But why an empty ring? black holes? (APOD 2008 Nov 04)

[Axel]

The wave of star formation following a collision I can understand, but by what mechanism is the middle of a galaxy emptied?

[bystander]

http://apod.nasa.gov/apod/ap081104.html

The wave of star formation following a collision I can understand, but by what mechanism is the middle of a galaxy emptied?

IMHO, the center wasn't emptied, the ring grew. The original core is the red knot of older stars at the lower left of the zero.

Space.com - 2008 Oct 30
Hubble Telescope Photographs a Perfect Cosmic 10
http://www.space.com/scienceastronomy/0 ... rfect.html

BTW: The YouTube video about when galaxies collide is amazing.

[JohnD]

Quote from APOD text: [It] "shows a ring because it has recently collided with the other galaxy in the frame"

Really?
When the blue galaxy is near enough for individual stars to be discriminated, and the red one isn't.
Surely the red one is ?twice? as far away, meaning that iteraction is unlikely.

There's another ring galaxy in the famous image of Hoag's Object, but that doesn't mean that they have interacted.
See: http://apod.nasa.gov/apod/ap020909.html

Jhn

[Chris Peterson]

Really?
When the blue galaxy is near enough for individual stars to be discriminated, and the red one isn't.
Surely the red one is ?twice? as far away, meaning that iteraction is unlikely.

There are no stars resolved in either galaxy. The "stars" you see in the blue galaxy are vast clumps of material, probably star forming regions. They happen in galaxies where shock waves cause a local collapse of material. Such shocks can be internally produced by galaxy dynamics (like arm motion), or externally produced by collisions. The fact that one galaxy shows such structure and the other doesn't can't be taken as an indication of their relative distances. Red shift measurements do show these galaxies to be interacting.

There's another ring galaxy in the famous image of Hoag's Object, but that doesn't mean that they have interacted.

No, but an old collision is the leading theory for how Hoag's Object formed; likewise for ring galaxies in general. Numerical simulations of collisions produce rings as one possible outcome.

[Ted]

Looking at today's APOD (11/4/08), I wonder if the blue galaxy still has a black hole. If it does, it seems strange that the nearby stars were all tossed out of it's orbit, and if it doesn't, isn't that highly unusual? Does anyone have an explanation?
Thank you.
Ted

[bystander]

Looking at today's APOD (11/4/08), I wonder if the blue galaxy still has a black hole. If it does, it seems strange that the nearby stars were all tossed out of it's orbit, and if it doesn't, isn't that highly unusual? Does anyone have an explanation?

http://apod.nasa.gov/apod/ap081104.html

Not unusual at all. See the YouTube video from the APOD about when galaxies collide. It has a simulation of colliding galaxies interspersed with actual pictures of interacting galaxies.

[Chris Peterson]

Looking at today's APOD (11/4/08), I wonder if the blue galaxy still has a black hole. If it does, it seems strange that the nearby stars were all tossed out of it's orbit, and if it doesn't, isn't that highly unusual? Does anyone have an explanation?

The supermassive black hole found at the center of most galaxies doesn't have much impact on a galaxy's dynamics. With a typical mass of a few million solar masses, the gravitational effects are dwarfed by the vastly greater number of stars making up the entire structure. A galactic collision can whisk away a little old supermassive black hole like there's nothing there, and the rest of the galaxy will continue on its way.

[Nereid]

Thread split; tangential discussion moved to here.