Structure of Elliptical Galaxies

Note: we will almost exclusively be talking about normal ellipticals. Dwarf ellipticals (dE's) and dwarf spheroidals (dSph's) are different animals entirely...
 
 
Elliptical galaxies are generally smooth and relatively featureless spheroidal galaxies.

Like the bulges of (some) spirals, ellipticals are often characterized by a surface brightnesses profile which follow the de Vaucouleur or r-to-the-quarter law: log(I) ~ r1/4

Are ellipticals and spiral bulges the same thing? No! The similarity ends there...

But using this luminosity profile, we can characterize the effective radius (re) as the radius containing half the total light of the galaxy, and the mean surface brightness of the galaxy inside that radius (<Ie>). One measures the size of the galaxy, the other measures the luminosity density.

In detail, though ellipticals are found to have diverse surface brightness profiles, and people often use a more generalized Sersic profile to describe galaxies. This mathematical profile is log(I) ~ r1/n, and is characterize by re, <Ie>, and a Sersic index n:


left: mu vs r
right: mu vs r0.25


How do these properties correlate with luminosity?

 

Brighter ellipticals are bigger ...

...and lower in surface brightness


Also note how dwarf E's and normal E's define different relationships on these plots. In particular, look how extreme the dwarf spheroidals are.

 
 
 
Those relationships apply to the global structure of elliptical galaxies (on the scale of kiloparsecs). 

Recent studies of elliptical galaxies with the Hubble Space Telescope revealed a quite unexpected result - similar relationships hold for the nuclei of ellipticals (on scales of parsecs). Why did it take HST to reveal this?

The nuclei of ellipticals show a power law surface brightness profile which sometimes becomes flat  (constant density) at a break radius rb



 

When you plot break radius against galaxy luminosity you see this:


Think this through. The nucleus of the galaxy knows about the galaxy's global properties (ie, its luminosity). More luminous galaxies have more diffuse cores. How could this be true?

 
 

The Dynamical Effect of Central Massive Black Holes

If massive (luminous) galaxies harbored central black holes, how would this affect the structure of the galaxies nucleus?

Single Black Hole:

The presence of a single massive black hole can trap stars in the nuclear regions -- they don't have enough orbital energy to escape the black hole's gravity. This effect causes the central density of the galaxy to be larger -- which is the opposite of what we see!


Black Hole Binaries:

As stars in the nucleus interact with a pair of orbiting black holes, the stars can gain energy from the black hole binary and leave the nucleus. Because the binary has lost energy, it must contract, and the black holes move closer to each other. As time passes, the nucleus becomes "scoured out" of stars, and the binary may ultimately merge.

How do we get binary black holes? If ellipticals form by mergers with other galaxies, or accreting smaller galaxies, those galaxies may bring another black hole into the galaxy, forming a binary pair.

 

The black hole - galaxy relationship

 


Plotted on the left is the black hole mass vs  the absolute magnitude of the "bulge" of a galaxy, where in this case "bulge" refers to any spheroidal part of a galaxy -- ie, for a spiral galaxy it is the stellar bulge, while for an elliptical galaxy it is the entire galaxy.

Plotted on the right is the black hole mass vs the velocity dispersion of the galaxy. For more information, see John Kormendy's webpage, where this image comes from.