[Paper Review] The Cepheid Extragalactic Distance Scale: Past, Present and Future

Wendy L. Freedman, Barry F. Madore|arXiv (Cornell University)|Aug 4, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This paper presents new James Webb Space Telescope (JWST) data that significantly reduce scatter in the Cepheid period–luminosity relation—by a factor of two—compared to Hubble Space Telescope (HST) data, demonstrating improved photometric precision and reduced crowding effects. The results support the use of JWST for refining the extragalactic distance scale and testing the Hubble tension, with implications for whether new physics beyond the standard cosmological model is required.

ABSTRACT

Cepheids have been the cornerstone of the extragalactic distance scale for a century. With high-quality data, these luminous supergiants exhibit a small dispersion in their Leavitt (period-luminosity) relation, particularly at longer wavelengths, and few methods rival the precision possible with Cepheid distances. In these proceedings, we present an overview of major observational programs pertaining to the Cepheid extragalactic distance scale, its progress and remaining challenges. In addition, we present preliminary new results on Cepheids from the James Webb Space Telescope (JWST). The launch of JWST has opened a new chapter in the measurement of extragalactic distances and the Hubble constant. JWST offers a resolution three times that of the Hubble Space Telescope (HST) with nearly 10 times the sensitivity. It has been suggested that the discrepancy in the value of the Hubble constant based on Cepheids compared to that inferred from measurements of the cosmic microwave background requires new and additional physics beyond the standard cosmological model. JWST observations will be critical in reducing remaining systematics in the Cepheid measurements and for confirming if new physics is indeed required. Early JWST data for the galaxy, NGC 7250 show a decrease in scatter in the Cepheid Leavitt law by a factor of two relative to existing HST data and demonstrate that crowding/blending effects are a significant issue in a galaxy as close as 20 Mpc.

Motivation & Objective

• To address systematic uncertainties in the Cepheid extragalactic distance scale that affect the precision of the Hubble constant measurement.
• To evaluate the impact of crowding and blending on Cepheid photometry in nearby galaxies using high-resolution JWST data.
• To compare the scatter in the near-infrared Leavitt law between HST and JWST observations to assess improvements in photometric accuracy.
• To combine JWST F115W data with re-analyzed HST optical data to reduce systematics in distance measurements.
• To test whether the Hubble constant discrepancy (Hubble tension) requires new physics beyond the standard ΛCDM model.

Proposed method

• Utilizing single-epoch F115W imaging from the James Webb Space Telescope (JWST) to observe Cepheids in the nearby galaxy NGC 7250.
• Comparing JWST F115W photometry with archival HST WFC3 F160W (H-band) data to assess differences in signal-to-noise ratio and angular resolution.
• Re-analyzing SH0ES HST optical data (F350LP white light) to improve Cepheid identification and photometric calibration.
• Measuring the scatter in the period–luminosity relation for Cepheids in NGC 7250 using both HST and JWST data to quantify photometric precision.
• Applying improved photometric techniques to minimize blending and crowding effects in high-density regions of galaxies.
• Combining Cepheid, tip of the red giant branch (TRGB), and carbon star distance measurements in the same galaxies to cross-validate results.

Figure 1: Left panel: Leavitt laws at JHK wavelengths (McGonegal et al. 1983) demonstrating the power of infrared photometry for the extragalactic distance scale. Right panel: Leavitt laws at BVRI wavelengths based on CCD photometry (Freedman 1988) (open circles); filled circles illustrate photoelec

Experimental results

Research questions

• RQ1 To what extent does JWST’s higher angular resolution and sensitivity reduce scatter in the Cepheid period–luminosity relation compared to HST?
• RQ2 How significant are crowding and blending effects on Cepheid photometry in galaxies as close as 20 Mpc?
• RQ3 Can single-epoch JWST F115W photometry achieve comparable or better precision than multi-epoch HST F160W data for distance scale applications?
• RQ4 What is the impact of improved photometric precision on the uncertainty of the Hubble constant derived from Cepheids?
• RQ5 Do the new data support the existence of new physics beyond the standard ΛCDM model to resolve the Hubble tension?

Key findings

• JWST F115W data for NGC 7250 show a factor of two reduction in scatter (from 0.18 mag to 0.09 mag) in the near-infrared period–luminosity relation compared to HST F160W data.
• The signal-to-noise ratio (SNR) of JWST F115W photometry is on average nearly an order of magnitude higher than HST F160W, ranging from 36 to 121 compared to 1 to 23.
• JWST’s angular resolution is four times better than HST, enabling clear separation of Cepheids from nearby stars and reducing blending effects.
• Even with single-epoch F115W observations, JWST achieves lower scatter than HST’s multi-epoch corrected mean-light photometry.
• The improved photometric precision from JWST significantly reduces systematics related to crowding and blending in Cepheid distance measurements.
• The results suggest that JWST can reduce key uncertainties in the Cepheid distance scale, supporting a more accurate determination of the Hubble constant and testing the Hubble tension.

Figure 2: Hubble diagram showing the final results from the HST Key Project (Freedman et al. 2001) In the top panel velocity is plotted versus distance for the five secondary distance methods labeled in the plot. The solid line is for a slope of 72, flanked by dotted lines indicating a 10% uncertain