3.4 cd Galaxies. 20 a SJ log r (kpc) stoppmg mechanism.

Transcription

1 stoppmg mechanism. 3.4 cd Galaxies The term cd galaxy was introduced by Matthews, Morgan & Schmidt (1964) to describe a galaxy with the nucleus of a giant elliptical surrounded by an extended, slowly decreasing envelope. The cd galaxies are of large extent (they are the largest known galaxies) and high luminosity, and frequently contain multiple nuclei. Several lists of Abell clusters that contain cd galaxies have been published (e.g. Matthews, Morgan & Schmidt 1964, Morgan & Lesh 1965; see also Bautz & Morgan 1970, Bautz 1972, Rood & Sastry 1971, Leir & van den Bergh 1977). Approximately 20% of rich clusters of galaxies contain a dominant central cd galaxy. Recently Morgan, Kayser & White (1975)"and Albert, White & Morgan (1977) investigated the possible existence of cd galaxies in poor clusters and listed suspected cd galaxies in some small groups (cf also van den Bergh 1975). Since all previous examples of cd's were located in rich clusters, the newly reported objects are important if they are really cd's and if they are valid members of the a SJ log r (kpc) Figure 1 Surface brightness profile of the cd galaxy in A2670 (measured by Oemler 1973a). SJ is in mag (arc sec)z; open circles are green (J) magnitudes and filled circles are red magnitudes shifted by +1.1 mag. The solid line is the profile of a normal elliptical galaxy with a length scale a. The dashed line "represents the relation a(r) ex: r 1 6 suggested by Equation 4 (Section 3.4).

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3 26 Ī N 24 3:..J Q (/) ' Q )( l Q f4,. N 24 3:,.: 0:: > l (. _ "r...,. )'''......T.Q o. _ log LB. solar 42 ofo (/) log LB. solar Q x l < Q 39 h," T, * +...i' ; T.a (4)/0)(100 ;. + ; ; ; ; (4)/0)(100 Figure 5: The radio and Xray properties of elliptical galaxies against blue luminosity L B andisophote shape a4/a. n the ti.lx against 04/a diagram the contribution of the discrete sources to the total Xray emission (indicated by the line in the Lx against L B plot) is subtracted according to Canizares et al. (1987). L R gives the total radio luminosity at 1.4 GHz, values below W/Hz are nearly all upper limits. () Q ( / $r akdi x

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6 . PLATE 4.'. ;.. '. ' " " t...,.':. "....iit... :"...,, :.,. "...,.. "..i:..!.... 'j. ' ' "..::. ::'. " ",.. ;;'';>i;h' "8',,,... _'!:.;. f\, :..' ","",",,,..,.,:..'...",,0.:.:;0':">01\00:.04 FG. 4.NGC The top picture (Fig. 4a) was made from thrce UK Schmidt 1aJ plates, enhanced to show the NE shell. Scale bar is 10'. The lower pictures (Figs. 4h and 4(') were ma::!c from the same AAT plate printed through unharp masks with diffrent characteristics. About l shells can be counted, The dust cloud in Fig. 4(' is :cal. Scale bars are 5' and 2' in Fig.. 4h and 4c. respecllvc M L, AND CARTER (see page 539)

7 .... ""'. ""' 06 "tl r' MALN AND CARTER (see page 538)...

8 0 \D ltl... N,.;... ex> M :.:. "'';'.' ""....., FG..Time evolution of the radial, planar encounter between an exponential surface density disk and a 10 times more massive fixed and rigid isochrone potential. The cross denotes the center of the isochrone potential, and the disk is initially moving to the left. The evolution is viewed from above the orbit plane (X Y plane), and the times indicated are in units of the circular period at a radius of one scale length in the.isochrone potential. The bar under the initial time is 10 scale lengths long. The disk, as seen in the figure, is rotating in a counterclockwise sense ":.. FG. 2.Time evolution of the radia planar encounter of an exponential surface density disk and a 100 times more massive fixed and rigid isochrone potential. Other figure parameters are as in Fig American Astronomical Society Provided by the NASA Astrophysics Data System

9 o \D ltl 602 r N... co... 3 QUNN Vol. 279 '.:..'. '.:;;. :...! '0 1 :;;,ii1fl:;<< ;,.. ;. _ ;:...:....'. " o radius FG. 6.The radial velocityradius plane for particles in the model shown iii Fig. 2 at time 18. Radii are in units of the isochrone scale length, and velocities are in units of. (GM/10a)12, dimensional system oftest particles falling from rest into a fixed potential. The extent of the system in the field introduces a spread ofenergies across the system. For potentials other than the simple harmonic potential, the range in energies corresponds to a range in periods with the most bound particles having the shortest periods. Figure 5 shows the phase evolution of such a system. As time passes, the shorter period particles begin to lead the longer period ones, and the system starts to wrap in the phase plane. The wrapping proceeds at a rate determined by the range in periods present, and the number of wraps at time t after the infall begins is simply where Q is the radial frequency andq",ax a,ld Q",in are the maximum and minimum frequencies present, respectively. The spatial evolution of the system can be found by projecting the phase plot onto the spatial coordinate axis. The maximum spatial excursion of each phase wrap corresponds to a sharply defined density maximum. The density maxima occur near the turnaround points of the particle orbits and propagate slowly in radius to the outermost turning point set by the least bound particle. The density maxima are therefore propagating density waves and are similar. to the ring structures produced' by Lynds and Toomre (1976) in their ring galaxy simulations. A similar dynamical picture has been proposed for the evolution of largescale structure in the universe (Zel'dovich 1970; Doroshkevich et at. 1980). Figure 6 shows the radial velocity"radius plane for the model presented in Figure 2. Here the phase wrapping nature of the shells can be clearly seen. Note also that a much larger number of shells can be detected in this way thanfromjust the particle distribution. This is because the small number ofparticles used (1) is incapable of showing all the shells at a sufficiently high contrast to be seen. The phase wrapping interpretation of shell structures has a number of desirable features. First, the phase wraps are interleaved in radius, as has been observed for the shell galaxies. Second, the range of the number of shells present around ellipticals is a simple consequence ofthe age of the event. More shells will imply that a longer time has passed since the merger event, given similar rates of shells production. Shell production rates can be estimated from the range in radii of the shells and hence the spread in periods present. The position in space of a particular shell can be calculated for a given potential by appreciating that the shells occur close to the radial turning points of the orbits. Hence Q dm t ::::: 2n(m em), (2) where Qd is the radial frequency ora particle with its radial turning point at radius d m, d m is the distance of the shell from the center of the potential, em is the orbital phase of particles with turning points at d m, and m labels the shells by the number of orbital periods completed at time t by particles in the shell at that time. The orbital phase of a particle is defined to be the time required to travel around its orbit to the nearest turning point in units of the orbital period; hence 0.5:s; em :s; 0.5. f all the particles begin fromrest, then em = 0 for all particles and Qd m (t/2n) is an integer for all shells. The variable m is the proper shell number in that it is the total number of completed periods. A second number n, the observed number of shells, is defined to be n=m't+1, (3) where 't is the total number of completed periods for particles in the outermost shell. The outermost shell is therefore labeled American Astronomica,l Society Provided by the NASA Astrophysics Data System

10 4.3 Photometry of Elliptical Galaxies 203 ;;; S S ", Figure 4.41 Galaxies with fine structure have bluer colors. Here we plot the correlation between the finestructure parameter [equation (4.37)] and the color (BV) This is the galaxy's color after correction for the colormagnitude effect ( 4.3.4) to absolute magnitude 0.8 MB = 21. [After Schweizer & Seitzer (1992) from data kindly sup L: plied by F. Schweizer] very hard to detect in latetype systems, so we do not know how universal a phenomenon they are. A stellar system can display a sharp edge only if some parts of its phase space (see BT 4.1) are very much more densely populated with stars than neighboring parts of phase space. n the classical dynamical model of an elliptical (BT 4.4), phase space is populated very smoothly. Therefore the existence of ripples directly challenges the classical picture of ellipticals. One likely possibility is that ellipticals acquire ripples late in life as a result of accreting material from a system within which there are relatively large gradients in phasespace density. Systems with large density gradients in phase space include disk galaxies and dwarf galaxies: in a thin disk the phasespace density of stars peaks strongly around the locations of circular orbits, while in a dwarf galaxy all stars move at approximately the systemic velocity, so that there is only a small spread in velocity space. Numerical simulations suggest that ripples can indeed form when material is accreted from either a disk galaxy or a dwarf system see Barnes & Hernquist (1992) for a review. Moreover, simulations, have successfully reproduced the interleaved property of ripples described above. Despite these successes significant uncertainties still surround the ripple phenomenon because the available simulations have important limitations, and it is not clear how probable their initial conditions are. Schweizer et al. (1990) defined an index E that quantifies the amount of fine structure such as ripples that a galaxy possesses: E == S + 10g(1 + n) + J + B + X. (4.37) Here S measures the strength of the most prominent ripple on a scale of 0 to 3; n is the number of detected ripples; J is the number of optical 'jets'; B is a measure of the boxiness of the galaxy's isophotes on a scale of 0 to 4; X is o or 1 depending on whether the galaxy's image shows an X structure. For a sample of 69 nearby earlytype galaxies E varies from 0 to 7.6. Notice that

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13 r.. u..e '' 0.5 l l 0.5..'v j1mb Figure Normal ellipticals and dwarf systems define separate sequences of effective radius as a function of MB. The bulges of spiral galaxies lie along the sequence of normal ellipticals. (Data from Bender et al., Ap. J., 399,462, 1992.) c s;0,... de D. d'sra' )

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15 ;7JoJe. for 4 2) f..e. :& "? ;.. = cuj; velo c40o J:'vad:..sc... k..c.vu>s& 8 ('0/ _ xl r,.., ) J(e 0{ a: 9 UJ dalw.e.,) el,t.re >) ) L e /ev () e+z: c/aa. j ie :y A'e ::: ( /.* a; t c:... 8k..d J 71'

16 206 Chapter 4: Morphology of Galaxies 4.3 Photometry of Ellipti,..;.:"),.'.....,,1 : " rtj : '. :. " "' S ".:.:. 0 0 b 0,...; 0 co '....;..., : "0 "J"''. : ::;r., ::i. OJ /\,..:.:.:. "<... v 20. '.' " 'i' : ':?:'.;' :i:." R./kpc L e Figure 4.43 Correlations between four shapeindependent parameters of elliptical galaxies. The parameters are the effective radius Re, the mean surface brightness within Re, ()e, the central velocity dispersion 0'0 and Le, the luminosity in Djorgovski's G band interior to Re. The luminosity and the surface brightness are expressed in magnitudes and in magnitudes per square arcsecond, respectively. [From data published in Djorgovski & Davis (1987)J The lower right panel of Figure 4.43 shows the correlation of which this is the mean regression. More luminous ellipticals have larger central velocity dispersions. The upper right panel in Figure 4.43 illustrates this correlation, which is called the FaberJackson relation after its discoverers (Faber & Jackson 1976). Quantitatively one has (4.38) Since 0'0 is correlated with L e and L e is correlated with R e it follows that 0'0 must be correlated with R e. The top left panel in Figure 4.43 displays this correlation. Since 0'0 is strongly correlated with linestrengths and colors, the existence of a host of additional correlations involving Mgz, B V etc is implied by the top two panels of Figure One of the earliest of these to be discovered was the colormagnitude relation: Faber (1973) showed that more luminous elliptical galaxies have stronger absorption lines, and Visvanathan & Sandage (1977) showed that more luminous galaxies are redder. n all the correlations of Figure 4.43 there is cosmic scatter; the scatter of the points about the mean relations is larger than can be accounted Box 4.2: P Given M points x C <» in ddimensional unit vecto nearly as possible on th respect to p and the con subject to the constrair multipliers this problem 0= L M [(n <>=1 M 0= 2)n. <>=1 where >.. is the undeterr using the last equation t Then we find that 0= L(n <> This equation can be rel Thus the required vector A. One may show (se( values taken by S for n the desired n is the eige n order to minimiz have been scaled such th, for by measurement erron als between the positions correlated. For example, 1 relation in the top right pa magnitude relation in the of both the basic correlat is useful to imagine each

17 208 Chapter 4: Morphology of Galaxies 4.3 Photometry of Ellipti bd 0 '+ 1., 1\ V?D co ; :. '.. :. (;:": 7," Figure 4.44 An edgeon view of the fundamental plane as defined by the data of Figure Note how much narrower is the distribution of points 6 1 a 2 than in either of the lefthand panels R.lkpc of Figure dimensional space. The Cartesian coordinates Xi of a point in this space are the three numbers of the set (logre, (1)e,logO"O), where (1)e is in units of J.LB. 14 f there were no correlations between the variables, the points of individual galaxies would be fairly uniformly distributed within a cuboidal region of our threedimensional space. Correlations between the variables will confine the data points to a subvolume of this cuboid. For example, if two of the variables are genuinely independent and the third dependent on these two, the points will be confined to a plane, while if there is only one independent variable, the points will lie on a line. Our first step in analyzing such data is to ask whether the points are nearly confined to a plane; if we can find a suitable plane, we can further enquire whether the points lie on a line within that plane. n statistics books this type of investigation is called principal component analysis. Box 4.2 explains what has to be done. For the data in Figure 4.43 one finds that the points are as nearly confined to a plane as observational errors allow; in the absence of observational errors, they might lie precisely on a plane! f the position vector of a galaxy in our threespace is g = (log R e, ()e, log 0"0), where R e is measured in kpc, ()e in units of J.LB and 0"0 in km sl, then the equation of this fundamental plane is n g = 1 where n = (0.65,0.22,0.86). Naturally one wants to 'see' the fundamental plane. One way to do this is to choose an axis, for example the Reaxis, as the horizontal axis, and rotate the space about this axis until the plane appears edgeon. n this orientation, the plane's normal, n, lies in the plane of the projection, which is spanned by the unit vector, er, that runs parallel to Reaxis and some other, orthogonal, unit vector e. Thus n = o:er + {3e, where 0: and {3 are suitable numbers. Comparing this with the value of n given above we see that {3e = 0.22e[ e".. Thus the fundamental plane will be seen edgeon if we plot log Re against 0.22((1)e/ J.LB) log 0"0. Actually, it is conventional to plot 0.26((1)e/ J.LB) + log 0"0, which is simply 1/0.86 times the linear combination we have derived. Figure 4.44 shows this plot. The 14 L e need not be included in the set since it is related to R.e and ()e by Le = 1r()eR;, where L. is measured in W, e in Wm 2 sterad 1 and R.e in m. data points nearly lie on fundamental plane. The eo logr, is also an equation for the plying n g = 1 by 1/0.65. The D n 0"0 correlatio measured photometric pa virtue of the fundamental. which the mean surface bl terms of a fiducial surface surface brightness than fo luminosity. To quantify tl same surfacebrightness p 1(R) = 1ef(R/Re). Then From Figure 4.25 we dedu will come from radii at we mation to evaluate the int When equation (4.39) is u The weak dependence up' correlation between D n an f we adopt a distance of relation becomes D kr with a 15% scatter from g Dwarf elliptical galaxie brightness profiles of giant 15 Recall that ()e/jlb oc :