Elliptical galaxies: Ellipticals 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 New view! Some ellipticals have hot x-ray gas, some have dust! Ellipticals do rotate (speed varies)! Some contain decoupled (counter-rotating) cores! Some have weak stellar disks! Ellipticals formed by mergers of two spirals, or hierarchical clustering of smaller galaxies M89 E0 Some ellipticals are not so simple Elliptical galaxies: Cen A In general, ellipticals --! Pressure supported (little rotation), stellar motions are (mostly) random! No or very little disk component! No or very little star formation! No or very little cold (e.g., HI) gas, but contain hot, x-ray gas! Almost exclusively found in high density environments (clusters)! Populate a fundamental plane in luminosity-surface brightness-central velocity dispersion
Elliptical galaxies: Effective radius vs Absolute magnitude There are other correlations! Brighter ellipticals are bigger! Brighter ellipticals have lower average surface brightness! Can put the above two together to form the Kormendy relation larger galaxies have lower surface brightnesses -- µ B,e = 3.02 log r e + 19.74! Brighter ellipticals have lower central surface brightness! Brighter ellipticals have larger core radii -- the core radius is the radius where the SB drops to! that of the central SB, I(r=0) Average surface brightness vs Absolute magnitude Kormendy relation (1977)
Central surface brightness & core radius relations (Kormendy) Elliptical galaxies: With HST, we can study the nuclei of elliptical galaxies! Luminous ellipticals show central cores! Mid-sized ellipticals show central cusps, light continues to rise in SB towards center (power-law) Nuclei of Elliptical Galaxies, Faber et al. 1997 core cusp
Definition of Break radius Break radius vs. Absolute magnitude I( r) = I b 2 (! -" )/# (r b /r) " [1+(r/r b ) # ](" -! )/# r b = break radius where power-law changes slope and I b is the surface brightness at the break This is a five(!) parameter fit:!! is the outer power-law slope " is the inner power law slope #! defines the sharpness of the transition Shape of Ellipticals: Shape of Ellipticals: Ellipticals are defined by En, where n=10$, and $=1-b/a is the ellipticity. Note this is not intrinsic, it is observer dependent! 3-D shapes are ellipticals predominantly:! Oblate: A=B>C (a flying saucer)! Prolate: A>B=C (a cigar)! Triaxial A>B>C (a football)! Note A,B,C are intrinsic axis radii Want to derive intrinsic axial ratios from observed! Can deproject and average over all possible observing angles to do this! Find that galaxies are mildly triaxial: A:B:C ~ 1:0.95:0.65 (with some dispersion ~0.2)! Triaxiality is also supported by observations of isophotal twists in some galaxies (would not see these if oblate or prolate)
Observed axial ratio distribution: twisty disky boxy a/b Twisty isophotes: Shape of Ellipticals disky/boxy Galaxies do not have perfect elliptical isophotes typical deviations of a few % Deviations from ellipses can be classified as disky or boxy Measure difference between observed isophote and fitted ellipse as:! %r(t) & ' k(3 a k cos(kt) + b k cos(kt)! t = angle around ellipse, %r(t) is distance between fitted ellipse and observed isophote! a 3 and b 3 describe egg-shaped ellipses, generally small, b4 is also usually small! a 4 > 0, isophote is disky (pushed out)! a 4 < 0 isophote is boxy (peanut shaped) NGC 5831
Disky and boxy elliptical isophotes Shape of Ellipticals disky/boxy Disky/boxy correlates with 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 strong radio and x-ray emission! Boxy galaxies are slow rotators! In contrast disky galaxies are midsized ellipticals, oblate, faster rotators, less luminous x-ray gas Radio power and x-ray luminosity vs a 4 /a Kinematics of Ellipticals: Measured via the integrated absorption spectra ) is the velocity dispersion (random motion) observed via width of absorption line compared to template spectra V r is the mean radial velocity!rotation is evident as a gradient in V r across the face of the system
Kinematics of ellipticals (NGC 1399) Kinematics of Ellipticals: V rot /) correlates with luminosity! Lower luminosity ellipticals have higher V rot /) -- rotationally supported! Higher luminosity ellipticals have lower V rot /) -- pressure supported V rot /) correlates with boxy/diskiness! Disky ellipticals have higher V rot /) -- rotationally supported! Boxy ellipticals have lower V rot /) -- pressure supported Rotation implies that ellipticals are not relaxed systems! Some have kinematically decoupled cores, or rotation along their minor axis (implies triaxiality) V rot /) vs luminosity V rot /) & minor axis rotation vs a 4 /a disky boxy
Faber-Jackson relation: Faber-Jackson Relation In 1976, Faber & Jackson found that:! Roughly, L * ) 4! More luminous galaxies have deeper potentials! Can show that this follows from the Virial Theorem! Why is this relationship useful??! There is a large scatter a second parameter? Fundamental Plane: Surface brightness vs. luminosity The missing parameter is effective radius (discovered in 1987). There are four observables (but only 3 independent parameters):! Luminosity! Effective radius! Mean surface brightness! Velocity dispersion You can fit a FUNDAMENTAL PLANE through the observables! r e * ) 1.24 <I> -0.82 Any model of galaxy formation has to reproduce this relation Can also define the D n -) relation for use as a distance indicator
Velocity dispersion vs. effective radius Fundamental Plane Stellar Populations: Stellar Populations of Ellipticals In 1944, Walter Baade used the 100 inch Mt. Wilson telescope to resolve the stars in several nearby galaxies: M31, its companions M32 and NGC 205, as well as the elliptical galaxies NGC 147 and NGC 145 Realized the stellar populations of spiral and elliptical galaxies were distinct:! Population I: objects closely associated with spiral arms luminous, young hot stars (O and B), Cepheid variables, dust lanes, HII regions, open clusters, metal-rich! Population II: objects found in spheroidal components of galaxies (bulge of spiral galaxies, ellipticals) older, redder stars (red giants), metal-poor Ellipticals are full of old, red stars Ellipticals follow a color-magnitude relation such that more luminous galaxies are redder!is this due to age or metallicity? Age/metallicity degeneracy!!
Typical Elliptical galaxy spectrum Evolution of a single burst population B-V for different metallicity populations Stellar Populations of Ellipticals There is also a strong correlation between Mg 2 and velocity dispersion such that galaxies with higher velocity dispersions have stronger Mg 2 absorption Thus, more luminous/massive galaxies are more metal rich -- deeper potentials hold ISM longer allowing metals to build up There are also color & abundance gradients in elliptical galaxies
Color luminosity relation Mg2 vs velocity dispersion Abundance gradients in ellipticals Many ellipticals have extended x-ray halos of gas Optical M49 X-ray
Hot Gas in Ellipticals Hot Gas in Ellipticals Many ellipticals have extended x-ray halos of gas. T~10 6 K Where does it come from? Why is it hot? Many ellipticals have extended x-ray halos of gas Where does it come from?!mass loss of AGB stars! Why is it hot?!motions of stars heat the gas: 1/2m) 2 ~ 3/2 kt T = 10 6 K!!! M (gas) ~ 10 8 10 9 M! Globular clusters in Ellipticals Globular cluster color distributions Ellipticals are surrounded by numerous globular clusters (about twice as many as a similarly luminous spiral) Globular cluster colors in ellipticals show a bimodal distribution E2 S0 Rhode & Zepf 2004 This is probably a metallicity effect, so there is a population of metal poor and a population of metal rich GCs E3 E5 What does this mean? B-R color
Globular clusters in Ellipticals Many (all?) ellipticals (& bulges) have black holes Globular cluster colors have implications for formation process: Either!Merger of two galaxies metal poor clusters are old, metal rich clusters formed during merger process!hierarchical formation metal rich population builds up during accretion of gas rich clumps First direct detection Ford et al (1994) Many (all?) ellipticals (& bulges) have black holeseven compact ones like M32! Black holes Currently there are observations of at least 40 BH masses in nearby ellipticals and spiral bulges There is a strong correlation between black hole mass and galaxy luminosity and velocity dispersion Can measure BH masses for galaxies without central disks via their velocity dispersion
Black hole mass vs. galaxy luminosity & velocity dispersion Black hole formation From Kormendy (2003) review Observations imply BH mass directly tied to the formation of bulges and ellipticals Either!All proto-galaxy clumps harbored an equal sized (relative to total mass) BH, and BH merged as galaxy formed!bh started out small and grew as galaxy formed e.g., central BH is fed during process of formation and is the seed of the formation process (all galaxies have BHs?) Dark matter in elliptical galaxies Dark matter in elliptical galaxies Expected mass to light ratio of the stellar population implies M/L V ~ 3-5 Orbital motions of the stars in the centers of ellipticals imply they are not dark matter dominated In those (few!) ellipticals containing cold gas, we can measure the circular orbits of the gas we find M/L ~ 10 20! But are these galaxies typical?? Also can use the amount of mass required to retain the hot x-ray gas, find M/L~100 for galaxies with large x-ray halos! Are these typical? Need a tracer particle that can be easily measured kinematically at large galactic radii! 2 possibilities globular clusters and planetary nebulae! Recent results of PN dynamics around (a few) elliptical galaxies show NO dark matter, the galaxies are naked! Recent results of GC dynamics around (a few) elliptical galaxies show large dark halo. Are we measuring the galaxy potential or the potential of the cluster it lives in??
Planetary nebulae dynamics Planetary nebulae dynamics Romanowsky et al. 2003 Romanowsky et al. 2003