張江水，李海坤，王 金，劉智偉

(廣州大學(xué)天體物理中心，廣東廣州 510006)

0 Introduction

Since the first detection in 1969［1］，H2O molecular maser emission(～22 GHz)was reported in more than 2 000 sources in our Galaxy.Observations show that H2O maser emission can trace dense(≥107cm－3)and warm(≥400 K)gas circumstance associated with high mass star-forming regions in our Galaxy and very nearby galaxies.While in galaxies with relative large distance，this kind of masers could not be detected due to the limits of our instrument’s sensitivity.However，maser emission was found unexpectedly in the nuclear regions of active galaxies.And this kind of masers has surprisingly large luminosities of ～102－104L⊙，which is about million times more luminous than typical Galactic H2O maser sources(so these masers are called the“megamasers”)．H2O megamasers have become a very important subject and a promising tool for addressing a wide variety of astrophysical problems，such as mass estimation of supermassive black holes，determination of the distance of maser host galaxies，and providing a perspective to improve the accuracy of the Hubble constant and to constrain the equation of state for the elusive dark energy［2］． To date， among targeted～4 000 galaxies，H2O maser line emission has been detected in～160 galaxies①https://safe．nrao．edu/wiki/bin/view/Main/MegamaserCosmologyProject，with a low detection rate of～4%．To search more new H2O megamaser in various types of AGN，especially disk megamasers，is very helpful for us to eliminate those issues．

The H2O megamasers are believed to be related to Active Galactic Nuclei(AGNs)for their extremely large luminosities(assumed isotropic luminositiesLH2O＞10L⊙)，which was supported by all so far interferometrically studies of megamasers(a small fraction of the known ones)．Observations show that those maser spots locate preferentially in heavily obscured(gas column densityNH＞1023cm－2)or even Compton-thick nuclei region(NH＞ 1024cm－2，e．g．，［3］－［6］)．Conclusive identifications of Compton-thick AGN are made through spectroscopic X-ray observations performed atE＞ 10 keV(e．g．，Beppo-SAX，Swift，Suzaku，INTEGRAL)，where the relatively unabsorbed high-energy emission can be directly detected［7－8］．Among all unambiguously identified bona-fide Compton-thick AGN［8］，we found that more than half of them have been detected with H2O megamaser emission．So heavily obscured or even Compton-thick AGNs are good targets for searching H2O megamaser emission．

Previous surveys always focused on optically identified AGN samples．The optical identification of AGNs is always based on the relative strength of forbidden and permitted emission lines［9－10］or on a broad permitted line(FWHM ＞1 000 km·s－1)．However，these criteria are insensitive to the identification of heavily dust-obscured AGNs．X-ray observations have revealed potential AGNs in many galaxies lacking optical AGN signatures［11］．MIR spectroscopy(e．g．，Spitzer，AKARI)provides an unambiguous indicator of AGN activity in nearby galaxies［12］．For relatively lower optical depth，the Mid-IR fine structure lines are significantly less affected by extinction than UV，optical or near-IR lines．Even heavily Compton-thick AGNs that are weak at X-ray energies can be identified using mid-IR spectroscopy［13－14］．Unlike Low excitation ionic fine structure lines，high excitation ionic emission lines(e．g．，［Ne V］14.32 μm;24.32 μm;［O IV］25.89 μm)require the very hard radiation fields of AGN．Thus，the identification of high-ionization emission lines provides a relatively optically thin means by which to probe the central engine of nearby AGN［12］．In addition，MIR color criteria and MIR-excess were found to be quite effective for finding obscured AGN or Compton-thick AGN missing in the optical surveys［15－16］．

Here we present a pilot survey to search H2O maser emission from one small sample of heavily obscured AGNs，which was identified from MIR spectroscopy and colors，but lacking AGN features in optical spectra．

1 Observations

Our observations in the 616－523transition of H2O molecular(the rest frequency:22.235 08 GHz)were performed in January 2014，with Effelsberg 100 m radio telescope of the Max Planck Institut für Radioastronomie(MPIfR)located in Bonn，Germany．We used the P13mm receiver(18～26 GHz)and XFFT spectrometer with 500 MHz bandwidth and 32 768 channels，which provide～15.3 kHz frequency resolution(～0.21 km·s－1at 22 GHz)．Adual channel HEMT receiver provided system temperatures of 15～40 K on a main beam brightness temperature scale．The observations were obtained in a position switching mode，with the off-position offsets of 900 arcsecs in Right Ascension．Signals from individual on-and off-source positions were integrated for 150 seconds each，and 10 repeats gives the typical on-source integration time of ～60 minutes．Our targeted sources are listed in Table 1．

2 Results and discussion

The data were reduced using the CLASS and GREG packages of the GILDAS software①http://www．iram．fr/IRAMFR/GILDAS．Flux calibration was carried by frequent measurements of NGC7027，which was typical calibration source with one reported ～ 22 GHz flux densities of 5.53 Jy［17］．The ratio of its flux(5.53)and its measurement value can be used to determine the flux density of our target sources，i．e．，this ratio times the measurement value of each source．Their spectra were presented in Fig．1 and their system velocities were also marked with arrows．Observational parameters and results are displayed in Table 1．None of the observed sources was detected，although some show possible emission feature．The root mean square(rms)flux density is from 2 mJy to 8 mJy after being smoothed to a velocity resolution of ～20 km·s－1(see Fig．1 and Table 1)．

Based on the sensitivity of our observations，the upper luminosity limit can be derived［18］:

Where ∫F'dν is the integrated flux in the observed frame，F(xiàn)'peakand Δν are the peak flux density(in Jy)and the frequency width of spectra line，zis the redshift，DLis the luminosity distance in Mpc of the source．

where Δvis the velocity width in km·s－1，the rest frequency of H2O maser line is 22.235 08 GHz，the speed of light c～3*105km·s－1．Thus

Assuming a characteristic linewidth of 20 km·s－1andF'peak～ 3*rms，the upper luminosity limit was estimated for our targeted sources and listed in the last Column in Table 1．

Table 1 Source list

Fig.1 Spectra of our obscured-AGN sample observed by Effelsberg 100 m telescope.The velocity resolution is smoothed to be ～20 km·s－1.And the system velocity of each source is also marked in one vertical arrow

3 Summary

H2O megamaser spots were found to prefer to be in heavily obscured AGNs.Through Effelsberg 100 m radio telescope in Germany，we performed one pilot survey in January 2014 to search H2O maser emission among one obscured AGN sample，which was identified from MIR spectroscopy and colors，but missing in optical identification.Our observation results show no significant maser emission among our targets，though some of them show possible maser signals.Based on the noise level of our observations and the distance of our targets，we estimated the upper limit of H2O maser isotopic luminosity for our sample，which should be helpful for guiding future H2O megamaser searching with high sensitivity.

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