The formation and survival of discs in a ΛCDM universe

Abstract

We study the formation of galaxies in a Λ cold dark matter (ΛCDM) universe using high‐resolution hydrodynamical simulations with a multiphase treatment of gas, cooling and feedback, focusing on the formation of discs. Our simulations follow eight isolated haloes similar in mass to the Milky Way and extracted from a large cosmological simulation without restriction on spin parameter or merger history. This allows us to investigate how the final properties of the simulated galaxies correlate with the formation histories of their haloes. We find that, at z= 0, none of our galaxies contains a disc with more than 20 per cent of its total stellar mass. Four of the eight galaxies nevertheless have well‐formed disc components, three have dominant spheroids and very small discs, and one is a spheroidal galaxy with no disc at all. The z= 0 spheroids are made of old stars, while discs are younger and formed from the inside‐out. Neither the existence of a disc at z= 0 nor the final disc‐to‐total mass ratio seems to depend on the spin parameter of the halo. Discs are formed in haloes with spin parameters as low as 0.01 and as high as 0.05; galaxies with little or no disc component span the same range in spin parameter. Except for one of the simulated galaxies, all have significant discs at z≳ 2, regardless of their z= 0 morphologies. Major mergers and instabilities which arise when accreting cold gas is misaligned with the stellar disc trigger a transfer of mass from the discs to the spheroids. In some cases, discs are destroyed, while in others, they survive or reform. This suggests that the survival probability of discs depends on the particular formation history of each galaxy. A realistic ΛCDM model will clearly require weaker star formation at high redshift and later disc assembly than occurs in our models.

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).…”

Simulations of galaxy formation and evolution

“…In particular, the results discussed here correspond to the simulations presented in Scannapieco et al (2009) and extensively described in Scannapieco et al (2009Scannapieco et al ( , 2010Scannapieco et al ( , 2011. Additional papers describe the effects of baryons on the dark matter distributions (Tissera et al 2010) and the chemical patterns of simulated galaxies (Tissera, White & Scannapieco 2012).…”

“…For more details on the simulation code, we refer the reader to Scannapieco et al (2005Scannapieco et al ( , 2006. The code has been already applied to the study of Milky Way-mass galaxies (Scannapieco et al 2008(Scannapieco et al , 2009, as discussed in this paper, and to the study of dwarf galaxies (Sawala et al 2010(Sawala et al , 2011.…”

“…Navarro et al 1995;Navarro & Steinmetz 1997). More recent simulation codes including feedback processes, in particular from SN explosions, have shown to be successful in reproducing disk-like systems, suggesting that feedback processes are indeed a crucial ingredient in galaxy formation (e.g., Governato et al 2004Governato et al , 2007Okamoto et al 2005;Scannapieco et al 2008Scannapieco et al , 2009Ceverino & Klypin 2009;Stinson et al 2010). SN feedback, through the return of energy and mass to the interstellar medium, is efficient in regulating star formation, preventing overcooling and helping baryons to retain their angular momentum (e.g.…”

“…These simulations were first presented in Scannapieco et al (2009) and correspond to galaxies similar in mass to the Milky Way, and with a variety of formation and merger histories. This sample allows us to investigate the most relevant processes shaping the properties of galaxies as they evolve.…”