• OH megamasers: dense gas & the infrared radiation field

• # Fulltext

https://www.ias.ac.in/article/fulltext/joaa/039/03/0034

• # Keywords

Maser – galaxies; nuclei; galaxies; LIRGs; galaxies; dense gas; galaxies.

• # Abstract

To investigate possible factors related to OH megamaser formation (OH MM, $L_{{\rm H}_2{\rm O}} > 10L_{\odot}$), we compiled a large HCN sample from all well-sampled HCN measurements so far in local galaxies and identifiedwith the OH MM, OH kilomasers ($L_{{\rm H}_2{\rm O}} > 10L_{\odot}$, OH kMs), OH absorbers and OH non-detections (non-OH MM). Through comparative analysis on their infrared emission, CO and HCN luminosities (good tracers for the low-density gas and the dense gas, respectively), we found that OH MM galaxies tend to have stronger HCN emission and no obvious difference on CO luminosity exists between OH MM and non-OH MM. This implies that OH MM formation should be related to the dense molecular gas, instead of the low-density molecular gas. It can be also supported by other facts: (1) OH MMs are confirmed to have higher mean molecular gas density and higher dense gas fraction ($L_{\rm HCN}/L_{\rm CO}$) than non-OH MMs. (2) After taking the distance effect into account, the apparent maser luminosity is still correlated with the HCN luminosity, while no significant correlation can be found at all between the maser luminosity and the CO luminosity. (3) The OH kMs tend to have lower values than those of OH MMs, including the dense gas luminosity and the dense gas fraction. (4) From analysis of known data of another dense gas tracer HCO$^+$, similar results can also be obtained. However, from our analysis,the infrared radiation field can not be ruled out for the OH MM trigger, which was proposed by previous works on one small sample (Darling in ApJ 669:L9, 2007). On the contrary, the infrared radiation field should play one more important role. The dense gas (good tracers of the star formation) and its surrounding dust are heated by the ultra-violet (UV) radiation generated by the star formation and the heating of the high-density gas raises the emission of the molecules. The infrared radiation field produced by the re-radiation of the heated dust inturn serves for the pumping of the OH MM.

• # Author Affiliations

1. State Key Laboratory of Geodesy and Earths Dynamics, Institute of Geodesy and Geophysics, CAS, Wuhan 430077, China
2. University of Chinese Academy of Sciences, Beijing 100049, China.
3. Physics Department, School of Science, Wuhan University of Technology, Wuhan 430070, China.
4. Center for Astrophysics, Guangzhou University, Guangzhou 510006, China.

• # Journal of Astrophysics and Astronomy

Current Issue
Volume 40 | Issue 6
December 2019

• # Continuous Article Publication

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