Carnegie hubble program: A mid-infrared calibration of the Hubble constant

Using a mid-infrared calibration of the Cepheid distance scale based on recent observations at 3.6μm with the Spitzer Space Telescope, we have obtained a new, high-accuracy calibration of the Hubble constant. We have established the mid-IR zero point of the Leavitt law (the Cepheid period-luminosity relation) using time-averaged 3.6μm data for 10 high-metallicity, Milky Way Cepheids having independently measured trigonometric parallaxes. We have adopted the slope of the PL relation using time-averaged 3.6μm data for 80 long-period Large Magellanic Cloud (LMC) Cepheids falling in the period range 0.8 < log(P) < 1.8. We find a new reddening-corrected distance to the LMC of 18.477 ±0.033 (systematic) mag. We re-examine the systematic uncertainties in H ₀, also taking into account new data over the past decade. In combination with the new Spitzer calibration, the systematic uncertainty in H ₀ over that obtained by the Hubble Space Telescope Key Project has decreased by over a factor of three. Applying the Spitzer calibration to the Key Project sample, we find a value of H ₀ = 74.3 with a systematic uncertainty of ±2.1 (systematic) kms ^-1Mpc^-1, corresponding to a 2.8% systematic uncertainty in the Hubble constant. This result, in combination with WMAP7 measurements of the cosmic microwave background anisotropies and assuming a flat universe, yields a value of the equation of state for dark energy, w ₀ = -1.09 ±0.10. Alternatively, relaxing the constraints on flatness and the numbers of relativistic species, and combining our results with those of WMAP7, Type Ia supernovae and baryon acoustic oscillations yield w ₀ = -1.08 ±0.10 and a value of N _eff = 4.13 ±0.67, mildly consistent with the existence of a fourth neutrino species.