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Home , CfA Redshift Survey, Coma Cluster, Elliptical galaxy, Galaxy cluster, Hydra (constellation), NGC 4874

Astronomy 218 Clusters of Hierarchical Structure The shows range of patterns of structures on decidedly different scales. (typical diameter of d ~ 106 km) are found in gravitationally bound systems called clusters (≲ 106 stars) and (106 ‒ 1012 stars). Galaxies (d ~ 10 kpc), composed of stars, star clusters, gas, dust and , are found in gravitationally bound systems called groups (< 50 galaxies) and clusters (50 ‒ 104 galaxies). Clusters (d ~ 1 Mpc), composed of galaxies, gas, and dark matter, are found in currently collapsing systems called . Superclusters (d ≲ 100 Mpc) are the largest known structures. The Three large spirals, the Galaxy, (M31), and Galaxy (M33) and their satellites make up the Local Group of galaxies. At least 45 galaxies are members of the Local Group, all within about 1 Mpc of the Milky Way. The mass of the Local Group is dominated by 11 11 10 M31 (7 × 10 M☉), MW (6 × 10 M☉), M33 (5 × 10 M☉) Cluster The nearest large cluster to the Local Group is the at a distance of 16 Mpc, has a width of ~2 Mpc though it is far from spherical. It covers 7° of the sky in the Virgo and . Even these The 4 brightest very bright galaxies are giant galaxies are elliptical galaxies invisible to (M49, M60, M86 & the unaided M87). eye, mV ~ 9. Virgo Census The Virgo Cluster is loosely concentrated and irregularly shaped, making it fairly M88 M99 representative of the most M100 common class of clusters. M90 M89 M86 Because of its proximity, at M59 M84 least 3500 galaxies are known, M87 M58 mostly dwarf ellipticals and M60 spheroidals.

Among the brightest 207 M49 galaxies are 128 bright spirals, 49 lenticulars and 30 ellipticals, also typical of large irregular clusters. Giant The giant elliptical galaxy M87 M87 is truly a monster, with a mass of 2.4 × 1012 M☉ within 32 kpc but ~2 13 × 10 M☉ within 240 kpc. M87 has an active nucleus, with a jet that was first observed in 1918 and is one of the brightest 5 kpc radio sources in the sky (Virgo A). M87’s central is one of the most massive known, 9 with MBH = 6.6 × 10 M☉. It is accreting matter at a rate �̇ ~ −1 0.1 M☉ /yr . Denser, richer clusters tend to be regularly shaped. An example is the Coma cluster, located just north of the Virgo Cluster in the Coma Berenices on the sky. At an average of z = 0.023, it lies 100 Mpc away.

The center of the Coma cluster is dominated by 2 giant elliptical galaxies, NGC 4889 & NGC 4874. The galaxy population, especially near the center, is dominated by ellipticals and S0, only 15% of bright galaxies are spiral. Cluster Kinematics Galaxy clusters are gravitationally bound systems, thus all of the galaxies orbit the common center of mass. For the Local Group, the typical velocity is 150 km s−1. The Milky Way and Andromeda are approaching each other at a combined 119 km s−1. At this rate, they will cover the 770 kpc separating them in 6.3 Gyr. For the Virgo cluster, the spiral galaxies show a much larger (888 km s−1) than that typical of the elliptical, lenticular and dwarf galaxies (573 km s−1). The regular shapes of regular galaxy clusters suggest they are in dynamical equilibrium, with uniform velocity dispersions of 800 ‒ 1000 km s−1. For Coma cluster, the velocity dispersion σ = 880 km s−1. Cluster Mass For a system in dynamical equilibrium, the applies, thus the velocity dispersion of galaxies in a rich cluster can be used to estimate the cluster’s mass. , in 1933, showed that for Coma, σ = 880 km s−1 and the half-light radius rh = 1.5 Mpc. 15 ≈ 2 × 10 M☉ ≈ 4 × 1045 kg 10 In contrast, the LB ≈ 2 × 10 LB,☉ ≈ 400 LB,MW. This gives a mass to light M/LB ≈ 250 M☉/LB,☉. Intergalactic Gas The space between galaxies in clusters is filled with hot (~10 million K), low density (100‒1000 m−3), X-ray 36‒38 emitting (Lx ~ 10 W) intracluster gas and stray stars. The gas maps the gravitational potential of the cluster.

Abell 1835 Source of the Gas The intracluster gas is predominantly hydrogen and helium, however X-ray spectra reveal emission lines of heavy elements, like Fe, S, Si, Mg & O in high ionization states, for example Fe XXV, Fe XXVI. These heavy elements come from supernovae and represent a nearly ⅓ of that in the . Ram stripping While supernovae are capable of ejecting gas directly from galaxies, the can also remove gas from galaxies directly via ram stripping. As a galaxy moves through the ICM, interactions between the ICM and the gas in the galaxy strip the gas from the galaxy. This ’s swept- back lobes provide a direct observation of interaction with intracluster gas. Gas Mass For the Coma cluster the average gas temperature is 8 〈Tgas〉 ≈ 10 K and the total mass of X-ray emitting gas 14 is Mgas ≈ 2 × 10 M☉. This represents ~ 5 times more mass than is present in the stars of the cluster.

Maintaining with the hot gas requires kBTc ≈ 2 G M μmp /R

This can be reorganized to 45 ~ 10 kg calculate the mass, using ~ 0.5 × 1015 M ☉ 〈Tgas〉 = Tc and rh =R/2, Lensing Clusters The dark matter present in the cluster affects not only the galaxies of the cluster and its intracluster medium. The light of more distant galaxies is also bent as it passes the foreground cluster’s mass, distorting the image of distant galaxies into arcs of light. Lens Reconstruction The distorted images of an extended object reveal much about the defects in a glass lens. In a similar fashion, the multiple distorted images of a galaxy disclose the mass of the lensing 15 cluster, typically ~ 10 M☉.

Numerical reconstruction of the lens unveils a map of the dark matter in a . Though smoother than the distribution of light, the dark matter is nonetheless clumpy. Beyond the Neighborhood Despite the large numbers of galaxies in rich clusters like the Virgo cluster and Coma cluster, 80% of galaxies reside in groups or poor clusters. The environment beyond the Local Group is illustrative. The next nearest galaxies are members of the group (6 galaxies, at an average distance of 1.8 Mpc), (8 galaxies, 3.1 Mpc away) and group (17 galaxies, 3.5 Mpc). Because these groups are nearby, they appear large on the sky. For example, the covers 20°. Within 10 Mpc lie 4 more groups; M101 (5 galaxies, 7.7 Mpc), M66 (3 galaxies, 9.4 Mpc), I (8 galaxies, 9.4 Mpc) & NGC 1023 (6 galaxies, 9.5 Mpc). Virgo There are about 20 groups of galaxies closer to us than the Virgo cluster. Their spatial distribution forms a plane which intersects the Virgo cluster. This establishes our membership in the Virgo (or Local) supercluster. The forms a flattened ellipsoid (r ~ 20 Mpc), centered on the Virgo cluster. Nearby superclusters include -Centaures (d ~ 70 Mpc) & Persus- (d ~ 40 Mpc). Supercluster membership Superclusters are only now collapsing under their own . To estimate the density of the supercluster, one can equate the time for gravitational freefall to the age of the Universe

11 −3 ⇒ ≈ 3 × 10 M☉ Mpc −26 −3 −3 ≈ 2 × 10 kg m ≈ 14 atom m Virgocentric Infall Because the members of a supercluster are gravitationally bound to the supercluster, they “orbit” the supercluster’s barycenter. By mapping departures in the velocity field for galaxy clusters from the pure “Hubble flow”, we can measure the motion of the Local Group.

Though values as large as 300 km s−1 have been suggested, the current value for the Virgocentric of the Local Group is 168 ± 50 km s−1 compared to a Hubble flow −1 of H0d = 1140 km s . Looking Deeper As we look deeper into the universe, we see a repeating pattern of more and more clusters and superclusters. For example, the cluster of galaxies, lies 670 Mpc on the far side of the Virgo cluster. However the distribution of clusters is not uniform. Images like this Herschel space observatory observation of the Lockman hole, a 1° dust-free lane in , show the galaxy distribution is clumpy, with filaments rich in galaxies and voids. Mapping the Galaxies Understanding this pattern of filaments and voids requires a 3-dimensional map. Distances at this scale are impossible, but are available, thus these maps are called redshift surveys. The first example of these were the CfA redshift surveys of the 1980s. They revealed a structure to the Universe dominated by walls and voids.

This map shows the redshifts of all galaxies brighter than mB = 15.5 as a function of right ascension (8h ‒ 17h) for a narrow range in declination (26.5°‒ 32.5°). As early as 1878, it was noted that there was a gap in the distribution of galaxies (then nebulae). It was named the Zone of Avoidance by Hubble, who mapped it in 1931. The result of interstellar extinction due to gas and dust, the only means of mapping galaxies in the Zone of Avoidance is by radio or X-ray observations. Pencil-Beam Surveys Practical matters limited galaxy surveys to a couple thousand galaxies in the 1980s. Kirshner, Oemler, Schechter & Schechtman took a complementary approach to the CfA of Gellar & Huchra, the pencil-beam survey. This provided a measure of large-scale structure that was narrow but deep. It revealed the first (the void in Boötes) but no structure larger than 200‒300 Mpc. Finger of God The most noticeable feature in the original CfA redshift survey was the stick man. Lines pointing towards are a common features in redshift surveys, the result of peculiar motions. The velocity dispersion of galaxies in a cluster allow two neighboring galaxies to have velocities different by 2σ. This introduces a difference in redshift Δz = 2σ/c. This equates to a different in distance of For Coma cluster, σ = 880 km s−1 ⇒ Δd ~ 25 Mpc, which is much larger than the true diameter d ~ 3 Mpc. Total Local Group Motion Thus far, we have seen that the Milky Way and Andromeda are approaching each other at 119 km s−1. The Local Group is approaching the center of the Virgo Supercluster at 170 km s−1, but this seems not to be the only impact on the LG velocity, as the total velocity from the local supercluster (LSC) is 323 km s−1. One theory is an effective “repulsion” of 258 km s−1 away from the Local Void. Beyond the Local Supercluster, the -Hydra-Centaurus and the Shapley Concentration contribute 455 km s–1, bring the total to 631 km s−1, indicated by the Cosmic Microwave Background. Filling the Voids The greatest surprise to come from the redshift surveys was the large voids, essentially devoid of bright galaxies. In the center of a dense cluster (the red regions at right), ~1045 kg is packed into a radius of a few Mpc, so ρ ~ 10−23 kg m−3, while ρ ~ 10−26 kg m−3 when averaged over a supercluster. The density in the voids is harder to determine, but their −30 transparency at optical wavelengths requires ρdust < 10 kg m−3. Limits on Lyman α absorption by neutral hydrogen −36 −3 requires ρHI < 10 kg m . Limits on the density of ionized gas and dark matter are more difficult. Mapping Further Early redshift surveys revealed structures up to the scale of the survey, 100 ‒ 200 Mpc, but could not reveal the maximum scale of such structures. Later surveys, with 10 times more galaxies, confirmed 100 ‒ 200 Mpc structures but showed no sign of structure on larger scales. The decreasing density of galaxies at the farthest distance is due to the difficulty of observing them. Malmquist Bias Most population surveys in astronomy

Average Luminosity with are limited by the distance apparent brightness. Gunnar Malmquist, (1893-1982) in 1922, developed the statistical basis to study this sort of selection bias. In such magnitude-limited samples, intrinsically dimmer objects are under-represented. Thus the measured average intrinsic brightness will be higher true average and will increase with distance. Luminosity Function Studies of globular clusters revealed the distribution of stars as a function of luminosity, but we have no similar group of co- evolved galaxies. However, if the Universe is similar on the scale of 100 Mpc, a local census over this volume will serve as “typical”.

Empirically, the resulting luminosity or Schecter function is 10 −3 where L∗ ≈ LMW ≈ 2 × 10 L☉, Φ∗ ≈ 0.01 Mpc & α ≈ −1.2. How Bright is the Universe Integrating Φ(L) provides the total luminosity density 8 −3 = Φ∗ L∗ Γ(α + 2) ≈ 2 × 10 L☉ Mpc ≈ 10 W AU−3

For comparison, the has a volume VSS ~ (2000 3 26 AU) and a Luminosity L☉ = 4 × 10 W, thus 16 −3 ρL,SS = L☉/VSS ~ 5 × 10 W AU 3 9 The Milky Way has a volume, VMW ~ (35 kpc) = (7 × 10 3 10 36 AU) and a Luminosity LMW = 2 × 10 L☉ = 8 × 10 W, so 7 −3 ρL,MW = LMW/VMW ~ 2 × 10 W AU The larger the scale, the dimmer the Universe. Next Time Large Scale Structure