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A Search for High-z Water Vapor Masers in Obscured AGNs
Tapasi Ghosh 1 , Smita Mathur 2 , and Chris Salter 1
1 Arecibo Observatory, HC 3 Box 53995, Arecibo, PR 00612
2 The Ohio State University, 140 West 18th Avenue, Columbus, OH 43210
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 engines 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 extragalactic
distance ladder. For example, one of the best determinations of a black hole mass has resulted from the
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 3:510 7 M o in
NGC4258. 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 extragalactic 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 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 extragalactic distance
scale, and set constraints on the values of Hubble Constant and the cosmological constant . The rst
step toward reaching these objectives, naturally, is to nd the high redshift masers.
Is it at all possible to detect H 2 O-maser emission 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 accepted 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)
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,
low luminosity AGNs such as NGC4258, (and other Seyfert galaxies and LINERs), maser production is
believed to take place in obscuring torus/ disk, beyond the broad emission line region (BELR), with Dm
about 0.1 { 1 pc (Herrnstein et al.,1997, ApJ, 475, L17). For high-luminosity AGNs (L  10 45 47 ergs 1 ),
however, the size of the BELR is much larger, with RBELR / L 0:620:02 (Peterson et al. 2000, ApJ, 542,
161). Thus, the region between 0.1 and 1 pc is occupied by the BELR material itself, and the obscuring
torus is well beyond. The BELR region is too hot (T> 10 4 K) to support water masers. By about 10 pc
from the \central engine", the temperature drops to about 200 { 1000 K, creating suitable conditions for
water-maser formation. At about the same distance, dust can also form (e.g. Netzer & Laor, 1993 ApJ,
404, L51), shielding the gas from the central ionizing radiation, and promoting the formation of water
molecules.
If we substitute Dm  10 pc in equation (1), then we can re-write it as: S  = 2840=(D Gpc) 2 mJy.
Sources out to even 10 Gpc will thus have detectable (20 mJy or greater levels) ux density in their water-
maser emission, allowing the probe of the observable Universe out to redshift z=5 (for any reasonable
cosmological model,
including
M =
0:3;
 = 0:7).
Previous High-z Water Maser Searches:
To date, two searches of high-z water masers have been initiated using the 100-m Eelsberg telescope and
the VLBA (Barvainis 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.
Their observations are sensitive to ux densities of 20 and 40 { 50 mJy respectively. No detection has yet
been reported. We notice that one of the selection criteria of Barvainis et al. has been detectable X-ray ux
in their targets. X-ray emission is an ubiquitous property of quasars, and indeed, according to models of
Neufeld et al. (1994, ApJ, 436, L127), X-rays are responsible for keeping the molecular gas \warm" enough
for maser production. However, direct exposure to X-rays tend to make the gas too hot to form molecules,
and the necessary condition for making the environment conducive for maser production appears to be
substantial shielding from the X-ray source (Neufeld et al.). Indeed all the nearby maser sources, observed
in X-rays, show large absorption in soft X-rays (e.g. Wilkes, Mathur et al. 2001, ApJ, 549, 248). Thus, the
appropriate selection criterion appears to be \soft X-ray absorption" instead of \detectable X-ray ux"
which may, in fact, select against obscured sources.
Here, we propose to undertake a H 2 O-maser search in the redshift ranges 1.22 { 1.78 and 2.70 { 4.55 using
the X and C-band receivers of the 305-m telescope, down to a maser detection limit of about 2 mJy, an
order of magnitude deeper than the present limit.
Sample selection 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 Arecibo
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 radio information was obtained using a compilation of compact edge brightened sources, with steep
2

radio spectra, by Nilsson (1998, A&AS, 132, 31). We searched the literature and databases for X-ray
information (e.g. Fiore et al. 1998, ApJ, 492, 79; Reeves & Turner, 2000, MNRAS, 316, 234; Brinkman et
al. 2000, A&A, 356, 445). 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 observations with about 1 km s 1 velocity resolution. With the data smoothed to 5 km s 1 res-
olution, 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 3 mJy, roughly an order of magnitude improvement over 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 the Aug { Oct period) and during the night times
to avoid solar interference.
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