with its larger neighbor NGC 4725. Like AGC
208457, these stellar aggregates likewise
have relatively high metallicities, but they
are in an earlier stage of evolution. Thus,
they may represent a tidal dwarf galaxy in
the process of formation. By using a combi-
nation of radio data, wide-field imaging, and
GMOS spectroscopy to confirm the nature
of these objects, this work significantly ex-
pands the limited sample of well document-
ed tidal dwarf galaxies. mainly because the vast distances and ex-
treme redshifting will make any radio signal
difficult to detect. Moreover, powerful radio
jets, and the black holes that power them,
have not had sufficient time to grow to large
sizes at such early times. Now, an interna-
tional team of astronomers from Brazil, Italy,
the Netherlands, and the United Kingdom
has discovered the most distant radio galaxy
to date, observed just one billion light years
after the Big Bang, when the Universe was
roughly 7% of its current age.
Confirmation of the Most
Distant Known Radio Galaxy The team used spectroscopic data from
GMOS-North to measure a redshift of z = 5.72,
based on the Lyman-α line, for the radio gal-
axy identified as TGSS J1530+1049 (Figure 7).
This is the largest redshift of any known radio
galaxy. The object was selected as a high-
redshift radio galaxy candidate based on its
very steep spectral index at a frequency of
150 megahertz and its compact morphology
in radio imaging by the Very Large Array at
More than a dozen galaxies have been re-
ported at redshifts beyond 7. These tend to
be highly magnified star-forming objects
found at infrared wavelengths, seen when
the Universe was less than 5% of its current
age. However, radio emission from such
objects has not yet been detected. This is
January 2019 / 2018 Year in Review
GeminiFocus
Figure 7.
Top panel: The two-
dimensional GMOS spec-
trum showing the strong
emission feature detected
in the radio galaxy TGSS
J1530 + 1049. The size
of the emission region
is a bit less than 1 arc-
second. Lower panel:
One-dimensional plot
of the spectrum of TGSS
J1530 + 1049 (black line)
is compared to a simple,
symmetric Gaussian fit
(orange line) to the emis-
sion feature. The asym-
metry of the data with
respect to the Gaussian fit
indicates that the emis-
sion is Lyman-α at red-
shift of z = 5.72, making
TGSS J1530 + 1049 the
most distant radio galaxy
known to date.
[Figure reproduced from
Saxena, et al., MNRAS,
480: 2733, 2018.]
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