Astron. Astrophys. 356, 23-32 (2000)

4. Discussion

4.1. Interpretation of the large

What is the significance of the obvious discrepancy between models and the observed ? Clearly, the working hypothesis that the emitting and absorption gas phases are physically the same, is now ruled out and an alternative explanation must be sought for, based on our result that the two gas phases (absorption vs. emission) are physically distinct. We consider the two following explanations in Sects. 4.1.1 and 4.1.2.

4.1.1. The absorption gas is metal-poor and further out

Since the absorption gas in this picture is not spatially associated with the emission gas, its metallicity is unconstrained. It turns out that the value of is easily reproduced by simply using in the one-zone case (see Eqs. 4 and 5). The more rigorous stratified slab geometry would favor a value of to reproduce the same , assuming both gas phases to have equal excitation. Can we disentangle the absolute abundance values? We cannot rely on the emission spectra alone to derive a precise and independent value for as the emission lines are very model-dependent, with fluxes from lines like C IV depending critically on the temperature. It can realistically be argued, however, that a less than half solar could not reproduce the observed metal line ratios. On the other hand, a much higher than solar cannot be ruled out in absence of direct knowledge of the ionizing continuum distribution. We consider more plausible a near solar value for on the ground that the extended emission lines extend over 13 kpc and therefore sample a huge galactic region very distinct from that of the nucelar BLR (hidden here) which has been shown to be ultra-solar in high z QSOs (Hamann & Ferland 1999 and references therein). An attempt, on the other hand, to model separately the absorption columns observed in 0943-242 as described below in Sect. 4.2 is more dependable since temperature is much less of an issue. The value inferred below of is consistent with those observed in absorbers of comparable redshift along the line of sight of more distant QSOs (Steidel 1990a). Since measured galactic metallicity gradients are always negative and a function of the distance to the nucleus, such a contrast in metallicity between absorption and emission gas makes more sense if the absorption gas is located much further out than the emission gas which extends to at least 13 kpc in 0943-242.

We emphasize that this scenario does not entail that the absorption gas does not belong to the environment of the parent radio galaxy. As argued by v097, the high frequency of detection of H I aborbers in 9 out of 10 radio galaxies smaller than 50 kpc, much in excess of the density of absorbers along any line of sight to distant QSOs, is a compelling argument for concluding that the absorption gas is spatially related to the parent galaxy. Our postulate is that the large scale H I absorption gas is the same gas which is seen instead in emission in those radio galaxies with Ly sizes larger than 50 kpc. In effect, absorption troughs are not seen when the emission gas extends beyond 50 kpc. Such objects in general also have much larger radio sizes as shown by v097. Kinematically, the gas which is seen in emission at the largest spatial scales shows narrow FWHM. For instance a reresentative case is the radio galaxy 1243+036 () which was studied in great detail by van Ojik et al. (1996) and which reveals the presence of very faint Ly emission extending up to 136 kpc, a region labelled "outer halo". This emission gas has a FWHM of 250 km s^-1 and shows clear evidence for rotational support.

A straightforward explanation of why the same gas is seen in emission in some objects while in absorption in others might simply be the environmental pressure. A larger pressure, like the one adopted by v097 can cause the warm gas to condense and hence reduce his filling factor as compared to similar gas components in a low pressure environment. Due to this process, high pressures and consequently high densities lead to detectable Ly since emissivities scale proportionally to , but also to an overall smaller covering factor (hence no detectable absorption) while low pressures lead to large covering factors (hence absorption) as well as negligible emissivities. Differences in pressure in the outer halo would therefore naturally account for the reported dichotomy of detecting H I troughs exclusively in those emission Ly objects devoid of very large scale emission ( kpc)

Since absorption troughs tend to be absent in radio galaxies showing the largest radio scales, we propose that the gas which is seen in absorption must lie outside the zone of influence of the radio jet cocoon, a region with pressure of order K cm^-3 (v097). An unpressurized outer halo responsible for the absorption troughs ought to precede the regime in which the radio material has expanded sufficiently outward to pressurize the outer halo. The eventual increase in environmental pressure would either disrupt the gas or compresses it into small clumps (making it unobservable in absorption when the covering factor dwindles), which becomes visible in emission if it lies within the ionizing cone. v097 assumed that the absorption and emission gas were both immersed in zones of comparable surrounding pressure ( K cm^-3) and were therefore of comparable density ( cm^-3 for a photoionized gas). We propose instead that whenever aborption troughs are observed, the absorption gas must lie outside the radio jet cocoon, allowing for a lower density and high covering factor.

The clear-cut advantages of locating the H I absorber in an unpressurized outer halo are threefold:

1. We can now get the high excitation of the low density absorption gas for free. In effect, if the density of the absorption gas is as low as - cm^-3, the metagalactic background radiation suffices to photoionize the absorption gas to the high degree observed in 0943-242, whether it does or does not lie within the ionizing cone of the nucleus. Conversely, for the objects devoid of absorption, when a higher pressure has set in in the outer halo (as we presume to be the case in 1243+036), the gas is much denser and can be seen in emission only if it lies whithin the ionizing cone (since a high density gas of cm^-3 cannot be kept highly ionized by the background metagalactic radiation). This picture would be in accord with the findings of van Ojik et al. (1996) who detect Ly in emission in 1243+036 only along the radio axis (presumably the same axis as that of the ionizing radiation cone) and not in the direction perpendicular to it.

2. The much smaller velocity dispersion ( km s^-1) of the absorption gas as compared to the emission gas (FWHM km s^-1, cf. Table 1) is more readily explained if the absorption gas lies undisturbed at relatively large distances from the parent galaxy.

3. It explains why the absorption (yet ionized) gas in 0943-242 is not seen in emission while being more massive than the inner emission Ly gas observed within 13 kpc. In effect, the mass of ionized gas either in emission or absorption around 0943-242 inferred by v097 are and , respectively. Adopting the conservative value of (cf. panel b in Fig. 5), the total ionized mass of the absorption ionized gas therefore exceed that of the inner emission gas by at least a factor two and yet it is not seen in emission! This huge pool of ionized gas can remain undetectable in emission only if it has a very low density, as argued above. It is customary to assume a volume filling factor of for the gas detected in emission in radio galaxies and that this gas is immersed in a region characterized by a pressure of order K cm^-3 (v097; van Ojik 1996). If we suppose instances where the outer halo has much lower pressure than this, it can be shown that for the same outer halo mass, the luminosity in Ly would scale inversely to the volume filling factor. Hence, the gas would be weaker in emission by a factor of if its filling factor approached unity (with the mean density being lower by the same amount). This scheme would easily explain why the outer halo of 0943-242 is not seen in emission despite its huge mass (comparable incidentally to the outer halo mass measured in emission in 1243+036 of by van Ojik et al. 1996).

4.1.2. A two-phase gas medium

Due to radiative cooling (which goes as and rise steeply with T), density enhancements can condense out of the emitting gas and form a population of about 100 times denser and 100 times cooler clouds in pressure equilibrium with the ambient medium. If we maintain that the pressure characterizing the absorption and the emission gas is comparable ( K cm^-3) and that either gas phase has a temperature typical of photoionization, K, we obtain (adopting a similar notation to v097 but adapted to the case of 0943-242) that the size and the number of small homogeneous absorbing condensations required to cover the emission region would be pc and clouds, respectively, where [as above we adopt 0.03 as the reference neutral H fraction]. Can we find an alternative interpretation to (1) above for explaining the large that does not require low metallicities for the absorption gas? Such a possibility would arise if the column was not directly related to the column. For instance, in the auto-gravitating absorber model of Petitjean et al. (1992), which consists of a self-gravitating gas condensation with a dense neutral core surrounded by photoionized outer layers, could in principle give ratios between columns of H I and C IV which do not reflect the abundance ratio but represents rather the average impact parameter for our line of sight. Of course, these models have to be rescaled to a pressure of K cm^-3 implying much smaller sizes but requiring much higher ionizing fluxes (both by a factor ). This rescaling poses no conceptual problems if we assume that the photoionization is by the central AGN. Using their figures and Table 4 (Petitjean et al. 1992), we infer that the number of auto-gravitating condensations needed to achieve a covering factor of unity and a mean H I column of would have to be large, in excess of , for instance, for the model . However, after inspection of the various columns derived from their extensive grid of models, we did not find any model which would reproduce the observed C IV column without having a metallicity . The gain in Z is therefore insufficient to get and we conclude that this explanation for a high is unworkable.

4.2. Metallicity determination of the absorption gas

Our favoured interpretation of the large is that the absorption gas is of very low metallicity compared to the (inner/denser) emission gas. Furthermore, a close parallel in the physical conditions of the absorption gas could be made with those adopted for the study of QSO absorbers (e.g. Steidel 1990a,b; Bergeron & Stasinska 1986), namely the densities, the metallicities and the excitation mechanism (photoionization by a hard metagalactic background radiation). The observed column of would position the 0943-242 absorber in the category of "Lyman limit system" according to Steidel (1992). The coincidence in physical conditions might be fortuitous and it does not imply per se a common origin or correspondance between QSO absorbers and outer halos of radio galaxies. Under the sole assumption of similar physical conditions, what estimate of the metallicity can we derive for C? From the ratio, we cannot determine the ionization parameter and therefore directly apply the results and models of Steidel (1990b) who determined for each Lyman limit system a probable range of U from upper limits or from measurements of other species than C IV . It is nevertheless reasonable to assume that the excitation degree in 0943-242 is comparable to that encountered in high excitation QSO absorbers. To determine an appropriate value for U, we adopted the set of data provided by the three Lyman limit systems observed in the spectrum of the QSO HS1700+6416 by Vogel & Reimers (1993) who successfully measured the columns of up to 3-4 ionization species of each of the three elements C, N and O. Amongst our model sequence (Sect. 3.4), we selected the model which had the same U () as Vogel & Reimers (1993) and inferred that the observed columns in 0943-242 implied that the Carbon metallicity of the absorption gas was 1% solar (that is C/H ), which is broadly consistent with the range of values favored in Sect. 4.1.1.

4.3. Mean density and cloud sizes

What would be the minimum density assuming the absorption gas to be uniformly distributed? If our proposed picture was correct, a representative size for the absorption gas volume is that given by the outer halo as seen in emission in other HZRG. Let us adopt the value measured for 1243+036 by van Ojik et al. (1996) of 136 kpc. Assuming the same mean ionization parameter as used above (0.007), we derive a total gas column of . Hence the mean density for a volume filling factor unity on a scale of the 1243+036 outer halo would be cm^-3 which is a value sufficiently low to allow photoionization by the feeble ionizing metagalactic background radiation.

4.4. Comparison with the metallicity of BAL QSOs

Our estimate of the metallicity for the outer halo of 0943-242 is at odds with the super-solar metallicities (e.g. Hamann 1997, Papovich et al. 2000) of the "associated" absorbers seen in high redshift QSOs. The QSO emission gas itself (the BLR) is similarly characterized by super-solar metallicities (cf. Hamann & Ferland 1999 and references therein). If we consider QSOs and HZRG as equivalent phenomena observed at different angles, it may appear at first surprising that the metallicities of the absorption components are so different. However, we show below that this contradiction is only apparent as we are probably dealing with totally different gas components.

1. Kinematics. The HZRG large scale absorbers are kinematically very quiescent. In effect, the modulus of the velocity offset between the absorbers and the parent galaxy is usually less than 400 km s^-1 for the dominant absorber (v097) ⁶. A substantial fraction of HZRG absorbers are actually infalling (Binette et al. 1998). This is far from being the case for QSO "associated" absorbers whose ejection velocities can extend up to many thousands km s^-1 (Hamann & Ferland 1999). For instance, the two associated systems (with detected metal lines) recently studied by Papovich et al. (2000) are blueshifted by -680 and km s^-1, respectively.

2. Selection effect. QSOs are spatially unresolved with a size of the source light beam less than a few light-weeks across. In the case of HZRG absorbers, the backgound source is the emission gas which extends over a scale kpc. This huge difference in scale results in a totally different bias on what is preferentially observed. In effect, the extended absorbers of HZRG are weighted towards the largest volumes and hence towards the most massive gas components (the total mass of the absorption component exceeds in 0943-242). By contrast, in the case of QSO associated absorbers, the mass of gas directly seen in absorption is tiny (e.g. if one considers a background light beam one light-month diameter and a total gas absorption column of ).

3. Coexistence with the BLR. To the extent that QSO associated absorbers represent gas components expelled from the BLR, we should not be surprised that their metallicity turn out comparable to the BLR. Given that in HZRG we do not directly see the pointlike AGN, we cannot expect to see any BLR component in absorption. As for the extended gas detected in HZRG, there exists no evidence in favour of super-solar metallicities on large scales kpc (N V when detected is strong only in the nucleus) If a fraction of associated absorbers correspond to intervening galaxies close to the QSO, we might expect to see amongst counterpart HZRGs one or more C IV or Ly absorbers of small spatial extent relative to the size of the extended emission gas. The weak H I absorption found by Chambers et al. (1990) in 4C41.17 might be such occurrence given its partial coverage of the Ly background.

We conclude that HZRG absorbers, when their size is comparable to galactic halos (as those found by v097), have probably little to do with QSO associated absorbers. A more suitable analogy to the absorption gas of HZRG is that of the Francis cluster of galaxies at which is characterized by large scale absorption gas on a scale of Mpc (Francis et al. 2000).

4.5. Constraints on radio galaxy evolution

The size of the radio source can be used as a clock that measures the time elapsed since the start of the radio activity. A number of observed characteristics of distant radio galaxies change as a function of radio size, - i.e. as function of time elapsed (cf. Röttgering et al. 2000). For 3CR radio sources, these include optical morphology (Best et al. 1996), degree of ionisation, velocity dispersion and gas kinematics (Best et al. 2000). At higher redshifts (), only the smaller radio galaxies are affected by H I absorption (v097). All these observations seem to dictate an evolutionary scenario in which the radio jet has a dramatic impact on its environment while advancing on its way out of the host galaxy (Röttgering et al. 2000, Best et al. 2000).

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
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