Simulating disc galaxy bulges that are consistent with observed scaling relations

ABSTRACT We present a detailed comparison between the photometric properties of the bulges oftwo simulated galaxies and those of a uniform sample of observed galaxies. This anal-ysis shows that the simulated galaxies have bulges with realistic surface brightnessesfor their sizes and magnitude. These two field disc galaxies have rotational veloci-ties ∼ 100 km/s and were integrated to a redshift of zero in a fully cosmological Λcold dark matter context as part of high-resolution smoothed particle hydrodynamicsimulations. We performed bulge-disc decompositions of the galaxies using artificialobservations, in order to conduct a fair comparison to observations. We also dynami-cally decomposed the galaxies and compared the star formation histories of the bulgesto those of the entire galaxies. These star formation histories showed that the bulgeswere primarily formed before z= 1 and during periods of rapid star formation. Bothgalaxies have large amounts of early star formation, which is likely related to the rel-atively high bulge-to-disc ratios also measured for them. Unlike almost all previouscosmological simulations, the realistically concentrated bulges of these galaxies do notlead to unphysically high rotational velocities, causing them to naturally lie along theobserved Tully–Fisher relation.Keywords: methods: numerical, galaxies: bulges, galaxies: formation, galaxies: spi-ral, galaxies: structure