I've heard/read a couple of astronomers calling elliptical galaxies "all-halo". What do they mean by this?

Answer 1

Spiral galaxies such as the Milky Way and Andromeda (M31) have two components: a rotating disk of stars, dust, and gas and a spherical halo of more or less randomly orbiting stars. In the disk, compression waves passing through the clouds of gas and dust compress the clouds and cause new stars to be born. Galactic disks are distinguished by hot, massive, young, blue stars. The halo is mostly free of gas and dust and consists of very old, red stars. All the blue stars in the halo have long since used up all their fuel and died.

Elliptical galaxies lack the disk. They tend to have relatively little gas and dust and little star birth, consisting almost entirely of old, red stars, like the halo of a spiral galaxy. Most ellipticals tend to have some net angular momentum and so are flattened somewhat due to rotation.

Answer 2

That is an usual expression of astronomers to mean that
most of elliptical galaxies are made up just of halo, which is made up mostly of old, reddih, low-mass stars. A brief explanation follows:

Lack of spiral arms is not the only difference between spiral and elliptical galaxies. Most ellipticals also contain little or no gas and dust. In most cases there is no evidence of young stars or of any ongoing star formation. Like the halo of our own Galaxy, ellipticals are made up mostly of old, reddish, low-mass stars. Indeed, having no disk, gas, or dust, elliptical galaxies are, in a sense, "all halo." Again as in the halo of our Galaxy, the orbits of stars in ellipticals are disordered, exhibiting little or no overall rotation; objects move in all directions, not in regular, circular paths as in our Galaxy's disk. Apparently all, or nearly all, of the interstellar gas within elliptical galaxies was swept up into stars (or out of the galaxy) long ago, before a disk had a chance to form, leaving stars in random orbits, with no loose gas and dust for the creation of future generations of stars.

For more info, take a look at:

http://physics.fortlewis.edu/Astronomy/astronomy%20today/CHAISSON/AT324/HTML/AT32401.HTM

paragraph elliptical, specifically.

Answer 3

The old professor says: Many galaxies, like our own Milky Way, have a low density cloud of stars surrounding the central hub. This is called the "halo".

Elliptical galaxies lack such a halo, and they are low density (in terms of stellar populations) when compared to spiral galaxies. Thus they seem to be made of just halo density and halo type stars.