ASTR 200 : Lecture 25 Galaxies: internal and cluster dynamics 1
Galaxy interactions Isolated galaxies are often spirals One can find small galaxy `groups' (like the Local group) with only a few large spiral galaxies It is obvious that in some cases the galaxies are 'interacting' Distorted shapes `bridges' between them In rare cases there are 3 or more large galaxies in a group. At right: Stephan's quintet 2
Small groups of galaxies take roughly the age of the universe to reach their first interactions Paul Hickson (UBC) is well known for his catalog of `compact groups' These are useful for studying the first phases of group mergers 3
Galaxy 'collisions' 4
What happens in galaxy collisions Amazingly, the mutual impact cross section of STARS is completely negligible! Thus, as galaxies cross, the stellar components `go right through each other', but each STAR always feels all the other gravitational tugs of all mass from both galaxies. The GAS, on the other hand, has huge cross-section and can `collide' and dissipate energy. The gas `smacks into other gas'. This triggers massive amounts of star formation in the initial stages of a merger Gas can also be stripped and heated and end up far from the galaxies (especially in clusters) 5
Galaxy clusters. The Virgo Cluster. ~ 16 Mpc away. ~1300 galaxies 6 apod.nasa.gov/apod/ap110422.html Click on image there to zoooom in. Incredible
Ram pressure stripping Stellar component continues unimpeded 7 galaxy's motion into Virgo Cluster NGC 4402, entering Virgo cluster
Temperature and spectrum of hot gas between galaxies We can estimate what the temperature of stripped gas would be by assuming that all the kinetic energy of the gas cloud orbiting in the cluster is converted to thermal energy. Typical speed of motion of Virgo cluster galaxies is v ~ 400 km/s For an individual atom of mass m, the kinetic energy is just mv2/2 The average thermal energy of the atom in a gas of temperature T is going to be 3kT/2 2 2 mv m v 6 Equating this gives: T = 6.5 x 10 K ( )( ) 3k mh 400 km/s At a few million degrees Kelvin, the thermal emission peaks in what is called the 'soft' X ray region (energies per photon of 0.1 5 kev). The observed peak photo energy is ~ 0.5 kev, corresponding to a gas temperature of 7 million K 8 The energy source was the collision of the galaxies
X-ray emission in galaxy groups and clusters There is a new phenomenon found when galaxy groups and clusters are examined Large scale X ray emission <<< At left : X ray map of Stephan's quintet Right, the Xray image (in blue) is superposed on the optical image. As is typically the case, the X ray emitting gas is mostly between the galaxies Gas has been stripped out! 9
So, that's the gas. What about the stars? The passing galaxies produce `tidal tails' of stars that take away orbital energy, allowing the central portions of the two galaxies to merge after a couple mutual orbits At right: snapshots (top to bottom) of a merger simulation Shown in class: www.youtube.com/watch?v=c0xnytp5brm Generic outcome: the merger remnant is an elliptical galaxy 10
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The Coma Cluster ~100 Mpc away ~10,000 galaxies Mostly ellipticals 12
Morphology Density Relation, in clusters This all fits into a pattern. One sees that as one examines first isolated galaxies, then groups, and then clusters of increasing density, the fraction of galaxies that are spirals drops and the fractions that are ellipticals and S0's (spirals with very little recent star formation because they they have little gas) rises. Conclusion: Galaxy interactions strip gas and convert spirals to ellipticals 13
Galaxies may intially grow by accreting many small galaxies This is called `galactic cannibalism' 14
An example in the Milky Way The globular cluster named `Palomar 5' has a halo orbit that takes it through the Milky Way's disk During its last close pass to the galactic center, the Milky Way's gravity pulled tidal tails off Palomar 5 Eventually the cluster will be disrupted 15
Supergiant ellipticals in centers of clusters have done this many times 16
Supergiant ellipticals in centers of clusters have done this many times 17
Cluster dynamics Galaxies in a cluster are in a gravitationally bound system. But this is not like a solar system; no single galaxy dominates. Instead, the galaxies all buzz around in the gravitational potential created by all of them. In such a situation of mutually interacting objects where one single mass does not dominate, the objects exchange kinetic and potential energy and (for the ones that don't collide) come to 'virial equilibrium', where 2K= U That is, they transfer speeds until the kinetic energy N K = i=1 1 1 2 2 mi v i M v 2 2 And the potential energy of the system is 1 G M2 U 2 rh rh is the 'half mass' radius (a sphere inside of which half the cluster's mass in contained). In this case: 2 18 v rh M virial 2 G M total system mass <v2> mean square speed
The first evidence for dark matter: galaxy clusters In 1933 Fritz Zwicky was examining the Coma cluster of galaxies, measuring the speeds of the galaxies He realized that the computed virial mass (~2x1015 solar masses) needed to keep the galaxies in a bound group greatly exceeded the mass one estimated by adding up the luminous galaxies He realized there must be a large amount of mass that was not emitting light, and invented the term 'dark matter' 19 Galaxy rotation curves was the 2nd
A third argument for dark matter: That X-ray gas First of all, examining the total flux of gas allows an estimation of the total amount of gas that is between the galaxies in a cluster; the total mass of this gas is far too little for it to be all the dark matter (only about 10% of the cluster's mass). However, the X-ray emitting gas in clusters is hot, so atoms are moving fast because their temperature is high If you calculate the speed of atoms, it exceeds the escape speed from the cluster if you calculate the latter based only the mass that you can see. Therefore, because the gas is clearly remaining in the cluster, there must be more (dark) matter than is emitting light. 20
The 'Mass to Light' ratio Important simple quantity used in discussions of galaxy dynamics and dark matter. The Mass-to-Light ratio of an object (examples: a star cluster, a galaxy, a cluster of galaxies) is just the estimated mass (in solar masses) divided by amount of energy output, in solar luminosities. Thus, by definition the M/L ratio of the Sun is 1. The M/L ratio of the Solar System is also just slightly bigger than one Globular star clusters have M/L ratios of 1-3 Typical galaxies have M/L ratios of 2-10. Because L~M4 expect M/L > 1 because most stars have M<1 However, even a few high-mass stars add lots of light, bringing it down Typical clusters have M/L in the range 10-100. 21