Activity in Galactic Nuclei

The properties of galaxies as a whole are profoundly influenced by the energy release processes which take place in their centres. The mass and spin of the central black hole and the rate of nuclear feeding are thought to play dominant roles in establishing the properties of galactic nuclei, and differences in these parameters may lead to the various forms of nucleus observed: active radio-loud, active radio-quiet, LINERs, normal galaxies etc.

The process of accretion releases energy as gamma ray, X-ray and UV continua which, in active nuclei, can photo-ionize material orbiting within the central tenth of a parsec producing broad optical emission lines, and further out, up to a few kpc, narrow optical emission lines. In radio-loud nuclei, relativistic jets of material are ejected along the polar axis of the black hole and in some cases have sufficient energy to escape into intergalactic space as classical double-lobed sources. On their way out through the narrow line region the jets may play a role in shock-exciting the lines observed. In less active nuclei like LINERs, the photo-ionizing field is probably much more dilute.

Orientation effects also play a major role in the appearance of galactic nuclei. For more than a decade, it has been postulated that geometrically thick and optically thick dusty molecular “tori” are located in the accretion plane of a nucleus which are capable of obscuring the central region from direct view. Only recently has it become clear through HST imaging observations and VLBI observations of HIabsorption and maser emission that the postulated ring-like gas/dust structures are indeed found in the equatorial planes of galactic nuclei on scales of pc to kpc.

The phenomenology of nuclei is very varied, and their nuclear properties are distributed over many orders of magnitude. Producing a unified picture of the factors governing the distribution of mass and mass motions in the inner few kpc in galactic nuclei and how this picture evolves with cosmological epoch are major tasks confronting us. Specific questions which may be asked are (e.g. Lawrence, 1999):

The SKA and Active Galactic Nuclei

The Square Kilometer Array with its spectacular increase in sensitivity over conventional radio telescopes will play a major role in answering some of these questions, particularly the question of how widespread is nuclear activity in galaxies. Does every normal galaxy contain a black hole? The sensitivity of the SKA will also be crucial in greatly enlarging the number of objects in which the environment of the central engines can be studied through VLBI HI absorption measurements and in which the mass of the central engine can be estimated through water megamaser studies (see chapter 2.5.1). It will greatly increase the size of VLBI surveys for cosmological purposes, and allow studies of edge effects in compact radio emitting flows for the first time.

Black holes in normal galaxies

The radio signature of a black hole in a galactic nucleus is generally taken as the presence of an unresolved core component with a flat radio spectrum and variable flux density, and a jet, or jets, emanating from the central core. In normal galaxies, detection of a jet(s) should resolve any confusion with starburst galaxies dominated by a central supernova remnant. Clearly, high angular resolution is an instrumental prerequisite for the radio detection of black holes in a large sample of normal galaxies, in addition to sensitivity (see Fig. 1.12 from Wrobel, Condon and Machalski for a display of current sensitivity limits). The ability to observe at frequencies up to 20 GHz is also a prerequisite in order to maximise the detection rate by observing near the peak in the radio spectrum.

**Figure 1.12:** Dependence of peak VLA power on distance for 374 UGC galaxies
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The sensitivity required is of order micro Janskys and the angular resolution of order milli arcseconds. The “strawman” SKA configuration with 500 km maximum baselines has the sensitivity and frequency coverage required, but not the angular resolution. Using the SKA as the major element in a global VLBI array or increasing the baseline lengths of the SKA itself are options that need to be examined in subsequent studies of the concept. The sensitivity of the SKA in VLBI configurations is summarised in the final section.

The environments of central engines: the structure and kinematics of gas in the nucleus

Once lines are detected, the higher angular resolution afforded by global VLBI (10 milli arcsec) is needed to make detailed studies of the structure and velocity field of the gas. Baselines to a space borne antenna are required to obtain sufficient resolution for the continuum structure. Work done so far by Conway and collaborators and Peck and Taylor have shown that HIgas in nuclei can be found in circumnuclear tori, amorphous clouds, and in distinct off-nucleus clouds. An example of the latter is shown in Fig 1.13 which displays a cloud apparently blocking the SE-going jet $\sim$ 1 kpc from the nucleus in the giant radio galaxy, 3C236 (Conway and Schilizzi, in preparation).

**Figure 1.13:** HIabsorption in 3C236
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Physics of Outflows

The most common parsec-scale radio structures in AGN have a so-called ”core-jet'' morphology. The “jets'' are collimated and connect a flat spectrum and usually brighter feature (“core”) with extended steeper spectrum ”lobes'' (e.g. Pearson 1996). Smaller groups of sources exhibit other types of morphologies, such as compact symmetric, compact doubles and ``complex' sources (Readhead 1995). At present, state-of-the-art VLBI images of these sources have noise levels of 50 microJansky per beam, where the beam size is of the order of one to several milli arcsec (Zensus et al. 1995). However, extensive studies of prominent targets (3C84, Dhawan et al. 1998; 3C236, Conway 1996) make it clear that the depth of these VLBI images is several orders of magnitude short of reaching the level at which the lowest brightness features in AGN structures will be blended by the background radiation (see Figure 1.14).

**Figure 1.14:** The 1995 image of 3C 84 at $\lambda$ = 2 cm. The contours are -10,10,14,20,...,2560 mJy/beam. The peak is 2.97 Jy/beam, the off-source rms residual is 0.33 mJy/beam, and the beam has a FWHM of 0.77 $\times$ 0.58 mas, at PA = 34 $^\circ$ . (from Dhawan et al. 1998).
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The SKA as a part of a global VLBI array will enable imaging with a sensitivity of micro Jy per milli arcsecond-size beam at centimeter and decimeter wavelengths. Such imaging sensitivity will allow us to ``see'' the edges of plasma flows along and across jets, and therefore to directly compare the predictions of various models with physical reality. This will be particularly interesting in regions where the jets pass through the optical emission line regions and in regions where jet-induced star formation is occuring. On larger scales, the SKA as a stand-alone instrument will provide data on the structure of plasma flows across jets on kiloparsec scales. A global ground-based VLBI array enhanced by the SKA will make it possible to study core-jet physics in new classes of extragalactic sources, previously unreachable by VLBI due to their faintness. A combination of the SKA with the largest ground-based radio telescopes (VLA, GBT, Effelsberg, WSRT) in the 5 - 8 GHz bands would allow the study of parsec-scale structures in cores of spiral galaxies similar to our Galaxy at cosmological redshifts (mini-AGN).

Polarization VLBI observations have proved to be one of the most powerful tools of studying the geometry of magnetic fields and plasma flows on parsec scales (e.g. Wardle et al. 1994). At present, only several tens of highly polarized sources (with linearly polarized flux density of the order of several percent of the total flux density at cm-dm wavelengths) are within the reach of VLBI systems. To substantially enlarge the number of extragalactic radio sources measurable with VLBI polarimetry, one needs to increase the sensitivity of the VLBI systems by a factor of 10 to 100.

Cosmological Tests with Parsec-scale Radio Structures

An increase in recent years in the amount of VLBI data on milliarcsecond structures in AGN has rekindled interest in the classical idea of using radio sources as cosmological ``standard rods'' (Hoyle 1959). Based on a comparison of the predicted and observed dependence of the apparent angular size of a standard object as a function of redshift, one can derive cosmological parameters ( $\theta-z$ test). Recent attempts to use VLBI images for $\theta-z$ tests have been made by Kellermann (1993), Gurvits (1994,1998), Pearson et al. (1994), and Wilkinson et al. (1997). In several follow-up publications, authors have critically analyzed these attempts, pointing out the necessity to include in the models not only the deceleration parameter q_o but also the cosmological constant $\Lambda$ (see Krauss and Schramm 1993, Stelmach 1994, Kayser 1995, Jackson and Dodgson 1996). It has also been noted that statistical confidence in the results, in particular estimates of the value q_o, is still in need of improvement (e.g. Dabrowski et al. 1995, Stepanas and Saha 1995).

While the background principles of using parsec-scale sources as cosmological ``standard rods'' are simple and undisputed, a practical implementation of these tests faces serious difficulties, most of which are imposed by various selection and masking effects (eg. Wilkinson et al. 1998 and Vermeulen 1996). The most obvious selection effect is illustrated by Fig. 1.15, in which the luminosities of 330 AGN used for the $\theta-z$ test (Gurvits Kellermann and Frey. 1999, in press) are plotted against redshift. Due to the flux density limits of that ad-hoc sample of about 1 Jy, the luminosity of the low- and high-redshift AGN counterparts are mismatched by several orders of magnitude. Although one can try to use such a luminosity mismatched sample for cosmological tests, a much better result could be achieved by composing a luminosity-matched sample. To do so, one needs to image with VLBI, AGN's at higher redshifts (z > 1) with total flux densities 10-1000 times lower than those imaged to date, i.e. at a level of 0.1 - 10 mJy. Furthermore, as shown by Dabrowki et al. (1995), a cosmologically conclusive result will require a sample of several thousand sources at this level of flux densities. If we add to this other masking effects (such as Malmquist bias, spectral properties, orientation bias, etc.), it translates into an extensive observing program of VLBI imaging of a sample of 10⁴ AGN's with total flux densities of milli- and sub-millijansky level.

**Figure 1.15:** Compact component luminosity as a function of redshift for a sample of 330 flat spectrum extragalactic radio sources. The solid lines show luminosities of sources with flux density of 1, 0.1 and 0.01 Jy, calculated under the assumption that the spectral index $\alpha = 0$ . (From Gurvits, Kellermann and Frey 1999.)
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Sensitivity of the SKA in VLBI Arrays

The sensitivity of the SKA as an element of a VLBI array can be characterised in two ways - single baseline sensitivity and image noise. Assuming an integration time of 120^s, a recorded bandwidth of 512 MHz, and $T_{sys}/A_{eff} = 5 \times 10^{-5}$ for the SKA and 10^-2 for a 70 m telescope, the sensitivity for a single baseline between SKA and a 70m telescope is $\sim 15 \mu$ Jy. The sensitivity to a 25 m telescope (e.g. a second generation space VLBI antenna) would be $\sim 40 \mu$ Jy.

If the SKA is an element of an array with ten 70m class telescopes, the theoretical image noise after an 8^h observation is $\sim$ 250 nJy. For $\mu$ Jy sources, only the ten baselines to the SKA would contribute and phase referencing would be required to phase up the array. Image simulation should be carried out to quantify the effects of such a configuration.

For comparison, were the SKA configuration itself to be extended to global dimensions, the sensitivity of a single baseline between two of the 200m diameter stations would be $\sim 25 \mu$ Jy and the image noise would be $\sim$ 60 nJy. Note that the brightness temperature of a 1 $\mu$ Jy radio source which is 1 milli arcsec in size is $\sim 10^{5}$ K.