Structural Analysis of Ultra-Faint Dwarf Galaxy Horologium I

We use Hubble Space Telescope (HST) photometry to study the morphology of the low luminosity Milky Way (MW) satellite Horologium I (Hor I). Previous literature values of Hor I are further refined through the fitting of galactic models using a Maximum Likelihood algorithm applied to HST data to fit two density profiles: exponential and Plummer. This ultra-faint dwarf (UFD) galaxy has structural parameters of elliptical half-light radius $a_{h, exp}= 1.99'^{+0.24}_{-0.20}$ for the exponential model and $a_{h, p} = 1.95'^{+0.22}_{-0.18}$ for the Plummer model, ellipticity $e_{exp}=0.31±0.06$ and $e_p=0.32±0.06$, and position angle $\theta=58 \pm 7$ degrees for both modeling profiles. To derive these values, we systematically examine Hor I data, with magnitudes fainter than $m_{F606W}=27.0$ excluded. Best-fit structural parameters are tabulated, including ellipticity, position angle, half-light radius, richness, and position. We investigate this newer HST data via structural analysis to explore the differences between Hor I morphology and that of a few of the most distant satellites in the halo of the MW, Pegasus III (Peg III), Pisces II (Psc II), and Eridanus II (Eri II). Similar positive correlations were found between richness (number of stars) and half-light radius as well as between the half-light radius and ellipticity. A negative correlation between richness and background density also exists. Additional information on Hor I brings us one step closer to unambiguously characterizing this UFD.