Stars and planets aren’t the only celestial bodies that may harbor satellites orbiting around them – galaxies may have them too. Certainly, in this case the conception of a satellite changes according to an adequate scale. If planets have moons, and stars have planets, then galaxies may carry their own little satellite galaxies. But what exactly is the likelihood to encounter these objects in a present-day Universe?

This idea served as a basis for the research conducted by Pablo Ruiz, Ignacio Trujillo and Esther Mármol-Queraltó, results of which were recently published at arXiv.org. The team decided to explore the satellite distribution around modern massive elliptical galaxies to determine the incidence of these objects in relation to their main physical characteristics, such as size/mass and the distance to the host galaxy.

The astronomers used the spectroscopic and photometric catalogs of the Sloan Digital Sky Survey (SDSS DR7) for this investigation. In total, approximately 1000 massive galaxies classified as ellipticals were selected as the study sample. The main inclusion criteria was the mass exceeding ~2×1011 solar masses.

Obviously, the following task was to define what kind of object could be considered as a satellite specific to any individual galaxy. The authors of the current study chose quite a simple although highly practical approach: they looked into the satellite and host galaxy mass ratio down to 1 to 400 (1:400), also exploring several intermediate steps. Another limiting factor was the projected radial distance to the host galaxy, which had to fall within the range of 100 kpc.

The results of the data analysis yielded some interesting results. It turns out, that the satellite galaxies are not so common after all, as one could imagine. When a mass ratio of 1:10 was considered, only 17-23% of all sample galaxies had at least one satellite. At a finer scale of 1:100, this proportion of satellite-carrying galaxies grew up to 40-52%. Such relatively low incidence of satellite galaxies could be probably explained by the fact that massive galaxies tend to ‘eat’ their smaller counterparts over long timescales.

Finally, at a mass ratio of 1:400, the likelihood for an elliptical galaxy to contain a satellite exceeded 55-70%. However, this also means that the mass of the satellite would be probably less than a few percent compared to the mass of the host galaxy. In fact, the team also estimated that the average fraction of mass which is contained in all the satellites down to a mass ratio of 1:400 represents as little as 7.4% of the total mass contained by their hosts.

Certainly, there are exceptions at the upper limit of the mass ratio: satellites with a mass ratio from 1:2 to 1:5 are the main contributors (~27%) to the total satellite mass. The authors concluded, that in case of the galaxy merger only the largest satellites could influence this process in a significant way. The scientists hope to refine this study in the future with aim to explain why their observations do not fully match the theoretical models, especially for the satellite galaxies with lower masses.

By Alius Noreika, Source: www.technology.org