lar populations is dominated by con-
tribution from red giant stars, which
have exhausted their core supply 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 neigh-
bor, 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.
Figure 11.
Orientation during
each observation
shown in Figure 10 of
the best-fitting triaxial
model derived from the
full set of deconvolved
AO images. The south
pole and equator are
visible in all cases. The
sub-Earth and sub-Sun
points are labeled as m
and b , respectively.
[Figure reproduced
from Drummond, et al.
Icarus, 305: 174, 2018.]
means that only four distinct geometries are
possible at opposition. Improving the shape
and orientation measurements for this
unique asteroid will require further AO ob-
servations at oppositions with geometries
not represented in Figure 11, as well as at
times when it is challengingly away from op-
position. The upcoming robotic rendezvous
will provide an exciting opportunity to test
the analysis methods used for the AO data
and will measure the object’s moment of
inertia, finally revealing the secret structure
beneath Psyche’s shiny surface.
APRIL 2018
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 where-
withal for any significant amount of star
formation. The optical light in such old stel-
34
GeminiFocus
A team of astronomers led by Fernanda
Urrutia (Universidad de La Serena and
now at Gemini Observatory) used an
observational technique called Multi-Slit Im-
aging Spectroscopy (MSIS), applied with the
Gemini Multi-Object Spectrograph (GMOS),
to pinpoint the locations 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 arcsec-
ond in width, and the slits within each group
were spaced by 8 arcseconds (Figure 12, top
panel). For the observations, the researchers
moved the telescope in a series of 1-arcsec-
ond steps — in the direction perpendicular
to the slits in order to cover the full field of
view — and took a spectroscopic exposure
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
January 2019 / 2018 Year in Review