Discovery of an Ultra-Faint Stellar System near the Magellanic Clouds with the DECam Local Volume Exploration (DELVE) Survey

• Main Authors: Cerny, W, Pace, A. B, Drlica-Wagner, A, Ferguson, P. S, Mau, S, Adamów, M, Carlin, J. L, Choi, Y, Erkal, D, Johnson, L. C, Li, T. S, Martínez-Vázquez, C. E, Mutlu-Pakdil, B, Nidever, D. L, Olsen, K. A. G, Pieres, A, Simon, J. D, Tollerud, E. J, Vivas, A. K, James, D. J, Kuropatkin, N, Majewski, S, Martínez-Delgado, D, Massana, P, Miller, A, Noël, N. E. D, Riley, A. H, Sand, D. J, Santana-Silva, L, Stringfellow, G. S, Neilsen, E. H, Tucker, D. L
• Format: Journal Article
• Language: English
• Published: 17-09-2020
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.2009.08550

We report the discovery of a new ultra-faint stellar system found near the
Magellanic Clouds in the DECam Local Volume Exploration (DELVE) Survey. This
new system, DELVE J0155$-$6815 (DELVE 2), is located at a heliocentric distance
of $D_{\odot} = 71 \pm 4\text{ kpc}$, which places it at a 3D physical
separation of 12 kpc from the center of Small Magellanic Cloud (SMC) and 28 kpc
from the center of the Large Magellanic Cloud (LMC). DELVE 2 is identified as a
resolved overdensity of old ($\tau > 13.3\text{ Gyr}$) and metal-poor (${\rm
[Fe/H]} = -2.0_{-0.5}^{+0.2}$ dex) stars with a projected half-light radius of
$r_{1/2} = 21^{+4}_{-3}\text{ pc}$ and an absolute magnitude of $M_V =
-2.1^{+0.4}_{-0.5}\text{ mag}$. The size and luminosity of DELVE 2 are
consistent with both the population of recently discovered ultra-faint globular
clusters and the smallest ultra-faint dwarf galaxies. However, its age and
metallicity would place it among the oldest and most metal-poor globular
clusters in the Magellanic system. DELVE 2 is detected in Gaia DR2 with a clear
proper motion signal, with multiple blue horizontal branch stars near the
centroid of the system with proper motions consistent with the systemic mean.
We measure the system proper motion to be $(\mu_{\alpha} \cos \delta,
\mu_{\delta})= (1.02_{-0.25}^{+0.24}, -0.85_{-0.19}^{+0.18})$ mas yr$^{-1}$. We
compare the spatial position and proper motion of DELVE 2 with simulations of
the accreted satellite population of the LMC and find that it is very likely to
be associated with the LMC.