Deep Photometric Observations of Ultrafaint Milky Way Satellites Centaurus I and Eridanus IV

We present deep Magellan+Megacam imaging of Centaurus I (Cen I) and Eridanus IV (Eri IV), two recently discovered Milky Way ultrafaint satellites. Our data reach ∼2-3 mag deeper than the discovery data from the DECam Local Volume Exploration Survey. We use these data to constrain their distances, structural properties (e.g., half-light radii, ellipticity, and position angle), and luminosities. We investigate whether these systems show signs of tidal disturbance and identify new potential member stars using Gaia EDR3. Our deep color-magnitude diagrams show that Cen I and Eri IV are consistent with an old (τ ∼ 13.0 Gyr) and metal-poor ([Fe/H] ≤ −2.2) stellar population. We find Cen I to have a half-light radius of r h = 2. ′ 60 ± 0. ′ 30 (90.6 ± 11 pc), an ellipticity of ϵ = 0.36 ± 0.05, a distance of D = 119.8 ± 4.1 kpc (m − M = 20.39 ± 0.08 mag), and an absolute magnitude of M_V = −5.39 ± 0.19. Similarly, Eri IV has r h = 3. ′ 24 ± 0. ′ 48 (65.9 ± 10 pc), ϵ = 0.26 ± 0.09, D = 69.9 ± 3.6 kpc (m − M = 19.22 ± 0.11 mag), and M_V = −3.55 ± 0.24. These systems occupy a space on the size-luminosity plane consistent with other known Milky Way dwarf galaxies, which supports the findings from our previous spectroscopic follow-up. Cen I has a well-defined morphology that lacks any clear evidence of tidal disruption, whereas Eri IV hosts a significant extended feature with multiple possible interpretations.