7 Conclusion

We have analysed deep HST images at 300nm and 600nm (Figs. 1 and 2) of the jet in 3C273 and constructed an optical spectral index map at 0 $.\!\!^{\prime\prime}$2 resolution (Fig. 3). The optical spectral index varies smoothly over the entire jet, indicating a smooth variation of the physical conditions across the jet. Unlike in M 87 (Meisenheimer et al. 1996a; Perlman et al. 2001), there is no strong correlation between optical brightness and spectral index (Figs. 4 and 6). The spectral index map thus shows no signs of strong synchrotron cooling at any location in the jet. Particle acceleration at a few localised sites in the jet is not sufficient to explain the absence of strong cooling. This does not preclude the possibility that the enhanced-brightness regions are shocks - but even if they are, re-acceleration between them is necessary to explain the observed spectral index features. We have further shown that relativistic effects cannot lead to significant enhancements of the electron lifetime in 3C273's jet, whatever the bulk Lorentz factor (Sect. 6.3.2), strengthening previous electron lifetime arguments. The need for a continuous re-acceleration of electrons emitting high-frequency synchrotron radiation in the jet of 3C273 is thus evident. Mechanisms have been proposed which can explain the apparent lack of cooling by distributed re-acceleration. These include acceleration by reconnection in thin filaments (Lesch & Birk 1998) and turbulent acceleration (Manolakou et al. 1999). Both processes manage to maintain the injection spectrum over distances much larger than the loss scales, although the latter so far only maintains cutoff frequencies in the range of $10^{12}\,$Hz-$10^{13}\,$Hz, i.e., below the values observed in 3C273. We note that for those jets which are bulk relativistic flows at high Lorentz factors, the increased inverse Compton losses form a further sink of energy that has to be filled by re-energization processes inside the jets. This requirement becomes more severe at higher redshifts. As has been suggested previously (Celotti et al. 2001), inverse Compton scattering off cosmic microwave background photons might explain the so far unaccounted-for X-ray flux from 3C273's jet (Röser et al. 2000; Marshall et al. 2001) and from Pictor A's jet and hot spot (Wilson et al. 2001). If this is true, the outward-decreasing X-ray flux from the jet in 3C273 indicates that the jet is still highly relativistic near region A and slows down towards the hot spot. Recently, the detection of extended X-ray emission from the jet in PKS 0637-752 by the new X-ray observatory CHANDRA has been reported and a similar explanation has been brought forward (Chartas et al. 2000; Schwartz et al. 2000; Tavecchio et al. 2000; Celotti et al. 2001). Like for 3C273, in situ re-acceleration is required in PKS 0637-752 to explain the mismatch between de-projected extent of the jet (>1Mpc) and inverse-Compton loss scale (10kpc) (Tavecchio et al. 2000). In order to understand the physical conditions in extragalactic jets like that in 3C273, it is necessary to detect even the most subtle variations in the parameters describing the synchrotron spectrum (that is, cutoff frequency, break frequency and especially low-frequency spectral index), which requires the deepest images at the highest resolutions and in many wavelength bands to detect variations at all. A theory of the physical processes at work in this jet initially has to explain both the simple overall spectral shape, as well as its constancy over scales of many kiloparsec. The details of this physical process will be constrained by the subtle deviations from the simple spectral shape, such as those tentatively identified in Sect. 6.3.1. We aim to find these deviations with the full data set, including new radio and near-infrared data in addition to the presented optical and UV images.

Acknowledgements

  We are grateful to D. van Orsow for his assistance with the HST observations. This research has made use of NASA's Astrophysics Data System Abstract Service.