The Tip of the Red Giant Branch Distances to Type Ia Supernova Host Galaxies and the Hubble Constant

Abstract

Measuring the value of the Hubble constant is a fundamental step in extragalactic astronomy and cosmology. A tremendous work to determine the value of the Hubble constant has been carried out over the past decades. However, the estimated values have been controversial until recently. Type Ia Supernovae (SNe Ia) are one of the most powerful indicators for distant galaxies in the cosmic expansion dominated field (d ≳ 100 Mpc). The value of the Hubble constant can be obtained from the absolute calibration of SNe Ia. Previous absolute calibrations of SNe Ia were carried out using Cepheids, a population I distance indicator. However, recent values of H0 show a significant discrepancy with the values from the Cosmic Microwave Background Radiation (CMBR) analysis. It is called "the Hubble tension", being an issue in the modern cosmology. The Tip of the Red Giant Branch (TRGB) is a precise and population II distance indicator for resolved stellar systems. It has several advantages over the Cepheids. An absolute calibration of SNe Ia with the TRGB will be useful for more accurate calibration of SNe Ia and the Hubble constant. We present the results of an absolute calibration of SNe Ia based on the TRGB. In order to derive an accurate calibration of SNe Ia, we refine the TRGB calibration. A modified TRGB calibration corrected for the color (metallicity) dependence of the TRGB, the QT magnitude, is introduced for better detection of the TRGB. We determine the color-magnitude relation of the TRGB from photometry of deep images of HST/ACS fields around eight nearby galaxies. The zero-point of the TRGB at the fiducial metallicity ([Fe/H] = -1.6, (V-I)0,TRGB = 1.5)) is obtained from photometry of two distance anchors, NGC 4258 (M106) and the LMC, to which precise geometric distances are known: MI,TRGB = -4.121 ± 0.067 mag from NGC 4258 and MI,TRGB = -4.033 ± 0:063 mag from the LMC. A weighted mean of the two zero-points is MI,TRGB = -4.074 ± 0.046 mag. The quoted uncertainty is 2 ~ 3 times smaller than those of the previous calibrations. With the revised TRGB calibration, we determine the distances to eight SN Ia host galaxies: M101 hosting SN 2011fe, M66 hosting SN 1989B, M96 hosting SN 1998bu, NGC 4038/39 hosting SN 2007sr, NGC 5584 hosting SN 2007af, NGC 3021 hosting SN 1995al, NGC 3370 hosting SN 1994ae, and NGC 1309 hosting SN 2002fk. Luminosity functions of red giant stars in the outer regions of these galaxies show the TRGB to range from I ~ QT = 25.083 ± 0.034 (M101) to 28.398 ± 0.048 (NGC 1309) mag. The TRGB distances to these galaxies are estimated to range from (m-M)0 = 29.168 ± 0.034 (M101) to 32.475 ± 0.048 (NGC 1309). By combining the TRGB distance estimates to SN Ia host galaxies derived from this study with the SN Ia calibration provided by Riess et al. (2011), we obtain a value of the Hubble constant: H0 = 71.15 ± 1.80(random) ± 1.51(systematic) km/s/Mpc (a 3.3% uncertainty including systematics) from all eight SNe, and H0 = 72.95 ± 2.03(random) ± 1.55(systematic) km/s/Mpc (a 3.5% uncertainty including systematics) from six low-reddened SNe. We obtain our best estimate, H0 = 70.45 ± 1.67(random) ± 1.65(systematic) km/s/Mpc (a 3.2% uncertainty including systematics), from the combination of the absolute magnitude of six lowreddened SNe with the recent SN Ia calibration given by Riess et al. (2016). This value is between those from Cepheid calibrated SNe Ia and those from CMB analysis.