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The Great Andromeda Galaxy (M31) is the nexus of the near-far galaxyevolution connection and a principal data point for near-field cosmology. Dueto its proximity (780 kpc), M31 can be resolved into individual stars like theMilky Way (MW). Unlike the MW, we have the advantage of a global view of M31,enabling M31 to be observed with techniques that also apply to more distantgalaxies. Moreover, recent evidence suggests that M31 may have survived a majormerger within the last several Gyr, shaping the morphology of its stellar haloand triggering a starburst, while leaving the stellar disk largely intact. TheMW and M31 thus provide complementary opportunities for in-depth studies of thedisks, halos, and satellites of L* galaxies. Our understanding of the M31 system will be transformed in the 2020s if theyinclude wide field facilities for both photometry (HST-like sensitivity andresolution) and spectroscopy (10-m class telescope, >1 sq. deg. field, highlymultiplexed, R~ 3000 to 6000). We focus here on the power of these facilitiesto constrain the past, present, and future merger history of M31, viachemo-dynamical analyses and star formation histories of phase-mixed starsaccreted at early times, as well as stars in surviving tidal debris features,M31's extended disk, and intact satellite galaxies that will eventually betidally incorporated into the halo. This will yield an unprecedented view ofthe hierarchical formation of the M31 system and the subhalos that built itinto the L* galaxy we observe today.

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 (DES J2038–4609). For the case of Grus i , we resolved the main sequence turn-off (MSTO) and ∼2 mags below it. The MSTO can be seen at g0 ∼ 24 with a photometric uncertainty of 0.03 mag. We show Grus i to be consistent with an old, metal-poor (∼13.3 Gyr, [Fe/H] ∼ −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 halflight radius more than two times larger (∼151+21−31 pc; ∼) and an absolute V-band magnitude ∼−4.1 that is ∼1 magnitude brighter than previous studies. We obtain updated distance, ellipticity, and centroid parameters that are in agreement with other studies within uncertainties. Although our photometry of Indus ii is ∼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. [ABSTRACT FROM AUTHOR]

Tidally stripped galaxy nuclei and luminous globular clusters (GCs) are important tracers of the halos and assembly histories of nearby galaxies, but are difficult to reliably identify with typical ground-based imaging data. In this paper we present a new method to find these massive star clusters using Gaia DR2, focusing on the massive elliptical galaxy Centaurus A (Cen A). We show that stripped nuclei and GCs are partially resolved by Gaia at the distance of Cen A, showing characteristic astrometric and photometric signatures. We use this selection method to produce a list of 632 new candidate luminous clusters in the halo of Cen A out to a projected radius of 150 kpc. Adding in broadband photometry and visual examination improves the accuracy of our classification. In a spectroscopic pilot program we have confirmed five new luminous clusters, which includes the 7th and 10th most luminous GC in Cen A. Three of the newly discovered GCs are further away from Cen A than all previously known GCs. Several of these are compelling candidates for stripped nuclei. We show that our novel Gaia selection method retains at least partial utility out to distances of ∼25 Mpc and hence is a powerful tool for finding and studying star clusters in the sparse outskirts of galaxies in the local universe. [ABSTRACT FROM AUTHOR]