Halos of Spiral Galaxies. I. The Tip of the Red Giant Branch as a Distance Indicator

We have imaged the halo populations of a sample of nearby spiral galaxies using the Wide Field Planetary Camera 2 on broad the Hubble Space Telescope with the aim of studying the stellar population properties and relating them to those of the host galaxies. In four galaxies, the red-giant branch is sufficiently well populated to measure the magnitude of the tip of the red-giant branch (TRGB), a well-known distance indicator. Using both the Sobel edge-detection technique and maximum-likelihood analysis to measure the $I$-band magnitude of the red giant branch tip, we determine distances to four nearby galaxies: NGC 253, NGC 4244, and NGC 4945, NGC 4258. For the first three galaxies, the TRGB distance determined here is more direct, and likely to be more accurate, than previous distance estimates. In the case of NGC 4258, our TRGB distance is in good agreement with the the geometrical maser distance, supporting the the Large Magellanic Cloud distance modulus $(m-M)_0 = 18.50$ that is generally adopted in recent estimates of the Hubble constant.