A morphological and spectral study of GPS galaxies and quasars

Adv. SpaceRes. Vol.26, No. 4, pp. 709-714,2OOO 0 2OOQ COSPAR. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain 0273-1177/00$20.00 + 0.00

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A MORPHOLOGICAL AND SPECTRAL STUDY OF GPS GALAXIES AND QUASARS R. T. Schilizzi’“, W. Tschage?, I. A. G. Snellen3, A. G. de Bruyn4>, G. K. Miley2, H. J. A. Rijttgering2, H. J. van Langevelde’, C. Fant?“, R. Fant?,’

‘Joint Institute for VLBI in Europe, PO Box 2,799O AA Dwingeloo, The Netherlands 2Sterrewacht, University of Leiden, PO Box 5413,230O RA Leiden, The Netherlands 31nstitute of Astronomy, University of Cambridge, Madingley Rd, Cambridge CB3 OHA, UK 4Netherlands Foundation for Research in Astronomy, PO Box 2,799O AA Dwingeloo, The Netherlands ‘Kapteyn Laboratory, University of Groningen, PO Box 800,970O AV Groningen, The Netherlands 6Department of Physics, University of Bologna, Via Imerio 46,40126 Bologna, Italy ‘Institute of Radio Astronomy, Via Gobetti 101,40129 Bologna, Italy ABSTRACT We have analysed HALCA and global VLBI data for 2021+614, the first of eleven GPS quasars and galaxies to be observed by HALCA. We show that 2021+614 is a compact symmetric object of overall size -40 pc, and confirm that the speed of separation of the two dominant lobes is approximately one tenth of the speed of light. Its apparent age of less than 1000 years provides additional support for the contention that compact symmetric radio objects associated with galaxies are young radio sources and the possible precursors of the classical FRl or FR2 radio galaxies. 0 2000 COSPAR.Publishedby Elsevier Science Ltd. INTRODUCTION Extragalactic radio sources have been studied for many years, but it is still unclear how they are formed and evolve. A crucial element in the study of their evolution is to identify the young counterparts of “old” FRl/FR2 extended objects, but until recently, it has not been clear which class of radio source represents that early evolutionary stage. Good candidates for young radio sources are those with peaked spectra (Gigahertz Peaked Spectrum (GPS) sources and Compact Steep Spectrum (CSS) sources), because they are small in angular size as expected for a young source. GPS sources are characterized by a convex radio spectrum peaking at a frequency of about 1 GHz and are typically 100 pc in size. CSS sources have a peak in their spectrum at lower frequencies and have projected linear sizes of < lo-15 kpc. For many years, however, it has also been thought possible that the small sizes of GPS and CSS sources could be due to confinement by a particularly dense and clumpy interstellar medium that impedes the outward propagation of the jets (O’Dea et al. 1991). This latter hypothesis now looks less likely sinde recent observations show that the surrounding media of peaked spectrum sources are not significantly different from large scale radio sources, and insufficiently dense to confine these sources (eg. Fanti et al. 1995). In addition, Owsianik and Conway (1998) have measured the propagation velocity of the hot spots of 07 10 + 3 19, a prototype:GPS source, to be 0.244 h-’ c. Assuming a constant propagation velocity, this corresponds to an age of 1400+150 years for this GPS source. Evidence also exists that the age of the GPS galaxy 2021+ 614 is also about 1000 years (Conway et al, 1994; Pearson 1995). It therefore appears

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that peaked spectrum sources are indeed young radio sources and are the objects of choice to study the initial evolution of extragalactic radio sources. Work done by our group (eg Snellen et al, 1998a) on a small sample of faint GPS sources, and compared with samples of bright GPS and CSS sources to disentangle redshift and radio power effects, has yielded interesting information on the morphological and luminosity evolution of young radio sources. Correlations were found to exist between peak flux density, peak frequency and the angular sizes of GPS and CSS sources: the higher the peak frequency, the smaller the angular size, and the higher the peak flux density, the larger the angular size. Assuming the spectral peak is due to syncbrotron self-absorption, these correlations demonstrate that the ratio between the overall angular size and the angular size of the dominant radio components producing the peaked spectrum is constant throughout the different samples of GPS and CSS sources, suggesting that peaked spectrum sources evolve in a self-similar way (Snellen et al. 1998b). It is obviously of interest to examine these young sources at a number of points along their evolutionary track, and to carry out that investigation, at radio wavelengths, with a range of angular resolution, limiting flux density, and observing frequency. As part of this programme we were granted observing time with VSOP in AO-1 at 5 GHz for a “mini-survey” sample of 11 GPS sources (project VO85). THE HALCA SAMPLE The 11 sources in our sample were all those known in November 1995 with: . declination 225 degrees . peak frequency - 5 Ghz . well determined spectra . known redshifts . sufficient flux density to be detectable on baselines to HALCA Table 1 lists the sources, thei

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All objects in the sample are scheduled for observation beam spectral index data.

with the VLBA at 15 GHz to obtain matched

At the time of the COSPAR General Assembly, 5 of the 11 objects had been observed, but only one, 202 I+61 4, carried through the complete data reduction sequence. In this contribution, we report on the results of the data reduction and initial data analysis. THE SOURCE 202 1+6 14 Gptical 2021+614 is identified with an elliptical galaxy whose ‘redshift is 0.2266M.0002. It is a highly reddened narrow line radio galaxy most probably with a considerable dust component within the galaxy (Bartel et al 1984a). The asymmetric shape of the [OIII] 5007 A emission line profile indicates that the nucleus contains clouds with velocity differences of 780 krn/sec. Deep CCD imaging by O’Dea et al (1990) shows that the galaxy has a prominent compact nucleus and two possible companions within 12 arcsec.

The spectrum of 2021+614 is shown in Figure 1. It has a broad, relatively flat peak centred at about 4 GHz, and falls off at lower and higher frequencies. The flux density at 5 and 15 GHz is variable; measurements by Aller et al (1992) from 1986 to 1991 showed that the flux density at 15 GHz, varied by 30 to 40 %, but there was no variation within the noise at 5 GHz. VLBI measurements of 2021+614 at 5GHz by Conway et al (1994) in 1982.93 and 1987.74 are interpreted by these authors as indicating an increase in the flux density of one the major components of the source structure leading to an increase in the total flux density of about 12 %.

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Figure I. Total radio spectrum of GPS galaxy 2021+614. The authors made use of the CATS database (Verkhodanov et al 1997, Special Astrophysical Observatory)

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The HALCA Observation at 5 Ghz HALCA observed 2021+614 on 6 November 1997 together with a 15 station ground-based array composed of 10 VLBA and 4 EVN radio telescopes plus the VLA in phased-array mode. The tracking stations used to downlink the data and relay the local oscillator signal to the satellite were located at the Deep Space Network sites at Goldstone in California USA and Tidbinbilla in Australia. The data were correlated at the NRA0 Array Operating Center in Socot-ro New Mexico USA. The images of the source for the ground array alone and for HALCA plus the ground array are shown in Figure 2. They were obtained following standard calibration procedures for space VLBI data reduction in the AIPS Cookbook. Self-calibration and imaging were carried out with DIFMAP. The images are shown with uniform weighting of the data points in order to emphasise the contribution of the spacecraft.The peak flux density of the ground based image is 896 mJy/beam and the rms noise level is - 120 nJy/beam giving a dynamic range of -7500. The peak flux density of the HALCA image is 426 mJy/beam and the rms’noise level is - 750 nJy/beam giving a dynamic range of -570. Previous VLBI Observations High angular resolution observations of 2021+614 at 2.3,5 and 8.4 GHz have been published by Wittels et al (1982), Bartel et al (1984b), Pearson and Readhead (1988), and Conway et al (1994). Cawthome et al (1993) determined that there is no significant polarised flux density from any of the components with upper limits of 5 mJy.

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levels are (-0.2,0.2,0.4,0.8,...,51.2) x 8.96 mlylbeam. Peak flux density is 896 mJy/beam. Restoring beam 0.90 x 0.83 milliarcsec. Figure 2 (right). Image of 2021+614 at 5 GHz from observations with HALCA and the 15 station ground array. Contour levels are (-1 ,1,2,4,...,64) x 4.26 mJy/beam. Peak flux density is 426 mlylbeam. Restoring beam is 0.50 x 0.23 milliarcsec.

Morphology and Spectra of GPS Sources

Discussion Following Bartel et al (1984b) we label the components A to D starting from the NE. At the redshift of the object, the overall linear extent is - 40 pc. As noted by the previous authors, the morphology of 2021+614 is dominated by components B and D. Decomposition of the total radio spectrum on the basis of multi-frequency VLBI by Bartel et al (1984b) and Conway et al (1994) makes it clear that the individual component spectra of B and D peak at slightly different frequencies causing the overall spectrum between 2 and 10 GHz to be relatively flat for a GPS source. On the basis of D being the most compact component, as well as being variable in flux density (Conway et al 1994), both groups suggested that it is the site of the nucleus of the galaxy. Our HALCA image, however, shows that at the higher resolution provided by the space baselines, a compact component is visible between B and D at the end of an oscillating low brightness jet in the direction of D. Its location on the line joining B and D suggests that it pinpoints the nucleus. This is confirmed by unpublished observations at 15 and 22 GHz, with similar beamsizes to our HALCA data, by Vermeulen et al and Wilkinson et al respectively (private communications); both sets of data reveal a compact component in the centre which is more prominent than in our observations at 5 GHz, thus indicating an inverted spectrum. A careful analysis of the positions of components B and D by Conway et al (1994) using model fitting techniques showed that the component separation changed by 69 microarcsec in 4.8 1 years, corresponding to a speed of separation of 0.13 h-‘c. We have made a first order estimate of the separation of B and D ten years after Conway et al’s observations (Figure 3), and find a rate of separation of -17 microarcsec/year, corresponding to a speed of separation of -0.19 h-/c. With respect to the weak central component at the nucleus, the speed of separation is -0.095 h“c, which means that the two components were ejected -400 years ago assuming constant velocity. Separation

vs Time of B-D

Figure 3. Separation of components B and D in 2021+614 as function of time. The separation in 1997 has been determined in two independent ways, shown here as two crosses. The noise in each determination is equal to the size of the cross. The change in separation between 1982 and 1987 was given by Conway et al (1994), but not the separations themselves at each epoch. We have placed these points on the diagram so as to give a tit with the least scatter.

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We confirm that 2021+614 is one of a small group of compact symmetric sources for which speeds of separation can be measured. All have apparent ages of a few hundred to a thousand years, and are thus good candidates for being young sources. In the case of 2021+614, further analysis needs to be done to make a more accurate measurement of the separation of B and D, and to try to estimate the speed of separation of component A. Rough estimates of the brightness temperatures of B and D give values of 1.6 and 2.6 x 10” K respectively. Using the spectral decomposition of Conway et al (1994), we derive the magnetic fields in B and D to be 2.5 and 1.O x 10e3gauss respectively, well out of equipartition. Further analysis will refine these estimates. REFERENCES Aller M. F. etal, Ap.J., 399,16 (1992) Bartel N. ef al, Ap. J. 279, 112 (1984a) Bartel N. er al, Ap. J. 279, 116 (1984b) Cawthome T. V. et al, Ap.J. 416,496 (1993) Conway J. E. et al, Ap. J. 425,568 (1994) Fanti C. et al, A&A 302,3 17 (1995) O’Dea C. P. et al, A&AS 82,261 (1990) O’Dea C. P. et al, Ap. J. 380,66 (1991) Owsianik I., Conway J. E., A&A 337,69 (1998) Pearson, T. J. in “VLBI and the VLBA” (ASP Conf. Series, vol 82), 267 (1995) Pearson, T. J, Readhead, A. C. S., Ap.J. 328, 114 (1988) Snellen I. A. G. et al, A&AS 131,435 (1998a) Snellen I. A. G. et al, in Proc IAU Coll. 164,297 (1998b) Verkhodanov 0. V. et al, in “Astronomical data analysis software and systems VI” (ASP Conf. Series, vol 125), 322 (1997) Wittels J. J. et al, Ap. J. 262, L27 (1982)