Hubble spies curious galaxy moving a little closer | Hacker News Reader

Hubble spies curious galaxy moving a little closer

phys.org

36 pointsdnetesn10 comments

As the article mentions, this is most likely due to the rotation of the cluster. Throwing some numbers out there:

The Virgo cluster is 60Mly away which is ~20Mpc (mega parsecs). Hubble's law [1] (v = Hd) tells us that for every 1Mpc away an object is, the expansion of the universe causes it to move away from us at about 70 km/s. So 20Mpc -> 1400km/s.

The true velocity is actually about 300km/s less than this because the local group is being pulled gravitationally towards Virgo. (If you are interested, this movement of the local group towards Virgo is the largest component of the earth's motion relative to the CMB. Some of the others being the earth's motion around the sun and the sun's around the milky way).

The velocity dispersion of large clusters is known to be on the order of 1000 km/s, so its not surprising that one or two objects whose orbits happen to be bringing them towards us are net blueshifted.

The long and the short of it is that this is more of a cool curiosity than evidence of something crazy going on.

[1] https://en.wikipedia.org/wiki/Hubble%27s_law

Ah, well. There goes my theory of galactic dark matter propulsion.

Perhaps the group is rotating rapidly in space, as it seems like other galaxies in the cluster are redshifted?

This is essentially what the article says.

> Astronomers think that this blueshift is likely caused by the cluster's colossal mass accelerating its members to high velocities on bizarre and peculiar orbits, sending them whirling around on odd paths that take them both towards and away from us over time. While the cluster itself is moving away from us, some of its constituent galaxies, such as Messier 90, are moving faster than the cluster as a whole, making it so that, from Earth, we see the galaxy heading towards us. However, some are also moving in the opposite direction within the cluster, and thus seem to be streaking away from us at very high velocity.

I don't think this quite amounts to "the cluster is rotating"–it only seems to imply that certain galaxies are orbiting at high velocity rather than the cluster as a whole.

AFAIU physics, there's no such thing as a rotating cluster of bodies in space that doesn't amount to just orbiting.

I would imagine that rotation implies the objects are all orbiting in roughly the same direction (eg. Clockwise) counterclockwise). It's possible that a group of objects (likely captured) would be orbiting in a very disorganized way.

And so it begins. Most scientific revolutions start with "that's odd". I would imagine this has significant implications for the current model of the expansion of space.

Edit: On second thought, galactic mergers are a thing, so perhaps not as significant as I thought?

The Virgo Cluster is pretty far away, so if anything from it does merge with our galaxy anytime soon I’d think it’d be because it was moving in a collision path rather than the two being gravitationally attracted together (as the Milky Way and Andromeda are).

> ...(as the Milky Way and Andromeda are)

Very neat, I never knew this

https://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_co...

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