Deep imaging of Virgo Z: not a nearby ultrafaint dwarf galaxy, but a distant compact group of galaxies

Context. The search for Milky Way faint satellite galaxies and star clusters has been intensified in recent years thanks to the Sloan Digital Sky Survey (SDSS), and other efforts to verify candidate objects. Aims. We observed the Milky Way satellite candidate Vir Z, reported by Walsh et al. (2009, AJ, 137, 450), from the detection of an overdensity in the SDSS database. Methods. Deep subarcsec seeing images from the New Technology Telescope (NTT) and Galileo telescopes were obtained in order to infer the nature of the proposed overdensity. Results. The image and colour-magnitude diagram of the Vir Z field shows no dwarf galaxy. Instead, we identified a compact group of galaxies near the center of the Vir Z field at redshift z ≈ 0.3 ± 0.2, based on the apparent magnitudes of the brightest galaxies assuming L = L∗, and on angular sizes of the spirals assuming physical sizes equal to M 31