|Elliptical galaxies are generally
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.
(Note that <Ie> measures in linear units (Lsun/pc2, for example), while <mue> 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
profile to describe galaxies. This mathematical
profile is log(I)
and is characterized by re, <Ie>, and a Sersic index
n. If we plot surface brightness (mu, in mags/arcsec2) versus r or r1/4, it looks like this:
left: mu vs r
right: mu vs r0.25
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 population synthesis tracks: B-V color vs age, for a population of evolving stars of 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
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 a color-magnitude (or color-mass) plot:
they form a distinct "red sequence" which is offset from the "blue
cloud" of star forming galaxies:
(from SDSS data, courtesy Kevin Schawinski)
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 Msun of hot gas in ellipticals. The metallicity of this gas is low, but not super low -- it's ~ 1/3 solar.
Questions to consider: