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2.1. Nature - Observations of Water Vapour Masers


The first astronomical molecular (OH) maser emission (9) was discovered in one of the star formation regions which populate Galactic interstellar space (Weaver et al. 1965). Since then, over 400 different astronomical molecular (OH, SiO and H2O) masers have been found. In recent years astronomers have been able to observe masers outside of our own Galaxy by virtue of their intrinsic power (typically 105 times more luminous than Galactic masers) and the fact that their emission lines are extremely narrow (due to their intrinsic monochromaticity). These ``mega-masers'' can greatly amplify background radiation, as well as their own intrinsic emissions thus making them readily observable if beamed towards the Earth.

In a very few cases, it has been possible to infer the mass and volume of the central mass concentration within an AGN by observing the orbital dynamics of the mega-masers within the galaxy. This is of great scientific interest as the quantities which are obtained can only be explained by invoking the presence of a black hole, thus providing the most definite evidence to date of these exotic objects (e.g. Miyoshi et al 1995). Aside from the possibility of black hole detections, with accurate knowledge of the masers' velocities, we can measure their proper motion as they pass in front of the nucleus (i.e. when their radial velocity is equal to the systemic velocity of the galaxy). By combining the measured Doppler velocities of the masers with the time to transverse an angular distance, we obtain the linear distance travelled. This angular to linear conversion factor allows a direct measurement of the distance to the galaxy. For example, in the case of NGC 4258, this method has reduced the uncertainty in the galaxy's distance from ~ 50% to 4% (Hernstein et al. 1999), thus having major implications in the determination of the cosmic distance scale.

Figure 3

Figure 3. Possible model of the maser emission geometry in NGC 4258. Courtesy of Lincoln Greenhill.

In Fig. 3 a possible geometry of the H2O maser emission on scales of ~ 0.1 pc (1 parsec [pc] = 3.086 x 1016 m = 3.26 light years) is shown. On this scale, the physical conditions are favourable (T gtapprox 100 K, rho ~ 1010 cm-3) for the molecules to mase (Tangiguchi & Murayama 1998). Since the amplification factor varies exponentially with the path length (e.g. Elitzur 1992), masers are seen where the path has large velocity coherence, i.e. where the velocity varies only slightly across the molecular disk, upon which the masers are located. This occurs at three points; directly in front of radio continuum source, where the radial velocity of the disk is equal to that of the galaxy, and at the two tangent points on the disk; where the projected velocity gradient across the disk is a minimum. Not lying directly between us and the radio continuum source, the tangent point masers appear much weaker than those directly in the path since they are the result of maser amplification of the molecules' own spontaneous emissions and not those of the central source. When the velocities of the masers are measured, unlike the molecular ring structures on larger scales (Section 3, Table 1), whose enclosed mass is comprised of stars and dust, the masers are seen to follow Keplerian orbits, implying that the dynamics are dominated by a compact object. The enclosed mass measured within the maser orbits then constrains the mass of the central object.


The first extragalactic H2O vapour maser was detected, at 22.23508 GHz - corresponding to the 616 -> 523 rotational transition, in the HII region of the galaxy IC133 (Churchwell et al. 1977). Since then there have been many detections of water masers in Seyfert galaxies. Examples of these are: NGC 4945 (dos Santos & Lépine 1979), the Circinus galaxy (Gardner & Whiteoak 1982), NGC 1068, NGC 3034, NGC 6946 (Claussen, Heiligman & Lo 1984), NGC 3079 (Henkel et al. 1984; Haschick & Baan 1985), NGC 5793 (Hagiwara 1997). Recently, Braatz, Wilson & Henkel (1997) have detected 10 new H2O mega-masing sources and a summary of all the water maser results can be found in Wilson (1998). Here we discuss those galaxies which, through the observations of water masers, are most indicative of housing super-massive black holes.

Nakai, Inoue & Miyoshi (1993) observed the LINER (10) NGC 4258 and found that the nuclear H2O masers were moving with velocity extremes of ~ ± 900 km s-1 relative to the nucleus, suggesting that the masers are orbiting a high mass concentration. Miyoshi et al. (1995) proposed that the maser ring is rotating according to Keplerian motion, around a central mass of 3.6 x 107 Msun, which is confined to a region less than 0.13 pc in radius. Haschick, Baan & Peng (1996) have also determined, by combining accelerated spectral features with previous Very Long Baseline Interferometry (VLBI) results, that the masers are located on a (possibly warped, e.g. Hernsteing, Greenhill & Moran 1996) ring located at the inner edge of a molecular torus (Section 3), orbiting a central mass of 2.2 x 10^7 Msun. A super-massive black hole is suggested as the only possible source of the mass concentration, since the high star cluster density (gtapprox 4 x 109 Msun pc-3) required would disperse the cluster in less than 108 yr (Miyoshi et al. 1995).

Braatz, Wilson & Henkel (1994) have found water vapour mega-masers in the Seyferts Mrk 1, Mrk 1210 and NGC 5506 and in the LINERs NGC 1052 and NGC 2639. In their survey, and the more extensive recent survey of Braatz, Wilson & Henkel (1997), no water vapour masers were detected in type 1 Seyferts, thus supporting the argument that due to obscuration, the maser disk is unfavourably orientated (i.e. face-on c.f. Fig. 3) with regard to the beaming of maser emission towards the Earth.

Water maser emission in 18 Seyfert galaxies has been searched for by Nakai et al. (1995), who found that the masers in the galaxies NGC 253, NGC 3079, M51, NGC 4945, the Circinus galaxy and NGC 1068 are consistent with disks rotating with velocities gtapprox ± 200 km s-1 around their respective centres. They confirm that the maser characteristics of NGC 4945 indicate that the central mass is a massive black hole.

From VLBI synthesis images of the water masers in NGC 1068, Greenhill et al. (1996) measured H2O maser emission of velocities of up to ± 300 km s-1, thus implying a central mass of ~ 1 x 107 Msun, which is located within 0.65 pc of the galaxy's centre. Gallimore et al. (1996) arrive at a similar mass from a maser disk which spans from gtapprox 1.3 - approx 2.5 pc and is perpendicular to the radio jets.

Finally, although not yet confirmed as black holes, the Milky Way, M31, M32, NGC 3377 and NGC 4594 are known to contain dark central objects of masses 106 Msun ltapprox M ltapprox 109.5 Msun (Kormendy & Richstone 1995). This implies that nuclear super-massive black holes may be a general feature of galaxies regardless of their radio activity (11), the lack of mega-masers as well as the low activity could be a consequence of a low feeding rate to the black hole (Blandford & Begelman 1999).

9 A brief review of the mechanism responsible for these masers can be found in Curran (1998), with the reader being referred to Elitzur (1992) for further details. Back.
10 While having much lower luminosities, these Low Ionisation Nuclear Emission-line Regions generally exhibit Seyfert 2 characteristics (Heckman 1980), although they can be distinguished (e.g. Peterson 1997). LINERS may represent a transition stage from star-burst galaxy to AGN (Wu et al. 1998). Back.
11 Indeed all galaxies could harbour dead AGNs (Woltjer 1959; Soltan 1982). Back.

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