Forming Realistic Late-Type Spirals in a Λcdm Universe: The Eris Simulation

Abstract

Simulations of the formation of late-type spiral galaxies in a cold dark matter (ΛCDM) universe have traditionally failed to yield realistic candidates. Here we report a new cosmological N-body/smooth particle hydrodynamic simulation of extreme dynamic range in which a close analog of a Milky Way disk galaxy arises naturally. Named "Eris," the simulation follows the assembly of a galaxy halo of mass M vir = 7.9 × 10 11 M with a total of N = 18.6 million particles (gas + dark matter + stars) within the final virial radius, and a force resolution of 120 pc. It includes radiative cooling, heating from a cosmic UV field and supernova explosions (blastwave feedback), a star formation recipe based on a high gas density threshold (n SF = 5 atoms cm −3 rather than the canonical n SF = 0.1 atoms cm −3 ), and neglects any feedback from an active galactic nucleus. Artificial images are generated to correctly compare simulations with observations. At the present epoch, the simulated galaxy has an extended rotationally supported disk with a radial scale length R d = 2.5 kpc, a gently falling rotation curve with circular velocity at 2.2 disk scale lengths of V 2.2 = 214 km s −1 , an i-band bulge-to-disk ratio B/D = 0.35, and a baryonic mass fraction within the virial radius that is 30% below the cosmic value. The disk is thin, has a typical H I-to-stellar mass ratio, is forming stars in the region of the Σ SFR -Σ H i plane occupied by spiral galaxies, and falls on the photometric Tully-Fisher and the stellar-mass-halo-virial-mass relations. Hot (T > 3 × 10 5 K) X-ray luminous halo gas makes up only 26% of the universal baryon fraction and follows a "flattened" density profile ∝ r −1.13 out to r = 100 kpc. Eris appears then to be the first cosmological hydrodynamic simulation in which the galaxy structural properties, the mass budget in the various components, and the scaling relations between mass and luminosity are all consistent with a host of observational constraints. A twin simulation with a low star formation density threshold results in a galaxy with a more massive bulge and a much steeper rotation curve, as in previously published work. A high star formation threshold appears therefore key in obtaining realistic late-type galaxies, as it enables the development of an inhomogeneous interstellar medium where star formation and heating by supernovae occur in a clustered fashion. The resulting outflows at high redshifts reduce the baryonic content of galaxies and preferentially remove low-angular-momentum gas, decreasing the mass of the bulge component. Simulations of even higher resolution that follow the assembly of galaxies with different merger histories shall be used to verify our results.

Subhalo demographics in the Illustris simulation: effects of baryons and halo-to-halo variation

“…They showed that baryonic processes can suppress the abundance of small subhaloes, while simultaneously increasing the abundance of massive subhaloes compared to DM-only runs. Similar results have also been found with AREPO-simulated Milky Way zooms (Zhu et al 2016) and the Eris Simulation (Guedes et al 2011, Annalisa Pillepich, private communication) Previously, Weinberg et al (2008) had also noted the enhanced survival of subhaloes in hydrodynamical simulations, although they did not observe any suppression in halo occupation, most likely due to their low-mass resolution (mDM = 7.9 × 10 8 M ). Macciò et al (2006) had also found from a resimulation of a MW-mass halo that the number density of substructure is enhanced up to an order of magnitude in the inner regions of the halo.…”

Spiral- and bar-driven peculiar velocities in Milky Way-sized galaxy simulations

“…The above results show that in addition to the bar only model, B15, the barred-spiral N -body simulation, K14, is able to reproduce the observed peculiar LOS power spectrum very well, in contrast to the cosmological N -body simulation (Guedes et al 2011) investigated by Bovy et al (2015). This suggests that the observed peculiar motions can be explained by either a combination of a bar and transient, co-rotating spiral arms (Grand et al 2012b;Kawata et al 2014) or the bar component only as found by (Bovy et al 2015, B15).…”

Early formation of massive, compact, spheroidal galaxies with classical profiles by violent disc instability or mergers

“…Vela07 is one case of a disc-dominated galaxy, but even in this case, the spheroidal component has a classical, de-Vaucouleurs-like profile, n = 4.8 ± 1.1. Therefore, simulations with strong feedback have smaller spheroids (Guedes et al 2011), but their structural properties at high-z are similar to the weak-feedback runs.…”