I-Band Asymptotic Giant Branch (IAGB) Stars: I. Exploring a New Standard Candle for the Extragalactic Distance Scale

Abstract

In the I-band color-magnitude diagrams (CMD) of resolved nearby galaxies, the reddest asymptotic giant branch (AGB) stars form a previously unremarked-upon, but nevertheless distinct and easily-identified population of high-luminosity stars. Hereafter we refer to this population as being comprised of I-Band AGB (IAGB) stars. Identifying these stars in the Large Magellanic Cloud (LMC), the Small Magellanic Cloud (SMC) and in NGC4258 (for all three of which there are published geometric distances) we find that the marginalized luminosity functions are each well approximated by single-peaked Gaussians, having one-sigma dispersions of +/- 0.22 mag, +/- 0.25 mag and +/- 0.24 mag, respectively. The zero points for the modal I-band absolute magnitudes of IAGB stars are found to be M_I = -4.49 +/- 0.003 mag (stat) in the LMC (4204 stars), M_I = -4.67 +/- 0.008 mag (stat), for the SMC sample (916 stars), and M_I = -4.78 +/- 0.030 mag (stat) for NGC4258 (62 stars). A global average over these three independent calibrations of the IAGB zero point (weighted inversely by squares of their systematic errors) gives <M_I> = -4.65 +/- 0.119 mag (stat) +/- 0.025 (sys). In Paper II we will show the results of applying the IAGB Method to 92 galaxies additional galaxies resolved by HST, reaching out to distances just short of 10 Mpc.