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 3.6 mu m 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 3.6 mu m 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 3.6 mu m 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) km s(-1) Mpc(-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.