The Araucaria Project. Near-Infrared Photometry of Cepheid Variables in the Sculptor Galaxy NGC 55

TL;DR

This study uses near-infrared photometry of Cepheid variables in NGC 55 to accurately determine its distance and reddening, improving the cosmic distance scale calibration.

Contribution

First multiwavelength near-infrared analysis of Cepheids in NGC 55 providing precise distance and reddening measurements, and assessing systematic uncertainties.

Findings

Distance modulus of 26.434 mag for NGC 55

Reddening E(B-V)=0.127 mag within NGC 55

Distance consistent with NGC 300, indicating possible association

Abstract

We have obtained deep images in the near-infrared J and K filters of four fields in the Sculptor Group spiral galaxy NGC 55 with the ESO VLT and ISAAC camera. For 40 long-period Cepheid variables in these fields which were recently discovered by Pietrzy{\'n}ski et al., we have determined mean J and K magnitudes from observations at two epochs, and derived distance moduli from the observed PL relations in these bands. Using these values together with the previously measured distance moduli in the optical V and I bands, we have determined a total mean reddening of the NGC 55 Cepheids of E(B-V)=0.127 $\pm$ 0.019 mag, which is mostly produced inside NGC 55 itself. For the true distance modulus of the galaxy, our multiwavelength analysis yields a value of 26.434 $\pm$ 0.037 mag (random error), corresponding to a distance of 1.94 $\pm$ 0.03 Mpc. This value is tied to an adopted true LMC distance modulus of 18.50 mag. The systematic uncertainty of our derived Cepheid distance to NGC 55 (apart from the uncertainty on the adopted LMC distance) is $\pm$4%, with the main contribution likely to come from the effect of blending of some of the Cepheids with unresolved companion stars. The distance of NGC 55 derived from our multiwavelength Cepheid analysis agrees within the errors with the distance of NGC 300, strengthening the case for a physical association of these two Sculptor Group galaxies.