1. INTRODUCTION 2. OBSERVATIONS

Transcription

1 THE ASTRONOMICAL JOURNAL, 117:1168È1174, 1999 March ( The American Astronomical Society. All rights reserved. Printed in U.S.A. OPTICAL POLARIZATION AND IMAGING OF HOT SPOTS IN RADIO GALAXIES A. LA HTEENMA KI Metsa hovi Radio Observatory, Helsinki University of Technology, FIN Kylma la, Finland AND E. VALTAOJA Tuorla Observatory, University of Turku, FIN Piikkio, Finland Received 1997 September 3; accepted 1998 November 17 ABSTRACT We present subarcsecond optical imaging and polarization observations of Ðve optical hot spot candidates in the classical double radio sources 3C 111, 3C 303, 3C 351, 3C 390.3, and PKS 2135[147. On the basis of positional coincidence, optical appearance, and polarization, all four 3C sources appear to have genuine optical counterparts to radio hot spots, whereas in PKS 2135[147 the hot spot candidate turns out to be an unrelated Ðeld galaxy. We also conðrm the Ðrst ever optical double hot spot in the source 3C 351. Key words: galaxies: active È polarization 1. INTRODUCTION A radio hot spot is a small region of high surface brightness located close to the far end of a radio lobe of an active galaxy. It is formed when a supersonic plasma jet from the core hits the intergalactic medium (IGM). The hot spot is a strong shock front produced by the collision. Some sources also have a di use secondary hot spot, usually o the source axis (the line connecting the hot spot, the core, and the hot spot in the opposite radio lobe). According to the splatter spot model (Williams & Gull 1985), the primary hot spot forms when the jet hits the IGM for the Ðrst time. Then the jet bounces o to another direction and forms a secondary hot spot in a repeated collision with the IGM. The dentistïs drill model of Scheuer (1982) suggests instead that the jet wobbles back and forth as a result of the precession and motion of the central black hole and in this way can form multiple hot spots, the di use secondary one being in this case an old remnant. The Ðrst radio hot spots were detected in Cygnus A in 1974 by Hargrave & Ryle (1974). An optical hot spot in 3C 303 was found soon after that, by Lelièvre & Wle rick (1975) and Kronberg (1976). Many other optical hot spot candidates have been detected, but so far only Ðve have been conðrmed, in 3C 20 (Meisenheimer et al. 1989), 3C 33 (Simkin 1978), 3C 111 (Meisenheimer et al. 1989), 3C 303 (Lelièvre & Wle rick 1975; Kronberg 1976), and Pictor A (Ro ser & Meisenheimer 1987). Most galaxies with radio hot spots seem to have no optical hot spots at all (Ro ser 1989), possibly because their spectra steepen so much between radio and optical frequencies that optical hot spots cannot be detected. The emission is optical synchrotron radiation, the most important evidence for this being high optical polarization. This was Ðrst established by Meisenheimer & Ro ser (1986), when they observed optical polarization from the southern hot spot in 3C 33. In general, polarization observations are the most reliable way of distinguishing between true optical counterparts to radio hot spots and chance positional coincidences of unrelated Ðeld objects. However, such observations exist for only three of the Ðve conðrmed optical hot spots (3C 20: Hiltner et al. 1994; 3C 33: Meisenheimer & Ro ser 1986; Pictor A: Ro ser & Meisenheimer 1987) In this paper we report optical imaging and polarization observations of Ðve optical hot spot candidates in classical double radio sources. A preliminary identiðcation existed for each source, but no polarization observations had been made to conðrm the synchrotron nature of the optical radiation. The published optical images were also rather poor by modern standards, only the best one (3C 303: Keel 1988) approaching arcsecond resolution. Our new observations resulted in the deepest images of these hot spot candidates yet obtained, with subarcsecond resolution for each source. Four of the hot spot candidates were found to be nonstellar, coinciding with the radio hot spots, whereas the Ðfth appears to be an unrelated Ðeld galaxy 5A away from the radio position. In addition, we obtained one strong and two marginal detections of high optical polarization in these hot spot candidates. 2. OBSERVATIONS 2.1. Optical Imaging and Polarimetry We present optical polarization observations of 3C 111, 3C 303, 3C 351, 3C 390.3, and PKS 2135[147. The CCD frames were taken with the 2.5 m Nordic Optical Telescope on La Palma in 1991 September and 1992 June. We used a rotating polarization Ðlter at four di erent angles and standard Ðlters I and V. The typical exposure time for a single CCD frame was 900 s, and the total integration times ranged between 4500 and 21,600 s. The typical seeing was 0A.9 or better. The resolution of the CCD camera was 0A.20 per pixel. Details of the observations are given in Table 1. The standard stars were taken from the list of Landolt (1983). The polarization calibrations were done according to the procedure of Piirola, Scaltriti, & Coyne (1988). The CCD frames were added together and analyzed with the NOAO IRAF package. The integrated magnitudes, the polarizations, and the polarization angles were calculated from the summed images using standard equations. During the observations in 1992, the bright half-moon caused a fairly high background in some images, and no V -band magnitudes or polarizations could be measured. The interstellar polarization was measured for each summed image from Ðeld stars and is already subtracted from the results (except in the Ðgures showing the measured P - and P - x y

2 HOT SPOTS IN RADIO GALAXIES 1169 TABLE 1 OBSERVATIONS Time Total Integration Time Seeing Source Date (UT) Band (s) (arcsec) 3C Sep È06.07 I È Sep È05.41 V C Jun È23.42 I Jun È00.45 I È Jun È00.54 V C Jun È04.33 I Jun È04.35 I È Jun È04.23 V È0.9 3C Sep È23.55 I È Sep È02.23 V PKS 2135[ Sep È01.57 I Sep È03.32 I values for the hot spot candidates and the corresponding Ðeld stars). For the weak hot spots, the dominant error source is the photon noise, calculated from the integrated pulse counts of the summed images in each polarizer angle. We have also estimated other errors by repeated measurements varying the routines o ered by the IRAF package (centering, background subtraction, etc.). We have further checked for possible magnitude-dependent bias in the polarization, as well as conðrmed the reliability of our error estimates, by measuring the polarizations and the position angles of a number of weaker, presumably unpolarized Ðeld sources. These range in magnitude from sources slightly fainter than the hot spot candidates to the bright Ðeld stars used to estimate the interstellar polarization. We Ðnd that our estimated errors in the hot spot candidate measurements are comparable to the errors in measurements of unpolarized objects of similar magnitude. The measured average interstellar polarization of faint Ðeld sources are also in agreement with values from bright stars. Some of these fainter source measurements are shown in Figures 4 and 6. In the case of 3C 303, 3C 351, and PKS 2135[147, the coordinates of the optical hot spots were measured relative to the parent galaxy visible in the same CCD frame and compared with the positions in the published radio maps. 3C 111 and 3C have no objects visible in either the radio maps or our optical frames. For 3C 111, the hot spot position was measured relative to a nearby infrared object. For 3C 390.3, comparison was made relative to the opticalradio overlay of Saslaw, Tyson, & Crane (1978). Considering that both the hot spots and the parent galaxies are extended objects with di erent arcsecond-scale radio and optical structures, we accept B1A positional coincidence as evidence for a true optical counterpart C 111 This source is an FR II radio galaxy having a redshift z \ A 1.4 GHz map shows a one-sided collimated jet emerging from the core and leading into the hot spot in the northeast lobe (LinÐeld & Perley 1984). No counterjet feeding the southwest lobe is visible. A near-infrared counterpart of the northeast hot spot was identiðed by Meisenheimer et al. (1989) and further studied by Meisenheimer, Yates, & Ro ser (1997). No optical images have been published. At 1.4 GHz the head of the hot spot is polarized less than 45% in P.A. 75. The polarization angle appears to be parallel to the source axis, and the projected magnetic Ðeld is thus perpendicular to the source axis (LinÐeld & Perley 1984). Figure 1 shows the CCD image of the optical hot spot candidate in the I band. The position of the radio hot spot is marked with a cross. The bright object below the hot spot is a star and has no connection with the hot spot. For the hot spot candidate a polarization of 30% ^ 5% at P.A. 76 ^ 5 was detected (Fig. 2), in good agreement with the radio values. A direct comparison between the optical and radio coordinates is not possible because there are no common features in the two images, save for the hot spot itself. The position of our optical hot spot, however, matches well the position of the optical/ir hot spot (compared with the bright Ðeld star visible in both maps) as given in Meisenheimer et al. (1997, Fig. 1e). The declination and the right ascension measured from Figure 1 as the distance between the star and the optical peak of the hot spot are *d \ 6A.3, *a \ 2A.5. The corresponding coordinates measured from Figure 1e in Meisenheimer et al. (1997) are *d \ 6A.2, *a \ 2A.4. The optical hot spot is clearly FIG. 1.ÈI-band map of 3C 111. The optical position of Meisenheimer et al. (1989) is marked with a cross. The bright object in the Ðeld is a star, unconnected to the hot spot. The scale in this and the other maps is given in pixels, with 1 pixel corresponding to 0A.20.

3 1170 LA HTEENMA KI & VALTAOJA Vol. 117 FIG. 3.ÈI-band map of 3C 303. The radio position is marked with a cross. The parent galaxy and some Ðeld stars are also visible. FIG. 2.ÈP - and P -polarization of 3C 111 in the I band. Filled circle: Hot spot. Open x circles: y Field stars used to measure the interstellar polarization. (Note that in this and the other Ðgures P and P for the hot spot are uncorrected for interstellar polarization.) x y extended, with the estimated angular size 1A.6] 2A.0 corresponding to a linear size of B1 kpc. (We use H \ 100 km 0 s~1 Mpc~1 and q \ 0.5 throughout this paper.) 0 Because the optical position and the polarization agree with the corresponding radio properties, we conclude that this object is a genuine optical hot spot C 303 3C 303 (z \ 0.141) is an unusual double radio source with a highly asymmetrical structure. A prominent one-sided jet leads to a complex multiple radio hot spot (Lonsdale, Hartley-Davies, & Morison 1983). An optical counterpart of the multiple radio hot spot was found by Lelie` vre & Wle rick (1975) and Kronberg (1976). The best previous optical images were obtained by Keel (1988) using the KPNO 2.1 m telescope, with up to 40 minute exposures and seeing generally between 1A.0 and 1A.3. Meisenheimer et al. (1997) observed the hot spot in the near-infrared, and they also detected the radio jet connecting the core and the hot spot. They suggest that, based on a steep radio-optical spectral index and a high cuto frequency typical of optically detected radio jets, the hot spot is actually a bright knot in the jet. At 8.1 GHz the head of the hot spot is only slightly polarized. At 5 GHz the polarization is 11% and the P.A. is about 67. At 2.7 GHz the polarization is almost the same as at 5 GHz, less than 11% in P.A. 61 (Kronberg et al. 1977). At 1.4 GHz the polarization is the smallest, 8% in P.A. 84 (Leahy & Perley 1991, Fig. 15). In Figure 3 we present an I-band CCD image of 3C 303. The radio hot spot is marked with a cross. We obtained a marginally signiðcant integrated I-band polarization of 14% ^ 5% in P.A. 98 ^ 10 (Fig. 4). The separation between the core and the radio hot spot A in Figure 2 of Lonsdale et al. (1983), measured in the middle 2 of the two components of A, is *d \ 2@@, *a \ 16A.5. In the near- 2 infrared image of Meisenheimer et al. (1997) the coordinates are approximately *d \ 2@@, *a \ 17@@. In our CCD image the values are *d \ 1A.3, *a \ 16A.6. The optical hot spot is extended (FWHM 1A.2 vs. 0A.9 for the nearby Ðeld stars). The total extent of the hot spot, as measured from the outermost 3 p contours, is 2A.3] 3A.1, corresponding to a linear size of B4 kpc. The radio Ñux of the other radio hot spot A 3A 1 away from A is about half the radio Ñux of A (Lonsdale et 2 2 al. 1983), and if we assume this to be also true for the optical Ñuxes, we should have detected A as well. However, no 1 optical emission is seen from A. This may be, for example, 1 due to di erent radio-optical spectral indices in the two components. The nonstellar nature of the image, the positional agreement, and the optical polarization (which is only marginally signiðcant but is in agreement with both the degree and the position angle of the radio polarization) lead us to conclude that the optical candidate is a genuine counterpart to the radio hot spot of 3C C 351 This source is a classical double quasar having z \ Both radio lobes have hot spots: the southern lobe has a FIG. 4.ÈP - and P -polarization of 3C 303 in the I band. Some of the open circles represent x y unpolarized faint Ðeld sources used for estimating the errors of hot spots (see text for details).

4 No. 3, 1999 HOT SPOTS IN RADIO GALAXIES 1171 FIG. 5.ÈI- and V -band maps of 3C 351. The radio positions of the two hot spots are marked with crosses. The radio galaxy itself is in the lower right corner. single di use one, and the northern lobe has a very prominent double hot spot. The more compact A component lies closer to the galaxy, o the source axis, whereas the more di use B component, the core, and the southern hot spots are aligned along the source axis (Kronberg, Clarke, & van den Bergh 1980; Leahy & Perley 1991). There are also traces of a curved jet entering the A hot spot (Bridle et al. 1994). It has been suggested that both the northern hot spots may have optical counterparts, but no images beyond the small Ðnding chart have been published (Ro ser 1989). Although double radio hot spots are rather common (see, e.g., Valtaoja 1984), no optical doubles are known. Therefore, 3C 351 is a highly interesting source. The lifetime of the electrons radiating in the optical is only about 100 yr, and so the optical emission pinpoints the regions where acceleration is currently occurring (Meisenheimer et al. 1989; Ro ser 1989). At 5 GHz the compact A hot spot is polarized B20% in P.A. 62 (Bridle et al. 1994). The magnetic Ðeld in the compact hot spot points toward the di use hot spot B. The degree of polarization in the B hot spot is small. Figure 5 shows the I- and V -band images of 3C 351. The radio hot spots are marked with crosses. The background in these two images is exceptionally high due to leakage of reñected light inside the telescope during these particular observations. Even with total integration times up to 6 hr, the only nonzero optical polarization we could measure is for the brighter A spot in the I band, 31% ^ 16% in P.A. 78 ^ 14 (Fig. 6). This is only a 2 p detection, but it is consistent with the radio value. (In the V band we obtained 32% ^ 23% in P.A. 124 ^ 18.) The coordinates of the hot spots relative to the core, measured from Leahy & Perley (1991, Fig. 19), are *d \ 17A.5, *a \ 17A.5 for the compact hot spot A and *d \ 22A.5, *a \ 15A for the di use hot spot B. In our images the corresponding values are *d \ 18A.8, *a \ 17A.6 and *d \ 23A.6, *a \ 15A.4, respectively. Figure 7 (left) shows an enlarged image of the two hot spots in the I band. Both hot spots are extended, the A hot spot having a size of 1A.2] 1A.8 and the B hot spot 1A.8 ] 2A.2, corresponding to linear sizes of B5 kpc. For comparison, Figure 7 (right) shows the same area in an enlargement from the 14.8 GHz radio map of Kronberg et al. (1980). The general shapes of the optical hot spots agree with the radio hot spots, although one must note that there are only a few contours in the CCD images. The positions and the shapes of the two optical hot spots, as well as the polarization of the A hot spot, agree with the radio values. Although there are several small galaxies in the Ðeld (as seen in the I-band image), an accidental double FIG. 6.ÈP - and P -polarization of 3C 351 in the I band. Some of the open circles represent x y unpolarized faint Ðeld sources used for estimating the errors of hot spots (see text for details).

5 1172 LA HTEENMA KI & VALTAOJA Vol. 117 FIG. 7.ÈL eft: I-band map of the hot spots in 3C 351. Right: The 14.8 GHz map of the two hot spots (Kronberg et al. 1980). coincidence is very unlikely. Moreover, the morphologies of the hot spots (Fig. 7) do not resemble those of galaxies. We thus believe 3C 351 to have the Ðrst conðrmed optical double hot spot. Optical synchrotron emission indicates that the electrons in the secondary B hot spot must be currently accelerated. The secondary hot spot cannot be a remnant of an older impact point of the jet, as the dentistïs drill model of Scheuer (1982) suggests. Valtaoja (1984) and Lonsdale & Barthel (1986) have shown that the radio properties of the secondary hot spots in general are incompatible with those of old remnants left behind by a wobbling jet. However, 3C 351 provides the Ðrst direct evidence of continuous particle acceleration occurring also in the secondary hot spots, supporting the splatter spot mechanism of Williams & Gull (1985) C This source is a classical double radio galaxy with z \ The northern hot spot has a double structure. A jet leads into the compact hot spot and continues into the secondary hot spot through a narrow neck (Leahy & Perley 1995). Leahy & Perley have argued that the more compact hot spot is the current termination of the jet Ñow. Therefore it is also the most likely location for optical emission. At 5 GHz the polarization is B20% in P.A. B130 (Leahy & Perley 1995). A possible optical counterpart to this hot spot was detected optically by Saslaw et al. (1978) at the confusion level in KPNO 4 m telescope photographic plates, but no new optical observations have been published. Figure 8 shows our I-band image. A signiðcant polarization measurement could not be obtained for the weak hot spot candidate: 38% ^ 38% in P.A. 23 ^ 38. Our CCD frame does not include any sufficiently bright optical objects, and so we cannot compare the optical and the radio positions directly. However, the optical hot spot in both our CCD image and in the photograph of Saslaw et al. (Fig. 6), coinciding with the Np radio peak, are at the same position. The angular extent of the candidate is 2A.0] 2A.7, corresponding to a linear extent of B2 kpc. FIG. 8.ÈI-band map of 3C The radio position is marked with a cross.

6 No. 3, 1999 HOT SPOTS IN RADIO GALAXIES 1173 FIG. 9.ÈI-band map of PKS 2135[147. The radio position of the hot spot is marked with a cross. The quasar is the bright object close to the right edge. After the Ðrst draft of this paper was submitted, we became aware of recent optical observations of 3C by Prieto & Kotilainen (1997), also obtained with the Nordic Optical Telescope. Their 80 minute I-band image (Fig. 1a in their paper) is very similar to ours, with both the size and the shape of the hot spot candidate in good agreement with our data. The spectrum of the candidate, as obtained by Prieto & Kotilainen, clearly shows the power-law nature of the emission, providing the Ðnal conðrmation for one of the Ðrst detected optical hot spot candidates PKS 2135[147 PKS 2135[147 is a classical double quasar with z \ An optical object has been reported close to the position of the eastern radio hot spot (Hawkins 1978). No recent optical images or radio polarization data have been published. Figure 9 shows the optical hot spot candidate. The radio hot spot is marked with a cross. We did not detect signiðcant optical polarization: 5% ^ 8% in P.A. 55 ^ 20. The coordinates measured from the quasar Q to the hot spot B in Figure 1a of Hawkins (1978) are *d \ 49A.6, *a \ 22A.5. These coordinates in our CCD image are *d \ 53A.2, *a \ 18A.8. The optical and radio positions do not coincide, and the extended object is clearly an unrelated Ðeld galaxy, similar to another one visible in our image. 3. SUMMARY Table 2 summarizes our results. Four of our Ðve optical hot spot candidates appear to be genuine, whereas the Ðfth in PKS 2135[147 is an unrelated Ðeld galaxy. In 3C 111, 3C 303, 3C 351, and 3C the radio and the optical positions coincide within the measuring accuracy, limited not by our image resolution but by the extended nature of both the radio and the optical hot spots. The excellent image quality of the Nordic Optical Telescope combined with subarcsecond seeing enabled us to obtain resolved images of the optical hot spot candidates. All the hot spot images were nonstellar, with angular sizes up to 3A. The corresponding linear sizes range between 1 and 7 kpc, similar to other hot spot size estimates between 0.8 and 5 kpc (Meisenheimer et al. 1989). The accuracy of our polarization measurements was limited by the faintness of the hot spots, with their approximate magnitudes ranging between I \ 20.7 and 22.3 mag. For 3C 111 a 6 p detection of high optical polarization, agreeing with the radio polarization, veriðes the nature of the hot spot candidate. For 3C 303 and 3C 351 we obtained only marginal (3 p and 2 p) detections. However, because the optical percentage polarizations and position angles agree with the corresponding radio values, the positions coincide, and the images are nonstellar, we consider the nature of these optical hot spots to be well established. In the case of 3C 390.3, our observations together with the new data presented by Prieto & Kotilainen (1997) also suffice to identify the candidate as a genuine optical hot spot. Of particular interest is the conðrmation of the Ðrst optical double hot spot in the quasar 3C 351. Because the lifetimes of the electrons emitting optical radiation are of the order of a hundred years, a constant supply of energy is required in both hot spots. Furthermore, as the separation of the hot spots is at least 20 kpc, the transition time from the primary to the secondary hot spot, even at the speed of light, is considerably larger that the electron lifetimes in the secondary hot spot. The electrons in the secondary hot spot must therefore be accelerated locally and continuously. 3C 351 o ers a unique opportunity to study the formation of multiple hot spots, their interactions with the IGM, and the details of the acceleration mechanisms. We wish to thank the hard-working and devoted Nordic Optical Telescope personnel at La Palma, and the Stockholm CCD group for providing an improved version of the CCD polarizer. Dr. H. Ro ser kindly provided details of unpublished optical hot spot observations and other valuable advice. We are also very grateful for the advice of Professor W. Saslaw and Professor M. Valtonen, from whom the idea of this observational program originated. TABLE 2 SUMMARY OF THE RESULTS IMAGE SIZE OPTICAL POLARIZATION RADIO POLARIZATION RADIO/OPTICAL POSITION Angular Size Value P.A. Value P.A. SOURCE (*a, *d) Appearance (arcsec) (%) (deg) (%) (deg) COMMENTS 3C , 0.5 a Extended 1.6 ] ^ 5 76^ 5 \45 75 Optical hot spot 3C , 0.7 Extended 2.3 ] ^ 5 98^ Optical hot spot 3C 351(A) , 1.3 Extended 1.2 ] ^ ^ Optical hot spot 3C 351(B) , 1.1 Extended 1.8 ] Optical hot spot 3C Extended 2.0 ] ^ ^ Optical hot spot PKS 2135[ , 3.6 Galaxy-like 2.8 ] ^ 8 55^ Field galaxy NOTE.ÈAll the optical data refer to the summed I-band images. For references to the radio data, see the main text. a Relative to IR map.

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