when mass transfer would be most efficient,
while the plateau phase of the Great Erup-
tion may have been the effect of a hydrody-
namic explosion of uncertain origin and its
consequent shock plowing through the cir-
cumstellar material. What is clear is that the
eruption involved enormous mass loss: the
bipolar “Homunculus Nebula” contains at
least 15 solar masses of material expanding
away from the star at about 600 km/s and
dates from this event.
Although the Great Eruption concluded 160
years ago, it is possible to observe the light
from that event reflected off cold clouds on
the far side of the extended Carinae Nebula
complex. A 2012 study of such “light echoes”
reported observations of Eta Car’s pre-1845
luminosity spikes illuminating a group of
background clouds. Now, the same team has
published two new papers dissecting light
echoes reflected by another cloud at a lesser
distance from the star. Based on the geom-
etry, the team believes the light is associated
with the enormous mass loss that occurred
during the plateau phase of the Great Erup-
tion in the 1850s. Figure 5 shows spectra
of the light echoes taken with the Gemini
Multi-Object Spectrograph (GMOS) at Gemini
South and the Inamori-Magellan Areal Cam-
era and Spectrograph (IMACS) at Magellan
Observatory.
The H-alpha emission lines in the new spec-
tra have wings that reach –10,000 km/s to
the blue and at least +12,000 km/s to the
red. The team argues that the wings span
the range of mass outflow speeds during the
plateau phase of the Great Eruption; such
speeds on stellar scales have only been seen
previously in supernova ejecta and outflows
from accreting compact stellar remnants.
The broad wings are absent in the previ-
ously studied echoes of the earlier phases
of the eruption. The extremely fast material
constitutes only a small fraction of the total
ejecta, the majority of which is expanding at
January 2019 / 2018 Year in Review
about 600 km/s. However, it provides strong
evidence in favor of the explosive outflow
explanation of the Great Eruption. The new
papers have been accepted for publication
in the Monthly Notices of the Royal Astronom-
ical Society.
Dwarfs Emerge from the Tidal
Debris of Interacting Galaxies
Large galaxies are produced through the
merging or accretion of smaller galaxies. If
the merging galaxies contain enough gas-
eous material, a burst of star formation may
cause the stellar mass of the final galaxy to
be substantially larger than the combined
mass of the stars of the two original galaxies.
This is the basis of hierarchical structure for-
mation, the standard paradigm in the field
of galaxy evolution for many decades.
Figure 5.
Gemini/GMOS and
Magellan/IMACS
spectra centered on
the Hα line of the
light echo believed
to correspond to the
latter part (circa 1855)
of Eta Carinae’s Great
Eruption. The spectra
show very broad Hα
line wings extending to
at least ±10,000 km/s,
indicating outflow
velocities typically seen
in supernovae. The
blue curve represents
a blackbody of
temperature 6,000 K.
[Figure reproduced
from Smith, Rest,
Andrews, et al., 2018.]
If two gas-rich galaxies exchange a glanc-
ing blow, rather than a head-on collision,
the encounter may give birth to one or more
GeminiFocus
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