A SEARCH FOR H2O MEGAMASERS IN HIGH-Z TYPE-2 AGNS Nicola Bennert1 Institute of Geophysics and Planetary Physics, University of California, Riverside, CA 92521 [email protected] Richard Barvainis2 National Science Foundation, 4301 Wilson Boulevard, Arlington, VA 22230; and Department of Physics, Gettysburg College, 300 North Washington Street, Gettysburg, PA 1732 [email protected] Christian Henkel Max-Planck-Institut f¨ur Radioastronomie, Auf dem H¨ugel 69, D-53121 Bonn, Germany [email protected] and Robert Antonucci Department of Physics, University of California, Santa Barbara, CA 93106 [email protected] ABSTRACT We report a search for H2O megamasers in 274 SDSS type-2 AGNs (0.3 < z < 0.83), half of which can be classified as type-2 QSOs from their [O III] 5007 luminosity, using the Robert C. Byrd Green Bank Telescope (GBT) and the Effelsberg 100-m radio telescope. Apart from the detection of the extremely luminous water vapor megamaser SDSS J080430.99+360718.1, already reported by Barvainis & Antonucci (2005), we do not find any additional line emission. This high rate of non-detections is compared to the water maser luminosity function created from arXiv:0901.0567v1 [astro-ph.GA] 5 Jan 2009 the 78 water maser galaxies known to date and its extrapolation towards the higher luminosities of “gigamasers” that we would have been able to detect given the sensitivity of our survey. The properties of the known water masers are summarized and discussed with respect to the nature of high-z type-2 AGNs and megamasers in general. In the appendix, we list 173 additional objects (mainly radio galaxies, but also QSOs and galaxies) that were observed with the GBT, the Effelsberg 100-m radio telescope, or Arecibo Observatory without leading to the detection of water maser emission. Subject headings: galaxies: active — galaxies: Seyferts — masers — quasars: general 1Current address: Department of Physics, University of tions expressed in this material are those of the author and California, Santa Barbara, CA 93106 do not necessarily reflect the views of the National Science 2 Any opinions, findings, conclusions, and recommenda- Foundation. 1 1. INTRODUCTION narrow emission lines. Indeed, with the advent of new extended surveys such as the Sloan Digi- The 22 GHz H2O maser emission line is of tal Sky Survey (SDSS), many type-2 QSOs have great astrophysical interest for its extreme re- recently been identified (Zakamska et al. 2003). quirements for density (>107 cm−3), temperature We conducted a search for water megamasers in (>300 K), and of course radial velocity coher- 274 of the 291 SDSS type-2 AGNs (Zakamska et al. ence. It is detected in both Galactic and extra- 2003) using the Robert C. Byrd Green Bank Tele- galactic star forming regions as well as in the cen- scope (GBT) and the Effelsberg 100-m radio tele- tral regions of active galaxies (AGNs). In AGNs, scope. With a redshift range of 0.3 < z < 0.83, isotropic luminosities commonly reach values of the sample covers significantly higher distances L > 10L and the objects are then classi- H2O ⊙ than most previous searches for H O megamasers fied as “megamasers” (see recent reviews by e.g. 2 (z ≪ 0.1; Braatz et al. 1996; Tarchi et al. 2003; Greenhill 2004; Morganti et al. 2004; Henkel et al. Braatz et al. 2004; Kondratko et al. 2006a,b; Braatz & Gugliucci 2005b; Lo 2005). 2008; Castangia et al. 2008) and is the first survey So far, water megamaser emission has been for water megamasers in objects with QSO lumi- detected in about 10% of active galactic nuclei nosities (except for the study of Barvainis & Antonucci (AGNs) surveyed in the local universe (Braatz et al. (2005) which is part of the larger survey presented 2004). The association of water megamasers with here). AGNs of primarily Seyfert-2 or Low-Ionization Such a search provides clues to whether the uni- Nuclear Emission-line Region (LINER) type (Braatz et al. fied model can indeed be extended to QSOs or 1997, 2004) and the fact that the emission often whether their powerful engines lead to a differ- arises from the innermost parsec(s) of their parent ent scenario. Do the high QSO luminosities re- galaxy have raised great interest in the study of sult in H O “gigamasers”? Or do they destroy 22 GHz maser emission. It suggests that the so far 2 the warm dense molecular gas needed to supply poorly constrained excitation mechanism is closely the water molecules? Are the molecular parts of related to AGN activity, probably irradiation by the accretion disks much farther away from the nu- X-rays (e.g. Neufeld et al. 1994). For Seyfert-2 clear engine, so that rotation velocities are smaller galaxies, in the framework of the so-called unified in spite of a potentially more massive nuclear en- model (Antonucci 1993), a dusty molecular disk gine than in Seyfert-2 galaxies? Finding mega- or torus is seen edge-on where the conditions and masers in type-2 QSOs may provide insights into velocity-coherent path lengths are favorable for QSO molecular disks and tori and enables us to the formation of megamaser activity. independently determine their BH masses. Even In those cases in which the emission arises more importantly, megamasers in type-2 QSOs from a nuclear disk and can be resolved spa- may provide the unique possibility to directly mea- tially using Very Long Baseline Interferome- sure their distances and thus verify the results try (VLBI), the central black hole (BH) mass from type 1a supernovae measurements on the ex- can be constrained, as has been successfully istence and properties of the elusive dark energy shown for NGC 4258 (e.g. Greenhill et al. 1995; (e.g. Barvainis & Antonucci 2005; Braatz et al. Miyoshi et al. 1995; Herrnstein et al. 1999, 2005). 2007; Reid et al. 2008). Moreover, using H O masers, distances to galax- 2 We summarize the sample properties in §2, de- ies can be obtained without the use of standard scribe the observations in §3, present the results in candles (Miyoshi et al. 1995; Herrnstein et al. §4, and discuss them in §5. After a brief summary 1999; Argon et al. 2004; Brunthaler et al. 2005; (§6), we list a sample of 171 additional objects (ra- Argon et al. 2007; Humphreys et al. 2008). Thus, dio galaxies, QSOs, and galaxies) in the Appendix finding new megamaser galaxies is of great inter- (§A). Throughout the paper, we assume a Hubble est. −1 −1 constant of H0 = 75kms Mpc . For the high- If the unified scheme for AGNs is to be equally z objects, we additionally assume ΩΛ = 0.73 and successful for objects of high as well as of low lumi- ΩM = 0.27 (Wright 2006). nosity, there should exist a large number of type- 2 QSOs whose optical spectra are dominated by 2 2. SAMPLE PROPERTIES bandwidth was covered with ∼0.5 km s−1 chan- nels. Antenna pointing checks were made roughly As already mentioned above, our sample con- every 2 hours, and typical pointing errors were less sists of 274 type-2 AGNs (0.3 < z < 0.83) selected than 1/10 of a full width to half power (FWHP) from the SDSS (Zakamska et al. 2003). Out of beamwidth of 48′′ at 15 GHz. GBT flux calibra- 8 these, 122 objects have L[OIII] > 3 10 L⊙, and can tion was done using standard antenna gain vs fre- thus be classified as type-2 QSOs (Zakamska et al. quency curves. We estimate the calibration uncer- 2003). About 10% of the SDSS type-2 AGNs tainty to be ∼20%. are radio-loud (Zakamska et al. 2004), compa- rable to the AGN population as a whole. A 3.2. Effelsberg few type-2 AGNs have soft X-ray counterparts (Zakamska et al. 2004). Spectropolarimetry was A total of 150 SDSS type-2 AGNs were ob- 2 carried out for 12 type-2 QSOs and revealed po- served with the Effelsberg 100-m radio telescope . larization in all objects. Five objects show po- The measurements were carried out with a dual larized broad lines expected in the framework polarization HEMT receiver providing system of the unified model at the sensitivity achieved temperatures of ∼36–45 K (for the observed fre- (Zakamska et al. 2005). Zakamska et al. (2006) quencies between ∼14.3 and 17.1 GHz) on a main studied the host galaxy properties for nine ob- beam brightness temperature scale. The observa- jects, finding that the majority (6/9) of the type-2 tions were obtained in a position switching mode. QSO host galaxies are ellipticals. All observations Signals from individual on- and off-source posi- support the interpretation of the type-2 AGNs se- tions were integrated for 3 minutes each, with lected from the SDSS as being powerful obscured the off-position offsets alternating between +900 AGNs. Table 1 summarizes the sample properties. and –900 arcsec in Right Ascension. The typi- cal on-source integration time was ∼70 minutes 3. OBSERVATIONS (possibly longer for objects with follow-ups) with variations due to weather and elevation. An auto- All sources were measured in the 616-523 line of correlator backend was used, split into eight bands H2O (22.23508 GHz rest frequency). The obser- of 40 MHz width and 512 channels, respectively, vations were carried out during several runs at the that were individually shifted in such a way that GBT of the National Radio Astronomy Observa- a total of ∼130–240MHz was covered. Channel tory (NRAO) in January and June 2005 as well as spacings are ∼1.5 km s−1.