Distances to Local Group Galaxies via Population II, Stellar Distance Indicators. II. The Fornax Dwarf Spheroidal

Kavli Affiliate: Wendy L. Freedman

| First 5 Authors: Elias K. Oakes, Taylor J. Hoyt, Wendy L. Freedman, Barry F. Madore, Quang H. Tran

| Summary:

We determine three independent Population II distance moduli to the Fornax
dwarf spheroidal (dSph) galaxy, using wide-field, ground-based $VI$ imaging
acquired with the Magellan-Baade telescope at Las Campanas Observatory. After
subtracting foreground stars using Gaia EDR3 proper motions, we measure an
$I$-band tip of the red giant branch (TRGB) magnitude of $I_0^mathrm{TRGB} =
16.753 pm 0.03_mathrm{stat} pm 0.037_mathrm{sys}$ mag, with a calibration
based in the LMC giving a distance modulus of $mu_0^mathrm{TRGB} = 20.80 pm
0.037_mathrm{stat} pm 0.057_mathrm{sys}$ mag. We determine an RR Lyrae (RRL)
distance from template mean magnitudes, with periods adopted from the
literature. Adopting a Gaia DR2 calibration of first overtone RRL
period-luminosity and period-Wesenheit relations, we find $mu_0^mathrm{PLZ} =
20.74 pm 0.01_mathrm{stat} pm 0.12_mathrm{sys}$ mag and $mu_0^mathrm{PWZ}
= 20.68 pm 0.02_mathrm{stat} pm 0.07_mathrm{sys}$ mag. Finally, we
determine a distance from Fornax’s horizontal branch (HB) and two galactic
globular cluster calibrators, giving $mu_0^mathrm{HB} = 20.83 pm
0.03_mathrm{stat} pm 0.09_mathrm{sys}$ mag. These distances are each derived
from homogeneous IMACS photometry, are anchored to independent geometric
zero-points, and utilize different classes of stars. We therefore average over
independent uncertainties and report the combined distance modulus $langle
mu_0rangle = 20.770 pm 0.042_mathrm{stat} pm 0.024_mathrm{sys}$ mag
(corresponding to a distance of $143pm3$ kpc).

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