Comparing galaxy morphology in hydrodynamical simulation and in semi-analytic model.

We compare galaxymorphology predicted by the Illustris hydrodynamical simulation and a semi-analytic model (SAM) grafted in the halo merger trees from the Illustris-Dark matter simulation. Morphology is classified according to the luminous profile and the kinematic bulge-to-total ratio for Illustris galaxy, and the bulge-to-total stellar massratio for SAM galaxy. For late-type galaxiesin the Illustris catalogue, most of their counterparts in the SAM model have the same type, and the consistency between two models is higher for lower mass galaxies. For early-type (ET) galaxiesin Illustris, the consistency is quite low for the counterparts except for most massive galaxies. By comparing in detail the growth histories of some matched galaxypairs of Milky Waymass in Illustris and the SAM model, we notice two aspects of differences in determining galaxymorphology between the two galaxyformation implementations. First, in the SAM, major merger and frequent minor mergers result in the growth of bulgesand turn the galaxyinto ET, while bulgeformation is not connected to mergers as tightly as in SAM for the Illustris galaxies. In addition, the satellite stellar masscan decrease significantly due to tidal stripping before merging into the central galaxyin Illustris, while it does not decrease in the SAM model. This results in less mergers with large ( stellar) mass ratiosin the Illustris simulation, and less effect of mergers on shaping galaxymorphology.