APOD: Virgo Cluster Galaxies (2023 May 26)

[Porter Wayfare]

A cosmic invitation

From wherever you are head for the Virgo Cluster. Once there you should be able to follow the scent of honeysuckle and lily of the valley to our place. It is particularly heavy this year and is the reason for this invitation. There is always the chance of a spring rain: another thing not to be missed.

If you stay overnight the cool morning air sharpens the individual fragrances and adds whisps of pine. The birds also start to sing.
We can bake bread for toast with coffee at sunrise. There are strawberries from last summer we can thaw or eat frozen. A couple of apples, one Fuji and one McIntosh, are in the root cellar still.

[VictorBorun]

My point is that they aren't "filaments" unless there's something binding them together. There is in the case of cosmological structure. There isn't in the case of stars or galaxies inside a cluster.

how many years must a filament exist before we call it a filament?

I think if a filament got to gravitationally form it's already is a filament if only for a moment.
Consider Milky Way's disk.
Galaxy orbital velocity is large, 200 km/s, compared with peculiar motion at 20 km/s.
The pressure oscillation of interstellar media arms-to-gaps waves is large, compared with clumps that trigger the filament formation.
Yet at a scale as small as 100 light years we can abstract off the galaxy orbital rotation and arms-to-gaps pressure waves and see the gravity/antigravity does it work. A cubic light year of positive or negative mass:

(mass of this cubic light year) minus (average mass of 1 cubic light year)

will gravitationally pull or repel its neighbour if their mass product M1*M2 is positive or negative.

So low-contrast initial clumps and bubbles get gravitationally amplified and form filaments and voids

[Chris Peterson]

My point is that they aren't "filaments" unless there's something binding them together. There is in the case of cosmological structure. There isn't in the case of stars or galaxies inside a cluster.

how many years must a filament exist before we call it a filament?

I think if a filament got to gravitationally form it's already is a filament if only for a moment.
Consider Milky Way's disk.
Galaxy orbital velocity is large, 200 km/s, compared with peculiar motion at 20 km/s.
The pressure oscillation of interstellar media arms-to-gaps waves is large, compared with clumps that trigger the filament formation.
Yet at a scale as small as 100 light years we can abstract off the galaxy orbital rotation and arms-to-gaps pressure waves and see the gravity/antigravity does it work. A cubic light year of positive or negative mass:

(mass of this cubic light year) minus (average mass of 1 cubic light year)

will gravitationally pull or repel its neighbour if their mass product M1*M2 is positive or negative.

So low-contrast initial clumps and bubbles get gravitationally amplified and form filaments and voids

Just pointing out that in conventional usage "filament" refers to dark matter filaments that form the background of the cosmological web, and at a smaller scale for lanes of dust and gas. So using that term for things light stars that happen to be in line because they formed from such structures is likely to be confusing.