Figure 6.
Anatomy of the
tidal dwarf galaxy
AGC 208457. The HI
emission contours
from the GMRT are
superimposed on a
CFHT MegaCam image
of the region between
the interacting galaxies
NGC 3166 and NGC
3169. The northwest
and southeast stellar
clumps of AGC 208457,
lying within the HI
tail, are indicated. The
green outline in the
zoomed view on the left
shows the orientation
of the GMOS-North
long slit (illustrated
with twice the actual
width). The yellow
arrows indicate the
approximate locations
of the extracted spectra
analyzed in the study.
[Figure reproduced
from Lee-Waddell, et
al., MNRAS, 480: 2719,
2018.]
28
smaller galaxies known as tidal dwarfs, rath-
er than a large merger remnant. Dwarf gal-
axies of this type are predicted to form when
gaseous material that is tidally stripped
from one of the larger galaxies condenses to
form a gravitationally bound stellar system.
The stripped gas may be highly enriched, in
which case the resulting dwarf will have an
unusually high metal content for its stellar
mass. Galaxies formed in this way are also
expected to have very little dark matter.
However, it is difficult to ascertain the past
history of any particular dwarf, and identify-
ing tidal dwarfs in the process of formation
has been quite tricky in practice.
A team of astronomers from Australia, Can-
ada, Argentina, Italy, and the United States
have used GMOS at Gemini North to obtain
long-slit spectroscopy of candidate tidal
dwarfs found near two pairs of large inter-
acting galaxies (Figure 6). The objects were
first identified as candidate tidal dwarfs
based on their neutral hydrogen emission
observed with the Giant Metrewave Radio
Telescope (GMRT) in India and their loca-
tions on the outskirts of larger gas-rich gal-
axies. Subsequent optical imaging with the
Canada-France-Hawai‘i Telescope identified
GeminiFocus
low-surface brightness stellar counterparts
of the HI tidal features and enabled stel-
lar mass estimates. The team proposed for
GMOS spectroscopy to determine if the ob-
served stellar components are physically as-
sociated with the HI tails.
The resulting study, published in Monthly
Notices of the Royal Astronomical Society,
confirms that the optically identified dwarf
galaxy known as AGC 208457 has a velocity
consistent with the HI velocity at its loca-
tion within the extended tidal feature asso-
ciated with the interacting galaxy pair NGC
3166/3169. The galaxy’s metal abundance
and star formation rate inferred from the
optical emission lines indicate that it formed
recently from enriched material processed
within the larger galaxies. In addition, the
study finds that there is no evidence for a
significant amount of dark matter. Thus, AGC
208457 has all the characteristics of a genu-
ine tidal dwarf galaxy.
Targeting a second system, the study con-
firms the physical association of gaseous
knots and star clusters with the extended
tidal tail of NGC 4747, a disturbed galaxy
that likely experienced a recent interaction
January 2019 / 2018 Year in Review