Young stellar distance indicators and the extragalactic distance scale

Abstract

The extragalactic distance scale is perhaps the most important application of stellar distance indicators. Among these, classical Cepheids are high-accuracy standard candles that support a 1.4% measurement of Hubble's constant, H 0 . The accuracy of Cepheid distances is thus directly relevant for understanding the implications of the Hubble tension, the > 5σ discord among direct, late-Universe H 0 measurements and H 0 values inferred from the early Universe observations assuming ΛCDM cosmology. This invited review aims to provide an accessible overview of the state of the art distance ladder that has established the Hubble tension, with a focus on Cepheids, their absolute calibration using trigonometric parallaxes from the ESA mission Gaia, and other Cepheid-related systematics. New observational facilities such as JWST and upcoming large surveys will provide exciting avenues to further improve distance estimates based on Cepheids.

Progress in direct measurements of the Hubble constant

“…Unfortunately, this unsupervised method in its current form, is susceptible to mistaking the asymptotic giant branch (AGB) for the RGB (e.g., NGC 4038), a problem that is known and has been addressed by many authors previously in the literature [55][56][57]. For NGC 4038 and NGC 4536, the TRGB distances that their unsupervised algorithm gives the highest weight to (having higher contrast values) are significantly closer than the published SHoES Cepheid distance measurements [9] by 0.7 and 0.6 mag, that is 30% and 40% offsets in distance, respectively, and ultimately contribute a higher H 0 value than other measurements based on the TRGB [10][11][12][13]. These results also differ appreciably from the excellent agreement between the published Cepheid distances in Riess et al [9] and TRGB JCAP11(2023)050 distances in Freedman et al [27], which in the mean, agree to 0.007 mag.…”

“…The SHoES program [9,13] has the goal of using HST/ACS and HST/WFC3 to extend and improve the Cepheid calibration of SNe Ia for a measurement of H 0 . Most recently, they have obtained NIR observations of Cepheids in 42 SN Ia host galaxies (figure 2) with the aim of reducing the systematic uncertainties due to reddening and metallicity.…”

“…Two additional decades of effort with HST, Spitzer, and many additional ground-based telescopes, subsequently improved the measurements of H 0 , with estimated accuracies currently falling in the 2-5% range [6]. The Cepheid calibration of H 0 [7][8][9] continues to yield values of H 0 ∼ 73 or 74 km s −1 Mpc −1 , whereas measurements using the tip of the red giant branch (TRGB) [10][11][12][13] yield slightly lower values, closer to 70-72 km s −1 Mpc −1 . Recent estimates of H 0 from CMB measurements have extremely high precision, with results from the Planck satellite [14] yielding H 0 = 67.4 ± 0.5 km s −1 Mpc −1 (better than 1%).…”