Investigating Anisotropies in the Local Universe with Type Ia Supernovae from the Nearby Supernova Factory

Abstract

The dipole anisotropy of the cosmic microwave background temperature is two orders of magnitude larger than its other multipole components, which is attributed to the Doppler shift due to the motion of our Local Group of galaxies with a velocity of 627±22 km s^-1. While velocities of this amplitude are possible within the standard model of cosmology, ΛDM, gravitational attraction by large mass concentrations is required to produce them. Even though the peculiar velocity field has been studied extensively, the source of this motion has still not been identified with certainty. If no sufficiently large mass concentrations can be found that cause the motion, it may stem from a fundamental anisotropy in universe, which would contradict the cosmological principle. In this context, one must investigate the "dark flow", i.e. the reported coherent motion of galaxy clusters at<br /> ∼ 1000 km s^-1 on distance scales, at which their velocities are expected to average out.<br /> This work presents an analysis of large-scale anisotropy using 279 type Ia supernovae (SNe Ia) at low-redshifts (0.015 < z < 0.1) from the Nearby Supernova Factory and the Union2 compilation of literature SNe. The Supernova Factory dataset, in particular, more than doubles the SN sample at the distance of the Shapley supercluster (z ∼ 0.046), the largest known mass concentration in the local volume up to z ∼ 0.06. The higher-redshift SNe in Union2 are also studied, though their constraints are weak due to the large uncertainties for each individual velocity measurement.<br /> The SN Ia dataset was analysed by determining the bulk flow velocity, i.e. the coherent dipole motion, for SNe binned in redshift shells. Furthermore a method of spatially smoothing the Hubble residuals was used to verify the bulk flow results. Additionally, a simple model for the infall towards a potential attractor (e.g. Shapley) was studied to constrain the mass responsible for the coherent peculiar velocities.<br /> The measured bulk flow is found to be compatible with the direction of the Local Group motion up to z ∼ 0.06, showing no evidence for a reversal of the dipole direction behind Shapley that would indicate a backside infall into it. At redshifts 0.06 < z < 0.1, the dipole in the dark flow direction is constrained to < 250 km s^-1 (68% confidence level), ruling out the reported large-amplitude flow. Mass estimates from SN data for an attractor in the proximity of Shapley are found to exceed those of independent studies. Instead, the data favour an additional attractor at greater distance, likely near the Sloan Great Wall.<br /> As a final step, the prospects for future surveys, e.g. the Zwicky Transient Facility (ZTF), are simulated. The bulk flow model is expanded by a shear (quadrupole) term that is shown to constrain the location of the main attractor in a less model-dependent way than using a matter overdensity as attractor. The simulation shows that ∼ 1000 additional SNe will be required to measure the shear due to Shapley and the Sloan Great Wall at a significance < 2σ. This number of SNe will become accessible with ZTF.