The First Galaxy Cluster Discovered by the VISTA Variables in the V'ia L'actea Survey.

We report the first confirmed detection of the galaxy cluster VVV-J144321-611754 at very low latitudes (l = 315 degrees.836, b = -1 degrees.650) located in the tile d015 of the VISTA Variables in the Via Lactea (VVV) survey. We defined the region of 30 x 30 arcmin(2) centered in the brightest galaxy finding 25 galaxies. For these objects, extinction-corrected median colors of (H-K-s) = 0.34 +/- 0.05 mag, (J-H) = 0.57 +/- 0.08 mag, and (J-K-s) = 0.87 +/- 0.06 mag; R-1/2 = 1.59 +/- 0 16; C = 3.01 +/- 0.08; and Sersic index n = 4.63 +/- 0.39 were estimated. They were visually confirmed showing characteristics of early-type galaxies in the near-IR images. An automatic clustering analysis performed in the whole tile found that the concentration of galaxies VVV-J144321-611754 is a real, compact concentration of early-type galaxies. Assuming a typical galaxy cluster with low X-ray luminosity, the photometric redshift of the brightest galaxy is z = 0.196 +/- 0.025. Follow-up near-IR spectroscopy with FLAMINGOS-2 at the Gemini-South telescope revealed that the two brighter cluster galaxies have typical spectra of early-type galaxies and the estimated redshift for the brightest galaxy VVV-J144321.06-611753.9 is z = 0.234 +/- 0.022 and that for VVV-J144319.02-611746.1 is z = 0.232 +/- 0.019. Finally, these galaxies clearly follow the cluster red sequence in the rest-frame near-IR color-magnitude diagram with a slope similar to a galaxy cluster at a redshift of 0.2. These results are consistent with the presence of a bona fide galaxy cluster beyond the Milky Way disk.