Deep VLT photometry of the faint stellar system in the Large Magellanic Cloud periphery YMCA-1

Abstract

We present FORS2@VLT follow-up photometry of YMCA-1, a recently discovered stellar system located 13\degr~from the Large Magellanic Cloud (LMC) center. The deep color-magnitude diagram (CMD) reveals a well-defined main sequence (MS) and a handful of stars in the post-MS evolutionary phases. We analyse the YMCA-1 CMD by means of the automated isochrone matching package {\tt ASteCA} and model its radial density profile with a Plummer function. We find that YMCA-1 is an old ($11.7_{-1.3}^{+1.7}$~Gyr), metal-intermediate ([Fe/H] $\simeq -1.12_{-0.13}^{+0.21}$~dex), compact (r${}_{\mathrm{h}}=3.5\pm 0.5$ pc), low-mass (M $=10^{2.45\pm 0.02}{M}_{\odot }$) and low-luminosity (M${}_V=-0.47\pm 0.57$~mag) stellar system. The estimated distance modulus ($\hat{I}{¼}_0=18.72_{-0.17}^{+0.15}$~mag), corresponding to about 55~kpc, suggests that YMCA-1 is associated to the LMC, but we cannot discard the scenario in which it is a Milky Way satellite. The structural parameters of YMCA-1 are remarkably different compared with those of the 15 known old LMC globular clusters. In particular, it resides in a transition region of the M${}_V$-r${}_h$ plane, in between the ultra-faint dwarf galaxies and the classical old clusters, and close to SMASH-1, another faint stellar system recently discovered in the LMC surroundings.

8 pages, 4 figures, Accepted for publication in The Astrophysical Journal Letters