Evaluación de acciones

whereas those in (high-luminosity) FRII sources are relatively faint until their ter- minal hotspots (see Section 1.3 and Figure 1.9). Over the years, evidence has been accumulated that jet flows in FRI radio galaxies are initially relativistic (e.g. Gio- vannini et al. 2001; Hardcastle et al. 2003) and then rapidly decelerate on kpc scales (e.g. Laing et al. 1999). Deceleration can be the result of either injection of mass lost by stars within the jet volume (e.g. Komissarov 1994; Bowman et al. 1996) or entrainment of the surrounding ISM (e.g. Baan 1980; Begelman 1982; Bicknell 1984, 1986; De Young 1996; Rosen et al. 1999; Rosen & Hardee 2000, see also Sec- tion 1.4.2). The idea that jets in FRI LERGs have initially relativistic speeds rests on various arguments:

• FRI sources are thought to be the side-on counterparts of BL Lac objects, in which relativistic motion on parsec scales is well-established (Urry & Padovani 1995).

• Superluminal motions have been seen on mas scales in several FRI jets (Gio- vannini et al. 2001) and on arcsec scales in M87 (Biretta et al. 1995).

• In FRI sources, the brighter jet (usually referred to as the main jet or simply the jet) is less depolarized than the fainter jet (called the counter-jet; Morganti et al. 1997).

The latter argument, in particular, can be explained if the main jet points toward the observer, suggesting that the asymmetry in brightness is caused by Doppler beaming (Laing 1988). The asymmetry decreases with distance from the nucleus, implying that the jets must decelerate (Laing et al. 1999).

In the last decades, the properties of straight, twin radio jets in typical FRI radio galaxies (i.e. physics, geometry, kinematics, etc. ) have been extensively explored in a series of works, coupling observations with accurate modelling (see Laing & Bri- dle 2014, for an overview of the subject). This analysis shows that the jets and counter-jets in FRI radio galaxies can be accurately modelled as intrinsically iden- tical, antiparallel, axisymmetric, decelerating relativistic flows, and the apparent differences between them are dominated by relativistic aberration. Furthermore, the inner parts of the jets in all of the modelled sources show several common features:

• an inner region, close to the core where the jets are launched, of well-collimated jet flow. In typical FRI radio galaxies (such as 3C31, the prototypical FRI LERG; Laing & Bridle 2002b) this region is typically observed as an initial gap, or extended region in which the radio emission is weak or undetectable. • a brightness flaring point, that is an abrupt increase in their intrinsic bright-

• A flaring region, where the jets first expand more rapidly and then re-collimate. This can be usually identified as the region where there is a significant in- creases in the apparent jet opening angle with increasing distance from the AGN.

Within the flaring region, the jets decelerate from ≈ 0.8c to sub-relativistic speeds and develop transverse velocity gradients. Immediately downstream of the bright- ness flaring point, before deceleration commences, there is little variation of velocity across the jets, and they can be approximated as uniform, constant-velocity flows.

The systematic differences in brightness between the main and counter-jet in the inner region, before the jets decelerate, can provide essential clues on their orientation to the line of sight. Indeed, for intrinsically symmetrical, relativistic jets, the jet/counter-jet flux density ratio (R, also known as sidedness ratio) can be written as: R= Ij Icj = 1 + β cos θ_{1 − β cos θ} !2−α (6.1) where θ (0 ≤ θ ≤ π/2) is the angle to the line of sight of the approaching jet, β = v/c (where v is the flow velocity) and α is the jet spectral index. For emission in total intensity which assumed to be is isotropic in the frame of the jet flow, Equation 6.1 does not allow β and θ to be determined independently. However, the detailed modelling of the relativistic jet flows in FRI radio galaxies carried out in Laing & Bridle (2014) has enabled accurate estimation of the jet inclination θ in a number of radio galaxies. This work also enabled a calibration of the relation between the jet/counter jet flux density ratio where the jets first brighten (within 1-2 kpc) and θ, which can be considered valid for FRI jets in general. The calibration relies on the inference that the dispersion in velocity just downstream of the flaring point is small both spatially across individual jets and between different sources means that we can determine the angle to the line of sight using a simple constant-velocity approximation. A version of that relation is presented in Figure 6.4, whereby the sidedness ratios were determined using the same method as used for our sources (see Section 6.4.1). The solid lines in Figure 6.4 are single-velocity models. The best-fit model (i.e. unweighted chi-squared minimisation; red solid line) is obtained for β = 0.75.

6.4.1

Jet sidedness ratios

Given the above considerations, a reasonable estimate of the jet inclination with respect to the line of sight can be obtained by the relation (following from Equa-

Figure 6.4: Jet/counter-jet flux density ratio versus inclination angle to the line of sight for the 10 sources in Laing & Bridle (2014). The solid lines are single-velocity models. Specifically, the red solid line is for β = 0.75 and is formally the best-fit. The blue solid lines below and above the best-fit are for β = 0.5 and 0.9, respectively, and roughly show the boundaries of the distribution.

tion 6.1): θjet = arccos   1 β R2−α1 −1 R2−α1 + 1   (6.2) (6.3) where β is assumed to be equal to 0.75 (according to the best-fit single-velocity model of Figure 6.4). A good approximation for α is to use the mean spectral index typically seen between 1.4 and 4.9 GHz in FRI jets, α = −0.6 (for our spectral-index definition; Laing & Bridle 2013).

Using the radio continuum maps in Figure 6.3, we aim at measuring the jet/counter- jet ratio (R) at the brightness flaring point, which usually has a well-defined onset in FRI jets (as explained above) and should be then identified with some precision in deep high-resolution radio images. The general approach adopted to carry out our analysis consists of the following steps:

• Set boxes covering 3 synthesised beams out from the flaring point, with widths covering the whole jet.

Table 6.4: Jet sidedness ratios and corresponding inclination to the line of sight. Target R θjet (deg) (1) (2) (3) IC 1531 ≥22 ≤46 NGC 3557 1.0 90 IC 4296 1.2 86 NGC 7075 99 19

Notes. − Columns: (1) Target name. (2) Jet/counter-jet brightness ratio. (3) Jet inclination angle to the line of sight.

• Divide the flux density of the (brighter) jet by that of the counter (fainter) jet to obtain the sidedness ratio. Equation 6.4.1 is then used to estimate the jet inclination angle.

This standard method, however, works well only in two of the observed objects, NGC 3557 and NGC 7075. In these two cases, the resolution of our observations allows us to resolve the inner jet structure (perfectly matching the typical FRI jet geometry described in Section 6.4) and the achieved sensitivity allows us to detect emission from the (fainter) counter-jet. In the other two cases a different approach is needed.

In IC 1531 (Fig. 6.3a) the inner jet structure is unresolved at the resolution of our observations (≈ 200 pc; see Table 6.2), then the brightness flaring point is not identifiable. Furthermore, emission from the counter-jet is not detected, although we cannot exclude the possibility that some emission from the inner counter-jet contributes to the unresolved core emission. In this case, given the observed struc- ture, we placed the two boxes as close to the core as possible, at the same distance with respect to the phase centre. We then assumed the rms integrated over the north-west box as an upper limit to the integrated flux density of the counter-jet.

In IC 4296 (Fig. 6.3d) emission from the brightness flaring point of both the jet and counter-jet is only marginally detected in our observations (≈ 3−5σ), thus not allowing a reliable estimation of the jet/counter-jet ratio. We then used the deep high-resolution archival radio image at 4.9 GHz presented in Chapter 2 (Fig. 2.7c) to measure the sidedness ratio of IC 4296, following the method described above.

As already introduced in Section 6.3.2, NGC 3100 is a peculiar case, showing a non-standard FRI jet geometry that does not allow us to obtain a reliable estimation of the sidedness ratio and, in turn, of the line-of-sight jet inclination. This will be discussed in detail in Section 6.5.1.

The measured sidedness ratios and corresponding jet inclinations are listed in Table 6.4.