Galaxies and the Universe - Components of Galaxies

Galaxies and the Universe - Components of Galaxies -...

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Unformatted text preview: 1/15/12 Gala ie and he Uni e e - Componen of Gala ie Structural Components of Galaxies Several morphologically (and sometimes kinematically) distinct components emerge from the images of galaxies: for disk galaxies, these may include nucleus, bulge, lens, and disk. The disk may itself contain arms, a bar, rings, and other embellishments. The relative roles of these in some cases change with Hubble type. Some of these (i.e. diskbulge luminosity ratio) were discussed by de Vaucouleurs (1977, Yale Conf. p. 43) under the rubric of "quantitative classification". Pho ome ic B lge and Di k Compone n : For S0 and spiral galaxies, we may broadly consider a disk and a central bulge. The disk is quite thin both in stars and gas (when present), with aspect ratio 10 or greater (beware photographic and visual effects in getting such numbers from pictures, however - the "real" axial ratio seldom exceeds 6). The bulge is more nearly spherical, though it is sometimes found to be a (triaxial) ellipsoid like a very stubby bar. A "classical" approach to separating the bulge and disk contributions for some galaxy with measured intensity I(x ,y) = I(r, ) starts by assuming a typical functional form for each. Bulges and elliptical galaxies usually follow similar radial distributions of intensity, parameterized by any of the following: Hubble and de Vaucouleurs profiles are purely empirical representations of typical data, without accompanying dynamical models. The last at least has a physical justification even if it has infinite total luminosity - the assumption of isothermal and isotropic stellar velocities, derived for globular clusters by King 1966 (AJ 71, 64). For an untruncated King model, the luminosity density (r) follows the simple form One frequently introduces a tidal truncation energy (radius) to remedy the infinite- luminosity problem, but then the intensity distribution becomes non- analytic. In these expressions, I0,r0,Ie are size and intensity scaling parameters - the central surface brightness, core radius, and intensity at effective radius Re respectively. These quite different functional forms fit most elliptical galaxies quite well over a range of 103 in surface brightness; systematic differences among them are less than real galaxy- to- galaxy changes in structure. For nonspherical systems, an ellipticity must be taken into account, so that the brightness profile is measured along nested ellipses of constant intensity. For some ellipticals, changes in ellipticity or position angle occur with radius, while for a disk one may fix a projected ellipticity from some assumed inclination angle and deproject. The King model incorporates a core radius which has interesting dynamical implications. Some ellipticals (such as M87) have unmistakeable cores, within which the intensity distribution is nearly flat, while others show scale- free profiles to the limit of even HST resolution. When distinct cores are present, they are very seldom of a pure King- law .a . a.ed /keel/gala ie /componen .h ml 1/6 1/15/12 Gala ie and he Uni e e - Componen of Gala ie form, flattening toward zero central slope, as shown in Fig. 6 of Lauer 1985 (ApJ 292, 104) (reproduced below by permission of the AAS). Driven by extensive HST imaging of galaxy cores, Lauer et al. (1995 AJ 110, 2622) have introduced a more complicated form, the "Nuker" profile, to represent such cores as two asymptotic power laws - an outer one of slope and an inner one of slope - shading into one another (over an area controlled by a parameter α) at a characteristic break radius b : This law may fail for large radii, where most ellipticals are well described by the 1 /4 law (which itself corresponds to a Nuker law with α = 1/4, =0, and limits b , asymptotically becoming infinite). Finite total flux requires that exceeds 2 at large radii. It is also worth mentioning that the form introduced by Jaffe (1983 MNRAS 202, 995) has the huge virtue of being analytic in both its density and surface- brightness L distributions as well as finite. In these expressions, the radius is given in terms of the radius within which half the light is emitted (Jaffe gives 1.31 times the effective radius as derived from the light profile), and the surface brightness is at projected distance a in the same units: The mechanics of measuring intensity profiles for galaxy images have been treated by Lauer 1985 (ApJSuppl 57, 473), Davis et al. 1985 (AJ 90, 169), and Jedrzewski 1987 (MNRAS 226, 747). The last algorithm is implemented .a . a.ed /keel/gala ie /componen .h ml 2/6 1/15/12 Gala ie and he Uni e e - Componen STSDAS isophot e .T (S , CCD - of Gala ie , 1979 A J 233, 23; 1981, AJ 86, 662) .A 5% . D T , E F 1970 (A J 160, 811), B 1979 (A JS 41, 435), Y W 1975 (AA 44, 363). P ( K A&A 95, 105) , .T , , 0.15- 0.2 .T 0 , , M W .A , , , O . S . S 1981 - - , , V : H , n .T , .R V K . T M : .K , .S ( ? 1985 (A JS 59,115) .T ) .F .L shapes , , , - ( , ). O ! B KK .a W , VV188=NGC 4438 ( 1993 (AJ 106, 236), . a.ed /keel/gala ie /componen .h ml ( ) .T - ). T AAS. 3/6 1/15/12 Gala ie and he Uni e e - Componen A of Gala ie , ( .A ). T ; F ' , 1988 (AJ 95, 1706 , E 1988 (AJ 95, 408). F , .S 96, 1336, 1352) .I , ( V ) 1990 (N ", ,D .D 239, 939) 346, 153) IR ( .F , NATO ASI &B ,K ,&C ( IR "T O ( . C, 469, K ). W , 1995) (2000, A J 542, 761) ( . ). T S - , .O / ( / .T Y , NGC 3521 7331, ' S0 ) " " V 1977 (Y C ADS) .a H -- .W A H - IR ,G . 1989 (N , ) D . ), H .N .E " ). C F .2 .) . a.ed /keel/gala ie /componen .h ml , ( " ( ,S Y S W . (1975 A&A 44, 363, : 4/6 1/15/12 Gala ie and he Uni e e - Componen of Gala ie T , - / , B .S - . / H .T = = .I= .N , P ( ) M33 , ( W , . II = B 1991 PASP 103, 609). , . 1981 (AA 104, 1) , . 1985 (A J 292, 78), B S , W 1984 (A J 278, 61). . T F , I , . 1982, A J 256, 103. T , E .T m=1,2,3... 2- .A .F 1995 A J 445, 591) , 1993 PASP 105, 644 , ( , . S ' r ~ log ( , , H II , ( H I). A - ) .T H ( )A H V ' ' , .T (M 2002 A&A 388,389). I , - ; M31 .G 2; .a . a.ed /keel/gala ie /componen .h ml . 5/6 1/15/12 Gala ie and he Uni e e - Componen of Gala ie Two- dimensional fitting can be used to constrain morphology at faint levels, or in the presence of complicated instrumental response functions, by asking how many components are needed to fit the full two- dimensional structure to a statistically meaningful level. Some examples are given in Keel & Windhorst 1993 (AJ 106, 455) and, for the HST Medium- Deep Survey, by Naim et al 1997 (ApJ 487, 510); and Casertano et al. 1995 (ApJ 453, 599). Bulges aren't always bulges. They may be separated into those with a de Vaucouleurs- like profile (Sersic index near 4, classical bulges) and those where the face- on profile of the bulge is more like an exponential (Sersic index near 1, pseudobulges). Simulations suggest that classical bulges form in the aftermath of major mergers, while pseudobulges can form gradually over long times from purely internal processes (instabilities in bars, scattering by molecular clouds, and so on). Rings are seen at various locations in some disk galaxies, generally marking locations of resonant orbits. These may be nuclear, at the inner Lindblad resonance where the pattern speed matches the orbital speed, or at the outer Lindblad resonance. The outer rings may be populated by stars leaking out of the inner disk through a barlike potential, so that rings would be a timer for the presence and perhaps dssolution of bars.. Wa e le ngth De pe nde nce of Clas s ification Parame te rs As shown in the M81 images atop the classification page, some of the quantities we've been decomposing are strong functions of wavelength, particularly bulge/disk ratio. Classically this has been done in the optical band, but sometimes (as in very dusty IR- bright galaxies, see Scoville et al. 2000 AJ 119, 991 and their data pages), we are driven to use wavelengths where there's actually something to be seen. Elliptical galaxies generally show only the most subtle structural changes, but of course spirals become more bulge- dominated at longer wavelengths, while bulges sometimes disappear in the UV. There have been several recent surveys which should eventually put this on a quantifiable bases (for example the Ohio State survey). Extension to the near- IR is especially important, since dust penetration and changes in stellar population can make the disks look quite different. One initial attempt at a near- IR Hubble diagram was done using 2MASS images (rather shallow in surface- brightness sensitivity) by Jarrett (2000, PASP 112, 1008). Once again, note that all these exercises can in principle be done in any passband. It's common to use differences in disk scale length at various wavelengths to measure age, metallicity, or extinction gradients (depending on the passbands and one's level of understanding of the system). Ryder & Dopita (1994 ApJ 430, 142) applied it to compare the distributions of current star formation (from azimuthally averaged H- alpha images) to the integral of past star formation (from optical continuum images), to test whether gas is being depleted by star formation uniformly or from the outside inward (they come down on a remarkably uniform behavior, by the way). « Galaxy classification | Global properties and systematics Course Home | Bill Keel's Home Page | Image Usage and Copyright Info | UA Astronomy k [email protected] . a.ed Ls cags 820 at hne: /09 .a . a.ed /keel/gala ie /componen .h ml 20009 6/6 ...
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This note was uploaded on 01/15/2012 for the course AY 620 taught by Professor Williamkeel during the Fall '09 term at Alabama.

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