Density-morphology relation Typical morphology of a galaxy depends on its location within the cluster central region: only giant ellipticals outer region: spiral galaxies dominate Conclusions galaxies in the center have collided and merged many times lost their galactic disks grew big via mergers
14 Formation and evolution of clusters Earliest clusters signs of clustering from as early as z 8 Hubble infrared and Chandra X-ray observations Butcher Oemler effect even clusters below z < 0.5 still evolve farther clusters contain more blue galaxies true for the larger clusters
15 Central galaxies of clusters In centers of clusters: a dominant giant elliptical galaxy morphological class: cd aka. brightest cluster galaxy (BCG) BCGs are slightly different from the rest of ellipticals 0,8 mag brighter than the second brightest of the cluster slightly different radial profile diffuse stellar halo extended to extreme distances How did they become this big? infall rate of galaxies seems to be too low is normal merging rate enough to explain BCGs? galactic cannibalism: is there an additional effect that helps devouring smaller galaxies? or is it stealing stars from other galaxies by tidal disruption?
16 Velocity dispersion Typical velocity dispersion of clusters: km s 1 significant Doppler shift comparable to Hubble expansion Fingers of god effect cluster members with highest redshifts are not further away but actually closer, falling at high peculiar velocity towards the cluster center Consequence a cluster cannot easily be mapped in 3D solely based on redshift
17 Dark matter Dynamical mass of clusters cannot be explained by visible matter considering the high velocity dispersion, additional constituent is needed to keep the cluster together we know that the cluster has been together for a very long time otherwise there wouldn t be giant ellipticals in it these formed via multiple mergers in billions of years clusters (at least their centers) are virialized Dark matter Fritz Zwicky (1933) 40 years latter: Vera Rubin observations of rotation curves of spirals
18 Distribution of dark matter Spiral galaxy rotation curves are constant: dark matter density is an isothermal sphere singular in the center ρ(r) = 3v 2 (R) 4πGR 2 Profile of DM halos of clusters from simulations: Navarro Frenk White profile ρ(r) = less pointed in the center, but still singular ρ 0 ( ) 2 R R S 1 + R R S
19 Gravitational redshift Redshift: Doppler shift: from line of sight peculiar velocity Transversal Doppler: relativistic effect Cosmological: from expansion of the Universe Gravitational: relativistic effect Expressively: light has to climb out of the potential well Lab confirmation: Pound Rebka experiment (Mössbauer spectroscopy) In galaxy clusters galaxies in the middle are in deeper potential larger redshift on average Doppler shifts average out need to stack data from thousands of clusters could not be shown with good enough significance
20 Galaxy clusters in X-ray Intracluster medium (ICM): extremely hot, tenuous gas very low density, transparent to its own X-ray radiation significant mass: M a few times the mass of all stars in the clusters still only a small fraction of dynamical mass Galaxy clusters are bright in X-ray intracluster plasma is extremely hot: K source of X-ray is bremsstrahlung Emission lines in X-ray ions of high mass ions, e.g. Fexxv, Fexxvi intracluster gas is not of primordial origin! must have gone there at early times today gas would not be able to escape from the galaxies
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22 Problem of (non existing) cooling flows Intracluster gas is highly luminous in X-ray should cool down on a 1 Gyr time-scale rapid cooling should cause infall of gas from outer regions only very rarely see such effect gas is hot even in nearby (old) clusters What s keeping the gas hot? cooling happens, so something has to keep the gas hot best candidates: active galactic nuclei keep the plasma hot via shock waves
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24 Warming effect of turbulent flows Source: Werner et al., Nature (2014)
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26 Automatic identification of clusters clusters appear as diffuse clouds in X-ray in the optical, identification is done by eye or algorithmically Redshift surveys by today, we have measured the redshift of about 1.5 million galaxies we als know the two angular coordinates Task: identify galaxy clusters and groups automatically friends-of-friends algorithm two galaxies belong to the same cluster if their spatial distance is smaller than a predefined limit
27 Clusters vs. superclusters Superclusters are much larges structures than clusters: 100 Mpc so big that they re not gravitationally bound their individual parts move with the Hubble flow by time, they will be disrupted by expansion Yet, it seems that there is large-scale collective motion of galaxies towards large concentrations of mass: dark flow the Local Group is moving away from the Virgo cluster this cannot be explained by Hubble expansion the mass be a large concentration of mass somewhere: the Great Attractor mass is a few time M, approx. 90% is DM Great Attrator is, unfortunately, obscured by the galactic disk, need to find in radio peculiar velocity of the Local Group wrt. the Hubble flow: v pec = 627 km s 1
28 A Laniakea supercluster Source: Tully et al., Nature (2014)
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