GeminiFocus April 2018 - Page 10

John Blakeslee Science Highlights A medley of results on a diversity of galaxies: multi-slit imaging spectroscopy with the Gemini Multi-Object Spectrograph reveals star clusters being born from enriched gas around an old galaxy, stellar velocities in the most massive galaxies rise or fall depending on galaxy surroundings, and an ultra-diffuse galaxy rich in globular star clusters is startlingly poor in dark matter. A New Generation of Star Clusters Adorning an Old Galaxy Elliptical galaxies are often described as “red and dead,” meaning that the stars within them are generally many billions of years old, and they lack the material and wherewithal for any significant amount of star formation. The optical light in such old stellar populations is dominated by contribution from red giant stars, which have exhausted their core sup- ply of hydrogen. For this reason, some galaxy enthusiasts might consider ellipticals bland and boring compared with the more showy grand design spirals such as the Whirlpool, or even star-forming dwarf irregulars, like the Magellanic Clouds. However, sometimes when an elliptical encounters a gas-rich neighbor, sparks fly, and a new generation of stars comes into being. This appears to be the case with the galaxy NGC 2865, a post-encounter elliptical with tell-tale shells, streams, and other tidal features. A team of astronomers led by Fernanda Urrutia (Universidad de La Serena and now at Gem- ini Observatory) used an observational technique called Multi-Slit Imaging Spectroscopy (MSIS), applied with the Gemini Multi-Object Spectrograph (GMOS), to pinpoint the loca- tions of newly formed star clusters scattered amidst the tidal debris surrounding NGC 2865. The MSIS technique uses a specially designed spectroscopic mask with multiple parallel long slits and a narrow-band filter to ensure that the spectra produced by the slits do not overlap. For this study, Urrutia’s team constructed a mask with 108 parallel long slits, split into three groups that each spanned a third of the length of the field. The individual slits were an arcsecond in width, and the slits within each group were spaced by eight arcsec- 8 GeminiFocus April 2018