Epicyclic Orbits. Epicyclic motion produces a spiral pattern (see figure, Sparke & Gallagher, and

Transcription

1 Hubble Heritage Team, S, S SSO-South (.Gilbert,D.Goldman,J.Harvey,D.erschatse) - POPT (D.eichart) D O D 4 53 picyclic Orbits X D G S (Toomre, 1977, ig. 2) picyclic motion produces a spiral pattern (see figure, Sparke & Gallagher, and Toomre 1977), with pattern speed Ωp = Ω κ/2 (4.47) Spiral rms 9 (Toomre, 1977, ig. 3) With this different types of spirals can be formed: Density wave theory

2 D O D 4 59 Bulges X D G S Bulges are among the densest stellar systems. Surface brightness approximated by Sérsic s formula (empirical!): () = (0) exp [ (/0) 1/n ]. (4.48) or n = 1, exponential decrease; for n = 4, de aucouleurs formula (developed for elliptical galaxies). 0:. Observed values reach thousands of stars per cubic parsec. Bulges of Disk Galaxies nglmaier P., Gerhard O., 1997, S 287, 57 Pagel B..J., 2009, ucleosynthesis and hemical volution of Galaxies, UP, ambridge, 2nd edition Prantzos., 2008, n:. harbonnel & J.-P. Zahn (ed.) Stellar ucleosynthesis: 50 years after B2H, ol. 32. S Publ. Ser., p.311 egan.w., Thornley.D., Bendo G.J., et al., 2004, pjs 154, 204 egan.w., Thornley.D., ogel S.., et al., 2006, pj 652, 1112 ybicki G.B., ightman.p., 1979, adiative Processes in strophysics, Wiley, ew York Sakai S., ould J.., Hughes S..G., et al., 2000, pj 529, 698 Seigar., arollo.., Stiavelli., et al., 2002, J 123, 184 Shu.H., 1991, The Physics of strophysics, ol.. adiation, University Science Books, ill alley, Toomre., 1977, & 15, 437 D O D 4 57 Observations, X D G S bout half of all disk galaxies show a central linear bar! Shape can be box-like or as extreme as 1 : 5 in ratio of short to long axis. dge-on view of disk galaxies tells us that bars are as thin as the disks themselves. n contrast to spiral arms, bars occur in gas-rich and gas-poor systems. Bars are not density waves! t is not well understood why some galaxies are barred, while others are not. Spiral arms usually start from the ends of bars Bars are rotating with the same pattern speed as spiral arms. Barred Disks 3 D O D 4 58 umerical alculation X D G S umerical calculation of particle distribution and velocity field of test particles in bars possible. Particle orbits close on themselves in corotating frame. elocity gradients along the orbits cause shocks Gas and dust are compressed Dust lanes along the bar major axis nergy dissipation leads to angular momentum transport Gas inflow toward galaxy center nglmaier & Gerhard (1997) Barred Disks 4

3 D O D 4 62 Photometry, X D G S The most luminous of all galaxies are the cd galaxies, the central galaxies of groups or clusters. 1/4 law fulfilled out to 20e. Beyond that, excess surface brightness from cluster stars or shredded debris of cannibalized dwarf galaxies. HST image of G 1399 in ornax cluster lliptical Galaxies 3 D O D 4 63 Photometry, X D G S Sparke & Gallagher ig 6.6 entral brightness and core radius are tightly linked to the luminosity for luminous and midsized ellipticals (and disk bulges): The more luminous the galaxy, the lower its central surface brightness and the larger its core. Opposite trend for dwarf ellipticals. lliptical Galaxies 4 D O D 4 60 Photometry, X D G S Bulges of disk galaxies are similar to elliptical galaxies in many respects. G 5846 and G (4.49) Surface brightness: modified Sérsic s formula: ( ) [ ( ) 1/n () log = b 1] (e) e where e: effective radius, i.e., radius containing in half of the total luminosity. lliptical Galaxies 1 D O D 4 61 X D G S Photometry, Three classes of elliptical galaxies: uminous giant ellipticals: > idsized ellipticals: < < Dwarf ellipticals: < Hubble type: n, where n = 10(1 b/a) (a: major axis of isophotes, b: minor axis; (1 b/a) = ǫ). 0 galaxies: circular; 5 galaxies: axial ratio 1 : 2. The Hubble type of an elliptical galaxy depends on our viewing direction! haracteristic parameters are largely determined by luminosity. ear- isophotes vs. diffuse X-ray emission of 32 (evnivtsev et al., 2007). lliptical Galaxies 2

4 D O D 4 66 The 3D Shapes of lliptical Galaxies X D G S The appearance (Hubble type) of an elliptical galaxy depends on the direction from which it is observed. near-circular does not guarantee that the ellipsoid has a true spherical threedimensional structure. Distribution of apparent shapes has to be studied. irst clues: There are no ellipticals in the sky more flattened than 7 (b 0.3a) and bright ellipticals on average appear rounder. Sparke & Gallagher ig.6.9 lliptical Galaxies 7 D O D 4 67 The 3D Shapes of lliptical Galaxies X D G S Try to understand apparent shapes with oblate spheroids: Sparke & Gallagher ig.6.9 Stellar density ρ(x) is ρ(x) = ρ(m 2 ), where m 2 = x2 + y z2 B 2 (4.50) with > B > 0 (the true major and minor axes of the oblate spheroid). lliptical Galaxies 8 D O D 4 64 Photometry, X D G S couple of complications: uminosity/entral-brightness correlation and uminosity/ore-adius correlation observationally similar to color-magnitude diagram relations for stars, but much harder to understand (in stars, the mass determines both luminosity and temperature; in galaxies, the processes in galaxy formation are most likely dominating) Surface brightness measurements are hampered by seeing and angular resolution. cd galaxies tend to have cores with approximately constant surface brightness but less-bright ellipticals have often central cusps (surface brightness keeps rising). ore-radius measurements are uncertain. The isophotes of some (luminous) elliptical galaxies twist from the inner to the outer isophotes. vidence for triaxiality. lliptical Galaxies 5 D O D 4 65 Photometry, X D G S Sparke & Gallagher ig.6.1b Twisted isophotes of a giant elliptical galaxy: the long axis of the inner isophotes is roughly horizontal, while the outer ones are near-vertical. lliptical Galaxies 6

5 D O D 4 68 The 3D Shapes of lliptical Galaxies X D G S q describes and ellipse for constant m 2, i.e., for constant stellar density, i.e., for isophotes. the oblate spheroid appears elliptical under all viewing angles. ollowing Sparke & Gallagher (Sect. 6.8), it can be shown easily that the observed axial ratio qobl is given by q 2 obl = (b/a) 2 = (B/) 2 sin 2 i + cos 2 i. (4.51) (or prolate galaxies, there is a fully analogous statement.) spheroidal galaxy never appears more flattened than its true axial ratio /B. lliptical Galaxies evnivtsev., hurazov., Sazonov S., et al., 2007, & 473, 783