Understanding the Nature of Ultra-faint Dwarf Galaxies through Characterization and Statistical Inference

We present deep $g$- and $r$-band Magellan/Megacam photometry of two dwarf galaxy candidates discovered in the Dark Energy Survey (DES), Grus I and Indus II. For the case of Grus I, we resolved the main sequence turn-off (MSTO) and $\sim 2$ mags below it. The MSTO can be seen at $g_0\sim 24$ with a photometric uncertainty of $0.03$ mag. We show Grus I to be consistent with an old, metal-poor ($\sim 13.3$ Gyr, [Fe/H]$\sim-1.9$) dwarf galaxy. We derive updated distance and structural parameters for Grus I using this deep, uniform, wide-field data set. We find an azimuthally averaged half-light radius more than two times larger ($\sim 151^{+21}_{-31}$ pc; $\sim 4\farcm16^{+0.54}_{-0.74}$) and an absolute $V$-band magnitude $\sim-4.1$ that is $\sim 1$ magnitude brighter than previous studies. Although our photometry of Indus II is $\sim 2-3$ magnitudes deeper than the DES Y1 Public release, we find no coherent stellar population at its reported location. The original detection was located in an incomplete region of sky in the DES Y2Q1 data set and was flagged due to potential blue horizontal branch member stars. The best fit isochrone parameters are physically inconsistent with both dwarf galaxies and globular clusters. We conclude that Indus II is likely a false-positive, flagged due to a chance alignment of stars along the line of sight. We present updated structural parameters of 13 UFDs in the DES footprint. We use the final Y6 coadded DES data that has a limiting magnitude of $g\sim24.7$. In all cases, where the UFD is resolved, it is consistent with an old, metal poor stellar population. Tuc II and Cet II are found to be larger than previously thought. We also present the slope of the Initial Mass Function (IMF), $\alpha$ of two UFD in the DES footprint. Both Ret II and Tuc III are well populated $\sim3$ magnitudes below the Main Sequence (MS). Reticulum II was found to have $\alpha=1.68\pm0.5$ and Tucana III was found to have $\alpha=1.56\pm0.8$. These findings contribute to the broader scope of expanding our ability to constrain the IMF with noisy data on low-mass objects