Circum-nuclear MegaMasers

Since the discovery of the first astronomical maser by Weaver et al. (1965), a great deal of effort has been expended on understanding their physics. However, even now, because of their complexity, in many maser scenarios we have only a vague understanding of the pumping mechanisms and the physical characteristics of the masers themselves. Despite this ignorance, masers have become powerful tools for probing the kinematics of gas in a variety of astrophysical environments. Maser regions are small and bright, and have a small velocity range, which makes them relatively easy to observe at extraordinarily high precision in location and velocity within their host environment. They have been used to study the astrophysics of both the birth and the death of stars, but recently a most spectacular result emerged from maser studies being the first conclusive evidence of a massive black hole in the center of a galaxy.

In this section, the extragalactic masers (OH, H₂O, formaldehyde, and methanol) will be discussed in the context of the locations in which they are found and of the impact of the SKA on their science. In conclusion, we consider in some detail the area in which the SKA may make its greatest in this field: the determination of the mass distribution of massive black holes in the nuclei of active galaxies.

Megamasers (so-called because their luminosity is about a million times greater than a standard Galactic maser) have been seen in both OH, first discovered in Arp 220 by Baan et al. (1982), and H₂O, first discovered in NGC 4945 by dos Santos and Lepine (1979). Only recently formaldehyde emission has also been found in a number of nearby (active) galaxies, whereas H₂CO emission has only been seen at two locations within the Galaxy (Baan et al. 1993). A review of megamaser characteristics can be found in Henkel et al (1993). While H₂O megamasers appear to provide information of the parsec-scale surroundings of the galactic nucleus, the OH and H₂CO megamaser emissions provide a view of the physics and dynamics of the inner few hundred. Curiously, extragalactic methanol masers have not yet been found although they are found widespread as Galactic masers (Phillips et al. 1998).

H₂O megamasers

Since the first H₂O megamasers were discovered (Dos Santos & Lepine 1979), they were suspected to be associated with accretion discs around black holes. Using VLBI techniques, Miyoshi et al. (1995) show that the H₂O masers in NGC 4258 are confined to a thin molecular disk, only 0.5 pc in diameter, surrounding a central engine. This data provides the best evidence to date for the existence of massive Black Holes (MBH) in some active galactic nuclei (AGN) of the Seyfert 2 type. As a result, H₂O megamaser studies are one of the most powerful tools available to us for probing the inner parsecs of active galaxies. For example, the rotation curve of the maser source in NGC 4258, which is Keplerian to high precision, has provided a mass estimate, accurate to a few percent, of the central engine, a well-defined geometric model as shown in Fig. 1.6.1, and the opportunity to measure the 3-dimensional velocity field of gas in the core of an AGN, using the proper motions of the masers. Recently, other authors (e.g. Herrnstein et al. 1997) have used these and related results to examine the turn-on of the radio jet at a distance of a fraction of a parsec from the MBH, as predicted by the standard Blandford & Königl model.

  

Figure 1.16: This graphic shows the geometrical relationships of the jet emission, the disk of water molecules and the black hole at the center of the galaxy NGC 4258. The pseudo-colors show the relative intensity of radio emission from the jets. The black dot indicates the location of the black hole. The dots in the disk indicate the location of water maser ``spots'' observed with the VLBA. All components are to scale; the scale bar indicates 5,000 Astronomical Units. (from Herrnstein et al. 1997)

Potentially, H₂O megamasers could be used to examine the relationship between MBHs and their host galaxies of various types, and as a function of their evolutionary state. However, we are severely limited at present by the small number of H₂O megamasers, and by the even smaller number that can be successfully observed with VLBI techniques.

The most complete survey for H₂O megamasers among Seyfert 2 and similar galaxies has so far been done by Braatz et al. (1996, 1997), made with a 1-$\sigma$ sensitivity of typically 30 mJy in a 0.8 km/s channel spacing. The total of 16 known megamasers discovered by Braatz et al and other authors have fluxes in the range 60-16000 mJy, corresponding to an isotropic luminosity of 24 to 6100 L$_{\odot}$. A typical VLBI array, such as the VLBA, has an rms imaging sensitivity in 0.8 km/s bandwidth of 7 mJy/beam in 8h (if there is a suitable nearby phase reference), or a baseline sensitivity of 700 mJy/beam in 2 min (i.e. without phase referencing). In practice, observations so far have been made without phase referencing, so that only the strongest few sources have been imaged. Such VLBI observations are essential if we are to use H₂O megamasers successfully to understand the processes surrounding the MBH.

OH Megamasers

OH megamasers probe somewhat larger scales within the nuclear region of their host galaxies. The OH megamaser galaxies are part of a much larger population of (ultra-) luminous FIR galaxies and the pumping of the OH molecules is done by the FIR radiation field in these galaxies (Baan 1989). The strongest OH megamasers have line strengths of several hundreds of mJy. The characteristics of OH megamasers suggest that the OH line luminosity increases quadratically with the FIR-luminosity, which implies that, as one samples larger volumes of space, one encounters increasingly luminous (but increasingly rare) ``gigamasers'' (Baan 1989; Staveley-Smith et al. 1989). This quadratic relation may in part be explained by maser amplification of (weak) radio continuum emission. Clearly the OH-luminosity function must turn over at some point as a result of the evolutionary paths of luminous FIR galaxies and of galaxy mergers, but we have not yet discovered that point. The fluxes of the OH megamasers lie in the range 6-300 mJy, corresponding to an isotropic luminosity of 3 to 1.4 x 10⁴ L$_{\odot}$ and with the source at the highest redshift of 0.265 also having the highest luminosity (Baan et al. 1992). Recent global VLBI studies of the prototype OH megamaser in Arp 220 (type Seyfert 2) have shown that the nuclear radio continuum is dominated by powerful (unresolved) SNR's spread over a region of order 50 parsec (Smith et al. 1998). On the other hand, the OH line emission originates in both compact and extended emission regions centered on these SNR clusters (Lonsdale et al. 1998).

The greatest contribution of the SKA for OH research will be its ultimate sensitivity, its RFI robustness, its speed in searching for gigamasers in large volumes of space, and its use as one element of a VLBI array. The array itself would be able to resolve a 50 pc disk at a redshift of 0.04, with a sensitivity of 1 $\mu$Jy in 8h, which allows imaging of all nearby megamasers.

Formaldehyde Megamasers

Formaldehyde emission has only been found in nearby prominent FIR galaxies (Baan et al. 1993). The known H₂CO emission lines are still rather weak and may be explained by maser amplification of weak continuum emission. The exact pumping mechanism is still not clear but their occurrence appears correlated with the FIR galaxy population; some FIR sources that are not ``warm'' enough to be OH megamasers but they do show H₂CO emission. The strongest formaldehyde emitter known to-date is Arp 220 with a line flux density of only 4 mJy, while the luminosity of the known sources ranges from 5-200 L$_{\odot}$. The emission in Arp 220 is located at the two nuclei as for the OH emission but it also closely mimics the peculiar NIR emission structure found with HST-NICMOS (Baan & Haschick 1995; Scoville et al. 1998). It is anticipated that many galaxies will show formaldehyde emission across their most active starburst/NIR/FIR regions providing a new diagnostic for these activity regions. The large sensitivity and its imaging capability will make SKA a unique instrument for H₂CO research.

The Impact of the SKA on Megamaser Studies

At present we are limited by the small number of known megamasers particularly those strong enough to be suitable for VLBI. Only with VLBI measurements can we unleash the full power of megamasers as a tool for understanding the nuclei of active galaxies.

The SKA will contribute to molecular and megamaser research in three ways:

• as a detection instrument it will be able to detect megamasers of all flavors, that are too weak to be detected with existing instruments. Assuming that the multi-beam capability and the high sensitivity allow routine phase referencing, the sensitivity of the SKA will enable detection (and imaging) of
  • all currently known megamasers in less than a second,
  • currently unknown megamasers in nearby galaxies, down to masers of strengths comparable with interstellar masers in our Galaxy,
  • moderate luminosity OH megamasers comparable to Arp 220 at the 20 $\mu$Jy level with 10 km/s resolution at up to a redshift of 2, and
  • moderate luminosity H₂O megamasers comparable to NGC 4258 at the 5 $\mu$Jy level with 1 kms^-1resolution up to a redshift of 0.15.

• as a stand-alone instrument the SKA will be able to resolve the nearby H₂O megamasers similar to NGC 4258 with a total extent of some 10 milliarcsec and OH megamasers similar to Arp 220 with total extent of about 0.2 arcsec.
• as an element of a VLBI array with existing antennas as the other elements, and assuming that the multiple beams and greater sensitivity enable routine phase- referencing, a baseline between a VLBA antenna and the would enable high-resolution imaging of
  • all currently known megamasers to a dynamic range of at least 300,
  • many currently unknown megamasers in nearby galaxies, and
  • H₂O megamasers comparable in luminosity to NGC 4258 at a redshift of 0.06, at which the maser disk is just resolvable by the longest Earth-based baseline, and similarly
  • OH megamasers comparable in luminosity and structure with Arp 220 at redshift 0.2.

For megamaser studies the effect of the SKA is both to vastly increase the number of objects to be studied, thereby increasing our knowledge of the workings of active nuclei with or without MBH's, and to extend the volume of space to be studied. In this manner megamasers may be used as diagnostic tools for studying the evolution of galaxies in the Universe.

As a particular example of H₂O megamasers, most currently known sources occur in Seyfert 2 and LINER galaxies. Given sufficient sensitivity, one may expect to find them in other types of galaxies as well and thus be able to compare the occurrence and mass of the MBH's as a function of galaxy type. Specifically, one should be able to make significant inroads on answering questions such as:

• What is the mechanism for fuelling a black hole, and what are the kinematics of the accretion disk?
• What is the relationship between the mass of the black hole and the type and evolutionary history of the host galaxy?
• How do black hole masses vary as a function of redshift? Are the massive black holes a result of many mergers of small black holes, or do small gas-rich galaxies already contain such black holes?
• Can we see accretion disks around black holes in merging galaxies? If so, can we trace the kinematics of the circum-nuclear material as the black holes merge?