A Green Bank Telescope Search for Water Masers in Nearby Active Galactic Nuclei

Abstract

Using the Green Bank Telescope, we have conducted a survey for 1.3 cm water maser emission toward the nuclei of nearby active galaxies, the most sensitive large survey for H 2 O masers to date. Among 145 galaxies observed, maser emission was newly detected in eleven sources and confirmed in one other. Our survey targeted nearby (v < 12,000 km s −1 ), mainly type 2 AGNs north of δ = -20 • , and includes a few additional sources as well. We find that more than a third of Seyfert 2 galaxies have strong maser emission, though the detection rate declines beyond v ∼ 5000 km s −1 due to sensitivity limits. Two of the masers discovered during this survey are found in unexpected hosts: NGC 4151 (Seyfert 1.5) and NGC 2782 (starburst). We discuss the possible relations between the large X-ray column to NGC 4151 and a possible hidden AGN in NGC 2782 to the detected masers. Four of the masers discovered here, NGC 591, NGC 4388, NGC 5728 and NGC 6323, have high-velocity lines symmetrically spaced about the systemic velocity, a likely signature of molecular gas in a nuclear accretion disk. The maser 1 The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement with Associated Universities, Inc.2 -2source in NGC 6323, in particular, reveals the classic spectrum of a "disk maser" represented by three distinct groups of Doppler components. Future single-dish and VLBI observations of these four galaxies could provide a measurement of the distance to each galaxy, and of the Hubble constant, independent of standard candle calibrations.

New H2O masers in Seyfert and FIR bright galaxies

“…Observations were made using two 3.125-MHz IFs centered at the velocity of the two maser features detected with the GBT (692 and 1127 km s −1 , respectively; Braatz et al 2004). Each IF was subdivided into 128 channels that provide a channel spacing of 24.4 kHz corresponding to ∼0.33 km s −1 .…”

“…NGC 4151 is a grand-design, weakly barred spiral located at 13.3 Mpc (Braatz et al 2004), which is known to host an active galactic nucleus (AGN). Although it is often classified as a type 1.5 Seyfert (Osterbrock & Koski 1976), its broad optical emission lines are variable and sometimes give a type 2 Seyfert spectrum (Sergeev et al 2001).…”

“…The identification of one of these components with the AGN is still uncertain. In Braatz et al (2004), the detection of water maser emission with an isotropic luminosity of 0.7 L was reported. The maser emission is primarily confined to two very narrow components, one at 692.4 km s −1 and the other at 1126.6 km s −1 , blushifted and redshifted, respectively, w.r.t.…”

Masers in AGN environments

“…The incidence rate of maser emission among known Seyfert-2 objects and LINERs is on the order of 20% for cz < 5000 km s −1 , within which past surveys are believed to be reasonably complete. The incidence rate drops with increasing velocity, probably because of incompleteness and sensitivity limitations (Braatz et al 2004;Kondratko et al 2006a). Today, there are ∼ 90 known masers in AGN (published and unpublished) with cz < 20000 km s −1 (Figure 1), plus the maser seen toward quasar J0804+3607 at z ∼ 0.66 (Barvainis & Antonucci 2005).…”

“…See review by Greenhill (2002a). References: (1) Churchwell et al (1977), (2) Dos Santos & Lépine (1979), (3) Gardner & Whiteoak (1982), (4) Claussen & Lo (1986), (5) Claussen et al (1988), (6) Nakai, Inoue, & Miyoshi (1993), (7) Braatz et al (1996), (8) Greenhill et al (2002b), (9) Greenhill et al (1995), (10) Miyoshi et al (1995), (11) Gallimore et al (1996), (12) Claussen et al (1998) Greenhill et al (2003a), (14) Greenhill et al (2003b), (15) Kondratko et al (2006a), Braatz et al (2004), (17) Macri et al (2006), (18) Barvainis & Antonucci (2005), (19) Humphreys et al (2005), and (20) Braatz et al (this volume).…”