The Parallax of ω Centauri Measured from Gaia EDR3 and a Direct, Geometric Calibration of the Tip of the Red Giant Branch and the Hubble Constant

Abstract

We use data from the ESA Gaia mission Early Data Release 3 (EDR3) to measure the trigonometric parallax of ω Cen, the first high-precision parallax measurement for the most massive globular cluster in the Milky Way. We use a combination of positional and high-quality proper motion data from EDR3 to identify over 100,000 cluster members, of which 67,000 are in the magnitude and color range where EDR3 parallaxes are best calibrated. We find the estimated parallax to be robust, demonstrating good control of systematics within the color–magnitude diagram of the cluster. We find a parallax for the cluster of 0.191 ± 0.001 (statistical) ±0.004 (systematic) mas (2.2% total uncertainty) corresponding to a distance of 5.24 ± 0.11 kpc. The parallax of ω Cen provides a unique opportunity to directly and geometrically calibrate the luminosity of the tip of the red giant branch (TRGB) because it is the only cluster with sufficient mass to provide enough red giant stars, more than 100 one magnitude below the tip, for a precise, model-free measurement of the tip. Combined with the preexisting and most widely used measurements of the tip and foreground Milky Way extinction, we find M I,TRGB =−3.97 ± 0.06 mag for the I-band luminosity of the blue edge. Using the TRGB luminosity calibrated from the Gaia EDR3 parallax of ω Cen to calibrate the luminosity of Type Ia supernovae results in a value for the Hubble constant of H 0 = 72.1 ± 2.0 km s−1 Mpc−1. We make the data for the stars in ω Cen available electronically and encourage independent analyses of the results presented here.

Detecting globular cluster tidal extensions with Bayesian inference – I. Analysis of ω Centauri with Gaia EDR3

“…For these three objects, we calculate an time-averaged dereddened magnitude of 0 = 14.2±0.02. Assuming the absolute magnitude of RRL (type AB) to be = 0.64 ± 0.25 , we calculate the distance to these stars to be 5.2 ± 0.6 kpc, which is entirely consistent with the measured distance of Cen (Harris 1996;Soltis et al 2021). The small number of candidate RRL stars that we have found coincident with the Cen tidal debris is in agreement with the earlier work of Fernández-Trincado et al (2015), who failed to find any strong RRL candidates in a search area of 50 sq.…”

“…We began by retrieving all stars within a five degree radius of Cen from the Gaia archive 1 using Table Access Protocol (TAP) facilities. With our adopted distance to the cluster of 5.2 kpc (Harris 1996;Soltis et al 2021), this corresponds to a physical radius of ∼450 pc. We first applied the several adjustments to the data as suggested by the EDR3 documentation.…”

A buyer’s guide to the Hubble constant

“…In a followup paper, Madore and Freedman (2020) present an upgraded methodology in which stellar photometry is itself used to separate metallicity and extinction effects (by exploiting their different sensitivities in J, H and K-band magnitudes), confirming their earlier results. Other results (Soltis et al 2021;Reid et al 2019;Capozzi and Raffelt 2020;Cerny et al 2021-see for example Table 3 in Blakeslee et al 2021) cluster evenly around these two values. We also note the difference between SN Ia zero points (Fig.…”

“…They are also abundant in the solar neighbourhood, meaning great numbers are available for calibration by parallax (by contrast, Cepheids with good parallaxes in Gaia DR3 will number in the hundreds). Sufficient numbers to resolve the tip can be seen in globular clusters such as x Centuari, where well-formed colour-magnitude diagrams can resolve metallicity and extinction effects, and an absolute magnitude calibration can be made either from Gaia DR3 parallaxes (Soltis et al 2021) or DEB distances (Cerny et al 2021).…”

Can Hubble Tension Be Eased by Invoking a Finite Range for Gravity?

“…These methods involve calibration of luminosities of standard candles such as Cepheid variables, type Ia supernovae, tip of red giant branch (TRGB), etc. The late Universe measurements give a value of Hubble constant that is significantly different from that measured from the CMBR [1][2][3][4][5][6][7]. It is evident with these observational results that this discrepancy in measured value of Hubble constant is not due to observational errors.…”