I-Band Asymptotic Giant Branch (IAGB) Stars: II. A First Estimate of their Precision and a Differential Zero Point

TL;DR

The paper assesses the precision and zero-point of using I-band AGB (IAGB) stars as distance indicators by performing a differential calibration against the TRGB using 92 HST-observed galaxies. The TRGB–IAGB offset is $-0.589 \pm 0.003$ mag with an inter-method scatter of $\pm 0.119$ mag, yielding an IAGB absolute magnitude of $M_I(IAGB) = -4.64 \pm 0.012$ mag when anchored to $M_I(TRGB) = -4.05$ mag, consistent with the independent anchor-based estimate of $M_I(IAGB) = -4.64 \pm 0.15$ mag. The analysis shows no detectable metallicity or star-formation history dependency within the current scatter and constrains nuisance contributions to about $\pm 0.12$ mag, suggesting that IAGB distances can be measured out to at least 9 Mpc with existing data. The study demonstrates the method's viability for calibrating the extragalactic distance scale and underscores the potential for future refinement with uniform, targeted observations and JWST capabilities. The appendix explores sub-sample effects when excluding disk-dominated galaxies, finding a modest shift in the mean offset but a larger dispersion, highlighting the importance of sample selection for precision.

Abstract

Hubble Space Telescope (HST) observations of 92 galaxies that have a strong showing of I-band Asymptotic Giant Branch (IAGB) stars in their color-magnitude diagrams (CMDs), are used to measure the relative offset between the mean apparent I-band magnitudes of the IAGB population and the corresponding apparent I-band magnitudes of the TRGB as measured in the same frames (and CMDs) of those individual galaxies. This first exploratory, large-sample comparison is independent of any extinction (foreground or internal) that may be shared by these two populations. The marginalized luminosity functions used to determine the modal value of the {\it IAGB } population are well fit by a single, symmetric Gaussian. The difference in the two apparent magnitudes (in the sense IAGB minus TRGB) is -0.589 mag, with a combined standard deviation of +/- 0.119 mag. Adopting M_I = -4.05 mag for the TRGB stars, the modal absolute magnitude of the IAGB is then calculated to be M_I(IAGB) = -4.64 +/- 0.12 mag. The ensemble dispersion quoted above gives a standard error on the mean of +/- 0.012 mag (based on the full sample of 92 galaxies). Independently, the three geometry-based zero points for I-band AGB stars are found (in Paper I) to be M_I = -4.49 +/- 0.003~mag in the LMC (4204 stars), M_I = -4.67 +/- 0.008 mag, for the SMC (916 stars) and M_I = -4.78 +/- 0.030 mag for NGC4258 (62 stars), leading to a global zero-point (weighted) average of <M_I> = -4.64 +/- 0.15 mag (stat). The scatter found in the anchors is comparable to the scatter in the field sample discussed here, but the calibration sample is small. The application of this method to galaxies well outside of the Local Group, shows that these standard candles can readily be found and measured out to at least 9 Mpc, using already available archival data

Figures (15)

• Figure 1: A differential comparison of the TRGB and IAGB distance scales. The apparent magnitude of the TRGB discontinuity is plotted on the horizontal axis. The apparent magnitude of the mode of the IAGB luminosity function is plotted on the vertical axis. The red dashed line has a slope of unity and has been fit to minimize the scatter of the data points around it. This minimization occurs at a value of -0.589 $\pm$ 0.003 mag (sigma on the mean) in the sense that the IAGB stars are brighter than the level of the TRGB. The measured inter-method scatter is $\pm$0.119 mag. The solid red lines are $\pm$ two sigma bounds. The residuals are plotted as a function of the IAGB apparent I-band magnitude the lower panel. The marginalized histogram of residuals is shown in the small inset found in the upper left corner of the main plot.
• Figure 2: Differences between the TRGB and IAGB true distance moduli as a function of the width of the IAGB luminosity function (expressed as a sigma). Broken lines are the one-sigma uncertainties on the statistically flat slope. The lack of a correlation with sigma suggests that star formation history over the age interval sampled by the IAGB stars is not perceptibly imprinted on the mode of the IAGB luminosity function.
• Figure 3: DDO Galaxies: I-Band AGB color-magnitude diagrams (left half of each panel) and I-band (F814W) luminosity functions (right half of each panel). Horizontal black lines in the left panel mark the mode of the luminosity function (center line), flanked by lines at $\pm$ one-sigma in the Gaussian fit to the smoothed luminosity function. A shorter, yellow-on-black line marks the measured level of the TRGB. Thick (smooth) black lines in the right panels are symmetric Gaussian fits to the peak and wings of the smoothed/main luminosity function, also shown in black. Blue and Red luminosity functions correspond to color cuts that are shown in the CMD (left panel) on either side of the adopted color cut (blue vertical line). Blue luminosity functions use the bluer color cut (larger numbers) and the red luminosity functions uses the redder color cut (smaller numbers). See text for additional details.
• Figure 4: IC galaxies (cont.) See Figure 3 for description.
• Figure 5: UGC Galaxies. See Figure 3 for description.