12.1 Elliptical Galaxies

Transcription

1 12.1 Elliptical Galaxies

2 Elliptical Galaxies Old view: ellipticals are boring, simple systems Ellipticals contain no gas & dust Ellipticals are composed of old stars Ellipticals formed in a monolithic collapse, which induced violent relaxation of the stars, stars are in an equilibrium state Modern view: Most/all ellipticals have hot x-ray gas, some have dust, even cold gas Ellipticals do rotate, but most of the kinetic energy support (and galaxy shapes) come from an anisotropic velocity dispersion Some contain decoupled (counter-rotating) cores, or other complex kinematics Some have weak stellar disks Ellipticals formed by mergers of two spirals, or hierarchical clustering of smaller galaxies 2

3 Dust lanes in E galaxy NGC 1316 Dust is surprisingly common in E s Probably it originates from cannibalized spiral galaxies

4 Elliptical Galaxies: Surface Photometry Surface brightness of elliptical galaxies falls off smoothly with radius. Measured (for example) along the major axis of the galaxy, the profile is normally well represented by the R 1/4 or de Vaucouleurs law: I(R) = I(0) e -kr1/4 where k is a constant. This can be rewritten as: I(R) = I e e 7.67 [( R R e ) ] { } where R e is the effective radius - the radius of the isophote containing half of the total luminosity. I e is the surface brightness at the effective radius. Typically, the effective radius of an elliptical galaxy is a few kpc. 4

5 De Vaucouleurs Law R 1/4 profile fits the data very well over some 2 decades in radius 5

6 Other Common Profiles Sersic profile: where Σ is the surface brightness in linear units (not magnitudes), b n is chosen such that half the luminosity comes from R < R e. This law becomes de Vaucouleurs for n = 4, and exponential for n = 1. Hubble s profile: with Σ 0 the central surface brightness, and r 0 the core radius interior to which the surface brightness profile is approx. constant. Note that the integral under the Hubble profile diverges! 6

7 Breaking the Homology: Density Profiles Sersic profile index (r 1/n ) correlates with the galaxy size (D Onofrio et al. 1994) More luminous ellipticals have shallower profiles (Schombert 1986) 7

8 The Cores and Nuclei of Ellipticals Profiles of elliptical galaxies can deviate from the R 1/4 law at both small and large radii. Close to the center: Some galaxies have cores - region where the surface brightness flattens and is ~ constant Other galaxies have cusps - surface brightness rises steeply as a power-law right to the center A cuspy galaxy might appear to have a core if the very bright center is blurred out by atmospheric seeing. Thus, HST is essential to studies of galactic nuclei! It turns out that: The most luminous ellipticals have HST-resolved cores Low luminosity ellipticals have power law cusps extending inward as far as can be seen 8

9 Central Surface Brightness Profiles To describe these observations, we can use a new profile suggested by the Nuker team: I(R) = I b 2 β γ & ( ' ( ) /α R R b ) + * γ, & 1+ R ). ( + -. ' R b * It is a broken power law: Slope of -γ at small radii Slope of -β at large radius Transition between the two slopes at a break radius R b, at which point the surface brightness is I b Remaining parameter α controls how sharp the changeover is α / 1 01 ( γ β ) /α 9

10 The Nuker Profile Changing inner slope Changing outer slope γ = 0, 0.5, 1, 1.5 β = 1, 1.5, 2 10

11 12.2 Elliptical Galaxies: Shapes

12 Typical Elliptical Isophotes Note the apparent ellipticity gradient NGC 1533 NGC

13 Shapes of Ellipticals Ellipticals are defined by En, where n=10ε, and ε=1-b/a is the ellipticity Note this is not intrinsic, it is observer dependent! The observed distribution 13

14 Triaxial Ellipsoids In general, the 3-D shapes of ellipticals can be triaxial (A,B,C are intrinsic axis radii): Oblate: A = B > C (a flying saucer) Prolate: A > B = C (a cigar) Triaxial A > B > C (a football) Studies find that ellipticals are mildly triaxial, with typical axis ratios: A:B:C ~ 1 : 0.95 : 0.65 (with some dispersion, ~0.2) Triaxiality is supported by observations of isophotal twists in some galaxies (would not see these if galaxies were purely oblate or prolate) It is due to the anisotropic velocity dispersions, which stretch the galaxies in proportion along their 3 principal axes 14

15 Triaxial Ellipsoids in Projection In a triaxial case, the orientation in the sky of the projected ellipses will not only depend upon the orientation of the body, but also upon the body s axis ratio (from Binney & Merrifield) 15

16 Isophote Twisting Since the ellipticity changes with radius, even if the major axis of all the ellipses have the same orientation, they appear as if they were rotated in the projected image. This is called isophote twisting: 16

17 Here is an example of twisted isophotes in a satellite galaxy of Andromeda (M31): Isophote Twists And another: 17

18 Isophotes: Deviations From Ellipses Isophotes are not perfect ellipses. There may be an excess of light on the major axis (disky), or on the corners of the ellipse (boxy): The diskiness/boxiness of an isophote is measured by the difference between the real isophote and the best-fit elliptical one: δ(φ) = <δ > + Σ a n cos nφ + Σ b n sin nφ where the terms with n < 4 all vanish (by construction), and a 4 > 0 is a disky E, while a 4 < 0 corresponds to a boxy E. 18

19 Disky and Boxy Elliptical Isophotes 19

20 Disky and Boxy Ellipticals Disky/boxy shapes correlate with various other galaxy parameters: Boxy galaxies more likely to show isophotal twists (and hence be triaxial) Boxy galaxies tend to be more luminous Boxy galaxies have stronger radio and x-ray emission Boxy galaxies are slow rotators, more anisotropic In contrast, disky galaxies are midsized ellipticals, oblate, faster rotators, less luminous in radio and x-ray Some believe that more dissipationless mergers lead to more boxy galaxies, whereas any embedded disks imply some dissipative collapse, but the real picture is probably more complicated 20

21 12.3 Elliptical Galaxies: Kinematics

22 The Kinematics of E-Galaxies Stars in E galaxies have some ordered motions (e.g., rotation), but most of their kinetic energy is in the form of random motions. Thus, we say that ellipticals are pressure-supported systems To measure the kinematics within galaxies we use absorption lines. Each star emits a spectrum which is Doppler shifted in wavelength according to its motion. Random distribution of velocities then broadens the spectral lines relative to those of an individual star. Systemic motions (rotation) shift the line centroids. 22

23 Kinematical Profiles of E-Galaxies dispersion rotation Rotation is present, but generally not a dominant component of the kinetic energy Velocity dispersion tends to be higher closer to the center 23

24 2-Dimensional Kinematics of E-Gal s Intensity (surface brightness) Rotation velocity Velocity dispersion 24

25 Velocity Anisotropy in Elliptical Galaxies The ratio of the maximum rotational velocity V m and the mean velocity dispersion σ indicates whether the observed shapes of E s are due to rotation or anisotropic pressure Galaxies on this line are flattened by rotation ellipticity Galaxies below it are flattened by anisotropy 25

26 Velocity Anisotropy in Elliptical Galaxies Now normalize by the isotropic rotator line More luminous ellipticals tend to be anisotropic This can be understood as a consequence of merging 26

27 Stellar Populations in Ellipticals Ellipticals are made mostly from old stars, ages > 1 Gyr and generally ~ 10 Gyr They have a broad range of metallicities (which indicate the degree of chemical evolution), up to 10 times Solar! More metal rich stars are found closer to the center This is observed as line strength gradients, or as color gradients (more metal-rich stars are redder) Mg line strength vs. radius 27

28 Metallicity-Luminosity Relation also known as the Color-Magnitude Relation There is a relation between the color (a metallicity indicator) and the total luminosity or velocity dispersion for E galaxies: R e d Bright Faint Cold Hot B l u e Brighter and dynamically hotter galaxies are redder. This could be explained if small E galaxies were younger or more metal-poor than the large ones. More massive galaxies could be more effective in retaining and recycling their supernova ejecta. 28

29 Hot Gas in Elliptical Galaxies M49 Optical M49 X-ray The gas is metal-rich, and thus at least partly a product of stellar evolution It is at a virial temperature corresponding to the velocity dispersion of stars Another good probe of dark matter in ellipticals 29

30 Stellar vs. Dynamical Mass M /M M dyn = M M dyn = 6 M Recall that Ω m ~ 6 Ω b Early-type galaxies from the SPIDER collab. (de Carvalho et al.) M dyn /M 30

31 More Massive Galaxies Have Larger (M/L) (M/L) vs. Luminosity a ~ Stellar mass ` (M/L) vs. Mass (Cappellari et al. 2006) 31

32 (M/L) vs. Mass From Gravitational Lensing ` Total Mass ` Stellar Mass red = V band, blue = B band (Auger et al. 2010, the SLACS collab.) 32

33 (M/L) Increases With Radius From X-ray profiles and modeling (Fukazawa et al. 2006) Fully consistent with dynamical modeling of optical data 33

34 12.4 Massive Black Holes in Galactic Nuclei and Dwarf Galaxies

35 Massive Black Holes in Galactic Nuclei It turns out that they are ubiquitous: nearly every non-dwarf galaxy seems to have one, but only a small fraction are active today; these super-massive black holes (SMBH) are believed to be the central engines of quasars or other AGN They are detected through central velocity dispersion or rotation cusps near the center - requiring more mass than can be reasonably provided by stars Their masses correlate very well with many of their host galaxy properties, suggesting a co-formation and/or co-evolution of galaxies (or at least their old stellar spheroid components) and the SMBHs they contain Understanding of this connection is still not complete, but dissipative mergers can both drive starbursts and fuel/grow SMBHs 35

36 Many (all?) ellipticals (& bulges) have black holes- even compact ones like M32! Can measure BH masses for galaxies via their velocity dispersion 36

37 Fundamental Corrrelations Between SMBH Masses and Their Host Galaxy Properties log M BH M 8 χ r 2 = 23 = ( 0.36± 0.09)B 0 T + (1.2±1.9) Kormendy & Richstone 1995 = (1.7± 0.3) 10 8 σ c M 200km s 1 M BH χ r 2 = ±0.5 Ferrarese & Merritt 2000; Gebhardt et al

38 The SMBH - Host Galaxy Correlations M ~ M M ~ σ 4.4 M ~ M H

39 Dwarf Galaxies Dwarf ellipticals (de) and dwarf spheroidals (dsph) are a completely different family of objects from normal ellipticals - they are not just small E s In fact, there may be more than one family of gas-poor dwarf galaxies Dwarfs follow completely different correlations from giant galaxies, suggestive of different formative mechanisms They are generally dark matter (DM) dominated, especially at the faint end of the sequence One possible scenario is that supernova (SN) winds can remove baryons from these low-mass systems, while leaving the DM, while the more massive galaxies retain and recycle their SN ejecta 39

40 Parameter Correlations Kormendy

41 Mean Surface Brightness vs. Absolute Mag. 41

42 Dwarf Spheroidals Mass to Light Ratios 42

43 12.5 Galaxy Scaling Relations

44 Galaxy Scaling Laws When correlated, global properties of galaxies tend to do so as power-laws; thus scaling laws They provide a quantitative means of examining physical properties of galaxies and their systematics They reflect the internal physics of galaxies, and are a product of the formative and evolutionary histories Thus, they could be (and are) different for different galaxy families We can use them as a fossil evidence of galaxy formation When expressed as correlations between distance-dependent and distance-independent quantities, they can be used to measure relative distances of galaxies and peculiar velocities: thus, it is really important to understand their intrinsic limitations of accuracy, e.g., environmental dependences 44

45 The Tully-Fisher Relation A well-defined luminosity vs. rotational speed (often measured as a H I 21 cm line width) relation for spirals: L ~ V rot γ, γ 4, varies with wavelength Or: M = b log (W) + c, where: M is the absolute magnitude W is the Doppler broadened line width, typically measured using the HI 21cm line, corrected for inclination W true = W obs / sin(i) Both the slope b and the zero-point c can be measured from a set of nearby spiral galaxies with well-known distances The slope b can be also measured from any set of galaxies with roughly the same distance - e.g., galaxies in a cluster - even if that distance is not known Scatter is ~ 10-20% at best, better in the redder bands 45

46 Tully-Fisher Relation in Diferent Bands Sakai et al

47 Why is the TFR So Remarkable? Because it connects a property of the dark halo - the maximum circular speed - with the product of the net integrated star formation history, i.e., the luminosity of the disk Halo-regulated galaxy formation/evolution? The scatter is remarkably low - even though the conditions for this to happen are known not to be satisfied There is some important feedback mechanism involved, which we do not understand yet Thus, the TFR offers some important insights into the physics of disk galaxy formation 47

48 Low surface brightness galaxies follow the same TF law as the regular spirals: so it is really relating the baryonic mass to the dark halo Zwaan et al

49 The Faber-Jackson Relation for Ellipticals Analog of the Tully-Fisher relation for spirals, but instead of the peak rotation speed V max, measure the velocity dispersion. This is correlated with the total luminosity: L V = σ 200 kms -1 4 L sun 49

50 The Kormendy Relation for Ellipticals Mean surface brightness R e ~ I e -0.8 Larger ellipticals are more diffuse Effective radius 50

51 Can We Learn Something About the Formation of Ellipticals From the Kormendy Relation? From the Virial Theorem, mσ 2 ~ GmM/R Thus, the dynamical mass scales as M ~ Rσ 2 Luminosity L ~ I R 2, where I is the mean surface brightness Assuming (M/L) = const., M ~ I R 2 ~ Rσ 2 and I R ~ σ 2 Now, if ellipticals form via dissipationless merging, the kinetic energy per unit mass ~ σ 2 ~ const., and thus we would predict the scaling to be R ~ I -1 If, on the other hand, ellipticals form via dissipative collapse, then M = const., surface brightness I ~ M R -2, and thus we would predict the scaling to be R ~ I -0.5 The observed scaling is R ~ I Thus, both dissipative collapse and dissipationless merging probably play a role 51

52 Deriving the Scaling Relations Start with the Virial Theorem: Now relate the observable values of R, V (or σ), L, etc., to their true mean 3-dim. values by simple scalings: k R One can then derive the virial versions of the FP and the TFR: Where the structure coefficients are: Deviations of the observed relations from these scalings must indicate that either some k s and/or the (M/L) are changing 52

53 Fundamental Plane of Elliptical Galaxies Commonly expressed as a bivariate scaling relation R ~ σ 1.4 I -0.8 Where R is the radius, I the mean surf. brightness, σ the velocity disp. Face-on Edge-on (Pahre et al. 1988) 53

54 Different Views of the FP Luminosity instead of radius Mg abs. line strength index (a measure of metallicity) instead of velocity dispersion FP connects star formation histories and chemical evolution of stellar populations with dynamical and structural parameters of ellipticals 54

55 Scaling Relations for Ellipticals Kormendy rel n Faber- Jackson rel n Cooling diagram Fundam. Plane 55

56 From Virial Theorem to FP Virial Theorem connects mass, density, and kinetic temperature, and is thus an equation of a plane in that (theoretical) parameter space. Assumptions about the dynamical structure of ellipticals and their (M/ L) ratios then map the VT into the tilted FP in the observable parameter space of measured quantities such as R, σ, I, L, 56

57 Fundamental Plane and M/L Ratios If we assume homology and attribute all of the FP tilt to the changes in (M/L), (M/L) ~ L α, α ~ 0.2 (vis) or ~ 0.1 (IR) Possible causes: systematic changes in M visible /M dark, or in their relative concentrations; or in the stellar IMF Pahre et al. 1995: K-band FP Cappellaro et al. 2006: SAURON dynamical modeling log (M/L) + const. log M + const. 57

58 For any elliptical galaxy today, big or small, Just Two Numbers determine to within a few percent or less: Mass, luminosity (in any OIR band), Any consistently defined radius Surface brightness or projected mass density Derived 3-d luminosity, mass, or phase-space density Central projected radial velocity dispersion OIR colors, line strengths, and metallicity Mass of the central black hole and maybe other things as well And they do so regardless of the: Star formation and merging formative/evolutionary history Large-scale environment (to within a few %) Details of the internal structure and dynamics (including S0 s) Projection effects (the direction we are looking from) 58

59 How Can This Be? The intrinsic scatter of the FP is at most a few %, and could be 0 The implication is that elliptical galaxies occupy only a small, naturally selected, subset of all dynamical structures which are in principle open to them Numerical sim s can reproduce the observed structures of E s, and the FP, but they do not explain them Understanding of the origin of the small scatter of the FP (or, equivalently, the narrow range of their dynamical structures) is an outstanding problem (Robertson et al. 2006) 59

60 The Galaxy Parameter Space A more general picture Galaxies of different families form 2-dim. sequences in a 3+ dimensional parameter space of physical properties, much like stars form 1-dim. sequences in a 2-dim. parameter space of {L,T} - this is an equivalent of the H-R diagram, but for galaxies 60

61 The Dark Halos Many of galaxy scaling relations may be driven by the properties of their dark halos It is possible to infer their properties from detailed dynamical profiles of galaxies and some modeling Numerical simulations suggest a universal form of the dark halo density profile (NFW = Navarro, Frenk & White): ρ(r) ρ crit = (but one can also fit another formula, e.g., with a core radius and a finite central density) δ c ( r r ) s 1+ r r s ( ) 2 61

62 Evidence for the NFW profile from simulations 62

63 Dark Halo Scaling Laws ρ o r c ρ o ~ L B r c ~ L B 0.37 σ ~ L B 0.20 (fits to Sc-Im only) are dsph, dirr σ so expect the surface density Σ ~ ρ o r c to be ~ constant over this range of M B, and it is Kormendy & Freeman