Observational properties of ellipticals

Transcription

1 Observational properties of ellipticals Ellipticals are deceptively simple it is so tempting to treat them as a pressure supported gas of stars but this is not correct. Too bad that only dwarf ellipticals are members of the Local Group! 1

2 Structure of Galactic Spheroids Recall: radial structure if we approximate spheroids as relaxed, purely stellar systems, stellar dynamical theory provides a semi-empirical description (King 1962, AJ, 67, 471) isothermal core, with projected brightness I(r ) = I 0 / (1 + r/r c ) r c --> core radius King model also requires outer truncation, parametrized by a tidal radius c --> log (r t / r c ) King 1966, AJ, 71, 64 2

3 King profile fit to NGC 4472 King 1972, ApJ, 333, 1 3

4 Empirical luminosity laws Hubble (1926) profile I(r ) = I 0 / (1 + r/r o ) 2 de Vaucouleurs r 1/4 law log I(r )/I(r e ) = [(r/r e ) 1/4-1] where r e --> effective radius (radius containing half of integrated luminosity) although the r 1/4 law is empirical in origin, n-body simulations of merging/forming spheroids can reproduce the profile 4

5 de Vaucouleurs, Capaccioli 1979, ApJS, 40, 699 5

6 Structure of elliptical galaxy cores Lauer et al. 1995, AJ, 110, 2622 ( Nuker team ) Jaffe et al. 1995, AJ, 108, 1567 subject revolutionized by HST luminous galaxies show strong turnover in inner ~100 pc, cores or shallow cusps lower luminosity galaxies show continued (1/r) increase in brightness into the galactic centers few ellipticals show asymptotic flat brightness profile in center older results influenced by seeing 6

7 Lauer et al. 1995, AJ, 110,

8 Many spheroids possess nuclear disks with gas, dust Nuclear dust ring in NGC3379, Gebhardt et al

9 2 2 2 x y z + + = a a b c Possible Shapes for Elliptical Galaxies Oblate a > c a = equi radius c = polar radius x + y z + = a c Prolate c > a Triaxial x y z + + = a b c Projection effects make it difficult to disentangle the true shape of ellipticals can t simply invert the observed brightness distribution to get the 3-D luminosity density Need to know the shape to interpret kinematics amount of dark matter that one deduces depends on the expected distribution of shapes of stellar orbits. Much trickier than for spirals where rotation of the disk leads to a clear prediction of the amount of matter 9

10 Photometric and kinematic tests more consistent with oblate geometry Many galaxies show evidence of at least slight triaxiality (eg. isophote twists would not be seen in axisymmetric galaxies) Sandage, Freeman, Stokes 1970, ApJ, 160,

11 R Deprojecting Surface Brightness Profiles z r I(R)=observed surface brightness j(r) = luminosity density Calc indicated by (2) works poorly in practice due to noise problems (1) (3) (2) I(R) = dzj(r) = 2 j(r) = π r R j(r)rdr r di dr dr R r 2 2 R I 0 j0 I( R) = j( r) = ( R/ r0 ) 1 + ( r/ r0 ) Handy but bad total luminosity diverges! 2 Various analytical schemes have been used Hernquist model has been useful but (3), the modified Hubble Law, has its problems. 3 γ La (4) jr ( ) = γ=1 for Hernquist γ 4 γ 4 π r ( r+ a) L = total luminosity a = scale length 11

12 Can We Determine the True Shapes? Only in a few special cases Lack of knowledge of the inclination of a spheroidal shape means that a unique solution cannot be found If I(R) is circularly symmetric, then the galaxy is spherical but otherwise it might or might not be even axisymmetric Addition of kinematic data helps 12

13 High S/N images reveal significant departures from elliptical geometry isophotal twists ellipticity variations boxy isophotes disky isophotes NGC 1600 Barbon et al. 1984, A&A, 137,

14 Capaccioli et al. 1988, AJ, 96,

15 At the 1-2% level most spheroids show systematic deviations from elliptical isophotes parametrize in terms of Fourier terms: +... I(θ) = a 0 + a 2 cos(2θ) + a 4 cos(4θ) a 4 = 0 pure ellipse a 4 < 0 boxy a 4 > 0 disky a 4 correlates with kinematic properties (i.e., disky galaxies rotate faster) it also correlates with most other properties along the fundamental plane 15

16 Bender et al. 1989, A&A, 217, 35 16

17 Shells Malin & Carter 1983, ApJ, 274, 534 faint(!) azimuthal shells are observed around a large fraction of elliptical galaxies shells are characterized by sharp edges, caustic structure galaxies with prominent shells have systematically bluer colors, evidence of intermediate age stars in spectra numerical simulations suggest shells are tidal remnants of satellite galaxy accretion, minor mergers cd Galaxies some luminous elliptical galaxies possess very extended powerlaw envelopes, extending up to >>100 kpc in radius predominantly seen in central galaxies in rich clusters multiple nuclei very common probably built up from multiple captures and mergers with neighboring galaxies --> galactic cannibalism Evidence for the importance of merging 17

18 Focus on the cd galaxy! 18

19 19

20 Malin & Carter 1983, ApJ, 274,

21 Internal Kinematics of Ellipticals Faber & Jackson

22 Kinematics of Elliptical Galaxies and Bulges Observational technique: obtain high S/N absorption line spectra for galaxies and template stars, use crosscorrelation techniques to measure velocity distribution, velocity dispersion Faber & Jackson 1976, ApJ, 204,

23 LOSVD Characterization Just as projection effects make extraction of a galaxy s shape from surface brightness difficult, the conversion of a measured line shape into a determination of the true velocity distribution, F(v LOS ) is difficult. LOSVD=line of sight velocity dispersion Must take account of the composite nature of a galaxy s stellar population (ideally would allow different velocity distributions for different stellar types) Cannot assume that LOSVD is Gaussian even if local regions of a galaxy have Gaussian velocity distributions, their superposition will not be Gaussian Could parameterize LOSVD by moments but higher order moments get weighted by large values of ( v v ) n LOS Van der Marel & Franx 1993 have devised a method expanding the LOSVD in terms of truncated Gaussian-Hermite polynomials that has proven to be robust in the face of observational errors 23

24 Mean velocity: Velocity dispersion: LOSVD Gaussian-Hermite expansion: w = (vlos v)/ σ vlos = dvlosvlosf(v LOS) σ = dv (v v ) F(v ) 2 2 LOS LOS LOS LOS LOS 1 2 n w 2 FGH e 1+ hkh k (w) k= 3 For distributions close to Gaussian, v, σ v LOS, σlos h 3 related to third moment of velocity distribution which is a measure of skewness, departure from symmetry in distribution h 4 related to fourth moment, kurtosis, departures from Gaussian (rectangular versus peaky) Hs are polynomials of order k 24

25 Ordered Motions and Rotation Parametrize degree of rotational support by: v m /σ = (maximum rotation velocity) / (velocity dispersion) luminous ellipticals: v m /σ < 0.2 independent of flattening(!) faint ellipticals: 0 < v m /σ < 1 bulges consistent with rotational flattening Davies & Illingworth 1983, ApJ, 266,

26 Carollo et al ApJL. Ellipticals rotate but not at a rate consistent with their flattening. 26

27 How can a galaxy be flattened without rotation? stellar systems need not behave like ideal gases, with isotropic velocity dispersion relaxation time for galaxies typically of order years gravitational potential in a flattened or triaxial system is highly non-spherical, so velocity dispersion may be anisotropic equilibrium configurations with non-spherical stellar distributions and anisotropic velocity ellipsoids possible anisotropic velocity dispersions also characteristic of rotating stellar disks 27

28 Variations in rotational support are strongly correlated with other parameters in the fundamental plane more luminous ellipticals tend to have more anisotropy, for example. Bender, Saglia, Gerhard 1994, MNRAS, 269,

29 velocity dispersion and rotation velocity tend to remain roughly constant with radius (small radial increase in v m /σ Fried & Illingworth 1994, AJ, 107,192 29

30 A small fraction of spheroids contain kinematically decoupled cores, often with counter-rotation (e.g., IC 1459) Franx & Illingworth 1988, ApJ, 327, L55 30

31 Scaling laws projected radial velocity dispersion (σ) tightly correlated with luminosity: Faber-Jackson relation similar correlations observed between σ and diameter (D n ) and linestrength (usually measured via Mg lines) Faber & Jackson 1976, ApJ, 204, 668 Pahre et al. 1998, AJ, 116,

32 correlations (L ασ 4, D n ασ ) are projections of the fundamental plane for elliptical galaxies, bulges slope of correlations wavelength dependent strongest correlations found in principal axes of fundamental plane define mass/light ratio M/L = mass/luminosity in units of M o /L o mean M/L B ~ 10 (M/L R ~ 3) for ellipticals/bulges Jorgensen et al. 1996, MNRAS, 280,

33 How Much Dark Matter is Required? Many claims and counter claims have been made as to the amount of dark matter in ellipticals As mentioned earlier, lack of a priori knowledge of orbit shapes is a major problem Lack of a probe similar to HI that can be measured at large distances from a galaxy s center is another problem There is no doubt that clusters of galaxies (dominated by ellipticals) contain dark matter both x-ray data and lensing show this result Evidence is accumulating that ellipticals have dark matter but that stars dominate the mass budget inside R e 33

34 X-ray Gas as a Probe Many but not all ellipticals contain a hot plasma Typically T~ K, readily observable at x-ray wavelengths Gas originates from stellar mass loss, likely heated by SN The assumption that such gas is in hydrodynamic equilibrium can be used to estimate the mass of the galaxy dp GM(r) ρ ρkt = p = 2 dr r µ m ktr d ln ρ d ln T M(r)= G µ m p dlnr dlnr Has the virtue that there is no worry about anisotropic velocities but does require spatially resolved x-ray T data Leads to ellipticals needing dark matter p 34

35 From Humphrey et al,

36 Dotted line = stars Dashed = dark matter Dot-dash = gas 36

37 Chandra data also are revealing that the x-ray gas is so disturbed in some ellipticals that it should not be assumed to be in hydrostatic equilibrium. Supernovae, episodic outflows from nuclear black holes may be the culprits. Statler & Diehl

38 Velocity dispersion and rotation velocity rise sharply inside inner 100 pc M/L ratio also increases sharply, indicating central black hole nuclear BHs appear to be ubiquitous feature in spheroids 38

39 Tremaine et al

40 BH mass strongly correlated with spheroid/bulge mass ( Magorrian relation ) as represented by velocity dispersion Magorrian et al. 1998, AJ, 115, 2285 Tremaine et al. 2002, ApJ, 574,

41 Dwarf Galaxies Dwarf spheroidal and irregular galaxies extend the trends seen in elliptical and spiral galaxies, respectively, except: dsph galaxies lie off the fundamental plane for E/S0 galaxies fraction of mass in dark matter (within visible galaxy) often much higher in dwarfs e.g., DDO 154 the dark galaxy Carignan & Freeman 1988, ApJ, 332, L33 41

42 Dwarf spheroidal galaxies show different radial structure radial profiles fitted better with King profiles (like star clusters) or exponential profiles (like disks) these galaxies also have distinct dynamical properties, and deviate from normal fundamental plane Star counts for six dwarf spheroidals. Faber & Lin 1983, ApJ, 266, L17 42