Angular Momentum and Vortex Formation in Bose-Einstein-Condensed Cold Dark Matter Haloes

Abstract

(Abridged) Extensions of the standard model of particle physics predict very light bosons, ranging from about 10^{-5} eV for the QCD axion to 10^{-33} eV for ultra-light particles, which could be the cold dark matter (CDM) in the Universe. If so, their phase-space density must be high enough to form a Bose-Einstein condensate (BEC). The fluid-like nature of BEC-CDM dynamics differs from that of standard collisionless CDM (sCDM), so observations of galactic haloes may distinguish them. sCDM has problems with galaxy observations on small scales, which BEC-CDM may overcome for a large range of particle mass m and self-interaction strength g. For quantum-coherence on galactic scales of radius R and mass M, either the de-Broglie wavelength lambda_deB <~ R, requiring m >~ m_H \cong 10^{-25}(R/100 kpc)^{-1/2}(M/10^{12} M_solar)^{-1/2} eV, or else lambda_deB << R but self-interaction balances gravity, requiring m >> m_H and g >> g_H \cong 2 x 10^{-64} (R/100 kpc)(M/10^{12} M_solar)^{-1} eV cm^3. Here we study the largely-neglected effects of angular momentum. Spin parameters lambda \cong 0.05 are expected from tidal-torquing by large-scale structure, just as for sCDM. Since lab BECs develop quantum vortices if rotated rapidly enough, we ask if this angular momentum is sufficient to form vortices in BEC haloes, affecting their structure with potentially observable consequences. The minimum angular momentum for this, L_{QM} = $\hbar M/m$, requires m >= 9.5 m_H for lambda = 0.05, close to the particle mass required to influence structure on galactic scales. We study the equilibrium of self-gravitating, rotating BEC haloes which satisfy the Gross-Pitaevskii-Poisson equations, to calculate if and when vortices are energetically favoured. Vortices form as long as self-interaction is strong enough, which includes a large part of the range of m and g of interest for BEC-CDM haloes.