Investigation On The Circumnuclear Environment And The Excitation Of Extragalactic Water Megamaser

Maser emission is one kind of common natural phenomena in the cosmos.H₂O maser is one of the most common astrophysical masers.ln the Galaxy,H₂O maser emissions have been detected since first detected toward Orion-KL,Sgr B2 and W49. Those masers originate in dense molecular gas in star formation regions.Moreover, H₂O maser emissions were also detected toward external galaxies,e.g.,first detection towards M33.Besides that similar masers were also found in star formation regions of nearby galaxies,in particular,maser emissions were also detected in the nuclear regions of active galaxies,with surprisingly large luminosities of～10²-10⁴ L_⊙,i.e.million times more luminous than typical Galactic H₂O maser sources.So these masers are called the "megamasers"(L_{H2O＞10L⊙},which have apparently different properties from the interstellar masers.Given their very high luminosities,this kind of nuclear masers are considered to be originated in circumnuclear dense clouds and the only possible energy source should be the active galactic nuclei(AGN),instead of stars in the case of the Galactic masers.Observations show that H₂O megamaser spots locate preferentially in nuclei with high gas column densities(N_H＜10²⁴cm^-2).AGN is considered to be the ultimate energy source of megamaser.Based on AGN unified model,a BH exists in the center and dominates the intense activity and strong radiation of AGN by accretion.The X-rays from AGN can heat the circumnuclear dense gas and give rise to a high density molecular layer in which powerful,collisionally pumped H₂O masers can exist.in this paper,the whole extragalactic H₂O maser sources with their observation data were investigated and analyzed in order to probe possible relation between maser emission and the X-ray emission,the central engine of maser host galaxy,and to discuss the excitation of water megamaser.