Proc. Natl. Acad. Sci. USA Vol. 92, pp. 11356-11359, December 1995 Colloquium Paper This paper was presented at a colloquium entitled "Quasars and Active Galactic Nuclei: High Resolution Radio Imaging," organized by a committee chaired by Marshall Cohen and Kenneth Kellermann, held March 24 and 25, 1995, at the National Academy of Sciences Beckman Center, Irvine, CA. Very-long-baseline radio interferometry observations of low power radio galaxies G. GIOVANNINI*t, W. D. COFTONt, L. FERETTI*t, L. LARAt§, T. VENTURIt, AND J. M. MARCAIDEI *Dipartimento di Astronomia, Universita di Bologna, Bologna, Italy; tIstituto di Radioastronomia, Consiglio Nazionale delle Richerche, Bologna, Italy; *National Radio Astronomy Observatory, Charlottesville, VA 22903-2475; §Instituto de Astrofisica de Andalucia, Consejo Superior de Investigaciones Cientificas, Andalucia, Spain; and 1Departamento de Astronomia, Universitat de Valencia, Valencia, Spain ABSTRACT The parsec scale properties of low power Table 1. VLBI radio galaxies radio galaxies are reviewed here, using the available data on Log 12 Fanaroff-Riley type I galaxies. The most frequent radio P408,t VLBI structure is an asymmetric parsec-scale morphology-i.e., Radio galaxy z* W/Hz morphology4: Ref.§ core and one-sided jet. It is shared by 9 (possibly 10) of the 12 mapped radio galaxies. One (possibly 2) of the other galaxies 0055+30 NGC 315 0.0167 23.95 One-sided (7) has a two-sided jet emission. Two sources are known from 0104+32 3C31 0.0169 24.50 One-sided This paper published data to show a proper motion; we present here 0206+35 4C35.03 0.0375 24.28 One-sided This paper evidence for proper motion in two more galaxies. Therefore, in 0755+37 NGC 2484 0.0413 25.04 One-sided (8) the present sample we have 4 radio galaxies with a measured 0836+29 4C29.30 0.0790 25.08 One-sided (9) proper motion. One of these has a very symmetric structure 1142+20 3C264 0.0206 24.85 One-sided This paper and therefore should be in the plane of the sky. The results 1144+35 0.0630 24.15 One-sided? This paper discussed here are in agreement with the predictions of the 1222+13 3C272.1 0.0037 23.27 Two-sided? This paper unified scbeme models. Moreover, the present data indicate 1228+12 3C274 0.0037 25.07 One-sided (10) that the parsec scale structure in low and high power radio 1626+39 3C338 0.0303 25.25 Two-sided (11) and galaxies is essentially the same. this paper 1637+82 NGC 6251 0.0230 24.55 One-sided (5) 2335+26 3C465 0.0301 25.39 One-sided (9) The knowledge of the structure of radio galaxies on the *Galaxy redshift. parsec scale is important to test current models of jet tTotal radio power at 408 MHz. dynamics and acquire new pieces of information to test the 4Parsec scale radio morphology. radio source unified schemes. Very-long-baseline radio in- §Reference for radio morphology. terferometry (VLBI) data on powerful radio galaxies and quasars show a strong evidence of relativistic jets and in many Radio galaxies in the same range of total radio power, but cases a proper motion with an apparent superluminal velocity unresolved on the arcsecond scale, have not yet been properly has been found (1, 2). VLBI observations of low power radio mapped. Therefore, they are not discussed here. Observations galaxies are also necessary to compare the parsec scale prop- to study this class of radio sources remain to be made, in order erties of radio sources with different radio powers and to test to understand their nature and connection with the more the unified scheme models, which predict also that low power radio galaxies should have parsec scale jets moving at a velocity powerful CSO and CSS sources (6). close to the speed of light. In this paper, we present and discuss A Hubble constant Ho = 100 km-sec- 'Mpc-1 (Mpc, the available VLBI data on extended low power radio galaxies megaparsec; 1 pc = 3.09 x 1016 m) and deceleration parameter [Fanaroff-Riley type I, hereafter referred to as FRI (3)]. We qo = 1 have been used throughout this paper. will use the radio galaxies of the sample currently under study by us (4) observed at 5 GHz with the very long baseline array Radio Morphology (VLBA) or the global array. This sample was obtained by selecting from the B2 and 3CR galaxy samples those objects The list of radio galaxies studied so far and discussed here is with a core flux density >100 millijansky (mJy; 1 Jy = 10-26 presented in Table 1. A morphological analysis based on the wm-2-Hz-1) at 6 cm at arcsecond resolution (4). The core flux available VLBI maps indicates that an asymmetric morphol- limit, imposed by observational constraints, could produce a ogy-i.e., core and one-sided jet-is the most frequent radio sample biased toward objects with jets pointing toward the structure (see, for example, Fig. 1). It is shared by 9 (possibly observer. This point is not important in discussing single 10) of the 12 mapped radio galaxies. A clear symmetric struc- objects but has to be taken in account for statistical studies. ture is found in 3C338 (Fig. 2), while 3C272.1 shows a complex The well known FRI galaxy NGC 6251 (see ref. 5 and structure with a possible counterjet close to the core (Fig. 3). references therein), not included in our sample, was added, for The one-sided jet is always well collimated and only small a total of 12 FRI radio galaxies. Some of them have been oscillations or bendings are visible. The nuclear emission is observed also at 1.6 and 8.4 GHz. Only 5 have observations at always the dominant component. When maps at two or more different epochs to search for a possible proper motion. frequencies are available, the core emission shows a flat or The publication costs of this article were defrayed in part by page charge Abbreviations: VLBI, very-long-baseline radio interferometry; FRI payment. This article must therefore be hereby marked "advertisement" in and -II, Fanaroff-Riley types I and II; Jy, Jansky; VLBA, very long accordance with 18 U.S.C. §1734 solely to indicate this fact. baseline array. 11356 Downloaded by guest on October 5, 2021 Colloquium Paper: Giovannini et al. Proc. Natl. Acad. Sci. USA 92 (1995) 11357 3C264 4974.989 MHz 3C338 4987.490 MHz I I ~I* I I 4 25 _A ;-> .I C~~-( 20 F 80 -2 15 F 20 10 0 -10 -20 10 F mas FIG. 2. VLBA map of 3C338 at 5 GHz. The HPBW is 2.2 X 2.2 5 , milliarcsec (mas). The peak flux is 44.4 mJy per beam; contour levels are -0.5, 0.5, 0.7, 1, 1.5, 2, 3, 4, 6, 8, 10, 20, 30, and 40 mJy per beam. oF. Q Two-sided arrow shows the direction of the symmetric kiloparsec scale I 0 jet. -5 F strongly variable. Several measurements of the flux density (4) , '. I ,t-. at 1.4 and 5 GHz show that it was "300 mJy in 1974 while now -10 it is 540 mJy after reaching a maximum of 610 mJy in 1991. -15 Simultaneous multifrequency observations show that the core 15 10 5 0 -5 -10 spectrum is flat between 1.4 and 5 GHz but strongly steepens mas between 5 and 8.4 GHz. The VLBI structure consists of two main components (A and C) with an inverted spectrum 3C465 8417.990 MHz between 1.7 and 5 GHz and low brightness, jet-like features departing from them (Fig. 4). A comparison between our data and the 5-GHz VLBI map obtained in the second Caltech- Jodrell Bank VLBI survey (14) shows that (i) component A is probably variable, and therefore we tentatively identify it as the core; in this case the parsec scale structure would be in the direction of the fainter kiloparsec scale jet. However, owing to the complexity of this source and the slight asymmetry of the faint kiloparsec scale jets, the definition of a main jet may be ambiguous. (ii) The separation between A and C increases between the two observing epochs. This proper motion is clearly visible also in comparing the model given (14) with our visibilities. The data are consistent with a proper motion of component C with respect to A with an apparent superluminal velocity = 1.2c. The snapshot data obtained by us (4) are in agreement with this motion. mas 3C272.1 4973.241 MHz FIG. 1. (A) VLBI map of 3C264 at 5.0 GHz. The half-power beam 40 width (HPBW) is 3.5 x 2.1 milliarcsec (mas) in position angle 11°. The peak flux is 135 mJy per beam; contour levels are -0.3, 0.3, 0.5, 0.8, 1.5, 3, 5, 10, 25, 50, and 100 mJy per beam. Arrow shows the direction of the main kiloparsec scale jet. (B) VLBI map of 3C465 at 8.4 GHz. 30 The HPBW is 2.52 x 0.83 mas in position angle -9.7°. The peak flux '0 is 132 mJy per beam; contour levels are -0.75, 0.75, 1.5, 2, 3, 5, 10, 20, 50, and 100 mJy per beam. Arrow shows the direction of the main kiloparsec scale jet. 20 _ inverted spectrum while the jet emission has a spectral index Io1 of ;0.5. A few sources are described in detail below. peculiar 10 _ 3C264.