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    {\small\rm\zihao{6}\songti\thepage\hfill
    \quad\quad ACTA ASTRONOMICA SINICA \hfill
    \hspace{2mm} Vol.44\vspace{2mm}}}} %%%%%%%
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    {\small\rm \zihao{6}\songti suppl.\hfill C. Henkel \& J. A. Braatz: Megamasers  %%�����ߣ���Ŀ���ƣ�
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    \zihao{6}\songti
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    Vol.44~ Suppl.
    \hfill\hspace{8mm} %%% if ``October/January'' then \hspace{8/10 mm} July
    \small ACTA ASTRONOMICA SINICA
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      Feb.,~2003 \\[-1.5 true mm] %%January
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\vs\vs\vs \bc {\Large\bf
Megamasers   %%�����⣩
%\\[1ex]
%$\!\!^*$
}

\vs\vs{\normalsize Christian Henkel\up{1}\hs\&\hs  James A.
Braatz\up{2}
}  %%���������ߣ�

\vs
 ({\footnotesize 1~{\it Max-Planck-Institute for Radio Astronomy, D-53121 Bonn,
  Germany. chenkel@mpifr-bonn.mpg.de}})\\
({\footnotesize 2~{\it National Radio Astronomy Observatory, Green
Bank, USA; jbraatz@nrao.edu}})
\vs\vs \ec

\normalsize
\begin{quotation}
\bigskip

 \no{\bf A{\scriptsize\bf BSTRACT }}~~~\small
The two decades following the serendipitous discovery of
megamasers have seen tremendous progress in the study of luminous
extragalactic maser sources. Single-dish monitoring and high
resolution interferometry has been used to elucidate physical
parameters of the nuclear starbursts triggered by spiral galaxy
mergers and to constrain properties of circumnuclear accretion
disks, including the degree of warping, density, temperature,
magnetic field, disk stability, mass acccretion rate, and mass of
the nuclear engine. To date, $\sim$100 OH megamasers and $\sim$25
H$_2$O megamasers have been detected, but only few have been
studied in detail.

\end{quotation}
\bigskip


\para{\large\bf{1}~~ Introduction}

\vs\vs In the thirty years since their discovery, the study of
cosmic masers has been an exciting and rapidly growing area of
research. Most galactic OH and H$_2$O masers are the size of our
solar system or smaller and thus require Very Long Baseline (VLB)
techniques to be resolved. Extragalactic masers are rarely
resolved, but can efficiently be used as signposts of massive star
formation and active galactic nuclei (AGN). Because their
luminosity can be truly outstanding, reaching
10$^{4}$\,$L_{\odot}$ (10$^{54}$ photons/s) in the most extreme
cases, they are observed out to large distances.

To date, five molecular species have been observed as masers in extragalactic space:
SiO, CH, H$_2$CO, OH, and H$_2$O. The SiO $J$=2--1, $v$=1 maser (86\,GHz), detected
in the Large Magellanic Cloud (LMC; van Loon et al. 1996), appears to be similar
to those observed in the circumstellar shells of galactic giant stars. CH (3264\,MHz)
emission was reported towards the LMC, NGC\,4945, and NGC\,5128 (Cen\,A) and CH
(3335\,MHz)
emission was measured towards the LMC (Whiteoak et al. 1980). These lines behave like
weak masers towards most galactic clouds (e.g. Hjalmarson et al. 1977) and it is
therefore
not farfetched to assume that the extragalactic lines are also caused by weak unsaturated
maser emission. The first extragalactic H$_2$CO maser (4829\,MHz) was discovered by
Baan et al. (1986) in Arp\,220. They proposed a pumping scheme based on infrared photons
from the dust and seed emission from radio photons from the nuclear continuum source(s).
Such 6\,cm H$_2$CO masers are rare in the Galaxy. They appear to be linked to OH
megamasers
(e.g. Henkel et al. 1991; Baan et al. 1993), possibly because infrared radiation plays a
role in pumping both maser types. For Arp\,220, a detailed comparison of interferometric
maps of OH, H$_2$CO, and the radio continuum is given by Baan \& Haschick (1995). The main
focus of astrophysical research is not on SiO, CH or H$_2$CO, however, but on extragalactic
OH and H$_2$O masers. Studies of these molecules provide insights into astrophysical
problems and processes that are otherwise difficult to discern. A few highlights are presented
in the following sections.

\para{\large\bf{2}~~OH Emission}

\vs\vs Interstellar OH maser emission, initially dubbed
`mysterium' because of its peculiar line profile, was first
detected by Weaver et al. (1965) and Gundermann (1965) towards
galactic star forming regions. Extragalactic OH masers were
observed by Whiteoak \& Gardner (1973, 1975), in the starburst
galaxies NGC\,253 and NGC\,4945. These are the first
(comparatively weak) known `kilomasers'. The first OH `megamaser',
with a luminosity surpassing any galactic OH maser by orders of
magnitude, was detected by Baan et al. (1982) in the ultraluminous
starburst galaxy Arp\,220. Line emission was measured in the `main
lines' of the ground state $\Lambda$-doublet (1665 and 1667\,MHz).
With the launch of the Infrared Astronomical Satellite (IRAS)
providing 12--100$\mu$m fluxes of many galaxies, it soon became
clear that OH megamasers are closely associated with Ultraluminous
Infrared Galaxies (ULIRGs), i.e. with merging gas-rich spirals. At
luminosities of 10$^{12}$\,L$_{\odot}$ every second (Baan 1989) or
third (Darling \& Giovanelli 2002a) ULIRG exhibits strong OH
emission. Assuming that every such system contains a megamaser but
some are not detected because of beaming and orientation effects,
a lower limit to the solid angle of the coherent radiation can
thus be determined.

Guided by the IRAS Point Source Catalog (e.g. IRAS 1989), $\sim$50 megamasers were
detected within a few years. Towards Arp\,220, not only the main lines but also
the satellite lines of the ground state $\Lambda$-doublet (1612 and 1720\,MHz) were
found in (weak) emission (Baan \& Haschick 1987), while a total of five lines from
higher excited $\Lambda$-doublets (the $^{2}\Pi_{1/2}$, $J$=1/2~4660, 4751, and
4766\,MHz transitions and the $^{2}\Pi_{3/2}$, $J$=3/2~6030 and 6035\,MHz transitions)
were observed in absorption against the nuclear continuum (Henkel et al. 1986, 1987).

The detection of a number of lines and the availability of a large number of infrared
photons strongly constrain any pumping scenario. Excitation by infrared emission was
thus proposed early (Baan 1985; Norris et al. 1995) and supportive numerical
calculations
were performed by Henkel et al. (1987), Burdyuzha \& Vikulov (1990), and Randell et al.
(1995). Further confirmation was obtained by the detection of strong 35$\mu$m OH
absorption
(Skinner et al. 1997) that alone is capable of pumping the megamaser emission seen
at radio
wavelengths. Nevertheless, more accurate calculations exploring the full parameter
space
would be highly desirable.

The standard OH megamaser model involves unsaturated maser emission amplifying the
nuclear
continuum background (Baan 1985; Henkel \& Wilson 1990). More recent VLBI studies of
Arp\,220 (Lonsdale et al. 1994, 1998) reveal that the 1665\,MHz flux and a part of
the 1667\,MHz flux is resolved out, apparently arising from extended sources with a low
maser gain that may be compatible with the standard model. However, the remaining
1667\,MHz flux has high gain and arises from possibly saturated maser emission in
compact clouds. Such compact emission regions may be responsible for
time variability observed in IRAS\,21272+2514 (Darling \& Giovanelli 2002b).
High resolution maps of other sources provide an even more complex picture (see
Montgomery \& Cohen 1992; Trotter et al. 1997; Diamond et al. 1999; Yates et al. 2000;
Pihlstr{\"o}m et al. 2001). Overall, OH and infrared luminosities are correlated, with
$L_{\rm OH}$ $\propto$ $L_{\rm FIR}^{\beta}$. Originally, $\beta$ was believed to be $\sim$2
(Baan 1989), which corresponds to unsaturated emission ($L_{\rm OH}$ $\propto$
$L_{\rm c}$\,$L_{\rm FIR}$ with $L_{\rm c}$$\propto$$L_{\rm FIR}$ and $L_{\rm c}$ denoting
the radio continuum luminosity). However, more recent studies give $\beta$ = 1.38 (Kandalyan
1998) and 1.2 (Darling \& Giovanelli 2002a). It may be that the detection of particularly
distant and luminous OH megamasers has significantly decreased the fraction of sources with
unsaturated emission, thus also reducing $\beta$.


\para{\large\bf{3}~~H$_2$O masers}

\vs\vs In contrast to OH, extragalactic water vapor (H$_2$O)
masers, first detected by Churchwell et al. (1977), have so far
been observed in only one line. Therefore pumping scenarios are
not well constrained. Nevertheless, collisional excitation at
densities 10$^{8}$--10$^{10}$\,cm$^{-3}$ and temperatures of
$\sim$250--500\,K appear to be adequate to produce such masers
(e.g. Elitzur 1998). The rotational energy states connected by the
highly time variable 22\,GHz line are high enough ($\sim$640\,K)
above the ground level not to be strongly affected by atmospheric
extinction. As in the case of OH, there are comparatively weak
kilomasers (up to a few solar (isotropic) luminosities) that can
be used to pinpoint sites of massive star formation and to obtain
distance estimates by a comparison of the scatter in radial
velocity and proper motion (e.g. Greenhill et al. 1993; Tarchi et
al. 2002). Proper motions can also be used to obtain three
dimensional velocity vectors in the Local Group (Brunthaler et al.
2002). Only one kilomaser is known to be associated with the
nuclear environment of a galaxy. It is observed toward the
Whirlpool galaxy M\,51 (Hagiwara et al. 2001).

Megamaser emission exceeding $\sim$20\,L$_{\odot}$ was first observed
towards NGC\,4945
(Dos Santos \& L{\'e}pine 1979) and then in four additional sources
(Gardner \& Whiteoak 1982;
Claussen et al. 1984; Henkel et al. 1984; Haschick \& Baan 1985).  Claussen and
Lo (1986) associated the maser emission with the active nucleus, but it took
some 10 years
until additional progress was made in our understanding of extragalactic
H$_2$O sources.
A large survey including $\sim$360 Seyfert and LINER galaxies (Braatz et al.
1994, 1996, 1997)
led to 10 new detections, all of them associated with galaxies containing a
Seyfert 2 or
LINER nucleus. This result and continuing interferometric observations
reinforced the nuclear
origin. The emission originates from the innermost parsec(s) of the parent
galaxy (e.g.
Claussen \& Lo 1986; Greenhill et al. 1995b; 1996, 1997b; Miyoshi et al.
1995; Claussen et
al. 1998; Trotter et al. 1998; Herrnstein et al. 1999; Peck et al. 2001).
Adopting a so-called
unified model in which Seyfert 2 nuclear disks are viewed edge-on, megamaser
activity must then be
related to the large line-of-sight column densities available only in edge-on
orientations.

To date, the number of known H$_2$O megamasers surpasses 20 (for more recent
detections, see
Koekemoer et al. 1995; Greenhill et al. 1997a, 2002; Hagiwara et al. 1997,
2002a; Falcke et
al. 2000; Henkel et al. 2002). The H$_2$O line provides the only opportunity
to map
accretion disks in AGN. It also permits the determination of the mass of the
supermassive
nuclear object, to obtain direct geometric distances to galaxies, and to study
the interaction
between nuclear jets and dense molecular material.  The masers appear to divide
into a few
classes: those found in nuclear disks, those originating along powerful radio
jets, and those
which are neither associated with disks or jets.  Among the so-called `jet masers',
NGC\,1052 (Claussen et al. 1998) and Mrk348 (Peck et al. 2001) are the most prominent.
The prototypical source of the third class is NGC\,3079 (e.g. Hagiwara et al. 2002b).

Most interesting are the circumnuclear `disk-masers', with NGC\,4258 being the prototype.
VLBI observations (Greenhill et al. 1995b; Miyoshi et al. 1995; Herrnstein et al. 1999)
provide
strong support for the presence of a thin (height to radius ratio $<$ 0.3\%;
Moran 1998), warped,
almost edge-on gaseous disk or annulus at a galactocentric radius of 0.16--0.28\,pc.
Maser emission
is observed both within 100\,km\,s$^{-1}$ of the systemic velocity, arising from the
clouds on
the near side of the disk and along our line of sight to the central core, and from
`satellite lines'
with velocities $\pm$900\,km\,s$^{-1}$ w.r.t. systemic velocity (Nakai et al. 1993),
arising from gas at the tangent points, with rotational velocities directed towards and
away from Earth. Haschick et al. (1994), Greenhill et al. (1995a), and Nakai et al.
(1995)
monitored the maser spectrum and found that the systemic maser features drift redward
with
time at a rate $\sim$9\,km\,s$^{-1}$. These velocity drifts are interpreted as
centripetal
acceleration ($V^2$/$R$) of clouds in the rotating disk as they orbit the central
black
hole. Systemic maser features may only be seen when gas is projected in front of the
innermost part of the southern jet, which the masers likely amplify. Satellite lines
are
believed to be self-amplified. The Keplerian disk requires a binding mass of
(39$\pm$1)$\times$10$^{6}$ ($D$/7.2\,Mpc) (sin $i$/sin 82$^{\circ}$)$^{-2}$\,M$_{\odot}$
inside of 0.16\,pc.

In NGC\,4258 the centripetal acceleration, $V^2$/$R$, is known from velocity drifts and
the
velocity of rotation, $V$, is known from the satellite emission lines. Thus the linear
radius,
$R$, is known and can be compared to the VLBI-measured angular size of the disk,
$\theta$,
to give the distance to the source, $D$ = $R$/$\theta$. This calculation can be further
tested by directly measuring the proper motion of the systemic maser components as they
pass the inner edge of the disk along the line of sight towards the innermost southern
jet (Herrnstein et al. 1997, 1999). The result is $D$ = 7.2$\pm$0.5\,Mpc. The original
distance estimate
obtained from Cepheids (Maoz et al. 1999) was discrepant with the distance obtained
from the
maser measurements, but a reevaluation of the Cepheid calibration schemes gives
$D$ = 7.8$\pm$0.3$\pm$0.5\,Mpc (Newman et al. 2001), consistent with the maser distance.

\begin{figure}[h]
\center {\includegraphics [bb=0 138 408
466,scale=0.6]{mrk1419tot.eps}}

   \parbox[tb]{10cm}
   %{\hspace{3.5cm}
   {%\center
   \footnotesize Fig.\,1~~ 22\,GHz H$_2$O spectrum showing the blue-shifted,
   systemic, and red-shifted
maser features (Henkel, Braatz, Greenhill \& Wilson 2002).}

    \end{figure}

Among the newly detected objects, one megamaser shows the same properties as the
prototypical accretion disk maser source NGC~4258 (Henkel et al. 2002): The spectrum
is characterized by a cluster of systemic ($V$$\sim$$V_{\rm sys}$) H$_2$O features,
two additional H$_2$O clusters, one red- and one blue-shifted with respect to $V_{\rm sys}$,
a likely acceleration of the systemic features and no similar drift in the blue- and
red-shifted emission. The source has a recessional velocity ten times that of NGC~4258
(see Fig.\,1).


\para{\large\bf{4}~~Future prospects}

\vs\vs With the completion of the GBT (Green Bank Telescope), the
E-VLA, and the recently improved system at the Effelsberg 100-m
telescope, new H$_2$O masers will be found. Hopefully, some of
these will also reveal a clear disk geometry as observed in
NGC\,4258, so that more nuclear masses and geometric distances can
be estimated. By including these highly sensitive telescopes in
VLB arrays, it will be possible to obtain interferometric
observations of the distant, weaker masers which is essential to
detailing their geometries and obtaining distances. Finally, one
H$_2$O megamaser host galaxy has been recently identified as a
Narrow Line Seyfert 1 (Nagar et al. 2002) and two new megamasers
were recently detected in luminous galaxy mergers (Hagiwara et al.
2002a; Peck et al. 2003). Thus the commonly adopted exclusive
association of H$_2$O megamaser emission with Seyfert 2 and LINER
nuclei may have to be modified.

OH and H$_2$O megamasers alike may be suitable tools to determine magnetic
field strengths (the
component parallel to the line-of-sight) through observations of Zeeman line splitting
in circumnuclear disks and regions close to merging galactic nuclei. So far, only upper
limits could be obtained. For OH in four sources including III\,Zw\,35, 3$\sigma$ values
for the magnetic field strength are 3--5\,mG (Killeen et al. 1996) and for the
circumnuclear
H$_2$O emission in NGC\,4258 it is 900\,mG (Herrnstein et al. 1998).

About 100 OH megamasers have been detected so far, half of them at redshifts
$z$=0.1--0.3
(Darling \& Giovanellli 2002a). Although OH megamaser emission appears to be a
starburst- and not
an AGN-phenomenon (e.g. Sturm et al. 1996), it may soon become, as in the eighties,
the main
focus of astrophysical megamaser research. This is a consequence of their enormous
luminosities
which make them detectable at cosmologically interesting distances, and their
association with
ULIRGs (e.g. Baan 1989; Darling \& Giovanelli 2002a). With the Arecibo telescope,
the most
luminous known OH masers would be detectable out to $z$$\sim$0.7. With the GMRT
(Giant Meter
Wave Radio Telescope) and LOFAR (Low Frequency Array), the most luminous sources
should be
detectable out to redshifts of $z$$\sim$6, while the construction of an SKA
(Square Kilometer
Array) would provide direct access to weaker OH megamasers as well, throughout
the universe.
Assuming that OH emission is closely related to the presence of ULIRGs, not only
`locally' but
also at high redshifts, the number of detected masers as a function of redshift
will then provide
a direct estimate of the number of merging spirals and star formation rate as a
function of time.
This could be compared with other interesting populations like that of quasars
to study the
evolutionary connection between the ULIRGs and QSOs (e.g. Sanders 1996).


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%\end{CJK}
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