The Extragalactic Distance Scale Key Project. VIII. The Discovery of Cepheids and a New Distance to NGC 3621 Using the Hubble Space Telescope

We report on the discovery of Cepheids in the field spiral galaxy NGC 3621, based on observations made with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope (HST). NGC 3621 is one of 18 galaxies observed as a part of the HST Key Project on the Extragalactic Distance Scale, which aims to measure the Hubble constant to 10% accuracy. Sixty-nine Cepheids with periods in the range 9-60 days were observed over 12 epochs using the F555W filter, and over four epochs using the F814W filter. The HST F555W and F814W data were transformed to the Johnson V and Kron-Cousins I magnitude systems, respectively. Photometry was performed using two independent packages, DAOPHOT II/ALLFRAME and DoPHOT. Period-luminosity relations in the V and I bands were constructed using 36 fairly isolated Cepheids present in our set of 69 variables. Extinction-corrected distance moduli relative to the LMC of 10.63 ± 0.09 and 10.56 ± 0.10 mag were obtained using the ALLFRAME and DoPHOT data, respectively. True distance moduli of 29.13 ± 0.18 and 29.06 ± 0.18 mag, corresponding to distances of 6.3 ± 0.7 and 6.1 ± 0.7 Mpc, were obtained by assuming values of μ0 = 18.50 ± 0.10 mag and E(V - I) = 0.13 mag for the distance modulus and reddening of the LMC, respectively.