The Araucaria Project. The Distance to the Sculptor Galaxy NGC 247 from Near-Infrared Photometry of Cepheid Variables

TL;DR

This study uses near-infrared photometry of Cepheid variables to accurately determine the distance to NGC 247, improving previous optical estimates and highlighting the importance of infrared data for extragalactic distance measurements.

Contribution

The paper presents the first near-infrared period-luminosity relations for Cepheids in NGC 247, providing a more precise distance measurement and assessing the impact of reddening.

Findings

Distance modulus of 27.64 mag for NGC 247

Reddening E(B-V) = 0.18 mag affecting Cepheids

IR-based distance is more accurate than previous optical estimates

Abstract

We have obtained deep near-infrared images in J and K filters of four fields in the Sculptor Group spiral galaxy NGC 247 with the ESO VLT and ISAAC camera. For a sample of ten Cepheids in these fields, previously discovered by Garc{\'i}a-Varela et al. from optical wide-field images, we have determined mean J and K magnitudes and have constructed the period-luminosity (PL) relations in these bands. Using the near-infrared PL relations together with those in the optical V and I bands, we have determined a true distance modulus for NGC 247 of 27.64 mag, with a random uncertainty of $\pm$2% and a systematic uncertainty of $\sim$4% which is dominated by the effect of unresolved stars on the Cepheid photometry. The mean reddening affecting the NGC 247 Cepheids of E(B-V) = 0.18 $\pm$ 0.02 mag is mostly produced in the host galaxy itself and is significantly higher than what was found in the previous optical Cepheid studies in NGC 247 of our own group, and Madore et al., leading to a 7% decrease in the previous optical Cepheid distance. As in other studies of our project, the distance modulus of NGC 247 we report is tied to an assumed LMC distance modulus of 18.50. Comparison with other distance measurements to NGC 247 shows that the present IR-based Cepheid distance is the most accurate among these determinations. With a distance of 3.4 Mpc, NGC 247 is about 1.5 Mpc more distant than NGC 55 and NGC 300, two other Sculptor Group spirals analyzed before with the same technique by our group.