A very dark stellar system lost in Virgo: kinematics and metallicity of SECCO 1 with MUSE

Abstract

We present the results of VLT-MUSE (Very Large Telescope-Multi Unit Spectroscopic Explorer) integral field spectroscopy of SECCO 1, a faint, star-forming stellar system recently discovered as the stellar counterpart of an ultracompact high-velocity cloud (HVC 274.68+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 H II regions in the main and secondary bodies of the system, and derived the metallicity for 18 of them. We provide the first direct demonstration that the two stellar bodies of SECCO 1 are physically associated and that their velocities match the H I velocities. The metallicity is quite uniform over the whole system, with a dispersion 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 Hubble Space Telescope (HST) images, we derive a total stellar mass of ≃1.6 × 10<SUP>5</SUP> M<SUB>☉</SUB> for SECCO 1, confirming that it has a very high H I-to-stellar mass ratio for a dwarf galaxy, M_{H I}/M<SUB>*</SUB> ∼ 100. The star formation rate, derived from the Hα flux, is a factor of more than 10 higher than in typical dwarf galaxies of similar luminosity.