Deviations from the Local Hubble Flow. I. The Tip of the Red Giant Branch as a Distance Indicator

Abstract

The properties of the velocity field in the local volume (cz < 550 km s-1) have been difficult to constrain because of a lack of a consistent set of galaxy distances. The sparse observations available to date suggest a remarkably quiet flow, with little deviation from a pure Hubble law. However, velocity field models based on the distribution of galaxies in the 1.2 Jy IRAS redshift survey predict a quadrupolar flow pattern locally with strong infall at the poles of the local supergalactic plane. We probe this velocity field and begin to establish a consistent set of galactic distances. We have obtained images of nearby galaxies in the I and V bands from the W. M. Keck Observatory and in F814W and F555W filters from the Hubble Space Telescope. Where these galaxies are well resolved into stars, we can use the tip of the red giant branch (TRGB) as a distance indicator. Using a maximum likelihood analysis to quantitatively measure the I magnitude of the TRGB, we determine precise distances to seven nearby galaxies: Leo I, Sextans B, NGC 1313, NGC 3109, UGC 3755, UGC 6456, and UGC 7577.