# Do we know two neighbor galaxies that are parallel?

Do we know two galaxies where the plane of their orbit is (almost) parallel and one is the neighbor galaxy of the other, or doesn't that exist?

Almost co-planar: +/- 5°

Neighbor galaxy: one galaxy is the closest galaxy of the other one, no matter of an absolute distance.

Galaxy: I'm looking for two big galaxies, like Milky Way and Andromeda. Not a small satellite galaxy.

Ideally something like this, where both objects are galaxies and their axes are co-linear:

• How do you define "almost parallel" (or co-planar)? What does constitute a "neighbour", how do you define it for this purpose? From which distance onward are they not neighbours anymore? Dec 11, 2023 at 15:05
• @planetmaker: I edited the question to address the issues. Thanks for asking. Dec 11, 2023 at 15:13
• well, the Milky Way in this universe and the Milky Way in a "parallel universe" for example (humor!)
– uhoh
Dec 12, 2023 at 14:18
• Two galaxies that close would be strongly interacting gravitationally, and would have pulled each other out of shape, Dec 12, 2023 at 20:19
• To limit the gravitational interaction, the diameters of the galaxies should be very small compared to their distance.
– Uwe
Dec 14, 2023 at 14:58

This is only half an answer, as I only address the likelyhood to find one such pair: If we define "neighbour" as "member of our local supercluster", there are investigations onto the relative alignment of the galaxies, e.g. Hu et al (2003): "Orientation of Galaxies in the Local Supercluster: A Review".

They looked at the local supercluster (LSC), thus the slightly wider galactic neighbourhood, of everything brighter than ~13mag.

From the abstract:

When the LSC is seen as a whole, galaxy planes tend to align perpendicular to the LSC plane with lenticulars showing the most pronounced tendency. Projections onto the LSC plane of the spin vectors of Virgo cluster member galaxies, and to some extent, those of the total LSC galaxies, tend to point to the Virgo cluster center. This tendency is more pronounced for lenticulars than for spirals. It is suggested that ’field’ galaxies, i.e., those which do not belong to groups with more than three members, may be better objects than other galaxies to probe the information at the early epoch of the LSC formation through the analysis of galaxy orientations. Field lenticulars show a pronounced anisotropic distribution of spin vectors in the sense that they lay their spin vectors parallel to the LSC plane while field spirals show an isotropic spin-vector distribution.

They looked carefully at the images of hundrets of galaxies, taking into account selection effects and biases due to them

Aryal and Saurer (2000) showed that the expected isotropic distribution curve is cosine for the polar angle θ and a straight line for the azimuthal angle φ only when there is no selection effect in (L, B), supergalactic position angle P A and inclination angle i.

The results with this taken into account look like: Thus using this neighbourhood definition, given the number of galaxies in each 10°-bin, there likely will be galaxies with the same orientation in our neighbourhood.

Whether they are immediately adjacent and not just interacting with each other by being member of the same (super)cluster... would need going through the data catalogue these authors (or others) compiled for these kind of works - however a quick search for such catalogue which lists these properties readily failed me, though.