Gemini South Adaptive Optics Imager (GSAOI)
Measuring H0 Out to 60 Mpc Using Red Supergiants:
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Despite intense efforts using the Hubble Space Telescope (HST) over the past decade, there is still fierce debate about the value of the Hubble Constant, although most people now agree that it has a value between 55 and 80 km/s/Mpc. The HST Key Project team has presented the most significant body of work. They used Cepheid variables out to 20 Mpc as the calibrating method for a host of secondary distance indicators such as Type Ia supernovae, surface brightness fluctuations, and the Tully Fisher method. However, the small numbers of objects, difficulties in HST CCD calibrations, large extinction corrections, and the difficulty of combining surface brightness fluctuation distances to early-type galaxies with Cepheid distances to late-type galaxies have all complicated this work. Luminous red supergiants provide a new method of measuring the Hubble constant that is independent of the Cepheid distances. The red supergiant method offers high precision, ease of use, low vulnerability to extinction, and a single method that can be applied to the LMC, SMC, nearby galaxies, and other late-type galaxies out to 60 Mpc with GSAOI.
The brightest red supergiants pulsate with periods of 400 to 900 days and have MK ~ -11. Typical full K band amplitudes are ~ 0.25 mag. These luminous red supergiants lie on a K-logP relation with scatter about the relation of ~ 0.25 mag. The Large Magellanic Cloud has roughly 20 of these variable red supergiants so that a galaxy of this relatively small size can give a distance modulus accurate to ~ 0.05 mag. Larger spiral galaxies will yield larger numbers of red supergiants and hence a more accurate distance modulus. Imaging in the K band maximizes the contrast between the red supergiants and the galaxy background. Red supergiants will be found by their variability, which distinguishes them from bright star clusters. |