Young, blue, and isolated stellar systems in the virgo cluster. II. A new class of stellar system

Abstract

We discuss five blue stellar systems in the direction of the Virgo cluster, analogous to the enigmatic object SECCO 1 (AGC 226067). These objects were identified based on their optical and UV morphology and followed up with H I observations with the Very Large Array (and Green Bank Telescope), Multi Unit Spectroscopic Explorer (on the Very Large Telescope) optical spectroscopy, and Hubble Space Telescope imaging. These new data indicate that one system is a distant group of galaxies. The remaining four are extremely low mass (M* ∼ 105 Me), are dominated by young blue stars, have highly irregular and clumpy morphologies, are only a few kiloparsecs across, yet host an abundance of metal-rich, 12 + log(O H) > 8.2, H II regions. These high metallicities indicate that these stellar systems formed from gas stripped from much more massive galaxies. Despite the young age of their stellar populations, only one system is detected in H I, while the remaining three have minimal (if any) gas reservoirs. Furthermore, two systems are surprisingly isolated and have no plausible parent galaxy within ∼30′ (∼140 kpc). Although tidal stripping cannot be conclusively excluded as the formation mechanism of these objects, ram pressure stripping more naturally explains their properties, in particular their isolation, owing to the higher velocities, relative to the parent system, that can be achieved. Therefore, we posit that most of these systems formed from ram-pressure-stripped gas removed from new infalling cluster members and survived in the intracluster medium long enough to become separated from their parent galaxies by hundreds of kiloparsecs and that they thus represent a new type of stellar system.