GeminiFocus April 2018 - Page 11

onds (Figure 1, left pan- el). For the observations, the researchers moved the telescope in a series of one-arcsecond steps — in the direction per- pendicular to the slits in order to cover the full field-of-view — and took a spectroscopic expo- sure at each position. By tuning the observed wavelength to that of the hydrogen-alpha (Hα) emission line, the MSIS technique makes it possible to find all the bright, actively star-forming regions within the GMOS field-of-view (Figure 1, right panel). Hα light is emitted when the ambient gas is excited by high-energy radiation from nearby young massive stars. Targeting six regions of Hα emission found in the MSIS study, the team then used standard GMOS multi-slit spectroscopy that affords much broader wavelength coverage. The data reveal that these regions are actively forming massive star clusters from gas rich in heavier ele- ments, or high metallicity. The gas was likely enriched with metals produced by genera- tions of stars that lived out their lives in an- other galaxy that has since been accreted by NGC 2865. “These high metallicities could be explained if the clusters were formed by the enriched gas coming from a merger event with a spiral galaxy, ” said Urrutia. “The fate of these clusters is unclear, how- ever. We cannot discard the possibility that these objects become globular clusters in the future,” adds team member Sergio Tor- res-Flores from Universidad de La Serena. Globular clusters are massive, compact, gen- erally old star clusters that are common in the halo regions of galaxies, especially ellip- ticals. This work may therefore provide a rare glimpse into their early evolution. April 2018 A paper presenting the discovery of the young massive clusters appears in a recent is- sue of Astronomy & Astrophysics; an e arlier pa- per on the MSIS observations was published in the same journal. Diversity in Dispersion Profiles of the Most Massive Galaxies Gravity is the glue that holds galaxies to- gether. In more massive galaxies with stron- ger gravitational fields, the stars must move faster in order to avoid being sucked toward the center. The gravitational tug felt by the stars, and thus their speed, also depends on the location of the stars within the galaxy and the spatial distribution of the galaxy’s mass. For instance, in a small galaxy with a very massive black hole in its center, the stars must orbit at high velocity near the black hole in order to resist its strong gravita- tional pull, but their orbital velocities would decrease at larger galactocentric radius (dis- tance from the galaxy’s center). On the other hand, stars within spiral galaxies like our Milky Way, embedded within extensive dark matter halos, have pretty much the same orbital velocity regardless of location within the galactic disk. Thus, the velocities of the stars within a galaxy, and the way those ve- locities change with radius, reveal important GeminiFocus Figure 1. Left: Design of the MSIS slit mask used to find sources of Hα emission around the galaxy NGC 2865. The red lines indicate the positions of the GMOS slits, which are one-arcsecond wide, overlaid on an r-band image of the field. A series of exposures are taken with the telescope offset by an arcsecond between exposures in order to obtain complete spectroscopic coverage of the entire field-of- view. Right: GMOS-South follow-up spectra of six emission line regions found with the MSIS technique in the outskirts of NGC 2865. These are regions of active star formation, and the majority appear to be young star clusters being born from chemically enriched gas. 9