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A Search for High-z Water Vapor Masers in Obscured AGNs
Smita Mathur 1 , Tapasi Ghosh 2 , and Chris Salter 2
1 Ohio State University........
2 Arecibo Observatory, HC 3 Box 53995, Arecibo, PR 00612
1. Introduction
Strong 22-GHz water vapor-masers have now been detected in about 20 nearby AGNs. These,
\megamaser" sources are mostly associated with Seyfert 2 and LINER galaxies whose active nuclei are
hidden by a large column density of optically obscuring, X-ray absorbing gas (e.g. Braatz et al., 1997,
ApJS, 110, 321).
Detailed studies of H 2
O masers with VLBI imaging and spectral monitoring, have become im-
portant tools for probing dense gas near the central engins of these sources, investigating the properties of
their accretion disks and measuring the masses of their (supermassive) black holes. Such studies have also
been employed to estimate geometrical distances, completely independent of the rungs of the extragalctic
distance ladder. For example, from their H 2 O maser observations of NGC4258, Miyoshi et al. (1995,
Nature, 373, 127) established the existence of a nearly edge-on, warped, extremely thin disk in Keplerian
rotation around a central mass of 3:5  10 7 M o . In addition, 5 epochs of VLBI monitoring of the proper
motion and the line-of-sight velocities of the maser spots in this source led Herrnstein et al. (1997, ApJ,
475, L17) to derive a geometric distance of 7:3  0:3 Mpc.
However, so far most of the detections of extragalctic H 2
O masers have been in relatively nearby
objects, and (hence) in low-luminosity AGNs. In fact, the farthest and most powerful source in Table 2 of
Braatz et al. (1996, ApJS, 106, 51), is TXFS 2226{184 at a systemic velocity of 7500 km s 1 , peak ux
density of 270 mJy, line width of 90 km s 1 , and an isotropic maser luminosity of 6100L ф . This is the
so-called, \Giga-maser" source discovered by Koekemoer. et al. (1995, Nature, 378, 697).
If higher-z sources with H 2
O maser emission could be found, that would open up the possibility
of testing some of the AGN models , provide an independent calibration for the extragalctic distance scale,
and even set constraints on the values of Hubble Constant and the Deceleration parameter.
Is it at all possible to detect H 2
O-maser emssions at high redshift ? For instance, if a source
similar to TXFS 2226{18 (with similar maser luminosity) were located at z=1.5, the expected ux density at
8.9 GHz would be just 0.125 mJy. Using the X-band receiver at Arecibo, we would need an integration time
of about 14 hours to detect that at 3-sigma level. However, if we consider the most aceepted mechanism
for producing such maser emission (Haschick et al., 1990, ApJ, 356, 149), and the standard model for
high-luminosity AGNs (Rees, Netzer, & Ferland, 1989, ApJ, 347, 640), our expectations change quite
considerably.
For instance, Haschick et al. (1990) state that the ux density (S  ) at which an amplifying maser
saturates is given by:
S   12 (Dm =0:15 pc) 2 (D=7:3 Mpc) 2 Jy (1)
where Dm is the distance between the masing material (part of the accretion disk or a cloudlet within the
disk) and the background source, and D is the distance of the source itself from the observer. In nearby,
1

low luminosity AGNs such as NGC4258,(and other Seyfert galaxies and LINERs), Dm is about 0.1 { 1 pc
(Herrnstein et al.,1997, ApJ, 475, L17)). In contrast, for high-luminosity AGNs (L  10 45 47 ergs 1 ), the
region between 0.1 and 1 pc is occupied by the Broad-Emission-Line Region (BLR) material, where the
gas is too dense and too hot (with number densities 10 12:5 10 cm 3 and optical line widths of several 1000
km s 1 ) to support water-maser. Several authors (e.g. Netzer & Laor, 1993 ApJ, 404, L51) have argued
that the outer edge of the BLR is caused by the formation of dust beyond 1 pc, shielding the gas from the
central ionizing radition, and promoting formation of water molecules. By about 10 pc from the \central
engine", the number density and temperature drop to about 10 8 9 cm 3 and 200 { 1000 K respectively,
creating suitable condition for water-maser formation.
If we substitute Dm  10 pc in equation (1), then we can re-write it as: S  = 237=(D Gpc) 2 mJy.
Sources out to even a Gpc will thus have detectable (mJy or greater levels) ux density in their water-maser
emission.
Previous High-z Water Maser Searches:
To date, two such searches have been initiated using the 100-m Eelsberg telescope and the VLBA (Bar-
vainis et al and Herrnstein in \Highly Redshifted Radio Lines", ASP Conference Proc., 156, 1999, pages
39, & 275, Eds. Carilli et al.), using the U, X, C and L-band receivers at those telescopes. The ux density
level to which their observations are sensitive, are 20 and 40 { 50 mJy respectively. No detection has yet
been reported. We notice that one of the slection criteria of Barvainis et al. has been detectable X-ray ux
in their targets. While the authors site the reference of Neufeld et al (1994, ApJ, 436, L127) in support
of their selection of X-ray bright sources, we wonder if that might have excluded heavily obscured objects
from their sample list.
Here, we propose to undertake a H 2
O-maser search in the redshift ranges 1.22 { 1.78 and 2.7 { 4.55 using
the X and C-band receivers of the 305-m telescope, down to a maser detection limit of about 2 mJy.
Sample selction and the Proposed Observation
We have selected radio-loud objects in the redshift ranges between 1.22{1.78 and 2.70 { 4.55, within the
declination range of the 305-m telescope as our targets. In addition the selected sources satisfy one or
more of the following criteria: (1) soft X-ray absorption, (2) narrow optical emission lines (these narrow
line radio galaxies are radio loud counterparts of Seyfert 2 galaxies), (3) steep radio spectrum, (4) compact
edge-brightened (FRII) radio morphology (angular size < 30 00 , so that they are unresolved for the x-band
beam). Together, these criteria ensure that the radio AGNs are viewed roughly edge-on, a necessary
condition for seeing through the masing disk/torus.
The X-ray data were obtained from... (ref. please, Smita), and the radio information was obtained using
a compilation of compact edge brightened sources, with steep radio spectra, by Nilsson (1998, A&AS, 132,
31). Additional information on individual sources was obtained from NED.
In our list, we have 35 objects satisfying the above criteria. We will perform standard total-power position-
switched observation with 10 km s 1 velocity resolution. For both X-band (SEFD  10 Jy) and C-band
(SEFD  5 Jy), 1 hr of total integration time will give a 3  detection limit of 2 mJy, an order of
magnitude improvemen on the existing work. Including over-head, we therefore ask for about 40 hours of
observing time.
As X-band observations are more aected by wet weather conditions, we would prefer to perform these
observations during the drier seasons at Arecibo (not in Aug { Oct period) and during the night times to
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avoid solar interferences.
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