# A 1.3% distance to M33 from HST Cepheid photometry

**Authors:** Louise Breuval, Adam G. Riess, Lucas M. Macri, Siyang Li, Wenlong Yuan, Stefano Casertano, Tarini Konchady, Boris Trahin, Meredith J. Durbin, Benjamin F. Williams  
**arXiv:** [2304.00037](https://arxiv.org/abs/2304.00037)  
**Date:** 2023-03-31  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.00037v1/

## Key points

- The study presents a low-dispersion period-luminosity (PL) relation based on 154 Cepheids in Messier 33 (M33), a nearby type Sc II-III spiral galaxy.
- Hubble Space Telescope (HST) photometry from the PHATTER survey is used, and high-quality ground-based light curves are employed to recover Cepheid phases and amplitudes for multi-epoch HST data.
- Template fitting is performed to derive intensity-averaged mean magnitudes.
- The HST observations in the SH0ES near-infrared Wesenheit system significantly reduce the effect of crowding relative to ground-based data, resulting in a final PL scatter of $\sigma$ = 0.11 mag.
- The absolute calibration of the PL is adopted based on HST observations in the Large Magellanic Cloud (LMC), and a distance derived using late-type detached eclipsing binaries is used to obtain a distance modulus for M33 of $\mu$ = 24.622 $\pm$ 0.030 mag (d = 840 $\pm$ 11 kpc), with a best-to-date precision of 1.3%.
- This result shows very good agreement with past Cepheid-based measurements, while several TRGB estimates bracket their result but disagree with each other.
- Star clusters hosting Cepheids in M33 do not have an impact on the distance measurement; only 3.7% of the sample is located in or nearby young clusters.
- M33 offers one of the best sites for cross-calibration of many primary distance indicators, making it an essential anchor point for measuring the Hubble constant accurately and precisely.
- Overall, this study provides crucial insights into improving our understanding of cosmic distances and refining our knowledge about fundamental cosmological parameters such as the Hubble constant.

## Summary

The study presents a low-dispersion period-luminosity (PL) relation based on 154 Cepheids in Messier 33 (M33), a nearby type Sc II-III spiral galaxy. The Hubble Space Telescope (HST) photometry from the PHATTER survey is used, and high-quality ground-based light curves are employed to recover Cepheid phases and amplitudes for multi-epoch HST data. Template fitting is performed to derive intensity-averaged mean magnitudes. The HST observations in the SH0ES near-infrared Wesenheit system significantly reduce the effect of crowding relative to ground-based data, resulting in a final PL scatter of $\sigma$ = 0.11 mag. The absolute calibration of the PL is adopted based on HST observations in the Large Magellanic Cloud (LMC), and a distance derived using late-type detached eclipsing binaries is used to obtain a distance modulus for M33 of $\mu$ = 24.622 $\pm$ 0.030 mag (d = 840 $\pm$ 11 kpc), with a best-to-date precision of 1.3%. This result shows very good agreement with past Cepheid-based measurements, while several TRGB estimates bracket their result but disagree with each other. The study also demonstrates that star clusters hosting Cepheids in M33 do not have an impact on the distance measurement; only 3.7% of the sample is located in or nearby young clusters. M33 offers one of the best sites for cross-calibration of many primary distance indicators, making it an essential anchor point for measuring the Hubble constant accurately and precisely. Overall, this study provides crucial insights into improving our understanding of cosmic distances and refining our knowledge about fundamental cosmological parameters such as the Hubble constant.