Uncertainties and Cosmological Constraints from the Megamaser Cosmology Project

This paper investigates sources of uncertainty within the Megamaser Cosmology Project (MCP) as well as cosmological implications of the results as of 2018. The contribution of uncertainty in the value of the matter energy density parameter, Omega_m, towards total uncertainty on the MCP H_0 calculation is found to be negligible (<2%) for galaxies at the redshifts currently used by the project. However, if future projects use angular-diameter distances at further redshifts to calculate H_0, the current Planck uncertainty in Omega_m would contribute significantly: it would provide ~30% of the uncertainty of a 3% H_0 measurement at a redshift of 1. Since peculiar velocity (V_P) uncertainties are another source of error in the H_0 measurement, this study also compares V_P distributions between different types of galaxies in the Illustris simulation. Galaxies identified through their black hole mass range as potential maser hosts show no significant difference in their peculiar velocity distribution than galaxies in different mass ranges. Disk galaxies also do not have a different V_P distribution than ellipticals in the samples tested. The V_P distributions are all approximately Gaussian with standard deviations of ~200 km/s. Lastly, the current MCP value of H_0 = 69.3 +\- 4.2 km/s/Mpc is applied as an external constraint to Cosmic Microwave Background data from the WMAP and Planck probes for open LambdaCDM and flat wCDM models. The joint constraints on the Planck data in flat wCDM, for example, are H_0 = 71.3 ^{+4.1}_{-3.9} km/s/Mpc and w = -1.11 ^{+0.14}_{-0.13} (68% confidence intervals). MCP results currently have larger uncertainties than both the Supernova/Cepheid variable and lensing time-delay methods, and the MCP is consistent with these studies as well as both the Planck and WMAP results.