Star formation in low density HI gas around the elliptical galaxy NGC 2865

Abstract

Context. Interacting galaxies surrounded by Hi tidal debris are ideal sites for the study of young clusters and tidal galaxy formation. The process that triggers star formation in the low-density environments outside galaxies is still an open question. New clusters and galaxies of tidal origin are expected to have high metallicities for their luminosities. Spectroscopy of such objects is, however, at the limit of what can be done with existing 8-10m class telescopes, which has prevented statistical studies of these objects. Aims. NGC 2865 is an UV-bright merging elliptical galaxy with shells and extended Hi tails. The regions observed in this work were previously detected using multi-slit imaging spectroscopy. Methods. We obtain new multi-slit spectroscopy of six young star-forming regions around NGC 2865, to determine their redshifts and metallicities. Results. The six emission-line regions are located 16-40 kpc from NGC 2865 and they have similar redshifts. They have ages of ∼ 10 Myears and an average metallicity of ∼ 12+log(O/H)∼8.6, suggesting a tidal origin for the regions. It is noted that they coincide with an extended Hi tail, which has projected density of N HI < 10 19 cm −2 , and displays a low surface brightness counterpart. These regions may represent the youngest of the three populations of star clusters already identified in NGC 2865. Conclusions. The high, nearly-solar, oxygen abundances found for the six regions in the vicinity of NGC 2865 suggest that they were formed by pre-enriched material from the parent galaxy, from gas removed during the last major merger. Given the mass and the location of the Hii regions, we can speculate that these young star-forming regions are potential precursors of globular clusters that will be part of the halo of NGC 2865 in the future. Our result supports the use of the multi-slit imaging spectroscopy as a useful tool for finding nearly-formed stellar systems around galaxies.