The Araucaria Project: The Distance to the Sculptor Group Galaxy NGC 55 from a Newly Discovered Abundant Cepheid Population1

We have detected, for the first time, Cepheid variables in the Sculptor GroupSB(s)m galaxy NGC 55. From wide-field images obtained in the optical V and Ibands during 77 nights in 2002-2003, we have found 143 Cepheids with periodsranging from 5.6 to 175.9 days. 133 of these objects have periods longer than10 days, making NGC 55 to-date the galaxy with the largest known number oflong-period Cepheids in the Sculptor Group. We construct period-luminosityrelations from our data and obtain distance moduli corrected for the smallforeground reddening to NGC 55 of 26.79 $\pm$ 0.04 mag (internal error) in V,26.66 $\pm$ 0.03 mag in I and 26.40 $\pm$ 0.05 mag in the reddening-independentV-I Wesenheit index. The trend of increasing distance moduli with shorterwavelength hints at the existence of significant reddening intrinsic to NGC 55which affects the measured Cepheid magnitudes. From our data, we determine theintrinsic mean reddening of the Cepheids in NGC 55 as E(B-V) = 0.102 mag whichbrings the distance determinations from the different bands into excellentagreement. Our best distance estimate for NGC 55 from the present opticalCepheid photometry is 26.40 mag $\pm$ 0.05 mag (internal error) $\pm$ 0.09 mag(systematic error). This value is tied to an assumed LMC distance of 18.50 mag.Our quoted systematic error of the present NGC 55 Cepheid distance does nottake into account the current uncertainty on the distance of the fiducial LMCgalaxy itself. Within the small respective uncertainties, the Sculptor Group galaxies NGC 55and NGC 300 are at the same distance of 1.9 Mpc, strengthening the case for aphysical association of these galaxiesComment: Astronomical Journal in press (December issue