A relativistic model of the radio jets in NGC 315

Abstract

We apply our intrinsically symmetrical, decelerating relativistic jet model to deep VLA imaging of the inner 140 arcsec of the giant low-luminosity radio galaxy NGC 315. An optimized model accurately fits the data in both total intensity and linear polarization. We infer that the velocity, emissivity and field structure in NGC 315 are very similar to those of the other low-luminosity sources we have modelled, but that all of the physical scales are larger by a factor of about 5. We derive an inclination to the line of sight of 38 degrees for the jets. Where they first brighten, their on-axis velocity is approximately v/c = 0.9. They decelerate to v/c = 0.4 between 8 and 18 kpc from the nucleus and the velocity thereafter remains constant. The speed at the edge of the jet is roughly 0.6 of the on-axis value where it is best constrained, but the transverse velocity profile may deviate systematically from the Gaussian form we assume. The proper emissivity profile is split into three power-law regions separated by shorter transition zones. In the first of these, at 3 kpc (the flaring point) the jets expand rapidly at constant emissivity, leading to a large increase in the observed brightness on the approaching side. At 10 kpc, the emissivity drops abruptly by a factor of 2. Where the jets are well resolved their rest-frame emission is centre-brightened. The magnetic field is modelled as random on small scales but anisotropic and we rule out a globally ordered helical configuration. To a first approximation, the field evolves from a mixture of longitudinal and toroidal components to predominantly toroidal, but it also shows variations in structure along and across the jets, with a significant radial component in places. Simple adiabatic models fail to fit the emissivity variations.Comment: 20 pages, 17 figures, MNRAS (in press

Is cold gas fuelling the radio galaxy NGC 315?

“…A multi-epoch study shows evidence for moving features, indicating an accelerating, mildly relativistic jet (Cotton et al 1999). The jets show deceleration at larger distances and detailed modeling indicates that the angle of the jet to the line of sight is ∼38 • (Canvin et al 2005). …”

NewChandraObservations of the Jet in 3C 273. I. Softer X‐Ray than Radio Spectra and the X‐Ray Emission Mechanism

“…This might provide a more natural explanation for the deceleration that is necessary to account for the changing X-ray : radio ratios along some extragalactic jets, including 3C 273's (see Hardcastle [2006] and in particular Marshall et al [2006] for the deceleration of this jet implied by a one-zone beamed IC model). The FR I flow modeling by Laing & Bridle (2004), Canvin et al (2005) shows that these jets decollimate as they decelerate, presumably by entrainment, while no sign of decollimation is observed in 3C 273 and other high-power jets.…”

“…Based on highresolution observations and a physical model of the jet flow, the presence of velocity shear has been established in a number of low-power jets: those in the radio galaxies 3C 31 (Laing & Bridle 2004), B2 0326+39 and B2 1553+24 (Canvin & Laing 2004), and NGC 315 (Canvin et al 2005). A ''spine-sheath'' shear structure has also been implied for low-power jets based on beaming statistics of FR I and BL Lac objects in the B2 survey (Capetti et al 2002), as well as from polarimetric observations of the jet in M87 (Perlman et al 1999).…”

“…If correct, the inferred values of À would constitute the first firm measurement of the velocity of high-power jets (for the velocities of low-power jets, see the modeling by Laing & Bridle 2004;Canvin & Laing 2004;Canvin et al 2005) with profound implications for the analysis of jet energetics and composition. Since the increase of the CMB energy density with redshift would cancel the surface brightness dimming with redshift, beamed IC-CMB X-ray jets at high redshift could be cosmic beacons that outshine their quasars (Schwartz 2002).…”

Relativistic spine jets from Schwarzschild black holes

“…However, in some FR I sources the structure of the jet in the kiloparsec scale appears more complicated, with an inner spine that remains relativistic and an outer shell that decelerates and becomes sub-relativistic (Canvin et al 2005); -FR II sources are more powerful than the FR I ones, with a threshold power ∼10 25 W Hz −1 sr −1 increasing with the radio galaxy luminosity (Ledlow & Owen 1996); -FR II are usually found in poor gas environments, with jets probably collimated by their helical magnetic fields (Hardcastle & Worrall 2000;Asada et al 2002;Zavala & Taylor 2005) and slightly interacting with the external gas. Rich environments harbor mostly FR I sources and their jets, thermally confined (at least partially) and appearing to strongly interact with the intracluster medium (Kaiser & Alexander 1997;Laing et al 1999;Gabuzda 2003).…”