20. The Local Universe PDF

20. The Local Universe 1.History of Astronomy 14.Stellar Remnants 2.Orbits & celestial mechanics 15.Special & General Relativity 3.The Earth-Moon system 16.The Milky Way 4.The Hydrogen Atom 17.Galaxies 5.Blackbody radiati...

20. The Local Universe 1.History of Astronomy 14.Stellar Remnants 2.Orbits & celestial mechanics 15.Special & General Relativity 3.The Earth-Moon system 16.The Milky Way 4.The Hydrogen Atom 17.Galaxies 5.Blackbody radiation 18.Active Galaxies & Quasars 6.Spectroscopy 19.Distance Indicators 7.Magnitudes 20.The Local Universe 8.The Sun 21.Galaxy Evolution 9.The Solar system 22.Newtonian Cosmology 10.Exoplanets 11.Star & Planet formation 12.Binary stars 13.Stellar Evolution 1 20. The Local Universe After we described the properties of galaxies as single objects, we now move to study how galaxies distribute in space. We are going to focus on the Local Universe, where we can study the distribution of galaxies in much higher details. 20.0 Luminosity and mass functions of galaxies 20.1 Structure in the nearby universe Nearby large-scale structures, groups, clusters, morphology segregation 20.2 The redshift space distortion and the two points correlation function 20.3 Crossing time, Dark matter Weighing galaxies and galaxy clusters 20.4 Modeling the large-scale structures N-body simulations (the millennium simulation) 2  Before moving on to see how galaxies distributes around us, we need to introduce two key A key observable: observables that helps the statistical analysis of galaxy samples. The first observable we introduce here is the luminosity function and we will then see the stellar mass function. The Galaxy Luminosity Functions Definition : “ The LF describes the relative number of galaxies of different luminosity by counting them in a representative volume of the Universe which then measures the comoving number density of galaxies per unit luminosity, L, such that : In the sky, as me move further out in space (towards higher redshifts), objects will become fainter and fainter to our eyes/telescope (they appear smaller and smaller in the distance). This means dN = Φ(L) dL dV that according to the capabilities of the telescope we are using at a time we will be able to observe where dN is the observed number of galaxies within a luminosity range [L, L + dL]” up to a certain apparent magnitude (or flux) or distance. In other words each telescope has limited capabilities in observing the sky such as that, in the end, we will only be able to observe a limited Volume of the sky dictated by the observing power of the telescope we use. Therefore, when we want to e.g. estimate how many galaxies we see in a certain area in the sky we always have to Luminosity and density evolution can take into account the Volume we are actually able to span. be seen as variations of L* and ϕ* The luminosity function is a function that describes the relative number of galaxies at different luminosity we observe in a determined Volume. The LF is characterise by the parameters here described and it can assume different parametric form. 3 The parametric form widely uses and here described is the Schechter function (which takes the name from Paul L. Schechter, the astronomer who first measured/described this function). Since Luminosity functions the luminosity of an object can also be described by its absolute magnitude we here also see how LF = # of gals/volume/mag; well-fit the Schechter function appear if expressed in absolute magnitude (the graphic corresponds to this by Schechter function. representation). When we observe a sample of galaxies in the sky, also called survey of galaxies, we can study the That can also be expressed in term luminosity functions per galaxy type: aka count the galaxies per galaxy type in each luminosity bin. of absolute magnitude When we do this we observe what is represented in the graphic on the top right: late-type spiral Bright Faint galaxies (blue) dominate the faint end of the luminosity function while the early-type elliptical galaxies (red) and intermediate type (aka lenticulars) dominate the bright end of the luminosity Because: function. At the bottom right you see the translation of the luminosity function in a simple histogram. When Note: Red galaxies dominate bright end, blue galaxies have steeper faint-end slope. we say that a specific type of galaxy dominate a region of the luminosity function we mean that 4 they are more numerous in the galaxy survey and thus dominate the number density of the galaxies in a survey.  Another important observable is the stellar mass function. This is like a luminosity function, but Stellar Mass Function now we are considering the number density of galaxies as a function of their stellar mass (instead MF = # of gals/volume/mass; of their luminosity). well-fit by a double The stellar mass function can also be broken up into “red” (ellipticals) and “blue” (spiral) galaxies. Schechter function. In this case the parametric form that best describe the mass function are two superimposed M* = 1010.64 M¤ Schechter functions, one for the red galaxies and one for the blue ones with different steepness. Massive (large) galaxies (ellipticals) are predominantly red, while blue galaxies have a steep faint- a1 = -0.43 end slope and hence dominate the small galaxy population. F*1 = 4.18 Mpc-3 ; + a2 = -1.50, Now that we have introduced this observable we can see how they can be used to describe the F*2 = 0.74 Mpc-3 galaxy in the Nearby Universe. Large/massive galaxies are predominantly red, while blue galaxies have a steep faint-end slope and hence dominate the small/low NOTE: these observable are also used at high redshifts, but there the observables are more luminous galaxy population. GAMA Survey uncertain and thus also the description of what we find complicates! 5 In order to study the local universe we are going to start a journey from where we are in space and we move further out. When we start our journey we immediately find some other galaxies around us. 2,500,000 light years between us and Andromeda 6  20.1 The Local Universe – Local Group of Galaxies We then realise that these galaxies are gravitationally bounded together to us. The milky way is part of a galaxy group called the Local group! Galaxies within 1 Mpc from us constitutes what we call The Local Group of Galaxies Major components of the Local Group are: Milky Way + M31 (Andromeda) + M33 A further 40 smaller galaxies complete the Local Group (dwarf ellipticals and irregulars) Side-view of the Local Group projected on the plane of the Galaxy 7 20.1 The Local Universe – Local Group of Galaxies Galaxies within 1 Mpc from us constitutes what we call The Local Group of Galaxies Major components of the Local Group are: Milky Way + M31 (Andromeda) + M33 A further 40 smaller galaxies complete the Local Group (dwarf ellipticals and irregulars) Side-view of the Local Group projected on the plane of the Galaxy Top-view of the Local Group projected on the plane of the Galaxy 8  20.1 The Local Universe – Local Group of Galaxies NOTE: In the local group some galaxies are moving towards us: local motion dominates over cosmological Selected members of the Local Group: expansion for galaxies at this proximity. Name Type MV Distance m–M Mass radial velocity (kpc) (mag) (MSun) (km/s) M31 = NGC224 Sb -21.1 690 24.6 7x1011 -267 Milky Way SBb/SBc -21 8.5 14.7 3x1011 ― M33 = NGC598 Sc -18.9 690 24.6 4x1010 -190 LMC Irr I -18.5 50 18.6 6x109 +275 SMC Irr I -16.8 60 19.1 1.5x109 +163 NGC205 E6p -16.4 690 24.6 ― -239 M32 = NGC221 E2 -16.4 690 24.6 2x109 -220 NGC6822 Irr I -15.7 460 24.2 1.4x109 -34 … Ursa Minor dE6 -8.8 67 19.5 105 ― Draco dE3 -8.6 67 19.6 105 ― Groups: 50 members, vrms ~ 800 km/s, M ~ 1x1015 Msun 9 As we move further out the local group we find that the local group itself is part of a larger structure named the Local supercluster which is composed of other small and big group/clusters of galaxy. Where we are 10  20.1 The Local Universe – Local Supercluster The Virgo cluster is the dominant component of the local supercluster and actually it sits in the centre of it. The local group is part of a Local larger structure called the Local Supercluster The Local Supercluster consists of the Virgo cluster with many group of galaxies around it (including the Local Group) preferred location in the supergalactic plane diameter ~ 30 – 40 Mpc Virgo cluster at the centre Local group at the outskirts M ~ 1015 MSun Ngal ~ few x 10 000 11 20.1 The Local Universe – Supergalactic Plane In this picture, colour indicates distance. Blue near, red “far” z>0.04. Far in this case is really close by! Remember this is still the local universe. The Supergalactic plane is a local ‘wall-like’ structure in which the nearby galaxy groups and the Virgo cluster reside. It even extends to the Great Attractor and the Perseus-Pisces supercluster. ↓ Supergalactic plane ← Galactic Plane → 12  20.1 The Local Universe – Supergalactic Plane Our “local” universe. 13 20.1 The Local Universe – Supergalactic Plane Tully and Fisher did a lot of work on characterising our local (Virgo) supercluster. A rich, convoluted structure that breaks into two main clouds with streamers appearing from above and below the central plane. Most of the supercluster is empty space: 98% of visible galaxies are contained in just 11 clouds that fill only 5% of the overall volume. The clouds delineate a disk structure, with a width:thickness of 10:1. 14  20.1 The Local Universe – Virgo Cluster Within the local supercluster the Virgo cluster dominates. Central concentration of ellipticals and spirals Gravitationally bound M90, m=10.2 2500 galaxy members M86, m=9.8 distance ~ 17 Mpc M87, m=9.4 radius ~ 3 Mpc (extends 20 degrees M86, m=10.0 on the sky) M89, m=10.7 The image of the Virgo cluster shown here only covers 3.2 x 3.2 degrees. 15 Well done! See you next time! END OF LECTURE 1/4 START OF LECTURE 2/4 16  20.1 The Local Universe – Galaxy Groups As we move even further out we find that galaxies form other groups and clusters and that actually the majority of the galaxies belong to at least a group or a cluster (a cluster is a bigger structure) The distribution of galaxies within 200 Mpc Clusters and superclusters (with dimensions varying between 10 Mpc and 100 Mpc) Majority of the galaxies reside in groups or clusters. Arp 319 = HGC 92 Stefan’s Quintet v = 6450 km/s Arp 320 = HGC 57 Copeland’s Septet v = 9110 km/s 17 Groups and clusters Clusters: 10’s-100’s of L* galaxies, mostly ellipticals Groups: a few L* galaxies, spirals and ellipticals Field: isolated L* galaxies, mostly spirals NGC6946 (field) Stephan’s Quintet (group) 18  The morphology-density relation is also known as morphology segregation Morphology-density relation Ellipticals/S0’s live in dense regions (groups, clusters), spirals live in “field” (isolated, poor groups) Groups Clusters Increasing densityà 19 20.1 distribution of galaxies We see here an illustration of how galaxies distributed in the different environments according to their galaxy type. We can describe this trends using the luminosity function we introduce earlier. In this way we can clearly see what type of galaxy dominates in the different environments. Quantifying the galaxy content of a galaxy cluster; the luminosity function. → large number of faint dwarf galaxies 20  As we already mentioned galaxies are generally not found in isolation. Most of them are Morphology-Density surrounded by a swarm of satellite galaxies and are often embedded in larger aggregates such as galaxy groups or clusters. Relation In these environments, interactions between galaxies are not uncommon and in general, can be Ellipticals prefer denser environments. divided into two basic types: Discovered in the 80 s, mergers and ‘fly-bys’. regarded as a fundamental Interactions involving high-speed fly-bys are often referred to as galaxy ‘harassment’. aspect of environment. Why does it occur? Harassment can disturb, or even radically change, the morphologies of the galaxies involved, often – Ram-pressure stripping? inducing new bursts of star formation. Asymmetrical galaxies, disturbed – Merging? galaxies, warps, bars and tidal tails can all be produced through galaxy harassment. – Harassment? Galaxy clusters are permeated by hot, X-ray emitting gas known as the intra-cluster medium. As – Starvation? individual galaxies move within such clusters, they experience this intra-cluster gas as a ‘wind’ – much like the wind experienced by a moving bicyclist, even on a still day. ‘Ram pressure stripping’ Goto et al. 2004 21 occurs if this wind is strong enough to overcome the gravitational potential of the galaxy to remove the gas contained within it. The result of ram pressure stripping is a galaxy which contains very little cold gas. This effectively halts star formation in the galaxy, supporting the belief that ram pressure stripping could be one of the processes responsible for the morphology density relation. (http://astronomy.swin.edu.au/cosmos/R/Ram+Pressure+Stripping)  20.1 The Local Universe – Galaxy Cannibalism Galaxy cannibalism in clusters High galaxy density → In clusters of galaxies, the spacing of galaxies is only ~100 times their diameter (compare this to the wider spacing of stars in a galaxy: 106 times their diameter) Large range in galaxy masses → Most massive galaxies (giant ellipticals) are at least 107 times more massive than the least massive ones (irregulars and dwarf ellipticals) → Tidal disruption frequently occurs: smaller galaxies get pulled apart by the more massive galaxies. The devouring of a smaller galaxy by a large one is called galaxy cannibalism. cD galaxies at the cores of clusters are likely due to the merging of many galaxies lie at the bottom of the potential well of the cluster multiple nuclei (> 50% of the cDs) dynamical friction reduces orbital energy of passing galaxies → bound to cD 22 20.1 The Local Universe – Galaxy Cannibalism As with earth’s tidal bulges, an interaction will result in matter being pulled out from both sides of a galaxy. Because galaxies rotates, and this angular momentum is conserved, the matter ends up forming tidal arcs or tails. Tidal bridges may join two tidally interacting galaxies (SMC & LMC) but Prime example of tidal interaction between two galaxies also streams of stars and gas Compression and redistribution of gas mean that mergers are Tidal bridges (conservation of angular momentum blamed for increasing star formation – you can see this in the blue emission from OB stars. in a rotating galaxy) NGC 4038 and NGC 4039 23  MOVIE TIME: This simulation shows how a merger of 2 milky way like galaxy could happen. We know this is our own future as the Milky Way and Andromeda galaxy are on the route to merge one day! The red Galaxy Major merger dot you see in this movie is us….yeah, a quite dramatic destiny…but very far away in time! 2 Milky Way galaxies stellar and gas disks, spheroidal bulges, central black holes and dark matter halos WATCH this movie in VULA! galaxies lose angular momentum as the halos orbit (dynamical friction) star formation starts as gas is driven to the centre after final coalescence, gas is heated, cold gas driven out of the galaxy star formation ends -> evolves to a massive elliptical 24 25  When we first start to study galaxies outside our own, we did not have the capabilities we have today, so all that we could observe was a very limited space. Moreover, we did not know about all CfA1 (1986) : First redshift survey the cosmological correction we need to do to what we observe. So at the beginning, when we were The Stick Man! observing the distribution of galaxies in space, we were puzzled to find what we then called the Galaxies are not uniformly distributed on the sky sticky man! Galaxies like to live together in groups & clusters We now know that the stick man was actually the results of the redshifts space distortions also called the “fingers of God”. 26 CfA1 (1986) : First redshift survey The Stick Man! Galaxies are not uniformly distributed on the sky Galaxies like to live together in groups & clusters 27  20.2 The Local Universe – Redshift space distortions Redshift-space distortions (RSDs) are an effect in observational cosmology where the spatial distribution of galaxies appears squashed and distorted when their positions are plotted in redshift- space (i.e. as a function of their redshift) rather than in real-space (as a function of their actual distance). The effect is due to the peculiar velocities of the galaxies causing a Doppler shift in Coma cluster’s finger of god addition to the redshift caused by the cosmological expansion. Zone of Avoidance Redshift-space distortions manifest in two particular ways. The Fingers of God effect is where the galaxy distribution is elongated in redshift space, with an axis of elongation pointed toward the observer. It is caused by a Doppler shift associated with the random peculiar velocities of galaxies bound in structures such as clusters. The large velocities that lead to this effect are associated with the gravity of the cluster by means of the virial theorem; they change the observed redshifts of the galaxies in the cluster. The deviation from the Hubble's law relationship between distance and redshift is altered, and this leads to inaccurate distance measurements. Another, closely related effect is the Kaiser effect. This time, the distortion is caused by the 28 coherent motions of galaxies as they fall inwards towards the cluster center as the cluster assembles. Depending on the particular dynamics of the situation, the Kaiser effect usually leads not to an elongation, but an apparent flattening ("pancakes of God"), of the structure. It is a much smaller effect than the fingers of God, and can be distinguished by the fact that it occurs on larger scales. NOTE in the pictures: - Fingers of god in the Come cluster: Elongated appearance of the Coma cluster due to velocity dispersion inside rich clusters. - Botton left shows Virgo supercluster. A top view corresponding to the plane of the galaxy.  20.2 The Local Universe – Redshift space versus real space We now know how to correct for these effects. The real space is then described by real voids, walls/filaments and clusters. Redshift-space versus real space: Important considerations: Apparent distortions due to motion of galaxies over and above the Hubble expansion clusters voids walls/filaments 29 Well done! See you next time! END OF LECTURE 2/4 START OF LECTURE 3/4 30  20.2 The Local Universe – Superclusters The Local supercluster is not the only superclusters out there! The largest connected structures in the Universe – consisting of various clusters interconnected by filamentary structures (the cosmic web - see next slide) Superclusters: Local supercluster Virgo cluster 1200 km/s Great Attractor Norma cluster 4880 km/s Perseus supercluster Perseus cluster 5400 km/s Coma supercluster Coma cluster 6650 km/s Hercules supercluster Hercules cluster 11200 km/s Shapley supercluster Abell 3562 15000 km/s Superclusters are not spherical; dominated by flattened structures All rich clusters lie in superclusters At least 95 – 99% (possibly 100%) of all galaxies lie in superclusters Voids are predominantly spherical Voids are empty of (at least) bright (M < M*) galaxies 31 20.2 The Local Universe – Superclusters & the cosmic web Representation of the cosmic web. 32  20.2 The Local Universe – Morphology segregation Hubble and Humason already knew that ellipticals are most often found in the densest cluster cores. This just shows it schematically. Morphology segregation in superclusters These provide some observational constraints on theories of galaxy evolution, which must then Extension of what Perseus supercluster predict larger numbers of ellipticals in rich cluster core, with spirals and irregulars more evenly was already known by Hubble and distributed (but without losing the voids). We will come back to galaxy evolution in the next Humason for module. galaxy clusters (elliptical galaxies mostly reside at the cores of rich clusters) Result from formation processes correlated over the entire supercluster, or just in individual high density regions? Clue about structure formation and the way galaxies form and evolve. 33 We have seen that the majority of galaxies are somehow bounded, but how do we find the Clustering: members of a group? We use the 2 point correlation function. 2 Point Correlation Function (2PCF) If galaxies are clustered, they are “correlated” This is usually quantified using the 2-point correlation function, ξ (r), defined as an “excess probability” of finding another galaxy at a distance r from some galaxy, relative to a uniform random distribution; averaged over the entire set: dN(r)=ρ (1+ξ(r))dV1 dV 2 Usually represented as a power-law: x=(r/r0)-g For galaxies, typical correlation or clustering length is r0~5 Mpc/h and typical slope is g~1.8 , but these are functions of various galaxy properties; clustering of clusters is stronger 34  How the 2PCF appears Clustering: 2 Point Correlation Function (2PCF) Actually… Not a pure power law. Red galaxies have steeper slope. Mild luminosity dependence, strongest at luminous end. Zehavi et al. 2003, 2004 35 MOVIE TIME! Virtual reality software developed by the IDIA visualisation Lab at UCT that enables you to explore, through graph theory, the aggregation of galaxies into groups and clusters in the local universe. The redder the line is, the higher is the probability of two galaxies (here represented as points) to belong to the same group. WATCH this movie in VULA! 36  20.3 The Local Universe – Galaxy clusters crossing time Gravitationally bound As a whole, the cluster has the same light profiles as elliptical galaxies:  relaxed Crossing time: For a cluster with diameter 1 Mpc, and galaxies moving at 103 km/s, the crossing time is:. 37 20.3 The Local Universe – Superclusters (crossing time) Crossing time: Can a cluster have travelled a supercluster in a Hubble time? Hercules supercluster extends from 9500 – 13500 km/s in radial direction ( ~ 57 Mpc) velocity dispersions of clusters: 400 – 1500 km/s For v ~ 400 km/s: tct = d / v = (50x106 pc) x (3.1x1013 km/pc) / 400 km/s = 1.2 x 1011 years (long) tct (for v = 1500 km/s) = 3.3 x 1010 years (short) Both times are larger than the Hubble time, therefore a cluster cannot have traversed the supercluster in the age of Universe → supercluster cannot have formed recently, present during the very earliest stages of the Universe. Hercules supercluster 38  Well done! See you next time! END OF LECTURE 3/4 START OF LECTURE 4/4 39 20.3 The Local Universe – Galaxy clusters mass NOTE: Recall that the kinetic energy is expressed as 1/2Mv^2 and U is = 3/5 GM2 / R. In applying the virial theorem to large clusters, it is assumed that the cluster is in a stationary state. With a gravitationally bound, relaxed system like a galaxy cluster we can apply the Virial theorem to The kinetic energy is obtained from an appropriately weighted determination of the space velocity calculate the mass: dispersion v^2 (Rood et al. 1970; see also Rood & Dickel 1978a). Choosing a proper weighting 2K = -U scheme is not a trivial point; for example, Chincarini & Rood (1977) show that the calculated mass Where K is the kinetic energy and U is the gravitational potential energy, thus: M = 5R/G (Virial mass) of Abell 194 varies by almost a factor of three among plausible weighting schemes. The issue is where σ is the observed average radial velocity dispersion of the members. whether to weight by observed luminosities or by masses inferred from mean mass-to-light ratios. The latter method introduces great uncertainty since mean M / L's are poorly known for early-type When we compare the Virial mass to the luminous mass of the cluster, there is a shortfall. Even including the dark matter required by galaxy rotation curves does not account for the shortfall galaxies, which are a significant fraction of the membership in large clusters.  hot gas and dark matter in galaxy clusters. As with small groups, a correction must also be applied to convert the observed projected velocity dispersion σ2 to v^2, which will in general depend on the types of orbits in the cluster. If the orbits are primarily radial and we determine σ2 from the cluster core, then v^2 may only slightly exceed 3σ2, while if the orbits are nearly circular with an isotropic distribution v^2 ~ 3σ2 (Abell 1977). 40 If we assume this latter type of orbits then the kinetic energy which is normally defined as 1/2Mv^2, in the case of a cluster become, 1/2M3σ2 which then applied in the viral theorem, gives the viral mass here reported.  20.3 The Local Universe – Galaxy clusters mass Hydra cluster Optical X-ray  A significant amount of mass in galaxy clusters is in the form of hot gas  heated through infall, collisions and galaxy mergers. Even including the gas, there is a shortfall of mass compared with the dynamical (Virial) mass!! Dark matter. The pressure of the gas can gauge the amount of dark matter in the cluster. 41 20.3 Dark Matter – Weighing Galaxies To study the mass of the cluster we actually have to recall how we weight its component : galaxies. Oort (in 1932) already noticed that the visible stars only have 50% of the density, necessary to explain the observed vertical motions (respective to the Galactic Plane) of stars in the Galaxy. The HI disk (neutral hydrogen) extends far beyond the visible part of a galaxy as shown here in the case of NGC3198 NGC 3198 (optical & HI contours) NGC 3198 (optical) 42  20.3 Dark Matter – Weighing Galaxies (Dynamical Approach) Recall what we have seen for the Milky Way and its rotational curve!! Method 1: The Virial Theorem (2 = – ) fundamental assumption: system is stable, not collapsing, nor flying apart. = 0.4 GM/rh where rh is the radius which encloses half the mass, v is the velocity dispersion, and M is the total mass of the galaxy. Method 2: Rotation curve (velocity as a function of distance) a simplified view of this method involves Kepler’s 3rd law (assuming that the galaxy has a strong central concentration of mass). This is true for the optical light distribution → also true for the mass distribution? All the galaxies behave like our own Galaxy, no Keplerian drop-off far from the centre of each galaxy → suggesting a large fraction of each galaxy’s mass comes from its outer regions. 43 20.3 Dark Matter – Weighing Galaxies (Dynamical Approach) Method 3: Double galaxies and Newton’s form of Kepler’s 3rd law in order to determine the mass of a (binary) galaxy, the distance, angular size of the orbit, the period and the position of the centre of mass must be known. Galaxies revolve too slowly to determine orbits, periods etc., so this only works on a statistical basis and one can only determine the sum of the mass of both galaxies. Results for a statistical sample of 279 binary systems: average mass of a spiral is 1012 MSun Dynamical mass estimates considerably higher than the luminosity-based mass estimates (see round 1 of the discussion). → Mass-to-light ratio (M/L) in galaxies is 5 to 30 times larger than the solar value consider: mass-components not adding to the luminosity of the system (dark objects) dust, planets, comets, asteroids (in our solar system, these have negligible masses). What kind of object can add 5 to 30 times the mass of all the stars in a galaxy without producing any light? … yes, Dark matter 44  20.3 Dark Matter – Weighing Galaxy Clusters We can now see how we can measure the mass of a cluster. Determining the mass of a galaxy cluster I. The dynamical mass Apply the Virial Theorem M = 5 σ2 Rv / G σ = velocity dispersions of the galaxies in a cluster Rv = virial radius of the cluster 45 20.3 Dark Matter – Weighing Galaxy Clusters The Norma cluster: σ ~ 900 km/s MNorma ~ 2.5 x 1015 MSun ~ 50 – 100 km/s M = 1012 – 1013 MSun Galaxy groups ~ 500 km/s M = 1013 – 1014 MSun Poor galaxy clusters ~ 1000 km/s M = 1014 – 1015 MSun Rich galaxy clusters Dark matter: spiral galaxies Mgal ~ 3 – 10 Mlum elliptical galaxies clusters Mcluster ~ 100 Mlum = 10 – 30 Mgal dark matter content scale-dependent: 90% of matter in the Universe is dark matter. 46  20.3 Dark Matter – Weighing Galaxy Clusters Norma cluster in X-rays (0.1 – 2 keV, ROSAT) and in the optical. The X-ray luminosity of a cluster is a good indicator of the cluster mass X-ray is a probe of the intergalactic matter (gas and dust) Dust has been looked for and not found (optically thin over distance of galaxy clusters) Low-density, ionised gas (H II), hot, so emits X-rays. Temperatures: ~ 107 – 108 K, densities: 1000 ions/m3 Fe lines seen in X-ray, Determining the mass of a galaxy cluster intergalactic gas must have been processed through stars II. X-ray mass (X-ray luminosity is proportional to its mass) and then removed from Lx ~ σ4 galaxies (through supernovae?) 47 Determining the mass of a galaxy cluster Dark matter in galaxy clusters contributes like luminous mass to the effect of strong gravitational III. Gravitational lensing lensing (we have seen early on). By measuring the distortion geometry, the mass of the cluster causing the phenomena can be obtained. In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters. NOTE: in the picture you ca notice all the distorted images of the galaxies in the background of the cluster due to the effect of strong lensing. Abell 1689 (785 Megaparsec, 500.000 billion Msun) 48  20.3 Dark Matter – Candidates By far the majority of mass in a galaxy is in the form of dark matter typically on the order of 90%, but in some cases even larger. This is not just matter that is invisible in the optical, but it is invisible across the entire electromagnetic spectrum (from radio to gamma rays). What is it then? Among the stellar contenders, there are the following possibilities: Black holes? Could supply some of the unseen mass End products of (relatively rare) very massive stars; it is unlikely that there are enough around to explain all of the missing mass; primordial black holes? Brown dwarfs? low-mass pre-stellar objects (that never reached the nuclear burning stage) Low-mass red dwarfs? possibility rejected by very deep Hubble Space Telescope observations of globular clusters (should and could have been detected). cut-off in the distribution around 0.2 MSun, very low-mass stars seems to form less frequently than previously supposed. 49 20.3 Dark Matter – Candidates Exotic subatomic particles? Believed by many theoretical astrophysicists to have formed in abundance during the very earliest moments of the Universe. WIMPS: Weakly Interacting Massive Particles The idea is very difficult to test because of difficulties in detecting such particles (none so far have been detected) Search for Dark Matter through gravitational lensing! Gravitational lensing is an effect which follows from Einstein’s theory of General Relativity where light rays are bent in the presence of a gravitational potential (a passing foreground star for instance) MACHOs: Massive Compact Halo Objects 50  20.3 Dark Matter – Modified Newtonian Dynamics Modified Newtonian Dynamics (MOND) see the article by Milgrom (Scientific American, 2002 August) Main argument: Newton’s laws of gravity and the proportionality of force to acceleration have been successful in explaining the motions of planets, but can they be extended to a Galactic scale? Modifications to Newtonian physics have happened before: special relativity, to deal with extreme speeds, and general relativity, to deal with extremely strong gravity MOND proposed a modified form of acceleration; the acceleration of the solar system towards the centre of the Galaxy is very small: 10-10 m/s2 Successful in explaining the galaxy rotation curves, but still many observational tests to pass. MOND has been around for the last 20 years and is still considered by quite a few astronomers as a real option. see also: http://www.astro.umd.edu/~ssm/mond 51 20.4 N-body simulations In Modern astronomy we have access to very complex simulations (B-body simulation) that incorporate all the various ingredients we know (DM included) and try to recreate what we see today. N-body simulations Millennium simulation, evolving the universe and its components over cosmic time NOTE: Matter (dark and visible) distribution. Top left picture shows a rich galaxy cluster, ellipticals in the core in red, and spirals in blue, tracing the outskirts of the cluster. 2Mpc/h where h is a number between 0 and 1, allows changes in the Hubble constant (as it becomes more accurately derived) to show up in the changes in the scale of these diagrams. Bottom right image shows the dark matter distribution in the same region as from the simulation. This simulation has been done on a Supercomputer at MPE for more than a month and it produced a 25 TB of outputs! The Millennium Run used more than 10 billion particles to trace the evolution of the matter distribution in a cubic region of the Universe over 2 billion light-years on a side 52  The cosmic web as out of the simulation. 53 Millennium sim was dark matter only. Another important simulation project that need to be mentioned is the illustris project. illustris-project.org Illustris tries to understand galaxies, gas and feedback from black holes. illustris 54  MOVIE TIME! Time evolution of four separate quantities from high redshift to z=0, in an over-dense region 10 Mpc on a side. Panels show stellar light (top left), gas density (top right), gas temperature (bottom left), gas metallicity (bottom right). WATCH this movie in VULA! 55 Well done! See you next time! END OF LECTURE 4/4 END OF MODULE 56