Detection of 183 GHz H2O megamaser emission towards NGC 4945

被引：20

作者：

Humphreys, E. M. L. ^{[1
]}

Vlemmings, W. H. T. ^{[2
]}

Impellizzeri, C. M. V. ^{[3
,4
]}

Galametz, M. ^{[1
]}

Olberg, M. ^{[2
]}

Conway, J. E. ^{[2
]}

Belitsky, V. ^{[2
]}

De Breuck, C. ^{[1
]}

机构：

[1] ESO, Karl Schwarzschild Str 2, D-85748 Garching, Germany

[2] Chalmers, Onsala Space Observ, Dept Earth & Space Sci, S-43992 Onsala, Sweden

[3] Joint Alma Off, Alsonso de Cordova 3107, Santiago 7630355, Chile

[4] Natl Radio Astron Observ, 520 Edgemont Rd, Charlottesville, VA 22903 USA

来源：

ASTRONOMY & ASTROPHYSICS | 2016年 / 592卷

关键词：

galaxies: Seyfert; masers; submillimeter: general; WATER MASER EMISSION; ACTIVE GALACTIC NUCLEI; GEOMETRIC DISTANCE; HUBBLE CONSTANT; GALAXY NGC-4258; UGC; 3789; REGION; STARS; GAS;

D O I：

10.1051/0004-6361/201629168

中图分类号：

P1 [天文学];

学科分类号：

0704 ;

摘要：

Aims. The aim of this work is to search Seyfert 2 galaxy NGC 4945, a well-known 22 GHz water megamaser galaxy, for H2O (mega) maser emission at 183 GHz. Methods. We used APEX SEPIA Band 5 (an ALMA Band 5 receiver on the APEX telescope) to perform the observations. Results. We detected 183 GHz H2O maser emission towards NGC 4945 with a peak flux density of similar to 3 Jy near the galactic systemic velocity. The emission spans a velocity range of several hundred km s(-1). We estimate an isotropic luminosity of > 1000 L-circle dot, classifying the emission as a megamaser. A comparison of the 183 GHz spectrum with that observed at 22 GHz suggests that 183 GHz emission also arises from the active galactic nucleus (AGN) central engine. If the 183 GHz emission originates from the circumnuclear disk, then we estimate that a redshifted feature at 1084 km s(-1) in the spectrum should arise from a distance of 0.022 pc from the supermassive black hole (1.6 x 10(5) Schwarzschild radii), i.e. closer than the water maser emission previously detected at 22 GHz. This is only the second time 183 GHz maser emission has been detected towards an AGN central engine (the other galaxy being NGC 3079). It is also the strongest extragalactic millimetre/submillimetre water maser detected to date. Conclusions. Strong millimetre 183 GHz H2O maser emission has now been shown to occur in an external galaxy. For NGC 4945, we believe that the maser emission arises, or is dominated by, emission from the AGN central engine. Emission at higher velocity, i.e. for a Keplerian disk closer to the black hole, has been detected at 183 GHz compared with that for the 22 GHz megamaser. This indicates that millimetre/submillimetre H2O masers can indeed be useful for tracing out more of AGN central engine structures and dynamics than previously probed. Future observations using ALMA Band 5 should unequivocally determine the origin of the emission in this and other galaxies.

引用

页数：4