*THE WANDERING ASTRONOMER*
Monday, November 13, 2023
Big, Small and Colliding Galaxies

Even though the Daily Astronomer no longer exists -and I am beginning to
miss it terribly- the DA still receives questions which we dutifully place
in our Pandora's Jar.    Today's question relates to galaxies: superlative
sizes and the inevitable "collision" of the Milky Way and Andromeda
Galaxies.

*"What is the biggest and smallest galaxy?  Also how can the Milky Way
galaxy be moving toward Andromeda since the Milky Way is the larger of the
two galaxies?"*

Two excellent questions.
No, galaxies have different sizes and structures, actually.

The largest and most massive galaxies tend to be giant ellipticals, those
believed to have been created by the merger of other galaxies. The most
massive known galaxy -so far- is ESO 146-IG 005, a supermassive galaxy
within the galactic cluster of Abell 3827, about 1.4 billion light years
away.

[Image credit: Gemini Legacy Image: R. Carrasco et al., Gemini
Observatory/AURA]

This immensely large galaxy is estimated to be as massive as 30 trillion
suns! For comparison, the material comprising our home galaxy, the Milky
Way, contains a comparatively meager four billion solar masses. In the
image above, captured by the Gemini south telescope in Chile, we can see
the nuclei of four galaxies which ESO 146-IG 005 is currently ‘devouring.’
Remarkably, this mammoth galaxy will only grow more massive as it
cannibalizes galaxies within its proximity. As it consumes more material
and consequently becomes more gravitationally powerful, ESO 146-IG 005 will
be able to incorporate many more galaxies into itself: a perfect example of
the ‘snowball effect.’

The smallest known galaxies are known as ultracompact dwarfs. The least
massive ultracompact dwarf yet discovered is M60-UCD1, a tiny galaxy
consisting of approximately 140 million stars within a diameter of 300
light years. (1/500th the diameter of the Milky Way.)* This ultracompact
dwarf galaxy, 49 million light years away within the Virgo Supercluster, is
likely the striped core of a massive galaxy that lost much of its material
when it encountered the galaxy M60 about 10 billion years away. Notice that
its name, M60-UCD1refers to its proximity to M60 and its size
classification.

An image of M60-UCD1 as captured by the Hubble Space Telescope


*Note: In this answer I am referring to those galaxies which contain a
supermassive black hole in their nuclei.       There are even smaller
systems, such as dwarf spheroidal galaxies such as the Fornax Dwarf and NGC
147.   However, I didn't consider this class because of the lack of the
SMBH at their centers.*

As for the second question: the Andromeda and Milky Way galaxies are
destined to “collide” within 4 - 6 billion years because the two galaxies
are relatively close to each other: 2.537 million light years. One can
consider this separation to be slight when viewed on the cosmological
scale. The mutual gravitational attraction is drawing them together at
about 500,000 miles an hour. (That velocity will increase as they draw
closer together.) One can watch a brilliant simulation of this merger here
<https://www.youtube.com/watch?v=4disyKG7XtU>

Also, the Andromeda Galaxy is actually larger and more massive than the
Milky Way. The Milky Way’s diameter is 100,000 -180,000 light years (still
uncertain), the diameter of Andromeda is nearly 220,000. One can see a
scale model depiction of both galaxies and their separation distance below:

[Image credit: Jan van der Crabben - Own work, CC BY-SA 4.0, File:Milky Way
and Andromeda in space, to scale.jpg - Wikimedia Commons
<https://commons.wikimedia.org/w/index.php?curid=47126811>

I hope this answer proves helpful.

*Were you to stand on the surface of a planet within this galaxy you’d see
about one million stars throughout the night. Compare this value  to the
10,000 stars visible throughout the night on Earth. And, yes, the
probability of life within this galaxy is reduced due to its density.