Structure and Stellar Populations 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) ~ r^1/4

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

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

(Note that <I_e> measures in linear units (L_sun/pc², for example), while <mu_e> measures in surface brightness units (mags/arcsec²). Either way, they measure the same thing.)

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) ~ r^1/n, and is characterized by r_e, <I_e>, and a Sersic index n. If we plot surface brightness (mu, in mags/arcsec²) versus radius, it looks like this:

Note: n=1 would be an exponential, while n=4 is the classical r-to-the-quarter law

For example, the massive galaxy at the center of the Virgo cluster (M87, a "cD galaxy") has a very extended n ~ 10-11 profile. Inner regions show rapid dropoff in surface brightness, outer regions fall off much more slowly, showing an extended faint halo of light (from Janowiecki et al 2010):

Stellar populations

We've seen that elliptical galaxies are much redder than spiral galaxies. They also typically have much less neutral hydrogen gas. What does this suggest about their star forming histories?

Population Synthesis reminder

Stellar population synthesis tracks: B-V color vs age, for an evolving "single burst" population of stars with different metallicities:

So two things make a stellar population red: old age, and high metallicity. The colors of a galaxy cannot distinguish between the two.

Review question: why would high metallicity make stars redder?

Elliptical galaxies (like spirals) show a color-luminosity relationship: brighter, more massive galaxies are redder. In elliptical galaxies, this is well-established to be a metallicity effect, not age. So brighter galaxies are more metal-rich.

The differences between elliptical galaxies and star forming spirals can be seen in plots of color versus luminosity or stellar mass: they form a distinct "red sequence" which is offset from the "blue cloud" of star forming galaxies:

Remember terminology: "early-type" means E/S0, "late-type" means Sb/Sc/Irr.

(from SDSS data, courtesy Kevin Schawinski)

Gas content

Actually ellipticals are not devoid of gas -- it's just not neutral hydrogen, or molecular gas. So what is it?

Look at the elliptical galaxy M49 in optical and X-ray:

(Thanks to Beth Brown, University of Michigan)

There is extended, X-ray emitting gas. How hot would it have to be to emit X-rays? Set the energy of an X-ray photon equal to the thermal energy of particles:

Solving for T, we get a temperature of ~ 10⁶ K or so.

Mass estimates range from 10⁸ to maybe as much as 10⁹ M_sun of hot gas in ellipticals. The metallicity of this gas is low, but not super low -- it's ~ 1/3 solar.

Questions to consider:

Where did this gas come from?
Why is it hot?
What holds it up?

Also, some galaxies do have neutral hydrogen -- check out the nearby elliptical galaxy Centaurus A (Schiminovich et al 1994):

optical starlight in greyscale
HI 21-cm emission in contours