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.
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