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4.1. Faraday Rotation Measures

Faraday rotation measures (RMs) where available are given in Table 5. There are many sources in the complete samples without measured RMs. Note that it is often difficult to measure reliable RMs in GPS sources for the following reasons. (1) The polarization is so low that it is often difficult to detect polarization at multiple wavelengths. (2) The existing observations tend to straddle the peak of the spectrum, so that (assuming the turnover is due to synchrotron self-absorption) the wavelength dependence of the polarization is contaminated by the effects of synchrotron opacity. Polarization measurements at high frequencies above the spectral peak are needed to determine if GPS sources generally have high RMs.

Table 5. Faraday Rotation Measures

      RM RM(1 + z)2  
IAU Name Catalog Name Class (rad m-2) (rad m-2) References
(1) (2) (3) (4) (5) (6)

0127+23 3C43 CSS -65 -393 1
0134+32 3C48 CSS -67 -126 2
0221+27 3C67 CSS -64 -110 1
0237-233 OD-263 GPS 19 195 3
0248+430   GPS 44 236 3
0319+121   GPS -15 -202 1
0429+41 3C119 CSS 1728 3400 4
0457+024   GPS 258 2954 5
0500+019 OG 003 GPS -7 -18 1
0518+16 3C138 CSS -2 -22 1
0538+49 3C147 CSS -1510 -3150 4
0552+398   GPS -658 4446 6
0658+38 3C173 CSS -11 -46 1
0738+313   GPS -813 -2160 5
0802+10 3C191 CSS 89 778 1
0941-08   GPS 35 53 1
1005+07 3C237 CSS 141 497 1
1019+22 3C241 CSS 18 123 1
1117+146 4C14.41 GPS -112 208 1
1127-145   GPS 33 158 1
1143-245   GPS 146 1270 5
1203+64 3C268.3 CSS 86 161 1
1245-197   GPS 942 4875 7
1354-17 OP-190.4 GPS -29 -499 1
1328+25 3C287 CSS -58 -245 1
1328+30 3C286 CSS -1 -3 2
1442+101 OQ172 GPS 1091 22400 4,8
1447+77 3C305.1 CSS -57 -259 1
1458+71 3C309.1 CSS 63 228 1
1517+20 3C318 CSS 456 1400 8
1641+17 3C346 CSS -32 -43 1
2223+21   GPS -125 -1094 1
2134+004   GPS 349 3008 5
2247+14 4C14.82 CSS -3 -5 1
2248+71 3C454.1 CSS -49 -395 1
2249+18 3C454 CSS -87 -661 1
2252+12 3C455 CSS 7 17 1

NOTES. - A compilation of Faraday Rotation Measures for GPS and CSS sources which are listed in Table 1. Columns: (1), IAU Name; (2), catalog name; (3), class (either CSS or GPS); (4), Observed RM; (5), RM in the rest frame of the source; (6), A reference for the observed RM. Note that the high RM for 0201+113 in O'Dea et al. (1990) was not confirmed by Stanghellini et al. (1997c).
REFERENCES. - 1. O'Dea et al. (1990). 2. Aizu et al. (1990), Kato et al. (1987). 3. Taylor et al. (1992). 4. Rudnick et al. (1983). 5. Simard-Normandin, Kronberg, & Button (1981). 6. Rudnick & Jones (1983). 7. Mantovani et al. (1994). 8. Stanghellini et al. (1998).

Saikia, Swarup, & Kodali (1985) suggested that the RMs of CSS sources tend to be slightly higher than those of large-scale 3CR sources. Since then, examples of GPS/CSS sources with extreme values of RM (gtapprox 1000 rad m-2 in the source rest frame) have been found (Kato et al. 1987; Aizu et al. 1990; Taylor, Inoue, & Tabara 1992; Inoue et al. 1995). Note, however, that there are also GPS/CSS sources with small measured values of RM. Lüdke et al. (1996) show that the CSS and large FR 2 sources display a trend of increasing Faraday dispersion with decreasing source size. Table 5 lists 11 sources with rest-frame RM gtapprox 1000 rad m-2. Figure 7 shows that (1) GPS and CSS sources have similar RMs; (2) there is a large range of RMs, possibly divided into three groups, small (RM ltapprox 100), intermediate (RM ~ 200), and large (RM gtapprox 1000); and (3) there is an upper envelope to the RM (defined mainly by the quasars) that decreases with increasing projected linear size (consistent with Lüdke et al.). Curiously, the quasars have a broader range in RM than the galaxies; the sources with the smallest and largest values are quasars.

Figure 7a
Figure 7b

Figure 7. (a) Histogram of rest-frame Faraday rotation measure for the subset with measurements of the Fanti et al. CSS sample and the Stanghellini et al. GPS sample (shaded). Sources in the rightmost bin have values gtapprox 103 rad m-2. (b) Rest-frame RM as a function of projected source size for the sources in (a). Galaxies are represented by solid squares, and quasars by crosses.

Currently, there are two classes of extragalactic radio sources that are found to have high RMs: GPS/CSS sources and radio galaxies at the centers of cluster cooling flows (Perley 1990; Taylor et al. 1992; Ge & Owen 1994). The high RMs in the cooling flow radio galaxies are thought to be produced in the intracluster medium (ICM) surrounding the radio galaxy. The inflowing cluster gas may compress, amplify, and radially align the intracluster magnetic field (Soker & Sarazin 1990), resulting in large Faraday depths. This raises the question of whether the GPS/CSS sources with high RMs are also in cooling flow clusters or whether the high RMs are produced in some other way, e.g., via interactions between the radio jet and the ambient medium.

The possible relationship between GPS/CSS sources and cooling flows should be investigated. At present, there is only mixed anecdotal evidence. O'Dea et al. (1994a) noted that PKS 2322-123, which is in the center of a large cooling flow in Abell 2597, is similar to many CSS sources and could be considered to be a low-redshift example of a CSS source. Forbes et al. (1990) argue that 3C 309.1 has a massive cooling flow based on pressures in the optical emission-line nebula. On the other hand, O'Dea et al. (1996b) used ROSAT observations to rule out a cooling flow greater than 3 Modot yr-1 in the GPS galaxies 1345+125 and 2352+495 (see section 9).

Recent VLBA polarimetry observations have clarified the location of the high RMs in the CSS quasars OQ 172 (Taylor et al. 1996b; cf. Udomprasert et al. 1997) and 3C 138 (Cotton et al. 1997a). OQ 172 has a very large rest-frame RM gtapprox 2 × 104 rad m-2 measured in its integrated polarization. Taylor et al. find that the high RM is confined to the nuclear region of the quasar, and that over a scale of about 50 pc (projected) the RM in the jet drops below 100 m-2. Similar results (i.e., very high RM near the core, much lower farther out) are found for 3C 138 (Cotton et al. 1997a). This suggests that the high RM is produced in the nuclear environment of the quasar and not in a cluster cooling flow. Thus, the high-RM phenomenon depends sensitively on the source structure and resolution.

What conditions in the ambient medium would produce RMs of order 103 rad m-2? For a uniform medium

Equation 5 (5)

where RM is the intrinsic RM in rad m-2, ne is the electron density in cm-3, B|| is the parallel component of the magnetic field in µG, and L is the path length in pc. For a path length of 100 pc, and a magnetic field of 10 µG in the ambient medium, the electron density is of order unity:

Equation 6 (6)

This calculation is meant to be merely illustrative; however, it does suggest that electron densities in the environment of GPS/CSS sources with large RMs could be very high. If the ambient medium has a low ionization fraction (f), then the ambient density is proportionally larger (nH appeq ne / f). Thus, at this point the existence of large RMs in some CSS/GPS sources supports the hypothesis of a dense ambient medium surrounding the radio source. Note that if the high RMs turn out to be produced mainly in the nuclei, then this support will be diminished.

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