3.5 Conclusions
5.4.3 Synchrotron / inverse-Compton model fitting
The results of the synchrotron / inverse-Compton model fitting presented in Section5.3.4have implications for radio galaxy physics. The first which must be discussed is the determination of the magnetic field strength which is found to be lower than those used in Chapters3and4. Whereas in previous chapters I have assumed that the lobes are in equipartition, the addition of X-ray observations allows the value of the magnetic field strength to be determined free from this constraint. For 3C223, the value of 0.34 nT is only∼ 75 per cent of the equipartition value of 0.45 nT. Deviation from equipartition is observed in similar FR-II sources byCroston et al. (2004,2005), so such a difference comes as no surprise, but as was discussed in Section3.4.3, these weaker field strengths may go at least some way to explaining the spectral / dynamical age discrepancy observed in some sources (e.g. Chapter3). It is of course not feasible to use X-ray observations for all sources and, for most powerful radio galaxies, equipartition still provides a reasonable estimate of the magnetic field strength. However, the potential effects of such devi- ations from equipartition values should be carefully considered, particularly when considering model fits over very large frequency ranges where the spectral ages are well constrained.
The most significant results of the synchrotron / inverse-Compton model fitting relates to the energetics of 3C223. Comparing the total energy content of the lobes presented in Section 5.3.4to the results ofCroston et al.(2004), I find an increase by a factor of 2 in the northern lobe and 2.3 in the southern lobe of the source. The reason for this significant increase lies in the assumptions made about the radio spectrum at low frequencies where observational data were not available in the original investigations. The new integrated flux measurements made at LOFAR frequencies show that the spectrum is much steeper than previously assumed, due to a greater electron population being present at low energies. Using the standard equation for a relativistic plasma ,Plobe = U/3, where Plobe is the lobe pressure andU is the total energy
density, I derive pressures of9.3 × 10−14and1.1 × 10−13J m−3 for the northern and southern lobe respectively. Comparing these values to the external pressure at the end of the lobe derived by Croston et al.(2004) of9.6 × 10−14±38.38.8 J m−3, I find, contrary to the original findings, that the northern lobe is approximately in pressure balance with the external medium, with the southern lobe being overpressured. From this one can infer that the southern lobe is currently still expanding into the external medium. A summary of these pressures is given in Table5.10.
WhileCroston et al.(2004) favour an underpressure interpretation in their original investi- gation, the large uncertainty of the external pressure at the end of the lobes means that both the
Table 5.10: Summary of lobe pressures for 3C223
Lobe Plobe Pext Ratio
(J m−3) (J m−3)
Northern 9.3 × 10−14 9.6 × 10−14 0.97 Southern 1.1 × 10−13 9.6 × 10−14 1.15 Table of derived lobe pressures for 3C223. ‘Plobe’ refers to the pressure of the lobe listed in
the ‘Lobe’ column. ‘Pext’ is the external pres-
sure taken fromCroston et al.(2004) and ‘Ratio’ is the ratio of the lobe to external pressures.
overpressured and pressure balance cases fall within the error of the measurements. Conversely, the large upper error bars mean that the true external pressure may be much higher, causing the lobes to be underpressured even when the revised internal pressures presented within this chapter are considered. While the results presented here therefore suggest that the lobes are approximately in pressure balance, improved X-ray measurements are required if this question is to be answered with any reasonable degree of certainty.
Perhaps the biggest implication of the increased total lobe energy content is that the same assumptions about the low frequency spectrum were made in later work byCroston et al.(2005) for a much wider range of powerful radio sources. If this additional low energy electron popu- lation is found to be a common feature of FR-IIs, this will cause the total energy density (hence lobe pressure) of all FR-IIs to be increased, potentially solving the problem of underpressured, but seemingly static or expanding, radio lobes.
5.5
Conclusions
I have presented within this chapter the low-frequency study of the FR-II source 3C223. Using LOFAR observations to determine the spectrum of this powerful radio source on small spatial scales, I have investigated whether the high injection indices found in Chapters 3 and 4 are physically realistic or an artefact of the methods used. I have also investigated how these mea- surements impact on the calculations of the energetics of FR-II sources. The key points made within this chapter are as follows:
5.5 Conclusions
1. The injection index found from both theBRATSminimization method used in the previous two chapters, and through the analysis of spectral index maps are consistent with the higher values of Chapters 3 and4. I discuss possible causes of bias but conclude that these values do have a basis in physical reality.
2. I show that whilst low-frequency observations provide excellent constraints on injection index values, GHz observations are vital if one wishes to simultaneously determine reli- able spectral ages.
3. I find though synchrotron / inverse-Compton modelling that due to the steeper than pre- viously assumed spectrum at low frequencies and subsequent underestimation of the low energy electron population, the total energy content of the lobe is increased by more than a factor of 2.
4. When the revised total energy content is accounted for, the northern lobe of 3C223 is in pressure balance with the external medium with the southern lobe being overpressured.
5. I suggest that if this additional low energy electron population is a common feature or powerful radio sources an revised estimates for the total energy content of the FR-II pop- ulation as a whole will be required.
In the next chapter, I use X-ray observations of jets in low power radio galaxies to investigate what relations exist to their host galaxy properties, and if current models provide an adequate description of particle acceleration in X-ray jets.