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

Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>r</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>2.</mml:mn> <mml:mspace width="-0.2em"/> <mml:mi>′</mml:mi> <mml:mn>60</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.</mml:mn> <mml:mspace width="-0.2em"/> <mml:mi>′</mml:mi> <mml:mn>30</mml:mn> <mml:mspace width="0.1em"/> </mml:math> (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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>r</mml:mi> <mml:mi>h</mml:mi> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>3.</mml:mn> <mml:mspace width="-0.2em"/> <mml:mi>′</mml:mi> <mml:mn>24</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.</mml:mn> <mml:mspace width="-0.2em"/> <mml:mi>′</mml:mi> <mml:mn>48</mml:mn> <mml:mspace width="0.1em"/> </mml:math> (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.