GeminiFocus July 2018 | Page 8

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). 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. 6 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. GeminiFocus 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), 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 July 2018