An Improved Distance to NGC 4258 and its Implications for the Hubble Constant (1908.05625v2)

Abstract: NGC 4258 is a critical galaxy for establishing the extragalactic distance scale and estimating the Hubble constant (Ho). Water masers in the nucleus of the galaxy orbit about its supermassive black hole, and very long baseline interferometric observations of their positions, velocities, and accelerations can be modeled to give a geometric estimate of the angular-diameter distance to the galaxy. We have improved the technique to obtain model parameter values, reducing both statistical and systematic uncertainties compared to previous analyses. We find the distance to NGC 4258 to be 7.576 +/- 0.082 (stat.) +/- 0.076 (sys.) Mpc. Using this as the sole source of calibration of the Cepheid-SN Ia distance ladder results in Ho = 72.0 +/- 1.9 km/s/Mpc, and in concert with geometric distances from Milky Way parallaxes and detached eclipsing binaries in the LMC we find Ho = 73.5 +/- 1.4 km/s/Mpc. The improved distance to NGC 4258 also provides a new calibration of the tip of the red giant branch of M_{F814W} = -4.01 +/- 0.04$ mag, with reduced systematic errors for the determination of Ho compared to the LMC-based calibration, because it is measured on the same Hubble Space Telescope photometric system and through similarly low extinction as SN Ia host halos. The result is Ho = 71.1 +/- 1.9 km/s/Mpc, in good agreement with the result from the Cepheid route, and there is no difference in Ho when using the same calibration from NGC 4258 and the same SN Ia Hubble diagram intercept to start and end both distance ladders.

• The paper presents a refined distance measurement to NGC 4258 of 7.576 0.082 0.076 Mpc by utilizing enhanced VLBI observations of water masers and improved modeling techniques.
• The refined NGC 4258 distance improves calibration of the cosmic distance ladder, resulting in updated Hubble constant estimates of around 72-73 km s-1 Mpc-1, depending on the combination of calibrators.
• The significant tension (4o) between the empirically derived Hubble constant and the value from Planck CMB data persists, suggesting the need for refinements in cosmological models or distance measurements.

Improved Distance Determination to NGC 4258 and its Implications for the Hubble Constant

The accurate determination of extragalactic distances is a pivotal element in the calibration of the cosmic distance scale and the subsequent calculation of the Hubble constant ($H_0$). The paper "An Improved Distance to NGC 4258 and its Implications for the Hubble Constant" concentrates on refining the distance measurement to the galaxy NGC 4258, a crucial anchor for distance scale calibration using water maser emissions from its nucleus.

Methodological Advancements

In the pursuit of enhancing precision, water masers in NGC 4258's circumnuclear accretion disk are observed using very long baseline interferometry (VLBI). This setup allows for angular-diameter distance determination via Keplerian motion modeling of maser features orbiting the central supermassive black hole. The paper reports a new distance measurement to NGC 4258 of $$7.576 \pm 0.082~\text{(stat.)} \pm 0.076~\text{(sys.)} \text{Mpc}$$, reflecting significant improvements in both statistical and systematic uncertainties compared to previous estimates. This was achieved by adopting an enhanced Markov Chain Monte Carlo (MCMC) modeling approach to mitigate past analytical limitations and refine error floor assessments, resulting in more reliable distance measurements.

Implications for the Hubble Constant

This refined calibration has direct implications for the determination of $H_0$ by improving the precision of the Cepheid-SN Ia distance ladder. When using the distance to NGC 4258 as the only calibrator, the Hubble constant is estimated at $H_0 = 72.0 \pm 1.9$ $\text{km s}^{-1} \text{Mpc}^{-1}$. Combined with other distance anchors such as Milky Way parallaxes and detached eclipsing binaries in the Large Magellanic Cloud (LMC), the estimation becomes $H_0 = 73.5 \pm 1.4$ $\text{km s}^{-1} \text{Mpc}^{-1}$.

The refined geometric distance further enables improved calibration of the tip of the red giant branch (TRGB) methodology, providing a critical bridge to other galaxies' distance measures under comparable photometric systems with minimal extinction. This results in an $H_0$ estimate of $71.1 \pm 1.9$ $\text{km s}^{-1} \text{Mpc}^{-1}$, confirming coherence with those obtained from Cepheid variable calibrations.

Theoretical and Practical Implications

The sustained difference between the empirically derived $H_0$ and the cosmic microwave background-derived $H_0$ from Planck data, at over 4$\sigma$, reaffirms tensions in current cosmological models. This highlights potential areas for new physics or reevaluation of systematics within distance measurements.

Practically, such precision enhancements in distance measurements can validate or challenge cosmological models that underpin assumptions around dark energy and universal expansion dynamics. Future investigations and technological advancements in VLBI and astronomical interferometry can further refine these distance measures, potentially bringing about resolutions to existing discrepancies.

As we advance our observational capabilities and analytical methodologies, the recalibration of the extragalactic distance ladder will remain a high priority, directly impacting our understanding of the universe’s rate of expansion. The convergence of results across varying distance measurement techniques will ultimately guide refinements in cosmological theory and measurement approaches.