Free Access
Volume 547, November 2012
Article Number A63
Number of page(s) 16
Section Galactic structure, stellar clusters and populations
Published online 30 October 2012

Online material

Appendix A: Additional galaxy properties

Here we describe some properties of the galaxies that are not directly discussed in the main paper body but are still useful as supplementary material. We begin with some general comments on particular features of each galaxy that are of interest.

  • Castor is the only galaxy in the sample to exhibit a bar, perhaps reflecting the greater resolution or the more isotropic nature of its group (compared with, for example, the filamentary structure of the Apollo group). It also presents the clearest example of spiral structure of the entire sample. The spiral structure presents challenges when quantifying the stellar scale length as the arms present a bump in the density profile. The young stars present in the arms mean that this is even more pronounced when measuring the brightness profile. Castor also has the most pronounced (and abrupt) disk warp, initially this was believed to be evidence of poor resolution at the disk edge but investigation has revealed no particularly favoured alignment of the disk warps in this sample. Analysis of Castor has been conducted on a snapshot slightly before z = 0 due to the irregular shape induced by a late merger in this snapshot.

  • Artemis is unusual in that it has a relatively massive dark matter halo (7.45×1011 M), reasonably low spheroid-to-disk fraction (0.32) and flat metallicity profile (–0.0068 dex/kpc), yet its disk scale length is only 1.87 kpc and is truncated at a radius of 7 kpc. There exists a gaseous polar disk (not dense enough to form stars) and yet the vertical velocity dispersion changes very little as a function of age. This suggests that the last major merger experienced by Artemis left star-forming gas with a similar velocity dispersion to the older stars, an effect not seen in the other galaxies.

  • Leto is the least massive galaxy within the Apollo group and the most spheroid dominated of the galaxies. This spheroid is not composed of older stars as in the other galaxies, there is a significant fraction of the spheroid stars that are young. This is the result of a low star formation rate at early times compounded by a recent, disruptive event that is evident in the velocity dispersion-age relation.

  • Eos undergoes a merger at late times that leaves it with an extremely irregular morphology at the last time step, analysis of this galaxy is performed on a snapshot prior to this event.

  • Helios is the most early type galaxy of all. Despite its great mass it is the reddest galaxy and has young stars with around twice the vertical velocity dispersion of much of the rest of the sample and a prolonged early star formation episode, this contrasts with the lack of an identifiable late merger to result in such a morphology.

  • Selene has few mergers in its history and is one of the most quiescent galaxies, forming the largest disk fraction of all the galaxies and presenting definite spiral structure.

  • Oceanus has the greatest stellar mass in the sample (though it is among others with comparable halo masses) and has a rotating gaseous disk that extends as far as 40 kpc from the centre. This disk is dense enough to form stars and hence this galaxy has an extremely long scale length (6.63 kpc) and one of the flattest metallicity gradients.

One of the key ways of understanding the formation of galaxies is by examining the star formation history of the different components. The distribution of star formation in time tells us a great deal about how the different components of a galaxy form. In the course of this work the signature of mergers were found to be identifiable in the bursts of star formation seen in Fig. A.1. These bursts can in some cases be associated with steps in the velocity dispersion described in Sect. 3.4, however we find that the magnitude of the star formation bursts is a poor indicator of the strength of the kinematic disturbance induced by the merger from which they both originate. The dichotomy of the star formation burst and the kinematic effects of mergers is yet more evidence that the signature of a merger depends on the gas fraction or phase space configuration of the merging bodies. Note the restrained recent star formation of the disrupted galaxy Artemis compared with the more disk dominated Apollo or Oceanus.

For the analysis of the simulated disk galaxies presented in this work to be as uniform as possible we performed a kinematic decomposition to examine the disk stars with minimal contamination from halo and bulge stars. The decomposition employed follows the Abadi et al. (2003) method of using the angular momentum of stars compared with the angular momentum expected for rotating stars. The distribution of the relative angular momentum is shown in Fig. A.2 with blue and red lines highlighting the distribution on disk and spheroid stars respectively.

thumbnail Fig. A.1

Star formation histories of the sample galaxies. The star formation rate of all stars within the virial radius at z = 0 are shown by the solid line, the dashed line is all stars tagged as disk stars at z = 0.

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thumbnail Fig. A.2

Jz / Jcirc distribution of stars within the virial radius (black). Blue and red lines shows the distribution of the disk and spheroid components respectively. Note the existence of a third intermediate component included in the distribution that is associated with the disk in some of the galaxies.

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© ESO, 2012

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