Australia Telescope National Facility

A great deal of effort has been expended on understanding the physics of astronomical masers. However, their complexity means that in most cases we have only a vague understanding of the pumping mechanisms and physical characteristics of the masers themselves. Despite this embarrassing ignorance, masers have become powerful tools for probing the kinematics of gas in a variety of astrophysical environments. This is because masers are small, bright, and have a narrow velocity range, which means that masers act as signposts throughout the interstellar medium, telling us about the physical conditions and kinematics of the gas that they sample. In particular, they tell us the velocity of a precisely known location within the interstellar medium with extraordinarily high precision.

I assume the SKA sensitivity after a 8-hour synthesis at 1.8, 6.7, and 22 GHz to be 1.6, 0.8, and 0.8 m Jy respectively, in a 30 kms^-1 channel, or 8.8, 4.4, and 4.4 m Jy respectively in a 1 kms^-1 channel. H₂O kms^-1

OH, H₂O, and methanol masers are frequently found in the cool molecular gas that surrounds newly formed massive stars, and have been used extensively to probe the physical conditions and kinematics of this gas. Numerous observations have shown them to consist of clusters of small maser components often associated with compact HII regions. The individual maser components have typical sizes of 10¹²m (corresponding to 0.01 arcsec at 1 kpc), and are arranged in clusters typically 3.10¹⁴ - 10¹⁵ m (~ a few arcsec) in diameter, although there are significant differences between the three species. Most of these masers appear to be situated in the warm gas accreting on to a pre-main sequence massive star. The properties of OH and H₂O masers are summarised by Elitzur [1].

Fig 1: the edge-on disk in G339.88. The left-hand image [3] shows the masers with a monotonic velocity gradient along them (indicated by red and blue dots) superimposed on the compact HII region. The right-hand image [4] shows the same object at 10 micron, showing a dust disk coincident with the masers

A recent development, shown in Fig.1, is that about 30% of methanol masers (at 6.7 and 12.2 GHz) appear to be located in edge-on circumstellar disks [2-4] around high-mass stars. The existence of these disks will require revision of those theories which assert that such disks should be destroyed by the strong stellar winds from high-mass stars. Interstellar masers tend to be so strong that present-day radio-telescopes are quite adequate for studying those in our Galaxy, and the SKA is unlikely to have a great impact on this field. However, the enormous sensitivity of the SKA opens up the possibility of using these masers to probe extragalactic star formation.

Star formation is now recognised as being one of the key energetic processes in the early Universe. For example, at sub-mJy flux densities the radio sky becomes dominated by starburst galaxies rather than classical radio galaxies. However, we still do not understand the major processes that drive and regulate a starburst galaxy, and we still do not know what the relationship is (if any) between starburst activity and the central nucleus. At present it is difficult to study star formation in the nuclei of active and starburst galaxies because the high dust extinction (which can be hundreds of magnitudes at optical wavelengths) prevent even mid-infrared observations from penetrating the dense shroud of dust. Whilst the far-infrared observations of ISO can penetrate this, ISO does not have sufficient angular resolution to measure the distribution of star formation on the parsec scale in the nuclei of these galaxies. Such resolution is important if we are to understand, for example, the potential role of star formation in feeding the massive black hole (MBH) at the centre.

At the distance of NGC253, individual methanol and H₂O maser spots corresponding to our own normal Galactic masers might have observed flux densities of 3 and 250 mJy respectively, while at Cen A they might have fluxes of 0.4 and 30 mJy. In both cases, individual maser spots would be detectable at the five-sigma level in a few minutes, and could be identified in the parsec-resolution images obtained from an 8-hour synthesis. SKA would therefore tell us about the structure and kinematics of star formation regions in nearby active and starburst galaxies at a level of detail approaching that which we have in our own galaxy. This would help enormously in solving questions such as the relationship between star formation and the AGN.

To be able to use these masers to study extragalactic star formation, the SKA will need to operate at 1.6 and 6.7 GHz for OH and methanol respectively. To study the H₂O masers, 22 GHz would be needed. The scientific value obtained from being able to study star formation in nearby starburst galaxies argues that these masers should be one of the SKA science drivers, particularly for 6.7 GHz.

The loss of mass which accompanies the departure of a star from the main sequence is often host to maser emission in the OH, H₂O, and SiO molecules, and this maser emission has been used by many authors to study the mass loss and chemistry of these stars [5]. However, equally important is the use of these stars as primary distance indicators. This is possible because the star varies periodically (on a time scale of ~ 1 year) in luminosity, which in turn changes the luminosity of the maser shell (assumed spherical) surrounding the star. Because this shell has a typical radius of tens of light-days, there is a measurable lag between the observed maxima of maser emission from the front and rear caps of the maser shell, from which the linear diameter of the shell can be measured. Combining this with the angular diameter of the shell (as measured from a synthesis image) then gives the distance to the star. This technique has been suggested by several authors as an independent way to measure distances within the Galaxy [6].

With the greater sensitivity of the SKA, it may be possible in principle to extend this to extragalactic distance measurement. A typical OHIR star in NGC253 would have a measured flux of S ~ 0.1 mJy in a few kms^-1 spectral channel, which is just detectable by SKA, enabling the linear diameter to be measured. However, terrestrial baselines (with 3 mas resolution) will be too short to measure the microarcsec diameter of the shell, and orbital VLBI baselines will have insufficient sensitivity. However, the H₂O shell could also be used for this purpose. For example, the H₂O masers in the supergiant star VY CMa at a distance of 1.3 kpc have a flux density of about 3000 Jy. Such a star in NGC253 would have a measured flux of S ~ 40 mJy in a 1 kms^-1 spectral channel at 22 GHz, which would be detected by SKA with S/N ~ 1000 in 8 hours, and so the linear diameter can easily be measured. However, in this case terrestrial baselines (with 0.2 mas resolution) will be sufficient to measure the diameter (~ 0.8 mas), provided that SKA-size elements are available with such long baselines, so that this technique could in principle be used to measure distances to nearby galaxies. This technique has the virtue of being independent of all other methods, and so would provide a useful crosscheck to the ensemble of techniques already used for measuring the extragalactic distance scale. However, because of the relatively low accuracy attainable with earth-based baselines, this technique is unlikely to make a major impact on this field and so should not be regarded as a major science driver for the SKA.

There has recently been a great deal of work [7] on shock-excited OH masers associated with supernova remnants (SNR). These masers offer the opportunity to study the interaction of the supernova with the interstellar medium, and in particular with molecular clouds. However, these masers are very weak (typically 500 mJy at 5 kpc, corresponding to 2 m Jy at the distance of NGC253) and so would be only marginally detectable in external galaxies, even with the SKA. In our own Galaxy, the sensitivity of the SKA would enable these masers to be studied routinely as part of SNR studies, so that we could directly measure the kinematics and magnetic field of the interaction zone between the SNR and a molecular cloud. However, this work, while significant, should probably not be a major science driver for SKA.

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 Arp220 [8]) and H₂O (first discovered in NGC 4945 [9]). Curiously, they are not seen in the other widespread Galactic maser, methanol [10].

5.1 H₂O Megamasers

Since the first H₂O megamasers were discovered in 1979 [9], they were suspected to be associated with accretion discs around black holes. However, the discovery [11] that the H₂O masers in NGC 4258 are confined to a thin molecular disk, only 0.5 pc in diameter, surrounding the central engine, provided the best evidence to date for the existence of massive Black Holes (MBH) in active galactic nuclei (AGN). As a result, H₂O megamasers are now becoming 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, 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 [12] have used these and related results to examine the turn-on of the radio jet a fraction of a pc from the MBH, as predicted by the standard Blandford & Konigl model, and to argue that the flow into the MBH is advective.

Fig. 2. (Left) The position-velocity diagram of the H2O megamasers in NGC4258 [11], showing a clear Keplerian rotation curve. (Right) the inferred position of the masers in a warped disk, surrounding the radio continuum jet [12].

Potentially, H₂O megamasers could be used to examine the MBH properties in a range of different galaxy types, and as a function of evolutionary state, which would enormously improve our understanding of the relationship between MBH and their host galaxies. However, we are severely limited at present by the small number of H₂O megamasers, and by the even smaller number that have been successfully observed with VLBI. Such VLBI observations are essential for successful unravelling of the maser and MBH properties.

The most complete survey for H₂O megamasers so far [13,14] was made with a 1-s sensitivity of typically 30 mJy in a 0.8 kms^-1 channel spacing. The 16 known H₂O megamasers discovered so far in this work have fluxes in the range 60-16000 mJy, corresponding to a luminosity of 24 to 6100 L_o. A typical VLBI array, such as the VLBA, has an rms imaging sensitivity in 0.8 kms^-1 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 MBH.

5.2 OH Megamasers

The impact of OH megamasers on other areas of astrophysics has not been as profound as H₂O megamasers, as they appear to occur on a larger scale within the host galaxy. Their luminosity distribution is curious in that the observed flux density of detected megamasers is independent of redshift - one of the highest redshift megamasers known is also one of the strongest (~ 0.3 Jy) [15]. This implies that, as we sample larger volumes of space, we encounter increasingly luminous (but increasingly rare) masers. Clearly the luminosity function must turn over at some point, but we have not yet discovered that point.

Thus it is not possible at present to predict a maximum redshift for these masers, and the greatest contribution of the SKA will be, not its ultimate sensitivity, but the speed with which it can search large samples of galaxies to discover the hypothetical "gigamasers", and its use as one element of a VLBI array. Such an array would be able to resolve a 5-pc disk at a redshift of 0.04, with a sensitivity of 1

Unlike the H₂O megamasers, we do not yet understand the physics of the OH megamasers sufficiently well to predict the astrophysical impact of this work. Thus although we can say with certainty that the SKA will contribute enormously to our knowledge of OH megamasers, and perhaps they will turn out to be as significant in their implications as the H₂O megamasers, it is not possible at present to quantify this any further. Therefore the remainder of this paper concentrates on the impact of H₂O megamasers.

At present we are limited by the small numbers of known megamasers, particularly those strong enough to be suitable for VLBI, because only with VLBI measurements can unleash the full power of megamasers as a tool for understanding AGNs. The SKA will be able to help in two ways: as a detection instrument and as an element of a VLBI array.

6.1. SKA as a detection instrument

As a detection instrument, the SKA will be able to detect megamasers that are too weak to detect with existing instruments. Assuming that the multibeam capability and 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
• megamasers comparable in luminosity to NGC4258 up to a redshift of 0.5.

However, the resolution of the SKA (0.07 arcsec) will not be able to resolve these megamasers - those in NGC4258 have a total extent of 10 milliarcsec.

6.2. SKA as an element of a VLBI array

If the SKA were used as part of a VLBI array (using 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 SKA will have a typical sensitivity (in 8h, in an 0.8 kms^-1 channel) of 70 m Jy. This 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
• megamasers comparable in luminosity to NGC4258 at a redshift of 0.06, at which redshift the maser disk is just resolvable by the largest Earth-based baseline.

At present, about twenty H₂O megamasers are known, of which only five have been imaged with VLBI, and only one of which has the level of detailed information that we need to study the physics of the accretion disk. The effect of the SKA will be to increase all these figures by about an order of magnitude. This will increase our knowledge of the working of the MBH, and also extend the volume of space that we can study, so that we can use megamasers as a tool for studying evolution of galaxies in the Universe. At present, most known H₂O megamasers lie in Seyfert 2 and LINER galaxies. Given sufficient sensitivity, we may expect to find them in other types of galaxy, and so be able to compare the occurrence and mass of the black holes as a function of galaxy type. Specifically, we should be able to make significant inroads on answering questions such as the following:

• What is the mechanism for fuelling the 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 large black holes a result of many mergers of small black holes, or do small gas-rich galaxies already contain large black holes?
• Can we see accretion disks around black holes in merging galaxies? If so, can we trace the kinematics of the circumnuclear material as the black holes merge?

The difficulty of building the SKA to work to a frequency as high as 22 GHz, compared with its earlier design goal of only a few GHz, should not be underestimated. Nevertheless, the importance of the 22 GHz H₂O megamasers as gauges of massive black holes should equally not be underestimated as being perhaps one of the major potential astrophysical advances to come from radioastronomy.

We may speculate that the SKA might generate a paper written in about fifteen years from now, in which the mass of the black hole, derived reliably from Keplerian rotation curves, is plotted as a function of mass of the host. On this "extragalactic H-R diagram", there may perhaps be a main sequence of spirals, with branches of other morphological types reflecting their age and evolutionary history. Such a plot might answer the burning questions of where the black holes came from, and what their role is in shaping the evolution of galaxies. If such a plot could be drawn, it would represent a major advance in our understanding of how the Universe evolves.