A Tip of the Red Giant Branch Distance to the Dark Matter Deficient Galaxy NGC1052-DF4 from Deep Hubble Space Telescope Data

Abstract: Previous studies have shown that the large, diffuse galaxies NGC1052-DF2 and NGC1052-DF4 both have populations of unusually luminous globular clusters as well as a very low dark matter content. Here we present newly-obtained deep Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging of one of these galaxies, NGC1052-DF4. We use these data to measure the distance of the galaxy from the location of the tip of the red giant branch (TRGB). We find a rapid increase in the number of detected stars fainter than $m_{F814W} \sim 27.3$, which we identify as the onset of the red giant branch. Using a forward modeling approach that takes the photometric uncertainties into account, we find a TRGB magnitude of $m_{F814W,\rm TRGB}=27.47 \pm 0.16$. The inferred distance, including the uncertainty in the absolute calibration, is $D_{\rm TRGB}=20.0 \pm 1.6$ Mpc. The TRGB distance of NGC1052-DF4 is consistent with the previously-determined surface brightness fluctuation distance of $D_{\rm SBF}=18.7\pm 1.7$ Mpc to NGC1052-DF2 and is consistent with the distance of the bright elliptical galaxy NGC1052. We conclude that the unusual properties of these galaxies cannot be explained by distance errors.