Dancing in the dark: galactic properties trace spin swings along the cosmic web

Abstract

A large-scale hydrodynamical cosmological simulation, Horizon-AGN , is used to investigate the alignment between the spin of galaxies and the cosmic filaments above redshift 1.2. The analysis of more than 150 000 galaxies per time step in the redshift range 1.2 < z < 1.8 with morphological diversity shows that the spin of low-mass blue galaxies is preferentially aligned with their neighbouring filaments, while high-mass red galaxies tend to have a perpendicular spin. The reorientation of the spin of massive galaxies is provided by galaxy mergers, which are significant in their mass build-up. We find that the stellar mass transition from alignment to misalignment happens around 3 × 10 10 M ⊙ . Galaxies form in the vorticity-rich neighbourhood of filaments, and migrate towards the nodes of the cosmic web as they convert their orbital angular momentum into spin. The signature of this process can be traced to the properties of galaxies, as measured relative to the cosmic web. We argue that a strong source of feedback such as active galactic nuclei is mandatory to quench in situ star formation in massive galaxies and promote various morphologies. It allows mergers to play their key role by reducing post-merger gas inflows and, therefore, keeping spins misaligned with cosmic filaments.

The role of mergers and halo spin in shaping galaxy morphology

“…Despite all the inevitable uncertainties in the modelling of feedback, the latest generation of cosmological hydrodynamic simulations (e.g. Vogelsberger et al 2014a;Dubois et al 2014;Schaye et al 2015) has been able to produce galaxy populations displaying a 'morphological mix' that agrees reasonably well with observations.…”

The rise and fall of stellar across the peak of cosmic star formation history: effects of mergers versus diffuse stellar mass acquisition

“…We briefly describe the cosmological hydrodynamical simulation used for analysis in this paper, Horizon-AGN, which is already described in more details in Dubois et al (2014). The Horizon-AGN simulation is run with a ΛCDM cosmology with total matter density Ωm = 0.272, dark energy density ΩΛ = 0.728, amplitude of the matter power spectrum σ8 = 0.81, baryon density Ω b = 0.045, Hubble constant H0 = 70.4 km s −1 Mpc −1 , and ns = 0.967 compatible with the WMAP-7 data (Komatsu 2011).…”

“…finder (Aubert et al 2004) operating on the distribution of star particles with the same parameters than in Dubois et al (2014). Unless specified otherwise, only structures with a minimum of Nmin = 100 particles are considered, which typically selects objects with masses larger than 2×10 8 M⊙.…”

“…The upper panels focus on galaxies with a stellar mass comprised between 10 9.5 M⊙ and 10 10.5 M⊙, the lower panel on more massive galaxies with Ms > 10 10.5 M⊙. This mass threshold corresponds to the transition mass above which galaxies embedded in filaments decouple from their vorticity quadrant and display a spin perpendicular to their closest filament (see Dubois et al 2014), and also to the transition in gas accreted onto the galaxy between cold and hot mode (e.g. Dekel & Birnboim 2006;Ocvirk et al 2008).…”

Star formation sustained by gas accretion

“…Pichon et al (2011) carry out simulations where the disk angular momentum comes from filaments and increases over time, building up disks from the inside out. Codis et al (2012), Laigle et al (2013), and Dubois et al (2014) show from cosmological simulations that low-mass galaxies, which tend to be disklike, spin in a direction that is aligned to their accretion filaments, but high-mass spheroidal galaxies spin perpendicular to their filaments. The difference arises because the disks accrete gas directly from their filament and inherit its angular momentum, while the spheroids grow from the mixture of other galaxies coming in along the filament (i.e, grow from mergers).…”