The similarity between the distributions of spins for galaxies (λ gal ) and for dark-matter haloes (λ halo ), indicated both by simulations and observations, is naively interpreted as a one-to-one correlation between the spins of a galaxy and its host halo. This is used to predict galaxy sizes in semi-analytic models via R e ≃ λ halo R vir , where R e is the half-mass radius of the galaxy and R vir is the halo radius. Utilizing two different suites of zoom-in cosmological simulations, we find that λ gal and the λ halo of its host halo are in fact only barely correlated, especially at z 1. A general smearing of this correlation is expected based on the different spin histories, where the more recently accreted baryons through streams gain and then lose significant angular momentum compared to the gradually accumulated dark matter. Expecting the spins of baryons and dark matter to be correlated at accretion into R vir , the null correlation at the end reflects an anti-correlation between λ gal /λ halo and λ halo , which can partly arise from mergers and a compact star-forming phase that many galaxies undergo. On the other hand, the halo and galaxy spin vectors tend to be aligned, with a median cos θ = 0.6-0.7 between galaxy and halo, consistent with instreaming within a preferred plane. The galaxy spin is better correlated with the spin of the inner halo, but this largely reflects the effect of the baryons on the halo. Following the null spin correlation, λ halo is not a useful proxy for R e . While our simulations reproduce a general relation of the sort R e = AR vir , in agreement with observational estimates, the relation becomes tighter with A = 0.02(c/10) −0.7 , where c is the halo concentration, which in turn introduces a dependence on mass and redshift.

We study the evolution of global shapes of galaxies using cosmological simulations. The shapes refer to the components of dark matter (DM), stars and gas at the stellar half-mass radius. Most galaxies undergo a characteristic compaction event into a blue nugget at z ∼ 2 − 4, which marks the transition from a DM-dominated central body to a self-gravitating baryonic core. We find that in the high-z, DM-dominated phase, the stellar and DM systems tend to be triaxial, preferentially prolate and mutually aligned. The elongation is supported by an anisotropic velocity dispersion that originates from the assembly of the galaxy along a dominant large-scale filament. We estimate that torques by the dominant halo are capable of inducing the elongation of the stellar system and its alignment with the halo. Then, in association with the transition to self-gravity, small-pericenter orbits puff up and the DM and stellar systems evolve into a more spherical and oblate configuration, aligned with the gas disc and associated with rotation. This transition typically occurs when the stellar mass is ∼ 10 9 M and the escape velocity in the core is ∼ 100 km s −1 , indicating that supernova feedback may be effective in keeping the core DM-dominated and the system prolate. The early elongated phase itself may be responsible for the compaction event, and the transition to the oblate phase may be associated with the subsequent quenching in the core.

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Photoelectron spectroscopy is a powerful method that provides insight into the quantum mechanical properties of a wide range of systems. The ionized electron wavefunction carries information on the structure of the bound orbital, the ionic potential as well as the photo-ionization dynamics itself. While photoelectron spectroscopy resolves the absolute amplitude of the wavefunction, retrieving the spectral phase information has been a long-standing challenge. Here, we transfer the electron phase retrieval problem into an optical one by measuring the time-reversed process of photoionization -photo-recombination -in attosecond pulse generation. We demonstrate all-optical interferometry of two independent phase-locked attosecond light sources.This measurement enables us to directly determine the phase shift associated with electron scattering in simple quantum systems such as helium and neon, over a large energy range. In addition, the strong-field nature of attosecond pulse generation resolves the dipole phase around the Cooper minimum in argon through a single scattering angle, along with phase signatures of multi-electron effects. Our study bears the prospect of probing complex orbital phases in molecular systems as well as electron correlations through resonances subject to strong laser fields. arXiv:1810.05021v1 [physics.atom-ph]

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