tokens: FR and SR and high-mass FR and SR are shown. 3 POPULATIONS OF FAST AND SLOW ROTATORS 3.1 Comparing to the ATLAS<sup>3D</sup> survey The first question we seek to answer is whether the Illustris simulation produces a representative sample of fast and slow rotators. The ATLAS 3D survey provides an excellent set of observational data to compare to Emsellem etal. ( 2011 , hereafter E11 ), examining the distribution of ellipticity and R for 260 ETGs in a volume- and magnitude-limited sample. We reproduce their results in the left-hand panel of Fig. 3 . Figure 3. Distribution of ETGs in the r plane, (a) reproduced from the ATLAS 3D survey ( E11 ) compared to (b) those created in the Illustris simulation, sampled based on the same selection criteria (see the text for more details). The magenta line shows the line on which edge-on elliptical galaxies with the same degree of rotational support would lie, as described in the text and re-derived in Appendix C . The green line follows equation ( 5 ), defining the cutoff below which galaxies are classified as SRs. Each galaxy is represented by a circle whose colour and size scale with the stellar mass. E11 selected ETGs based on whether they could clearly discern spiral arm structures in the galaxy. Taking into account the variation due to projection effects in observed galaxy properties, E11 suggested making a divide between FRs and SRs based on their and R properties using the curve above which galaxies are classified as FRs and below as SRs. Of their sample, 224 galaxies, or 862percent, were classified as FRs and the remaining 36, 142percent, as SRs, using equation( 5 ) to divide the population. They found FRs to be roughly oblate, with the ellipticity of a galaxy correlating linearly with its anisotropy, . In other words, the FRs are roughly rotationally supported with stars on mostly circular orbits. Thus, the motions of stars and the projected shape of the galaxy have a simple, linear correlation. SRs are instead galaxies with low degrees of rotational support, with many stars on radial or generally irregular orbits. Thus, we can identify galaxies whose shape is well represented by the assumption of rotational support, those with ellipticity () proportional to their degree of velocity anisotropy (). Cappellari etal. ( 2007 ) and E11 find an approximate expression for the variation of R with for galaxies with . We detail this expression in Appendix C , and follow their lead in showing the relationship for 0.7 as a magenta line in the left-hand panel of Fig. 3 . The E11 sample of galaxies is volume and magnitude limited to those galaxies within 42Mpc, brighter than mag, which are clearly visible from the William Herschel Telescope (declination 2935) and which are far from the Galactic plane to avoid contamination (galactic latitude b 15). Using the same criteria (arbitrarily setting the equatorial direction aligned with the positive z -axis and the galactic plane relative to that), we
ner_tags: O O O O O O O O O O O O O O O O O O O O O B-Survey B-Survey O O O O O O O O O O B-Model I-Model O O O O O O O O O O B-Survey I-Survey I-Survey O O O O O O O O O O B-Citation I-Citation I-Citation I-Citation O O B-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Survey I-Survey I-Survey O B-Citation O O O O O O O O B-Model I-Model O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-TextGarbage I-TextGarbage O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-TextGarbage O O O O O O O B-TextGarbage B-Citation I-Citation I-Citation I-Citation I-Citation O B-Citation O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O B-Citation O O O O O O O O O O O O O O O O O O O O O O B-Telescope I-Telescope I-Telescope B-CelestialObjectRegion I-CelestialObjectRegion O O O O O O B-CelestialObjectRegion I-CelestialObjectRegion O O O B-CelestialObjectRegion I-CelestialObjectRegion I-CelestialObjectRegion I-CelestialObjectRegion O O O O O O O O O O O O O O O O O B-CelestialObjectRegion I-CelestialObjectRegion O O O O