IRAS 13218+0552 is classified as a Seyfert type 1.5 galaxy[2] given its large [OIII] flux although XMM-Newton did not observe it. Further studies showed it as a Seyfert type 2 galaxy instead, as it harbors a highly obscured active galactic nucleus and not of Seyfert 1 type.[3] Moreover, it belongs to the ultraluminous galaxy classification, because according to IRAS, its luminosity range Lir = 1012-1013 L⊙ is found to be approximated by the power law of Φ(L) ~ L-2.35[Mpc-3 mag-1].[4][5]
Besides being a Seyfert galaxy and a luminous inflared galaxy, IRAS 13218+0552 also has a quasar nucleus which is notable for its extreme outflows and has strong star formations.[6] That being said, it resulted from a collision between two gas-rich disk galaxies.[7] Evidence showed both galaxies have orbited each other several times before merging with each other; signs left included distinct loops of glowing gas around the quasar's host.[8] Apart from the loop of gas, IRAS 13218+0552 has a tidal tail feature and possibly binary nucleus with its separation smaller than 1 kpc.[9]
Detected through targeted surveys,[10][11] observations find IRAS 13218+0552 hosts an OH megamaser (OHM), producing nonthermal emission from the hydroxyl (OH) molecules, with its two main lines situated at 1665/166 MHz and two satellite lines at 1612/1720 MHz.[12] This might be caused by OHM emission being pumped by infrared radiation from the galaxy's environment and also amplification of an intense radio continuum background. Through the observation, IRAS 13218+0552 has an OH spectrum showing two prominent broad emission peaks, having a separation of 490 km s−1 in its rest frame, suggesting it is associated with multiple nuclei.[13] This makes IRAS 13218+0552 among 119 OHMs found in ultraluminous galaxies right up to 2014.[14]
