Elliptical galaxies: Ellipticals



Elliptical galaxies: Ellipticals
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
•! 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
Some ellipticals are not so simple …
M89 – E0
Elliptical galaxies:
•! In general, ellipticals --
Cen A
–! 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
–! 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 -µB,e = 3.02 log re + 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,
Average surface brightness vs Absolute magnitude
Kormendy relation (1977)
Central surface brightness & core radius relations
Elliptical galaxies:
•! With HST, we can study the nuclei of elliptical
–! 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
Definition of Break radius
Break radius vs. Absolute magnitude
I( r) = Ib2(! -" )/# (rb/r)" [1+(r/rb)# ](" -! )/#
rb = break radius where power-law
changes slope and Ib 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
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
Shape of Ellipticals:
•! 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
–! 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 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
–! %r(t) & 'k(3 ak cos(kt) + bk cos(kt)
–! t = angle around ellipse, %r(t) is distance between fitted ellipse and
observed isophote
–! a3 and b3 describe “egg-shaped” ellipses, generally small, b4 is also
usually small
–! a4 > 0, isophote is disky (pushed out)
–! a4 < 0 isophote is boxy (peanut shaped)
NGC 5831
Disky and boxy elliptical isophotes
Shape of Ellipticals – disky/boxy
•! Disky/boxy correlates with other galaxy
–! 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
–! 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 a4/a
Kinematics of Ellipticals:
•! Measured via the integrated absorption
•! ) is the velocity dispersion (random
motion) observed via width of absorption
line compared to template spectra
•! Vr is the mean radial velocity
–! Rotation is evident as a gradient in Vr
across the face of the system
Kinematics of ellipticals (NGC 1399)
Kinematics of Ellipticals:
•! Vrot/) correlates with luminosity
–! Lower luminosity ellipticals have higher Vrot/) -- rotationally
–! Higher luminosity ellipticals have lower Vrot/) -- pressure supported
•! Vrot/) correlates with boxy/diskiness
–! Disky ellipticals have higher Vrot/) -- rotationally supported
–! Boxy ellipticals have lower Vrot/) -- pressure supported
•! Rotation implies that ellipticals are not relaxed
–! Some have kinematically decoupled cores, or rotation along their
minor axis (implies triaxiality)
Vrot/) vs luminosity
Vrot/) & minor axis rotation vs a4/a
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:
•! The missing parameter is effective radius (discovered
in 1987). There are four observables (but only 3
independent parameters):
Effective radius
Mean surface brightness
Velocity dispersion
•! You can fit a FUNDAMENTAL PLANE through the
–! re * )1.24<I>-0.82
•! Any model of galaxy formation has to reproduce this
•! Can also define the Dn-) relation for use as a distance
Surface brightness vs. luminosity
Velocity dispersion vs. effective radius
Stellar Populations:
•! 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),
Fundamental Plane
Stellar Populations of Ellipticals
•! 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
B-V for different metallicity populations
Evolution of a single burst population
Stellar Populations of Ellipticals
•! There is also a strong correlation between Mg2 and
velocity dispersion such that galaxies with higher
velocity dispersions have stronger Mg2 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
Hot Gas in Ellipticals
•! Many ellipticals have extended x-ray
halos of gas. T~106 K
•! Where does it come from?
•! Why is it hot?
Hot Gas in Ellipticals
•! 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 = 106 K !!!
•! M (gas) ~ 108 – 109 M!
Globular clusters in Ellipticals
•! 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
•! This is probably a metallicity effect, so
there is a population of metal poor and a
population of metal rich GCs
•! What does this mean?
Globular cluster color distributions
Rhode & Zepf
B-R color
Globular clusters in Ellipticals
Many (all?) ellipticals (& bulges) have black holes
•! Globular cluster colors have implications
for formation process:
•! Either
First direct
detection –
Ford et al (1994)
–! 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
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
•! Observations imply BH mass directly
tied to the formation of bulges and
•! Either
From Kormendy (2003) review
Dark matter in elliptical galaxies
•! Expected mass to light ratio of the stellar population implies M/LV
~ 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?
–! 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
•! 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
Romanowsky et al.
Planetary nebulae dynamics
Romanowsky et al.