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A very dark stellar system lost in Virgo: kinematics and metallicity of SECCO1 with MUSE

We present the results of VLT-MUSE integral field spectroscopy of SECCO1, a faint, star-forming stellar system recently discovered as the stellar counterpart of an Ultra Compact High Velocity Cloud (HVC274.68+74.0), very likely residing within a substructure of the Virgo cluster of galaxies. We have obtained the radial velocity of a total of 38 individual compact sources identified as HII regions in the main and secondary body of the system, and derived the metallicity for 18 of them. We provide the first direct demonstration that the two stellar bodies of SECCO1 are physically associated and that their velocities match the HI velocities. The metallicity is quite uniform over the whole system, with a dispersion sigma_12+log(O/H/)=0.08, lower than the uncertainty on individual metallicity estimates. The mean abundance, 12+log(O/H)=8.44, is much higher than the typical values for local dwarf galaxies of similar stellar mass. This strongly suggests that the SECCO~1 stars were born from a pre-enriched gas cloud, possibly stripped from a larger galaxy. Using archival HST images we derive a total stellar mass of ~1.6 X 10^5 M_sun for SECCO1, confirming that it has a very high HI to stellar mass ratio for a dwarf galaxy, M_HI/M_*~ 100. The star formation rate, derived from the H_alpha flux is a factor of more than 10 higher than in typical dwarf galaxies of similar luminosity.