Polarimetry of GPS and CSS Sources

Abstract

Abstract:The role of radio polarimetry in the understanding of GPS and CSS sources is explored. After an initial discussion of what can be learned from polarimetry, the expectations of a simple evolutionary sequence of GPS/CSO to CSS to FR i/FR ii sources are explored. Observational results are then compared with the expectations. Conclusions include: the GPS category is likely not a single homogeneous class of objects; Faraday depth effects are very strong inside the inner 3 kpc of CSS and CSO sources; in at least 3C 138 the Faraday screen has very fine scale (subparsec) structure; and there is evidence for increased ionisation near bends in some CSS jets probably due to jet-ISM interaction. New results on 3C 138, 3C 43, and 3C 454 are given.

Multifrequency VLBA observations of two compact symmetric objects

“…Figure 2.7: Fractional polarization at 1.4 GHz plotted versus the frequency of the peak in the spectrum for a GPS sample (Cotton et al, 2003). This is consistent with the picture of a small radio source with normal polarization properties whose radio emission is subject to severe Faraday rotation with inhomogeneities on very small scales, producing depolarization.…”

“…Such ISM may be responsible for the lack of polarization. Cotton et al (2003) found that small radio sources are unpolarized in the NVSS at 1.4 GHz ( Fig. 2.7), while at higher frequencies sources with size of about a few kpc start to show some fractional polarization at progressively smaller projected linear sizes (Fanti et al, 2004).…”

“…of such a dense gas (Cotton et al, 2003) which however is not dense enough to confine the source growth. Additionally, in the standard source model, the hot spot more distant to the core, should be the brighter one.…”

Radio properties of CSSs and GPSs

“…Fanti et al (2001) found that in the B3-VLA sample a large fraction of MSOs are polarized at 4.9 and 8.4 GHz, while CSOs mostly aren't. This was further strengthened by Cotton et al (2003c) with www.an-journal.org the plot reported in Fig. 5, obtained at 1.4 GHz from the NVSS (Cotton effect).…”

The B3-VLA CSS sample

“…This turns out in very low fractional polarization limits on the brightest features (hot spots), typically less than 0.5-1% depending on the peak brightness; the upper limit on the low surface brightest regions increases to 10-20%. If the very low level of polarization is due to an external Faraday screen (see Fanti et al 2004, Paper IV, for a Faraday screen model, and also Cotton et al 2003) the screen coherence length is required to be smaller than a few parsecs.…”