Luminous infrared galaxies (LIRGs) and
ULIRGs have luminosities that exceed 10¹¹
and 10¹² L Sun , respectively, in the infrared.
The source of their very large far-infrared
luminosities may reflect a quasar-like active
nucleus surrounded by a torus of dense gas
and dust (the latter absorbing the energetic
photons from the nuclear region and re-
emitting at infrared wavelengths), or a huge
burst of massive-star formation in dense
dusty clouds of molecular gas close to the
nucleus, which heats the surrounding dust
(Skinner et al., 1997).
When galaxies merge, the gas clouds close
to their nuclei are shocked and heated by
the collision, and the emission from certain
molecules especially OH is strongly ampli-
fied. These interaction environments seem
to supply all the requirements to originate
phenomena such as the emission of ma-
sers (microwave amplification by stimulated
emission of radiation).
OH megamaser galaxies are a subclass of ul-
traluminous infrared galaxies, which emit pre-
dominantly at microwave frequencies, in 1665
and 1667 megahertz (MHz). In the forties, mol-
ecules were detected in the interstellar medi-
um when absorption lines of CH, CH+, and CN
were evinced in the spectra of stars. These mol-
ecules show transitions in the visible and are
susceptible to detection by optical telescopes.
The hydroxyl radical (OH) was discovered in
1943 and was the only molecule detected at
radio wavelengths until 1968. The first OH
extragalactic megamasers were detected
in the spiral galaxy NGC 253 (Whiteoak &
Gardner, 1973; Gardner & Whiteoak, 1975)
and in M82 (Rieu et al., 1976). ARP 220 is an
example of an OH megamaser emission host
detected with the Arecibo radio telescope in
1982, presenting 400 L Sun , about 10 8 times
higher than the masers found in the W3 (OH)
molecular cloud complex. This discovery led
to the realization of a survey aimed at find-
ing other OH megamasers using telescopes
with big collecting areas.
Very-long-baseline interferometry also de-
tected OH megamasers hosted in III Zwicky
35, IRAS 17208-0014 (Diamond et al., 1999),
IRAS 12032 + 1707 (Londsdale et al., 2003a),
Markarian (Mrk) 231 (Klockner et al., 2003),
Mrk 273 (Klockner & Baan, 2004), and IRAS
14070+0525 (Pihlstrom et al., 2005). Darling
& Giovanelli (2002) performed a survey with
the Arecibo telescope that searched for OH
megamasers in 300 IRAS galaxies at z > 0.1,
resulting in the detection of 100 OH mega-
masers in LIRGs.
In a galaxy, microwave radiation can be
amplified in the interstellar medium in the
immediate neighborhood of young stellar
objects, or circumstellar envelopes around
Figure 1.
Left: HST large-scale
continuum-free
H-alpha+[NII] image.
Right: Closeup of
the region observed
with GMOS-IFU
(green box); the field
of view is 3.5 x 5.5
arcseconds, and the
color bars show the
fluxes in arbitrary
units.
January 2019 / 2018 Year in Review
GeminiFocus
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