Optical and Infrared Polarization of Active Extragalactic Objects

Annu. Rev. Astron. Astrophys. 1980. 18: 321-361
Copyright © 1980 by Annual Reviews. All rights reserved

II. STRONGLY POLARIZED OBJECTS

In this section, we summarize the optical-infrared and radio observations for all extragalactic objects that appear to be blazars and for which strong polarization has been measured, gtapprox 3%. We believe that the study of line-free. BL Lac objects has suffered from being too removed from the study of other polarized active objects. To redress that imbalance, the objects appearing in Table 1 have been included without regard to intrinsic luminosity or the presence of emission lines. A few faint objects, for which only single measurements of poor accuracy are available, have been omitted. Following Table 1, we present individual summaries of those interesting objects that bridge the compartments or subclasses of BL Lacs, QSOs, radio galaxies, etc. M87 and PHL 5200, which are strongly polarized but do not fall into the blazar class, are also summarized. A discussion of the only other compact extragalactic objects known, the strongly polarized Seyfert nuclei, is deferred until Section IV.

The information on blazars summarized in Table 1 is as follows. Coordinate designation and names are given in the first two columns, the redshifts where known and their sources are given in columns 3 and 4. If no emission lines are detected, and the redshift is that of the host galaxy, a superscript is given. If absorption lines in the continuous spectrum are the only indication of distance, z is given as greater than that of the absorption lines. Rough visual magnitudes and their sources are given in columns 5 and 6. In the next three columns we summarize the optical polarization values with the appropriate reference(s) in column 10. Column 9 gives the number of separate measurements. The ranges of percentage polarization and position angle of the electric vector appear in columns 7 and 8. In column 11, we note whether the sources are known to have strong (s), weak (w), or no detectable emission lines (no entry). References to spectra are in column 12. The optical spectral index, alpha (column 13), and the known range of B magnitude (in magnitudes, column 15) are derived from the references in columns 14 and 16 respectively. In the next eight columns (17-24), we list radio flux measurements at frequencies of 178 MHz, 408 MHz, 1.4 GHz, 2.7 GHz, 5 GHz, 8.08 GHz, 22.2 GHz, and 90 GHz. This may seem rather clumsy, but there is no simple parameter that can describe the spectrum shape over nearly three decades in frequency. References to the radio spectra are given in column 25, and to additional data in column 26.

In the following discussion of individual objects and throughout this paper we will assume that the redshifts are cosmological with H₀ = 75 km s^-1 Mpc^-1, and adopt q₀ = 1/2 when needed for objects at high z. The spectral index alpha will be defined by F propto ^- for both optical and radio spectra.

0316 + 413 = NGC 1275 = 3C 84 This source is of special interest in being the closest listed in Table 1, at z = 0.016. It is the nucleus of the dominant central galaxy of the Perseus cluster, and is distinguished by the presence of moderate strength emission lines (Table 2) and a radio halo of steep spectrum around the compact flat-spectrum core.

Optical polarization of the nucleus was first reported by Dibaj & Shakhovskoy (1966), and it has since been repeatedly measured by Walker (1968) and by Babadzhanyants, & Hagen-Thorn & Dombrovsky (references in Babadzhanyants & Hagen-Thorn 1975). This work showed the polarization to vary from month to month in strength up to 3% (26" aperture), while nearly all points lie between 100° and 150° in position angle. More recently Martin, Angel & Maza (1976) and Angel et al. (1978) found erratic changes in position angle of up to 20° from night to night. Through a small aperture (4") centered on the nucleus the polarization can be as strong as 6% in the blue (Maza 1979). The position angle during 1976-1978 showed again nearly all points in the range 100°-150°. Interstellar polarization from our own galaxy does not affect the results for NGC 1275 appreciably, the polarization of three nearby galaxies in the cluster being 0.4% at 101°. In addition to the polarization variability, rapid variability of the optical continuum strength also on a time scale of a day, has now been observed (Geller, Turner & Bruno 1979).

**Table 1.** Strongly polarized compact extragalactic objects

										Emission
						Optical polarization				lines
	Object	z	Ref.^b	V	Ref.^b	P		N	Ref.^b	Ref.^b		Ref.^b
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)

0048-097	OB-081	-	128	16	105	7-14	-	3	53, 143		.15	1.8	112
0109+224	GC	-	128	15.5	128	3-6	55-85	2	0, 90		85
0215+015	PKS	1.345	37	18.3	128	20	-	1	37		37	1.9	37
0219+428	3C 66A	.444?	69	15.5	128	6-15	170-45^p	41	2,53, 142	w	69	1.1^v	69, 112
0235+164	AO	.852	93	16.0	128	6-25	15-175	10	0, 2, 66, 93		93	4.0	93
0300+470	4C 47.08	-	228	18.0	128	12-24	70-85	2	0, 90		132
0316+413	NGC 1275	.0172	102	11.9	102	1-6	100-160^p	50	discussion	s	126
0403-132	PKS	.571	13	17.2	13	0-4	170-195	6	0, 106	s		1.1	13
0420-014	PKS	.915	13	18.0	13	8-20	150-175	7	0,68	s
0422+004	OF 038	-	128	16.0	128	6-22	140-210^p	36	2		132
0521-365	PKS	.055	128	15.0	128	6	155	1	0	w	28	1.0^v	28, 53
0548-322	PKS	.069^g	105	15.5	105	1.5-2	0-15	2	0			1.8	53
0735+178	PKS	.424	13	15.5	13	3-31	0-175	90	2, 16, 53, 74		69	1.3^v	53,69
									90, 94				112
0736+017	PKS	.191	13	16.5	13	.5-6	25-135	9	0, 106, 118	s		1.1	6
0752+258	OI 287	.446	13	17.0	13	8	145^p	8	0, 106	s
0754+100	OI 090.4	-	128	14.5	128	3-26	0-140	65	2, 27, 90, 94, 113	113
0808+019	OJ 014	-	128	17.5	128	4-14	-	5	53			0.9	53
0818-128	OJ-131	-	128	15.5	128	8-36	60-115^p	45	2, 27, 113		113
0829+046	OJ 049	-	128	16.5	128	12	110-115	2	0		100	2.3	53
0851+202	OJ 287	.306?	69	14.0	128	1-32	0-180^p	220	2, 30, 42, 49,	w	69	1.4	53,69
									53, 54, 73, 90,
									108, 121, 130
0906+430	3CR 216	.670	13	18.5	13	3-21	-	3	53, 0	s	104	1.8	53, 104
0912+297	OK 222	-	128	16.0	128	4-13	-	10	53		132	1.3	53, 112
0957+227	4C 22.25	-	128	18.0	128	2-4^a	-	2	53		95	.9	53
1057+100	HM	-	128	17.5	128	1-10	-	7	53,143		109	1.3	112
1101+384	Mkn 421	.030^g	69	13.5	128	0-7	150-185^p	25	53, 65, 66, 90,		69	1.1	69
									94, 114
1133+704	Mkn 180	.044^g	69	15.0	128	1-4	120-145	3	0		69
1147+245	OM 280	-	128	16.0	128	3-13	5-155^p	17	2, 53, 143		109	1.9^v	112
1150+497	4C 49.22	.334	13	16.1	13	0-4^a	20-180	7	0, 106	s
1156+295	4C 29.45	.729	13	15.6	13	1-9	0-120	6	0, 106	s		.93	91
1215+303	ON 325	-	128	15.5	128	4-17	120-180^p	62	2, 53, 74, 108			1.8	112

**Table 1.** - *continued*

				Radio spectrum fluxes in Jy at frequencies given in GHz
	Object	m	Ref.^b	0.18	0.41	1.40	2.70	5.00	8.08	22.2	90.0	Ref.^b	Comments^b
(1)	(2)	(15)	(16)	(17)	(18)	(19)	(20)	(21)	(22)	(23)	(24)	(25)	(26)

0048-097	OB-081	2.7	116		0.6	1.1	1.4^v	2.0^v	2.4^v		2.0^z	1, 32, 36, 47, 224	VLBI (45)
0109+224	GC	3.1	85	< 2	0.4^u	0.7	1.9	1.2^v	0.5^w	0.5	0.8^v	23, 80, 81, 86, 100
0215+015	PKS	3.5	37	< 2	0.8^u	0.5	0.4	0.4	< 1.2			9, 23, 39, 83,
												122, 123
0219+428	3C 66A	1.0	105		5.0	1.3					< 0.2	61, 81
0235+164	AO	5.2	87,93	< 2	1.5	2.6	1.5^v	2.8	2^v	2.3	2.2	1, 39, 81, 83, 110
0300+470	4C 47.08			2.2		2.1	2.3^v	2.2	2.8^v	3.3	2.2	1, 11, 39, 81	Compact radio (88)
0316+413	NGC 1275			63	29	13	16^v	30^v	50^v	36^v	36^v	1, 39, 44, 61, 81	Core-halo radio (10, 46)
												127, 133, 141	X-ray source
0403-132	PKS	.8	40	7.8^t	7.2	3.2	2.9	2.8^v				32, 36, 97, 101, 124	Compact radio (24)
0420-014	PKS	2.8	87	1.8	1.2	1.5^v	1.6^v	1.8^v	2.1^v	3.3	4.1	9, 32, 81, 82, 123	VLBI (24, 45, 46)
0422+004	OF 038	1.5	135	1.5	1.2^u	1.3^v	1.0^v	1.2^v	1.0^v	1.6	1.7	9, 22, 36, 81,
												83, 123
0521-365	PKS	1.4	117	67^t	26	15	11	10				32, 36, 98, 101	Ext. radio (28), X-ray (56, 96)
0548-322	PKS				< 2		0.3					32, 99	X-ray source (72, 92, 96)
0735+178	PKS	2.5	87	2.9^t	2.3^u	2.2^v	2.0^v	2.0	2^v	2.2	2.0	1, 10, 23, 32, 81,	VLBI (43, 45)
												101
0736+017	PKS	1.0	67	1.1	2.6	2.6^v	2.2^v	2.0^v	2.0^v	2.6	3.4	9, 32, 81, 98, 123	VLBI (24, 45)
0752+258	OI 287			< 2	1.4	0.5						20, 75, 86	Constant polarization (0)
0754+100	OI 090.4	1.0	27	< 2	< 2	0.7	0.8	0.9				32, 39, 75, 100
0808+019	OJ 014			< 0.5	0.5^u	0.5	0.7^v	0.9^v	< 1.2			9, 22, 36, 98,
												122, 123
0818-128	OJ-131	2.9	27	3.1^t	< 2	1.1	0.9	0.8				8, 32, 75, 101
0829+046	OJ 049	5.0	117	<2	< 2	0.6	0.6	0.7	0.7^v		0.5	9, 32, 39, 81, 100
0851+202	OJ 287	4.0	87	< 2	0.6^u	1.5	2.8^v	2.6	5^v	2.8	7^v	1, 23, 31, 82,
												86, 141
0906+430	3CR 216			20	12	3.7	2.4	1.8	1.4^w	1.0^y		33, 39, 44, 84, 133
0912+297	OK 222	1.9	117	< 2	0.5	0.6						19, 75, 86
0957+227	4C 22.25			3.7	2.7	1.1	0.7	0.4				23, 82, 86, 136	Extended radio (98)
1057+100	HM			< 2	1.2	0.6						32, 39, 136
1101+384	Mkn 421	4.0	105	< 2	1.1	0.6	0.6	0.7	0.5	0.4	0.5	59, 81, 86, 136	X-ray source (72, 96)
1133+704	Mkn 180			< 2			0.2	0.2	0.1	0.4^x	0.2	39, 59, 115
1147+245	OM 280			< 2	0.8	0.6		1.0		0.8^x	0.7	20, 75, 80, 82, 86
1150+497	4C 49.22	2.0	106	5.6	3.2	1.4	1.6	1.1	1.5^w	0.4^y		5, 84, 86, 133	Extended radio (24)
1156+295	4C 29.45	2.6	0	2.8	2.8	1.7		0.9				19, 75, 82, 86	Compact radio (24)
1215+303	ON 325	2.1	117	< 2	0.8	0.3		0.4				19, 75, 82, 86

**Table 1.** - *continued*

										Emission
						Optical polarization				lines
	Object	z	Ref.^b	V	Ref.^b	P		N	Ref.^b		Ref.^b		Ref.^b
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)	(12)	(13)	(14)

1219+285	W Com	-	128	16.5	128	2-10	30-95	14	53, 74, 108		108	2.3	112
1253-055	3C 279	.538	13	17.7	13	4-19	10-180	14	0, 34, 51, 120	s		1.6	76
1308+326	B2	.996	69	19.0	128	0-25	25-160	44	70, 89, 90	s	69	1.6	69, 89
1400+162	MC 3	.244	7	16.5	13	4-14	80-100^p	6	0, 7, 74	w	7, 69	1.5^v	7, 69
1418+546	OQ 530	-	128	15.0	128	2-19	50-105	5	27, 90		27
1514+197	GC	-	128	18.5	128	7-9	-	2	53			2.3	53
1514-241	AP Lib	.049	69	15.0	128	2-7	145-205^p	18	2, 14, 53, 108	w	69	2.7	53
1522+155	MC 3	.628	13	17.3	13	3-13	10-105	2	0	s		.20	103
1538+149	4C 14.60	-	128	15.5	128	22	-	1	53		131, 132	1.9^v	112
1641+399	3CR 345	.595	13	16.0	13	2-16	10-170	49	0, 3, 34, 51, 52,	s		1.1	76, 120
									55, 58, 107, 120
1652+398	Mkn 501	.034^g	69	13.8	105	2-4	125-145^p	36	2, 53, 65, 90, 114		69	2.5	112
1717+178	OT 129	-	128	18.5	128	27	-	1	27		132
1727+502	IZw 186	.055^g	69	16.0	128	4-6	-	6	53		69	1.9^v	112
1749+096	OT 081	-	128	17.0	105	3-9	-	3	53		109	2.2^v	112
1807+698	3CR 371	.050	102	14.8	102	0-12	65-100^p	68	0, 14, 53	w	69	1.3	69
1845+797	3CR 390.3	.056	102	14.5	102	1-4	155-165^p	30	0, 4	s	79
2032+107	MC	-	128	18.6	128	12	130	1	134
2155-304		.17?	17	14.0	128	3-7	150-170	4	41	w	17	1.0	41
2200+420	BL Lac	.069	69	14.5	105	2-23	0-180	> 500	0, 2, 53, 57,	w	69	1.6^v	69, 105
									74, 90, 119, 120
2201+171	MC 3	1.080	13	18.8	13	9.5	30	1	134
2208-137	PKS	.392?	13	17.0	13	5-9	100-170	3	0	s
2223-052	3C 446	1.404	13	18.4	13	4-17	10-160	16	0, 50, 68, 107, 120	s		1.8	76, 120
2225-055	PHL 5200	1.981	13	17.7	13	4	160^p	3	0, 107	s
2230+114	CTA 102	1.037	13	17.3	13	1-11	100-170	6	0	s		1.0	76
2251+158	3CR 454.3	.859	13	16.1	13	0-16	0-170	24	0, 107, 118, 120	s		1.5	76, 120
2254+074	OY 091	-	128	16.5	128	14-21	-	6	53, 143		109, 132	2.1^v	112
2345-167	PKS	.600	13	18.0	13	3-19	70-160	3	0	s

**Table 1.** - *continued*

				Radio spectrum fluxes in Jy at frequencies given in GHz
	Object	m	Ref.^b	0.18	0.41	1.40	2.70	5.00	8.08	22.2	90.0	Ref.^b	Comments^b
(1)	(2)	(15)	(16)	(17)	(18)	(19)	(20)	(21)	(22)	(23)	(24)	(25)	(26)

1219+285	W Com	4.0	117	0.8^u	0.3	1.4	1.5^v	0.7	1.6^v	1.2	1.0	1, 20, 23, 81, 82
1253-055	3C 279	6.7	31	21	11	10	12	16		9^v	6.6^v	32, 39, 47, 97, 141	VLBI (26, 45, 46)
													X-ray source (111)
													Superluminal (138)
1308+326	B2	5.6	38	<2	1.2	1.0		1.5	1.8^w	3.0	2.7	19, 81, 83, 86
1400+162	MC 3			2.2	1.9	0.8	0.6	0.4		0.3^x		7, 36, 39, 110	Double radio (7)
1418+546	OQ 530	4.8	27	< 2		0.8	0.9	1.1	1.4^w	0.7	0.7	39, 75, 81, 84
1514+197	GC			< 2	0.5^u		0.5	0.5				39, 100
1514-241	AP Lib	2.5	37		3.8	2.1	2.2^v	1.9	2.8^v			1, 32, 36	Core-halo radio (48)
1522+155	MC 3			< 2	0.6							39, 110
1538+149	4C 14.60	> 2.8	117	3.6	2.3	2.1	2.0	2.0			0.5	39, 44, 71, 81, 82
1641+399	3CR 345	2.0	40	12	9.0	6.6	9.0^v	8.0^v	9.0^v	9.0	7.7^v	1, 21, 44, 81, 84,	D2 (29),
												86, 141	VLBI (24, 29, 45, 46, 140)
													Superluminal (138, 139)
1652+398	Mkn 501			2.0	1.8	1.5	1.4	1.4	1.2	0.9	0.7	20, 59, 81, 84,	X-ray source (72, 96)
												86, 136
1717+178	OT 129			< 2	< 0.3^u	0.5	0.7	0.7^v				23, 39, 75, 100
1727+502	IZw 186	1.9	117	< 2		0.4		< 0.5				39, 75, 84	X-ray source (56)
1749+096	OT 081			< 2	1.3^u	1.3	1.0	1.8	1.5			1, 23, 36, 39, 82
1807+698	3CR 371	2.0	117	5.3		2.6	1.9	2.0^v	1.8^w	2.1	1.4	39, 44, 81, 84	N gal.X-ray source (96, 63)
													Extended radio?
													(12, 18, 33, 35)
1845+797	3CR 390.3	1.8	117	47	34	12	6.8	4.5				44, 60, 102	N gal. Double radio (60)
2032+107	MC			< 2	1.4	1.2	0.9	0.8				39, 75, 100, 110	X-ray source (63)
2155-304		1.4	41		2	0.3	0.3	0.3				32, 75, 136	X-ray source (41, 96)
2200+420	BL Lac	4.0	117	< 2	3.1	5.3^v	5.0^v	5.5^v	7.0	10^v	11^v	1, 10, 39, 46, 47,	VLBI (46, 48); X-ray
												81, 84, 133, 136, 141	upper limit (96)
2201+171	MC 3			< 2	1.1	0.8	0.6	0.7				39, 75, 83, 100, 110
2208-137	PKS				< 2		0.7	0.5				8, 32
2223-052	3C 446	3.4	87	17	8.5	5.8	4.6	4.3		4.8^y	3.0	32, 39, 47, 97	VLBI (45, 46);
													X-ray source (111)
2225-055	PHL 5200	0?	40		< 0.1		< 0.4					8, 137	Radio quiet;
													constant polarization
2230+114	CTA 102	1.0	0,40	5.5	8.0^v	6.5^v	4.5^v	3.5	2.5^v	1.3^y	0.6	1, 10, 32, 36, 39,	VLBI (43, 45, 46)
												47, 97, 129
2251+158	3CR 454.3	2.3	117	13	14	12^v	12^v	16^v	12^v	6.9^v	5.4^v	1, 32, 36, 39, 47,	D2 (29)
												97, 98, 141	VLBI (24, 43, 45, 46, 48)
2254+074	OY 091	1.6	117	< 2	0.4^u	0.4	0.7	0.8^v	1.9^w			23, 39, 75, 83, 100
2345-167	PKS	2.5	87	2.0^t	2.1	2.5^v	2.5^v	3.6				32, 36, 101	VLBI (45, 46)

Footnote to Table 1
^a A general description is given in the text. Table notes are as follows - a: the polarization maximum is uncertain, g: redshift derived from the host galaxy absorption lines, p: polarization has preferred angle, t: flux measurement at 160 MHz, u: 318 MHz, w: 10.7 GHz, x: 15.1 GHz, y: 31.5 GHz, z: 86.0 GHz, v denotes that the radio flux is variable, and a mean value of reported values is given. v against a spectral index indicates that different values are reported in the cited references, and the lower value is given. We should caution that she radio fluxes in this table are mostly obtained at different occasions at different frequencies, and so the shapes are not reliable for variable sources, simultaneous observations are given by Owen & Mufson (1977). Owen, Spangler & Cotton (1980), and O'Dell et at. (1978). Jones & Rudnick (1980) find that fractional variations at 90 GHz are probably greater than at lower frequencies. Absence of a v superscript should not be taken as an indication of stability, only that variability is not established from the limited published observations.
^b References :
0 Steward Obs, unpublished data	48 Kellermann et al. 1977
1 Altschuler & Wardle 1976	49 Kikuchi et al. 1976
2 Angel et al. 1978	50 Kinman, Lamla & Wirtanen 1966
3 Babadzhanyants et al. 1972	51 Kinman 1967
4 Babadzhanvants & Hagen-Thorn 1975	52 Kinman et al. 1968
5 Bailey & Pooley, 1968	53 Kinman 1976
6 Baldwin 1975	54 Kinman et al. 1974
7 Baldwin et al. 1977	55 Kinman 1977
8 Bolton, Shimmins & Wall 1975	56 Kinzer 1978
9 Brandie & Bridle 1974	57 Knacke, Capps & Johns 1976
10 Bridle et al. 1972	58 Knacke, Capps & Johns 1979
11 Bridle & Fomalont 1974	59 Kojoian et al. 1976
12 Broderick et al. 1972	60 MacDonald, Kenderine & Neville 1968
13 Burbidge, Crowne & Smith 1977	61 Mackay 1971
14 Capps & Knacke 1978	62 Mackay 1969
13 Carswell et al. 1973	63 Marshall et al. 1978
16 Carswell et al. 1974	64 Martin, Angel & Maza 1976
17 Charles, Thorstensen & Bowyer 1979	65 Maza, Martin & Angel, 1978
18 Cohen et al. 1971	66 Maza 1979
19 Colla et al. 1970	67 McGimsey et al. 1975
20 Colla et al. 1972	68 Miller & French 1978
21 Colla et al. 1973	69 Miller, French & Hawley 1978
22 Condon & Jauncey 1974a	70 Moore et al. 1980
23 Condon & Jauncey 1974b	71 Munro 1972
24 Conway et al. 1974	72 Mushotzky et al. 1978
25 Cooke et al. 1978	73 Nordsieck 1972
26 Cotton et al. 1979	74 Nordsieck 1976
27 Craine, Duerr & Tapia 1978	75 Ohio 1415 MHz survey
28 Danziger et al. 1979	76 Oke, Neugebauer & Becklin 1970
29 Davis, Stannard & Conway 1977	77 Oke 1966
30 Dyck et al. 1971	78 Osterbrock & Miller 1975
31 Eachus & Liller 1975	79 Osterbrock, Koski & Phillips 1976
32 Ekers 1969	80 Owen & Mufson 1977
33 Elsmore & Mackay 1969	81 Owen et al. 1978
34 Elvius 1968	82 Pauliny-Toth & Kellermann 1972
33 Fomalont & Moffet 1971	83 Pauliny-Toth et al. 1972
36 Gardner, Whiteoak & Morris 1975	84 Pauliny-Toth et al. 1978
37 Gaskell 1978	85 Pica 1977
38 Gottlieb & Liller 1976	86 Pilkington & Scott 1965
59 Gower, Scott & Wills 1967	87 Pollock et al. 1979
40 Grandi & Tifft, 1974	88 Porcas, Treverton & Wilkinson 1974
41 Griffiths et al. 1979	89 Puschell et al. 1979
42 Hagen-Thorn 1972	90 Puschell & Stein 1980
43 Jauncey et al. 1970	91 Richstone & Schmidt 1980
44 Kellermann et al. 1969	92 Riegler, Agrawal & Mushotzky 1979
45 Kellermann et al. 1970	93 Rieke et al. 1976
46 Kellermann et al. 1971	94 Rieke et al. 1977
47 Kellermann & Pauliny-Toth 1971	95 Schmidt 1974
96 Schwartz et al. 1979	120 Visvanathan 1973a
97 Shimmins, Manchester & Harris 1969	121 Visvanathan 1973b
98 Shimmins & Bolton 1972	122 Wall, Shimmins & Merkelijn 1971
99 Shimmins & Bolton 1974	123 Wall 1972
100 Shimmins, Bolton & Wall 1974	124 Wall, Wright & Bolton 1976
101 Slee 1977	125 Wampler 1967
102 Smith, Spinrad & Smith 1976	126 Wampler 1971
103 Smith et al. 1977	127 Wardle 1971
104 Smith 1978	128 Weiler & Johnston 1979
105 Stein, O'Dell & Strittmatter 1976	129 Williams, Kenderdine & Baldwin 1966
106 Stockman 1978	130 Williams et al. 1972
107 Stockman & Angel 1978	131 Wills & Wills 1974
108 Strittmatter et al. 1972	132 Wills & Wills 1976
109 Strittmatter stat. 1974	133 Witzel et al. 1978
110 Sutton et at. 1974	134 Zotov & Tapia, 1979
111 Tananbaum et al. 1980	135 Kinman 1976, IAU Circ. No. 2908
112 Tapia, Craine & Johnson 1976	136 From Ohio Master List of Radio Sources
113 Tapia et al. 1977	137 Mills & Little 1970
114 Ulrich et al. 1975	138 Seilestad et al. 1979
115 Ulrich 1978	139 Cohen et al. 1979
116 Usher, Kolpanen & Pollock 1974	140 Readhead et al. 1979
117 Usher 1975	141 Hobbs & Dent 1977
118 Visvanathan 1968	142 Puschell 1980
119 Visvanathan 1969	143 Serkowski & Tapia 1975

**Table 2.** Absolute luminosity of emission lines in units of 10⁴¹ ergs s^-1 in the rest frame of the source, computed using H₀ = 75 km s^-1 Mpc^-1, q₀ = ½.

		Mg II	[O II]	H	[O III]	H
	Object	2798	3727	4861	5007	6563	Ref.

0316+413	NGC 1275	-	1.5	0.8	3.0	8	Wampler 1971
0521-365	PKS	-	3	-	1.0	1.1	Danziger et al. 1979
0736+017	PKS	-	-	50	-	130	Baldwin 1975
0851+202	OJ 287	-	-	-	3.3	-	Miller et al. 1978
1308+326	B2	100	-	-	-	-	Miller et al. 1978
1400+162	MC3	-	0.6	0.3	0.6	-	Miller et al. 1978
1514-241	AP Lib	-	-	-	0.3	-	Miller et al. 1978
1641+399	3C 345	440	-	-	-	-	Visvanathan 1973a
1807+698	3C 371	-	0.4	0.1	0.6	< 0.5	Miller et al. 1978
1845+797	3C 390.3	-	0.7	11	11	70	Osterbrock et al. 1976
1958+407	Cyg A	-	1.8	0.7	10	5	Osterbrock & Miller 1975
2200+420	BL Lac	-	-	-	0.08	0.06	Miller et al. 1978
2230+114	CTA 102	1200	-	-	860	-	Oke 1966
2251+158	3C 454.3	380	-	-	-	-	Visvanathan 1973a

The structure of the very bright compact radio core of NGC 1275, which is not polarized, has been explored by VLBI measurements. At centimeter wavelengths the emission is concentrated in three very small components in aline about 3 pc in length, at position angle 170°. The relative motion of the components is not greater than 0.05 pc/yr, or 0.15 c.

PKS 0521-36 and 1807+698 = 3C 371 These two sources are very similar and are distinguished by their combination of forbidden emission lines and strong, steep-spectrum extended components to their radio emission. Both are strongly polarized variable nuclei located in giant elliptical galaxies at about the same distance (z ~ 0.05). In both sources, the narrow forbidden lines of [O III] are about 10 times stronger than in BL Lac (Table 2). PKS 0521-36 has been studied most recently in the optical by Danziger et al. (1979). After subtracting the galaxy, they estimate the spectral index in the optical to be ~ 1.0. No polarization data are reported in the literature, but we recently obtained one measurement through a 4 arcsec aperture, giving P = 6% at angle 155°. The low-frequency radio emission from 0521-36 is strong (67 Jy at 178 MHz), or about the same luminosity as 3C 390.3. The structure is known to be extended over 15 arcseconds or 15 Kpc (Fomalont 1968), while all the low frequency emission in 3C 390.3 is from the double lobes with 200 Kpc separation.

3C 371 does not have such a strong low-frequency component as 0521-36 (5 Jy at 178 MHz, and a flat 2 Jy component from 1.4-90 GHz) but its polarimetric properties have been extensively studied. Visvanathan (1967) found it to be polarized, and it has since been measured repeatedly by Dombrovsky et al. (1971), and Babadzhanyants & Hagen-Thorn (1975). Miller (1975) has recorded a high polarization of ~ 10%, using a small 2.4 × 4 arcsec aperture. The bulk of the available data obtained through a 26 arcsec aperture rarely exceeds 6%, presumably because of dilution by the galaxy, though Dombrovsky et al. did measure 10-12% through a 26 arcsec aperture in September 1970. At that time the source flared for a period of about a week by 0.6 magnitude to m_B = 14.5. The direction of polarization is certainly not constant, but nearly all measurements taken in each of seven years lie within 35° of position angle 85°.

Optical spectrophotometry of 3C 371 has been obtained by Miller (1975) who gives reference to earlier work. He identifies three optical components: a power-law continuum of index 1.35 and m_v = 15.4 at the time of observation; the absorption line spectrum of the galaxy; and the emission line spectrum given in Table 2.

PKS 0736+017 This is a source with strong permitted lines at z = 0.192, recently found to show variable optical polarization by Moore & Stockman (1980). The strength of polarization varies between 0 and 6%, with sharp changes from night to night. The optical continuum shows optical variability of more than a magnitude (McGimsey et al. 1975) and has a spectral index of 1.1 over the range 0.3-0.7 µ in the rest frame (Baldwin 1975). Line strengths by Baldwin are given in Table 2. The radio spectrum is nearly flat at ~ 2.5 Jy from 0.41-90 GHz.

B2 0752+258 = OI 287 = VRO 25.07.04 This object at z = 0.446 has recently been discovered by Moore & Stockman (1980) to show essentially constant polarization of 8% at position angle 143°. Wills & Wills (1976) found the optical spectrum to show strong sharp forbidden lines of [O II] and [O III] but no Balmer lines, and comment that from spectroscopic evidence alone it could be classified as a galaxy, though the appearance is stellar. The only permitted line detected is the resonance doublet of Mg II. The radio spectrum known only from 0.5-2 GHz appears steep, though there is no detection at 178 MHz (< 2 Jy).

1156+295 = 4C 29.45 = Ton 599 This object at z = 0.728 was found by Moore & Stockman (1980) to show strongly variable polarization, up to 10% in magnitude and with no preferred position angle. The optical spectrum shows strong Mg II and CIII lambda 1909 (Schmidt 1974), has a spectral index of 0.93 (Richstone & Schmidt 1980), and varies in brightness by at least 2 magnitudes (Moore & Stockman 1980). The radio spectrum drops slowly from 2.8 Jy at 178 MHz to 0.89 Jy at 5 GHz.

1228+127 = M87 Both the jet of polarized optical emission and the star-like nucleus of this central galaxy of the Virgo cluster are of particular interest. The optical jet is in the form of several unresolved knots out along a line from the nucleus, extending out to 25 arcsec (1.3 Kpc). Knot A, the brightest at B = 16.8, is about half way along the jet. Polarization was first studied photographically by Baade (1956) and photoelectrically by Hiltner (1959). Schmidt, Peterson & Beaver (1978) obtained a one-dimensional map of polarization and intensity along the jet with the 200-element Digicon detector, which allowed accurate determination of the diffuse galactic background. The knots themselves are polarized typically between 10% and 25%, with position angles apparently randomly oriented from knot to knot. Recently, Sulentic, Arp & Lorre (1979) have repeated Baade's photographs under similar conditions and find slight but significant changes in intensity and polarization of the knots, over a 22 year baseline.

In Turland's (1975) map of M87 made at 5 GHz, the radio emission from several knots is resolved, knot A having a flux of 1 Jy at 5 GHz and a spectral index of ~ 0.5. Comparing the optical and 5 GHz data, Schmidt et al. find the radio and optical polarization of individual knots agree closely in both strength and position angle, once a constant correction of 75° in angle is made for Faraday rotation. There is no detectable Faraday rotation, ltapprox 20°, within the knots themselves.

Sulentic et al. argue that the knots appear to be a type of BL Lac object that is ejected from the galactic nucleus. We note, though, that there are some characteristics that appear rather different: the optical variability is slower and the absolute optical flux weaker than for any known BL Lac; the radio emission is relatively steep and no compact cores are detected in the knots in VLBI observations. As in the Crab Nebula, the optical and radio emission can be interpreted as coming from a relatively large region of optically thin synchrotron radiation. In the absence of any evidence of compact, nuclear activity within individual knots, it seems likely that the energy is being supplied in a jet from the nucleus, where there is a compact core of flatter radio spectrum with alpha = 0.3 ± 0.2 and of strength ~ 3 Jy at 5 GHz.

Turning to the nucleus, it appears that optical polarization is present intermittently. In 1971-1972, Heeschen (1973) and Kinman (1973) found variable polarization of up to 6%, measured through a 3.2 arcsec diaphragm centered on the nucleus. This would appear to be associated with the stellar object at the nucleus which is distinct from the normal stellar core of the galaxy. In 1977, Schmidt et al. found no detectable polarization, obtaining an upper limit (3 sigma ) of 0.9% in a 1 × 3 arcsec aperture centered on the nucleus. In 1979-1980, R.L. Moore (private communication) obtained several measurements consistent with 0.3 ± 0.1% at 120° position angle through a 4 arcsec aperture. This last measurement was with a red-sensitive detector and would be subject to considerable dilution by the galaxy. The featureless continuum of the stellar object (Sulentic et al. 1979) and its apparent variability (de Vaucouleurs & Nieto 1979) are all consistent with weak blazar activity, but since it is not active now we shall not group it with the nearby objects of much more consistent activity. Obviously, continued polarimetric observations of the nucleus are needed. M87 is of particular interest because of the dynamical and photometric evidence of a large, central mass of 5 × 10⁹ M (Young et al. 1978, Sargent et al. 1978), similar to the black hole masses inferred for BL Lac objects from the time scale of variability (Angel et al. 1978).

B2 1308+326 and AO 0235+164 These two distant (z ~ 1) and extremely luminous sources have many properties in common. Both are flat- or inverted-spectrum compact radio sources with no steep, low-frequency components. Both have shown outbursts in optical and radio emission lasting months, with strong, rapidly variable optical polarization. The emission lines in both cases are weak or undetectable relative to the polarized optical continuum.

Detailed studies of the polarimetric behavior of B2 1308+326 during the 1978 outburst have been made by Moore et al. (1980) and Puschell et al. (1979). The strength of optical polarization ranged up to 20%, with large variations in the strength and angle of polarization ( Delta P ~ 7%, Delta theta = 30°), on a time scale of one day in the rest frame of the source. Repeated accurate measurements (typical errors of 0.25% in polarization) by Moore et al. over baselines of 2-5 hours showed only small variations consistent with the night-to-night trends. A recent reanalysis by Puschell (1980) of his data is also consistent with this result.

Moore et al. (1980) report infrared polarimetry obtained simultaneously with the optical on three different nights. On each of the three nights the strength and position angle at 2 µ and 0.6 µ are equal, indicating a common emission process. The angle of polarization is not always independent of wavelength though, since a rotation of 15° between 0.4 µ and 0.8 µ was observed on another night when only optical measurements were obtained. At centimeter wavelengths, the flux remained relatively steady during the 1978 outburst, and the polarization of a few percent was roughly constant in angle, with no Faraday rotation above 10 GHz (Puschell et al. 1979).

Only occasional polarization measurements have been made for AO 0235+164 but these show similar variation over all position angles and sometimes great strength (20%), even when the source was faint (Rieke et al. 1976, Angel et al. 1978).

1400+162 = 4C 16.39 This object is important because it and 3C 390.3 are the only extended radio sources with central compact nuclei showing strong optical polarization known to have classic double lobe structure. The optical and radio properties have been extensively studied by Baldwin et al. (1977). The redshift of the object from weak emission lines (Table 2) is z = 0.245, the same as that of the brightest galaxy in a small adjacent group. The optical continuum from 2.3 to 0.5 µ is well represented by a power law of index 1.3, but then steepens to the ultraviolet. In June 1976 the V magnitude was 17.4; the object was not studied for flux variability. Optical polarization measured at Lick Observatory with the Nordsieck device over three months in 1974 was approximately constant with strength 12% and position angle 96°. Measurements in June 1978 and January 1980 by R.L. Moore (private communication) both gave 12% at 85°. A single measurement in January 1976 by Anderson and Garrison (quoted by Baldwin et al. 1977) is discrepant, 4.4% at 81°. The polarization is thus nearly steady in position angle and could perhaps be constant in strength, if the last point were in error.

Coincident with the stellar optical object is a compact radio source with a spectrum flat from 1.67 GHz (0.15 Jy) to 15 GHz (0.14 Jy). The extended structure shows approximately symmetric lobes lying along a line at position angle of approximately 115°, inclined some 20° to the direction of optical polarization. The total extent at 5 GHz is 25 arcsec, or 100 Kpc (projected on the plane of the sky), and the measured flux at 178 MHz is 2.55 Jy, extending to higher frequencies with a spectral index of 0.7.

1641+399 = 3C 345 As will be discussed in Sections V and VI, Blandford, Rees, and others have suggested that the polarized optical emission in extremely luminous polarized objects may originate in a relativistic jet directed toward us. The same type of relativistic beaming is invoked to explain superluminal expansion in compact radio sources. Thus the violently variable polarized object 3C 345, with strong emission lines at z = 0.595, is of exceptional interest since it shows superluminal expansion very clearly. VLBI measurements reviewed by Kellerman (1978) show two components whose separation has increased linearly from 1969-1977, at a rate of 0.17 milli arcsec/yr, corresponding to a velocity of 5 c. Only one other source in Table 1 (3C 279) is known to show superluminal expansion. In 3C 345 the position angle of the expanding double is 106°. As is often the case in other similar sources (Readhead et al. 1978), larger scale structure is not collinear. The jet of low frequency emission lying 1-3 arcsec distant found by Davis, Stannard & Conway (1977) is at position angle 142°.

Polarization was discovered in 3C 345 by Kinman (1967), after it was found to be a radio (Dent 1965) and optical (Goldsmith & Kinman 1965) variable. The source has since been the subject of several polarimetric studies. Kinman et al. (1968) found the position angle to be about 80° during a period of high luminosity, with large changes in angle apparently correlated with short (10 day) bursts. Recently Kinman (1977) has examined all the data available over an eight-year period, and finds that the yearly polarization averages lie at approximately the same position angle (80-110°) when the source is bright. This indicates there is memory in the emission process and the angle may be related to the VLBI jet axis (105°). It is of interest that the polarization is not noticeably weaker when the source is faint.

The wavelength dependence of polarization of 3C 345 was explored by Visvanathan (1973a), who found strength and angle to be constant at any given time within errors. The same result was found by R.L. Moore (private communication) using filters centered at 4500 Å and 7500 Å. Recently Knacke, Capps & Johns (1979) have reported 22-32% polarizations at 2.2 µ over a three-day period in April 1978, with no polarization detected at 1.6 µm and P < 6% at 0.44 µ. While two-component models may account for this extreme behavior, we note that the variability at 2.2 µ and in the optical is well correlated (Neugebauer et al. 1979). The spectrum from 0.3-10 µ obtained by Neugebauer et al. is close to being a single power law of index 1.4, with only a trace of the excess emission at ~ 0.3 µ and little of the complex structure common in QSOs. However, 3C 345 does appear to have some episodes when the spectrum changes, shown by Visvanathan's (1973a) multichannel spectrophotometry in which the index across the optical spectrum is seen to flatten when the source brightens.

1845+797 = 3C 390.3 This is a second key object in making the bridge between double lobe radio sources and those with variable stellar polarized nuclei. It is a classical radio double lying at z = 0.056 whose central galaxy contains a violently variable compact optical source (Cannon, Penston & Brett 1971) with substantial polarization (~ 3.5%). Optical polarization data over the years 1968-1973 have been published by Dombrovsky et al. (1971) and Babadzhanyants & Hagen-Thorn (1975). We will not here consider data by Efimov & Shakhovskoy (1972) which are not accurate enough to be useful. The strength of polarization measured through a 26 arcsec aperture increases with increasing brightness of the source, while remaining reasonably constant in position angle. Babadzhanyants & Hagen-Thorn (1975) make a least-squares fit to the yearly averages over 1968-1973, and obtain a best fit by superposing a source of variable intensity but constant polarization (3.7 ± 0.6% at 155 ± 3°) with an unpolarized galaxy component of B magnitude ~ 16.8. In 1979, we measured the nuclear polarization in a 4 arcsec aperture to be 2.1% ± 0.4% at 165° ± 5°, consistent with the parameters given above since the remaining galaxy contribution has not been removed. An independent estimate of the underlying galaxy magnitude of B ~ 16.6, obtained by Penston & Penston (1973) from photometry with different aperture sizes, is also consistent with their separation model. In the radio map of 3C 390.3 given by Harris (1972) the central compact component had strength of 0.35 Jy at both 2.7 and 5.0 GHz, and is < 0.4 Jy at 1.4 GHz. Hine & Scheuer (1980) have found variability in this source on a time scale of a year. The north and south outer components are separated by 2 and 1.5 arcminutes (20 and 90 Kpc) and are unusually compact. Their strengths are respectively 7 and 17 Jy at 408 MHz and their spectral indices are both 0.85. The position angle of the source axis is 143°, quite close to the mean polarization angle of 155°. This alignment is the same as that seen in the much more luminous double lobe QSOs (see Sections III and IV).

The nucleus of 3C 390.3 shows strong, extremely broad Balmer emission lines. Spectrophotometry by Osterbrock, Koski & Phillips (1976) shows these lines having complex profiles with a full width at half-maximum of 13,000 km s^-1 (Table 2). Polarimetric measurements of these lines are now being undertaken and will clearly be of great value in locating the origin of the optical continuum polarization.

2200+420 = BL Lac Not only was this object the first blazar type with no emission lines to be recognized but it exemplifies the most rapid variability of flux and polarization in its optical, infrared, and radio flux (Angel et al. 1978, Aller & Ledden 1978). We will not give here a review of the general properties of BL Lac since this is already available in the literature (e.g. Stein, O'Dell & Strittmatter 1976, Miller 1978). It lies at the center of a giant elliptical galaxy of redshift 0.067, and Miller, French & Hawley (1978) find extremely weak emission lines (Table 2).

The variability of the optical polarization has recently been studied in some detail, and has been found to rotate in position angle at rates of 1-2°/hour during a night of observing, with changes of up to 30° from night to night (Angel et al. 1978, Angel & Moore 1980). The position angle has no preferred direction. In 1979 a group of observers in America, Europe, and Israel observed the optical polarization and intensity of BL Lac for one week, with continuous coverage for more than 12 hours on most days (R.L Moore et al., in preparation). During this week the source was being measured sometimes by four different observers simultaneously. All the data lie on a curve that varies smoothly hour by hour, but where again one day is the time scale for substantial changes. Infrared polarization measurements at 2.2 µ were obtained by Rieke and Lebofsky on two nights during this week, and also exactly tracked the optical in strength and angle. During one night when the polarization was ~ 3% a rotation of 30° during 6 hours was seen at both wavelengths. This shows that there cannot be much significant interstellar polarization arising in our own galaxy, despite its low galactic latitude. The strength and angle of polarization are not always independent of wavelength, as discussed in Section III.

2225-055 = PHL5200 cannot be classified as a blazar since it is radio-quiet (Mills & Little 1970) and shows no variation in its high polarization or optical flux. PHL 5200 is the prototype for a class of radio-quiet QSOs whose spectra show broad, deep absorption troughs blueward of strong resonance emission lines (Lynds 1967, Burbidge 1968). Spectropolarimetric observations show the continuum to be polarized with the lines essentially unpolarized (Stockman, Angel & Hier 1978). Thus the polarization must originate within the emission line region and is not due to dust or resonance scattering outside this region. High polarizations are not a general property of QSOs with intrinsic, broad absorption lines. We have observed three PHL 5200 type objects discussed by Turnshek et al. (1979) and all three show weak polarization, ltapprox 2%.