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Bars Galaxy and galactic system mergers Stefans Quintet impulse approximation

AST1420. Lecture 19 Bars, Mergers, Ellipticals. Bars Galaxy and galactic system mergers Stefans Quintet impulse approximation dynamical friction dynamical friction in Maxwellian velocity field applications: dissolution of globular clusters in Andromeda

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Bars Galaxy and galactic system mergers Stefans Quintet impulse approximation

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  1. AST1420. Lecture 19 Bars, Mergers, Ellipticals Bars Galaxy and galactic system mergers Stefans Quintet impulse approximation dynamical friction dynamical friction in Maxwellian velocity field applications: dissolution of globular clusters in Andromeda future merger with LMC and SMC and our Galaxy Elliptical galaxies: photometry M87 in Virgo as an example of a giant elliptical (cD) galaxy - jets, black hole, superluminal motions etc.

  2. BARS

  3. Simulations of spiral structure formation in a 3-component galaxy model (yellow=bulge stars, blue=disk stars, red=halo) Tidal forces applied at the beginning of the simulation Spontaneous growth of the pattern from noise by SWING N-body calculations by J. Barnes, time between frames ~orbital period.

  4. About 1/2 of all galaxies have a bar. Bar formation may be a by-product of a self-defence by a disk galaxy against the approaching gravitational instability.

  5. MW sun A Milky Way-like galaxy with non-linear waves (as opposed to ‘linear’, that is sinusoidal of very small amplitude; this term has nothing to do with the shape of the wave!). SPH (Smoothed Particle Hydrodynamics) model of gas disk response to the Milky Way’s force field including a stellar bar. Notice the non-linear response of gas: shock waves form, as opposed to a smooth and gradual density structure with a smaller density contrast in the stellar disk. Basic difference in response is due to velocity dispersion of 7 km/s in gas vs. 35-45 km/s in stars.

  6. Stephan’s Quintet is a small group of galaxies in the constellation Pegasus. The galaxies are not only close on the sky but are physically close in space. So close, in fact, that they interact gravationally with each other. A new Chandra X-ray observatory data details the results of this interaction. The image below (upper left) is a composite of an X-ray image (shown in blue), superimposed on an optical image showing the locations of the Quintet galaxies, which are marked A,B,D, E & F on the lower right image). The X-ray image shows a "blue glow" produced hot gas in a bow shock in front of the "intruder" galaxy B. The gas in the bow shocked is heated to ~1e6 K by the motion of galaxy B through the inter-galactic gas in the group. X-rays and optical B visible

  7. GALAXY MERGERS NGC4676 mice galaxies See links to work done at Canadian Inst. For Theoretical Adtrophysics (CITA) on St. George campus http://orca.phys.uvic.ca/~patton/openhouse/movies.html http://www.cita.utoronto.ca/index.php/index_items/nature_of_dark_matter

  8. Estimate of the temperature T of gas after virialization (thermalization): The mean energy per particle (ion or electron) and per one dimension, in a thermalized gas (gas which relaxed to Maxwellian equilibrium) is equal The colliding gas flows in a cluster have a specific kinetic energy of relative motion equal to which is also of order potential energy of the cluster (by the virial theorem). Temperature of the shocked gas will thus be of order Typical velocities in the center of a clusters reach V~500 km/s. Then, the temperature T must be of order T~1e7 K. Conversely, such a high temperature is measured in X-rays, because the photon energy are ~1 keV, which corresponds to T~1e7 K. From this observed T we can then derive the virial estimate of the mass M which binds the cluster: GM/R ~ kBT. That mass usually is significantly, for instance ten times larger, than the estimate based on the visible matter (stars and gas). The gravitational potential well and the high velocities seen in the galactic groups and clusters predominantly due to dark matter.

  9. Interaction of galaxies in pair and groups as a trigger of spirality While the optical image suggests separate galaxies, BIMA radio-telescope array image of M81+M82+NGC3077 shows gas bridges connecting the interacting galaxies Cf. Fig. 5.34 in textbook

  10. The future: Milky Way - Andromeda collision (several Gyr from now) simulation by John Dubinsky (CITA; McKenzie supercomputer, 512 cpus, 10 days)

  11. Another view… the merger remnant becomes nearly elliptical

  12. The Antennae Galaxies (also known as NGC 4038 / NGC 4039) are a pair about 20 Mpc away in the constellation Corvus. They were both discovered by Friedrich W. Herschel in 1785

  13. J.Dubinski (CITA)

  14. Movie of galaxy formation and mergers

  15. Why do collisions often end in mergers and not just fly-by’s? (cf. Fig. 5.37 and description in the book) Dynamical frictionprovides the braking force. Its basic physics is that of deflection (gravitational scattering) of stars from the host/target by the perturber body. The perturber may either be a point mass or a compact stellar system, all of which stars will be considered to move together. Let us denote the mass of the perturber as M, and that of a star from the target galaxy as m. The relative velocity of m and M at infinity is V, both before and after encounter (due to energy conservation). We treat all the encounters as independent scatterings, in the limit of weak encounter (trajectory only slightly bent). While it is sometimes easiest to talk and draw only the relative motion of the bodies m and M, we must be careful not to forget about the fact that the lighter body (m) undergoes a stronger deflection than the more massive one (M), in proportion to the ratio M:(M+m) vs. m:(M+m).

  16. The dynamical friction decelerates an object travelling through a sea of stars (perhaps a target galaxy in a merger). The impulse approximation predicts an inverse quadratic dependence of the friction force on the velocity of relative motion: dV/dt = -const.(…)/ V^2. This is because, for any given impact parameter b, the time of interaction is ~1/ V, V is large, and the interaction brief and weak. However, the situation may be quite different, if our chosen test particle travels around a center of some system together with the particles providing the dynamical friction. Then, our integration should not assume that all the stars encountered arrive from one direction. Rather, we should arrive in the limit of V --> 0 at a final result where the test particle is NOT subject to any friction force except the isotropic random kicks from the passing compact objects: dV/dt --> 0 as V-->0. This is a very different dependence, which looks like across the range of V like this: Here, is the velocity dispersion in the stellar system. Let’s understand this a little better. -dV/dt 0 V

  17. How dynamical friction from a stream of particles with Maxwellian velocity distribution affects a body of mass M

  18. (an explicit formula w/derivation can be found in Binney & Tremaine 1987; vM = V) (B&T 1987) Cf. Problem 5.17 in S&G erf(x) = error function, an integral of Gaussian curve (Maple, Mathematica…) Notice F~ -M*M

  19. Applications of dynamical friction

  20. Applications of dynamical friction

  21. Applications of dynamical friction OLD VIEW: SMC and LMC bound since long ago

  22. Applications of dynamical friction NEW VIEW: SMC and LMC bound very recently(?) http://arxiv.org/abs/astro-ph/0606240v2 • Is the SMC Bound to the LMC? The HST Proper Motion of the SMC • Nitya Kallivayalil (CfA), Roeland P. van der Marel (STScI), Charles Alcock (CfA) Jul 2006 • Abstract: We present a measurement of the systemic proper motion of the Small Magellanic Cloud (SMC) made using the Advanced Camera for Surveys (ACS) on the HST. We tracked the SMC's motion relative to 4 background QSOs over ~2 years. We combine this new result with proper motion of the LMC & investigate the orbital evolution of both Clouds over the past 9 Gyr. The current relative velocity between the Clouds is 105+-42 km/s. Our investigations of the past orbital motions of the Clouds in a simple model for the dark halo of the Milky Way imply that the Clouds could be unbound from each other. However, our data are also consistent with Clouds bound to each other for ~ Hubble time.

  23. Elliptical galaxies Ellipticals are not as simple as their projected shape suggests. Most are triaxial ellipsoids (3 different axial extents) They are not in a relaxed, Maxwellian state (Trelax >> age of the universe.) They are most common in dense clusters of galaxies rather than small groups like the Local Group. In the centers of dense clusters live the enormous cD-type galaxies like M87. They are significantly brighter than L* = 2e10 Lsun (L* corresponds to an absolute magnitude MB = -20 mag.)

  24. De Vaucouleurs’ formula is a purely empirical formula, there is no physical derivation or understanding of ubiquity.

  25. But it works here!

  26. Tricks played by projection

  27. Most galaxies in Virgo are spiral, except in the center which is dominated by ellipticals and giant ellipticals - a sure sign of environmental influences (probably mergers)

  28. M87 in the Virgo cluster (also: M84, M86) [such giant, central elliptical galaxies are given type designation cD]

  29. M87 sports a one-sided jet of relativistic electrons and plasma. It comes from a very small “central engine” region, probably a black hole. The one-sidedness of such jets is an illusion: the jet is actually two-sided (bipolar) and directed almost directly toward and away from us. Only the approaching side appears bright due to a relativistic boosting (a.k.a. gamma-factor). A fast-moving object (atom, electron) radiates mostly in the direction of motion, and little in the opposite direction.

  30. Can any object move faster than light? NO. Not according to physics. This trick of nature is connected with special relativity theory and the relativistic gamma factor, but it really just follows from the geometry of the problem involving a small angle (orientation of the jet to the line-of-sight) and a finite speed of light. Read about it on p. 330 of the textbook! Relativistic effects, as already mentioned, boost the jet luminosity by a large factor, while dimming the image of the receding jet to a point where we can’t see it at all!

  31. An explanation of apparent superluminal motions in BL Lac objects and quasars’ jets Apparent speed of blob S8 is 3 times c (!) Was Einstein wrong? Do we see tachions?

  32. The explanation is relatively simple: time delays plus trig V dt cosO Demonstrate that: more precisely, for

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