The Carnegie-Chicago Hubble Program. IX. Calibration of the Tip of the Red Giant Branch Method in the Mega-Maser Host Galaxy, NGC4258 (M106)

TL;DR

This paper calibrates the Tip of the Red Giant Branch (TRGB) luminosity in NGC 4258 using HST data and the galaxy's geometric maser distance, refining stellar distance indicators with high precision.

Contribution

It provides a new, precise calibration of the TRGB luminosity in NGC 4258, anchored by maser-based distance measurements, and compares it with previous calibrations.

Findings

TRGB apparent magnitude measured as F814W_0 = 25.347 ± 0.014 (stat) ± 0.042 (sys) mag.

Absolute TRGB luminosity calibrated as M_F814W^TRGB = -4.050 ± 0.028 (stat) ± 0.048 (sys) mag.

Calibration agrees within 1% with previous LMC-based TRGB calibrations.

Abstract

In the nearby galaxy NGC 4258, the well-modeled orbital motion of H$_2$O masers about its supermassive black hole provides the means to measure a precise geometric distance. As a result, NGC 4258 is one of a few "geometric anchors" available to calibrate the true luminosities of stellar distance indicators such as the Tip of the Red Giant Branch (TRGB) or the Cepheid Leavitt law. In this paper, we present a detailed study of the apparent magnitude of the TRGB within NGC 4258 using publicly-available HST observations optimally situated in the gas- and dust-free halo along the minor axis, spanning distances ranging from 8 to 22 kpc in projected galactocentric radius. We undertake a systematic evaluation of the uncertainties associated with measuring the TRGB in this galaxy, based on an analysis of 54 arcmin$^2$ of HST/ACS imaging. After quantifying these uncertainties, we measure the TRGB in NGC 4258 to be F814W$_0$ = 25.347 $\pm$ 0.014(stat) $\pm$ 0.042(sys) mag. Combined with a recent 1.5% megamaser distance to NGC 4258, we determine the absolute luminosity of the TRGB to be $M_{F814W}^{TRGB}$ = -4.050 $\pm$ 0.028(stat) $\pm$ 0.048(sys) mag. This new calibration agrees to better than 1% with an independent calibration presented in Freedman et al. (2019, 2020) that was based on detached eclipsing binaries (DEBs) located in the LMC.