The Aquila comparison project: the effects of feedback and numerical methods on simulations of galaxy formation

Abstract

We compare the results of various cosmological gas-dynamical codes used to simulate the formation of a galaxy in the Λ cold dark matter structure formation paradigm. The various runs (13 in total) differ in their numerical hydrodynamical treatment [smoothed particle hydrodynamics (SPH), moving mesh and adaptive mesh refinement] but share the same initial conditions and adopt in each case their latest published model of gas cooling, star formation and feedback. Despite the common halo assembly history, we find large code-to-code variations in the stellar mass, size, morphology and gas content of the galaxy at z= 0, due mainly to the different implementations of star formation and feedback. Compared with observation, most codes tend to produce an overly massive galaxy, smaller and less gas rich than typical spirals, with a massive bulge and a declining rotation curve. A stellar disc is discernible in most simulations, although its prominence varies widely from code to code. There is a well-defined trend between the effects of feedback and the severity of the disagreement with observed spirals. In general, models that are more effective at limiting the baryonic mass of the galaxy come closer to matching observed galaxy scaling laws, but often to the detriment of the disc component. Although numerical convergence is not particularly good for any of the codes, our conclusions hold at two different numerical resolutions. Some differences can also be traced to the different numerical techniques; for example, more gas seems able to cool and become available for star formation in grid-based codes than in SPH. However, this effect is small compared to the variations induced by different feedback prescriptions. We conclude that state-of-the-art simulations cannot yet uniquely predict the properties of the baryonic component of a galaxy, even when the assembly history of its host halo is fully specified. Developing feedback algorithms that can effectively regulate the mass of a galaxy without hindering the formation of high angular momentum stellar discs remains a challenge

The role of mergers and halo spin in shaping galaxy morphology

“…In fact, throughout this paper κrot will represent a whole class of kinematic morphological parameters, most of which are based on the distribution of the 'orbital circularity' parameter . This parameter is usually defined as = jz/j(E), where j(E) is the maximum specific angular momentum possible for a star with specific binding energy E (Abadi et al 2003), or as V = jz/rvc(r), where vc(r) = GM (< r)/r is the circular velocity at the distance r (Scannapieco et al 2009(Scannapieco et al , 2012. Some examples of such circularity-based morphological parameters include the discto-total ratio, defined as the fraction of stars with sufficiently circular orbits, typically D/T = f ( > 0.7) ≈ f ( V > 0.8) (Aumer et al 2013;Marinacci, Pakmor & Springel 2014), or the bulge-tototal fraction, usually defined as B/T = 2 × f ( < 0) (but see Martig et al 2012;Zavala et al 2016, for an improved B/T measurement).…”

Four phases of angular-momentum buildup in high-z galaxies: from cosmic-web streams through an extended ring to disc and bulge

“…In particular, they properly trace the cosmological streams that feed galaxies at high redshift, including mergers and smooth flows, and they resolve the violent disc instability that governs the high-z disc evolution and plays an important role in bulge formation Mandelker et al 2014). The AMR code is superior to the common SPH codes for properly tracing some of the high-resolution hydrodynamical processes involved (e.g., Agertz et al 2007;Scannapieco et al 2012;Bauer & Springel 2012), and it seems to be comparable in its performance to new codes using a moving unstructured grid (Bauer & Springel 2012). However, like other simulations, the current simulations are not yet doing the most accurate possible job in treating the star formation and feedback processes.…”

Dusty star-forming galaxies at high redshift

“…A number of code comparison projects have been performed, and are currently underway (e.g. Scannapieco et al, 2012;Kim et al, 2014).…”