evolved stars, resulting in cosmic maser
emission. Indeed, these emissions (intense
laser-like spectral lines at microwave fre-
quencies) seem to be found in star-forming
regions, close to young-stellar objects such
as protostars and compact HII regions. As a
number of ULIRGs are also a source of OH-
megamaser emissions, observations of them
may hold important clues as to the main
power source in these galaxies (Lo, 2005).
OH megamaser emission lines are commonly
associated with ULIRGs with the warmest IR
colors. Importantly, the OH luminosity is also
observed to increase with the IR luminosity:
the emission from these interacting galax-
ies is surprisingly brighter than that associ-
ated with galactic masers in non-interacting
galaxies. The OH megamaser emission can
be used to trace high density regions, and
separation of the megamaser emission com-
ponents could be linked to outflows of less
dense ma tter, but at high speed.
A general feature of many models is that
the masers are pumped radiatively by the
absorption of infrared photons. Identifying
the source of the maser pump may therefore
indicate whether the ultimate energy source
is a burst of star formation, or an active nu-
cleus (Lo, 2005).
Dissecting an OH Megamaser
Galaxy Merger
We used multiwavelength observations to
investigate the origin of the gas emission
and kinematics of the inner 6 kiloparsecs
(kpc) of the OH megamaser galaxy IRAS
F23199+0123. Located at a distance of 558
megaparsecs (Mpc), its OH megamaser was
first detected by the Arecibo OH Megama-
ser Survey (which observed 52 objects with
0.1< z <0.26). Previous optical spectroscopy
of the galaxy suggests that it also harbors a
type 2 active galaxy nucleus (AGN).
July 2018
The data comprise both optical and radio
imaging, as well as spectroscopy obtained
with the Gemini Multi-Object Spectrograph
(GMOS)/integral field unit (IFU) at the Gemi-
ni North telescope, Hubble Space Telescope
(HST), and Very Large Array (VLA). We used
the HST to make I-band, broad-band, and
[NII]+H-alpha narrow-band images. We used
the VLA to obtain 1.6- and 8-gigahertz (GHz)
continuum images and spectra centered at
the OH maser line. And we used Gemini’s
GMOS/IFU to conduct H-alpha and [N II;
(6583-Ångstrom) observations that would
match HST’s.
The HST observations revealed that IRAS
F23199+0123 is actually an interacting pair
with a tidal tail connecting the two galaxies
(Figure 1). The VLA observations indicate that
both nuclei present extended radio emission
at 3 and 20 centimeters (cm), with intensity
peaks at each nucleus. The 20-cm radio emis-
sion of the eastern nucleus is elongated in
the direction of the most extended emission
in the HST continuum image (northeast –
southwest), while in the western nucleus the
20-cm radio emission is tilted by about 45º
(Figure 2). The VLA spectra also reveal two
GeminiFocus
Figure 2.
Top panel: VLA L-band
(1.6 GHz) continuum
image of IRAS
F23199+0123, shown
as filled contours. The
contours are (black) 0.071
(3 σ), 0.15, (white) 0.32,
0.69, and 1.5 mJy beam-¹.
The OH1 and OH2 labels
identify the locations
where the OH maser
sources were detected.
Bottom panel: VLA
X-band (8 GHz)
continuum image of IRAS
F23199+0123, shown
as filled contours. The
contours are (black)
0.0278 (3σ), 0.0647,
(white) 0.150, and 0.349
mJy beam-¹.
7