Galaxies and the Universe - Extragalactic Distance Scale

Galaxies and the Universe - Extragalactic Distance Scale -...

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Unformatted text preview: 1/15/12 Gala ie and he Uni e e - E T D agalac ic Di ance Scale E D AGN S , - .H H ( " c = H0 D , ) " D, .T H , . F H0 , H ladde r, .T dis tance - / , .A I ( , : ) " " . R . A (. . C , - SN I ). , , M W T . . M V .E , .I , R -R , The Cosmological Dist ance Ladder (C 1987), Gala Dist ances and Dev iat ions f rom Univ ersal E pansion, . B. M ASI 180). W . Trigonome tric paralla . T Hipparcos , ( ) , .S .M R.B. T (NATO , E .T .S ' ( ) , secular paralla . T C C ( Hipparcos ). Clus te r conve rge nt points . F ( H ) , ' S .T ' ' , .M , -P .a . a.ed /keel/gala ie /di ance.h ml .T .T . (1998 A&A 331, 81): H , Hipparcos 1/6 1/15/12 Gala ie and he Uni e e - E agalac ic Di ance Scale M ain- e e nce fi ing. For even more distant star clusters (that might contain OB stars or Cepheids, for example) we estimate distances by assuming that main- sequence stars of identical spectral type have the same absolute magnitude. This amounts to, for example, shifting the main- sequence location of a cluster until it coincides with that of some reference cluster like the Hyades. The reddening must be reasonably well determined to make this work. This can be done for systems as distant as the Magellanic clouds, which is the easiest place to calibrate Cepheids. For this purpose, each Magellanic Cloud can be thought of as a giant cluster. Ce he id a iable . These are supergiants in the instability strip on the H- R diagram, undergoing regular pulsations that are expressed by luminosity and temperature variations. Their high optical luminosity makes them easy to pick out (though, being rather massive stars, they don't occur in elliptical galaxies). Recent data give a period- luminosity relation of the form <MV> = - 3.53 log P + 2.13 (<B0> - <V0>) + where ~ - 2.25 is a zero point. P is in days here, and the brackets denote averaging over a cycle of the light curve. The relations for the SMC and LMC are shown by Mathewson, Ford and Visvanathan 1986 (ApJ 301, 664) as follows, from their Fig. 3 (courtesy of the AAS): .a . a.ed /keel/gala ie /di ance.h ml 2/6 1/15/12 Gala ie and he Uni e e - E T C P agalac ic Di ance Scale , : <V> , HST , P .W C , , - P- L . .T ; ( ). R .T , ,W ( - IR . 1986 (A J 305, 583). N -C C , , . C ( HST HST K P .S . C ), M (C V 1411). N ,A M81 1986 A JL , I NGC 4751, ' NGC 4321=M100 V T , C . a.ed /keel/gala ie /di ance.h ml L S 301, L45), G ( H - , F , 7 M101 (P - .a IR ). R di ance mod l m-M = 5 log D - 5 , DM LMC 18.5. T C 25 M , C .T (1996 A J 464, 568), F . 1994 BAAS 26, , HST V 1994 (N .A 371, 757). , 3/6 1/15/12 Gala ie and he Uni e e - E agalac ic Di ance Scale was described by Kennicutt, Mould, and Freedman 1995 (AJ 110, 1476). Some of their Cepheid light curves are shown below - - for M100 alone, they already detect more Cepheids than are known in the LMC, so the LMC calibration becomes a weak link. The project has gotten all its data, and a recent summary (Mould et al. 2000 ApJ 529, 7867) gives a grand average value of H0= 71 6 km/s Mpc as based on HST Cepheid distances to 25 galaxies, in ridiculously close agreement with results of fitting the WMAP power spectrum of CMB fluctuation. This plot colects the Key Project Cepheid distances. Note the large peculiar motions within Virgo; the one galaxy lying right on the mean line at that distance is NGC 7331, almost opposite Virgo in the sky. .a . a.ed /keel/gala ie /di ance.h ml 4/6 1/15/12 Gala ie and he Uni e e - E agalac ic Di ance Scale RR L rae s tars . These are lower- luminosity stars, where the instability strip crosses the horizontal branch. They may appear on cluster H- R diagrams by omission in the "RR Lyrae gap", since variables are usually not plotted. The absolute magnitude of all RR Lyrae variables seems to be nearly constant at <MV = 0.75 0.1. There may be some poorlydetermined metallicity dependence. No period determination is needed here, just the determination that a star is of this type (which means you get the period anyway). Problems are: RR Lyraes are intrinsically about 2 magnitudes fainter than Cepheids, and similarly difficult to calibrate; only a couple are close enough for a parallax measurement with Hipparcos, so statistical parallaxes are still important. Automated image detection has proven fruitful in finding these stars throughout the Local Group, even before HST. Saha and Hoessel (1990, AJ 99, 97) report finding 151 in the small elliptical NGC 185, as seen in their Fig. 5 courtesy of the AAS: .a . a.ed /keel/gala ie /di ance.h ml 5/6 1/15/12 Gala ie and he Uni e e - E agalac ic Di ance Scale M o l mino (bl e / e d) a . There is an empirical relation between a galaxy's absolute magnitude and that of the brightest individual stars - this amounts to assuming a constant form for the upper end of the luminosity function and letting statistics operate. Conveniently, these are the first stars to be resolved. Possible problems: confusion with compact clusters (as in 30 Doradus), unknown variation with galaxy type. All of the stellar indicators listed above for other galaxies are easiest to use in systems with substantial population I components, and in rather open galaxies so that crowding is reduced. One therefore tries to deal with a galaxy's outer regions, and rather late- type galaxies (see the Sandage and Bedke atlas for illustrations of resolution into stars for such galaxies, which was the point of their producing this volume). There are also several temporary or indirect stellar distance indicators: No ae . There is a relation between absolute magnitude and fading rate for novae, as best we can tell from the Local Group. They can easily be picked out as transient H sources, and two seem to have been detected in this way as far away as M87 (Pritchet and van den Bergh 1987 ApJLett 288, L41); as well, data series sufficient to find Cepheids may find them as continuum sources. Ciardullo et al. (1990 ApJ 356, 472) discuss 11 well- observed novae in M31. The relation between fading rate and absolute B magnitude is only partially followed by H , so that a combination of H discovery, continuum observations near maximum, and H observations to faint levels seems the most effective approach. Faint con .a . a.ed /keel/gala ie /di ance.h ml 6/6 ...
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