The gravitational interactions of neighbours is indeed thought to have a significant influence on a galaxy’s morphology. However, galaxies move relative to one another, and it is the short-lived close-approaches which have the largest effect, as I’m sure you can appreciate. Motions of neighbours must therefore be included, although this is fairly simple in your framework, although to reliably simulate the effect the positions and motions of the neighbours must be realistic – not just random.

There are various other environmental effects, including mergers, gas-stripping and shocking, the gravitational interaction with the cluster’s own dark matter halo, and the changing distribution of galaxy masses with environment, which could all be important in explaining the observed trends. I’ll give more of a description of these mechanisms in a future post.

Finally, accurately simulating the evolution of just one galaxy, whether isolated or in a group, is currently well beyond our capabilities. The complex interplay between environment, galaxies, gas, star formation and active galactic nuclei, spans such a wide range of scales, easily a factor of a million, that the processes can not be followed consistently. Simplified models can, however, give us an understanding of some of the processes involved, and we can constrain the roles of some mechanisms by comparing with observations.

i’m a beginner to astronomy too, but might add my thoughts to the topic. please ignore, if it’s too stupid! (also i apologize for the bad english)

i agree with jim – the obvious difference between the two environments (dense / lesser dense regions of space) is the gravitational layout:

within an “isolated” galaxy the main sources of gravity seem to be the features of this very galaxy:
- the central bulge, usually with its big black hole in the center, binds the stars to the center
- the masses of the single stars influence their imediate neighborhood – but “level out” themselfs over the galaxy as a whole
- due to rotation of the system (=galaxy) we get this forces to form a disc of arms.

why? well, if i remember correctly from school physics, the gravitational (pulling) force is proportional to 1/r^2, meaning it reduces by the distance squared. (radius 1 ~ gravity 1, r=2 ~ g=.25, r=10 ~ g=.001 etc.) plainly spoken: you feel the gravity of nearby objects overproportionally strong. so
- local clusterings within a galaxy will attract more nearby stars to “gather together”
- the massive center will bind those to itself
(other organizing features, like star formation, gas clouds, novae etc. i will ignore, because this reply is about gravity)
(i skip the issue of dark matter here, but get to it later)

what happens, if we add some galaxies around?
well – distant galaxies have an influence on the galaxy as a whole, but either
- just “a slight pull” (remember the 1/r^2) to this or that direction, if the galaxies summed gravity is stronger in that direction, or
- a general “fluff up” of the galaxy, if the summed gravity is evenly distributed
but the influence will increase if the surrounding galaxies are closer. and especially the seemingly small irregularities in the “gravitational matrix/layout” will get greater importance. (same reason: 1/r^2, but this time r gets smaller -> g gets overproportionally larger)

what you could do is creating a small mathematical simulation of this gravity field/matrix/layout/how-ever-we-call-it:
– a galaxy consists of ~ 10^7..10^13 solar masses, radius ~ 10^2..10^6ly (both roughly values)
– lets take a medium one, say 10^10 s.mass by 10^4ly radius
– form a matrix/grid of mesuring points within 3D, doesn’t have to be that fine – 100×100x100 points should be enough for a start
– implement a rotation algorithm in one plane as a simple shifting of the values to the next grid in the direction of rotation (speed ~ 225km/s), stepping 10^1..10^3 y
– fill grid with the distribution of stars (populate the x/y plane and lets say planes 49..51 of the z axis), pre-form central bulge (of course in the z-axis, too) and some spiral arms
– calculate the gravity influences in all 3 directions for each rotation step and each of the grid points (skipping empty cells)
– transform the matrix by the resulting movement vector (circulation and gravitational pull)
– for otical study you can color code the points regarding the z part (like green: local gravity dominates, no z pull / red: z pull upwards / blue: z pull downwards) but thats just gimmicks – you only need the numbers
- then add a wider grid for the surrounding galaxies (lets say also 100×100x100 points over a larger space), where each galaxy is a single grid point
- implement a random fill alorithm for the second grid (within rational parameters of course)
- let the galaxy rotate for a while (some hundred million years)
- determine, if the z space populates (”elliptical”) or not (and if the general structure remains intact) – “spiral”
- save the outcome together with the neighborhood scenario
- repeat for different random neighborhood scenarios
you should come up with something like “a lot of near galaxies -> more likely elliptical”

to finetune the system you then can add some “self made dark matter”
- as a means to preserve the general structure (like an offset for the central galaxy’s gravity field)
and
- to match the systems outcome with reality:

if the simulation suggests, that the influence of the neighboring galaxies “kicks in” too late (=close), extend the offset field. this should “feel” the other galaxies gravitational pull sooner (as dark matter is influenced by gravity) and by deforming should transmit the effects to the central galaxy.

or may be you (or someone else) has already done that…

Very, very briefly: all these things can be measured, although it can be difficult!

You can find lots of information about galaxy astronomy at http://cas.sdss.org/dr6/en/astro/ and about the SDSS survey in particular at http://cas.sdss.org/dr6/en/sdss/

In fact, those links are so useful, I’ll add them to the list on the blog menu.

Question: The photos of the elliptical looks very much the same, fuzzy balls. How are sizes, distances, or number of stars involved in these galaxies be determined, or can they not be?