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

TL;DR

This paper identifies a distinct population of I-band AGB stars (IAGB) in the LMC, SMC, and NGC 4258 and demonstrates that their I-band luminosity functions are well described by single-peaked Gaussians. By photometrically selecting these stars and using geometric distances to calibrators, the authors derive absolute magnitudes for each galaxy: M_I(LMC) ≈ -4.49, M_I(SMC) ≈ -4.67, and M_I(NGC4258) ≈ -4.78, with a combined average zero point of M_I(IAGB) ≈ -4.64. The estimated statistical and systematic uncertainties are quantified for each calibrator, yielding an averaged zero point M_I(IAGB) = -4.644 with stat ±0.079 and sys ±0.028 mag. The work positions IAGB stars as a new standard candle for the extragalactic distance scale and sets up a larger application campaign to ~92 galaxies ( Paper II ), potentially offering a brighter, robust alternative to TRGB distances.

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.

Figures (6)

• Figure 1: I vs (V-I) Color-Magnitude diagram for the Large Magellanic Cloud. The newly identified AGB Plateau population of stars, discussed in this paper, is highlighted in black and yellow. Its flat luminosity function and unique color selection makes it an ideal candidate for being an extragalactic distance indicator.
• Figure 2: The central region around the mode of the smoothed IAGB luminosity function for the LMC (right-most panel: large, black dots, binned in 0.01 mag intervals) is well fit by the symmetric Gaussian shown in blue, having a dispersion of +/-0.20 mag around an apparent modal value of $I$ = 14.16 mag. Given the sample size of 4,204 IAGB stars (within two sigma of the mean), the error on the modal value is found to be +/-0.003 mag. The horizontal black line crossing the two panels, shows the mode of the IAGB. The horizontal yellow line below it marks the level of the TRGB for reference and comparison. One in ten stars are plotted in yellow so as to give a better sense of the density distribution along and across the RGB. All stars in the CMD redder than (V-I) = 2.5 mag are shown as red dots.
• Figure 3: Same as Figure 2 except that the upper panel shows the results for a color cut at (V-I) = 3.0 mag, and the lower panel shows a color cut at (V-I) = 2.0 mag. The mode of the IAGB is unchanged for the redder cut, while the mode for the bluer cut shifts fainter by only 0.04 mag. The blue horizontal line in the right half of the lower panel shows the modal value from Figure 2. The widths of the fitted Gaussian systematically change, with the redder cut being 0.18 mag and 0.26 mag for the bluer cut.
• Figure 4: Selection of IAGB stars in the Small Magellanic Cloud. The I-band CMD for stars in the SMC is shown in the left panel. The marginalized luminosity function of the stars with (V-I) $>$ 2.0 mag (black line) is shown in the right panel. The red luminosity functions are the result of changing the red color cutoff by +/-0.1 mag so as to give a visual indication of the sensitivity of the modal value to the adopted blue cutoff. The blue line is a Gaussian fit to the core of the black luminosity function having a sigma of 0.22 mag and a maximum occurring at an apparent magnitude of I = 14.37 +/- 0.22 mag. 916 IAGB stars are found within two sigma of the modal magnitude, resulting in an error on the mean of 0.25/$\sqrt(915)$ = 0.008 mag. The dashed blue line indicates the level of the TRGB discontinuity, emphasizing the brighter 0.5 mag advantage of the IAGB over the TRGB as a distance indicator.
• Figure 5: Detection and measurement of the IAGB population in NGC 4258 as marked by the circled red points in the left panel. Marginalized luminosity functions for the color-selected IAGB stars are shown in the right panel. The black line is the adopted curve fit by a Gaussian (the thick black line) centered at I = 24.65 mag, having a sigma of 0.24 mag. The red lines to either side of the adopted luminosity function are the result of shifting the blue cut-off by +/-0.1 mag, so as to illustrate the effect of that choice on the adopted modal value of the IAGB luminosity.