INTERFEROMETRIC IMAGING OF THE SUNYAEV-ZELDOVICH EFFECT AT 30 GHz

Transcription

1 THE ASTROPHYSICAL JOURNAL, 456 : L75 L78, 1996 January The American Astronomical Society. All rights reserved. Printed in U.S.A. INTERFEROMETRIC IMAGING OF THE SUNYAEV-ZELDOVICH EFFECT AT 30 GHz JOHN E. CARLSTROM, 1 MARSHALL JOY, 2 AND LAURA GREGO 1 Received 1995 March 23; accepted 1995 October 24 ABSTRACT We present high signal-to-noise ratio images of the Sunyaev-Zeldovich effect toward the clusters CL and Abell 773. A null result is reported for Abell The data were obtained with the millimeter array of the Owens Valley Radio Observatory outfitted with 30 GHz receivers. Isothermal models for the cluster gas with core radii spanning were fitted to the visibility data. The results suggest the central decrement T 0 lies within the range 700 to 820 K for CL and 575 to 760 K for Abell 773, corresponding to Compton y-values of and , respectively. A comparison of the image of the Sunyaev-Zeldovich effect toward CL with a ROSAT image of the X-ray emission shows good correspondence; the positions agree within the uncertainties, and both images show resolved structure elongated at a position angle of 150. Using the core radius and ellipticity derived from the X-ray data to constrain the model parameters resulted in a decrement for CL of T H 65 K. Subject headings: cosmic background radiation cosmology: observations galaxies: clusters: individual (CL , A773, A1704) techniques: interferometric 1. INTRODUCTION Observations of the Sunyaev-Zeldovich (S-Z) effect have been attempted since the effect was first predicted 25 years ago (Sunyaev & Zeldovich 1970, 1972). The effect is a spectral distortion of the 3 K cosmic microwave background radiation resulting from inverse-compton scattering of the microwave photons as they pass through the hot X-ray emitting gas in the extended atmospheres of clusters of galaxies. The spectral distortion appears as a temperature decrement at frequencies below 218 GHz ( 1.4 mm) and an increment at higher frequencies (see review by Sunyaev & Zeldovich 1980). Although the magnitude of the effect is small ( 1 mk at radio wavelengths), its promise of providing an independent estimate of the Hubble constant and of the peculiar motions of distant clusters of galaxies has continued to motivate increasingly sensitive observations. The first observations of the effect were made in the radio regime with single-dish telescopes. The reported results show considerable differences, reflecting the difficulty of the measurement (see review by Birkinshaw 1991 and references therein). These observations illustrated the need for telescopes and receiver systems designed explicitly to minimize differential atmospheric emission and ground pickup (see also Myers et al. 1996). More recently, using drift scans with a bolometric array receiver on a single-dish telescope to minimize systematic effects, Wilbanks et al. (1994) reported a detection at 2.2 mm. Interferometric observations offer several advantages over those made with single dishes for detecting and imaging weak emission. An interferometer measures only the correlated signal received by separate telescopes. Furthermore, the correlations are performed after compensation for the changing differential delays introduced as the array telescopes track the celestial source. This synchronous detection ensures that a 1 MS , Division of Mathematics, Physics, and Astronomy, California Institute of Technology, Pasadena, CA Space Science Laboratory, ES84, NASA Marshall Space Flight Center, Huntsville, AL L75 well-designed interferometer will not suffer from the systematic effects associated with atmospheric and ground emission that are so difficult to determine and control in single-dish observations. A further advantage of interferometry, especially for relatively low frequency observations, is the ability to image and then remove emission from point sources that otherwise contaminate the S-Z effect. Last, interferometry provides a two-dimensional image of the emission. However, using interferometry for cosmic background observations has its difficulties. The existing interferometric arrays were designed to obtain high-resolution images, typically of order 1. The large diameter, D, of the telescopes used to provide sensitivity to emission from such small scales limits the angular size scales that can be imaged; structures with angular scales larger than roughly /3D are suppressed. Since the angular temperature profile of the S-Z effect is expected to vary smoothly with a characteristic scale of about 1 even for distant clusters, these arrays are not suitable for imaging the effect. This problem is further aggravated by the fact that one wants to observe the effect at wavelengths shorter than 11 cm to minimize contamination from nonthermal emission from point sources. The eight 13 m antennas of the East-West Ryle Telescope were upgraded for observations of the S-Z effect at 15 GHz (Jones 1990), and a few clusters have been imaged (Jones et al. 1993; Grainge et al. 1993). At 15 GHz (2 cm) one still expects substantial emission from radio point sources. Although these contaminating sources are imaged with data from the longer baselines of the array and then subtracted from the visibility data, this adds noise. Also, man-made interference limits the available bandwidth and thus the sensitivity of the observations at 15 GHz. Observing at higher frequencies greatly reduces both of these problems. In this Letter we present images of the S-Z effect obtained with the millimeter-wave array of the Owens Valley Radio Observatory (OVRO) outfitted with centimeter-wave receivers. Using an array designed for millimeter wavelengths at centimeter wavelengths is ideal for imaging the S-Z effect and alleviates most of the problems discussed above. Additionally,

2 L76 CARLSTROM, JOY, & GREGO Vol. 456 the two-dimensional layout of the array makes it possible to obtain good u, v coverage and high brightness sensitivity, even for equatorial sources. 2. OBSERVATIONS Low-noise receivers utilizing cooled HEMT amplifiers that operate from 26 to 36 GHz were built and mounted at the Cassegrain focus of each of the 10.4 m telescopes of the Owens Valley millimeter array specifically to image the S-Z effect. The telescopes operating at 28.7 GHz provide a 4 FWHM primary beam. The resulting 1.0 k minimum baseline allows imaging angular scales as large as 1#7, although in practice angular scales only less than about 1#4 are imaged well. At these frequencies the surface accuracy is better than /100, and the pointing accuracy is better than 1/50 of the primary beam. The observations were obtained with five elements of the array during the period 1994 June 16 July 9. The system temperatures at zenith ranged from 60 K to 100 K with a mean of 75 K. Following the HEMT amplifier and bandpass filter, a cooled mixer driven by a phase-locked YIG oscillator provides the IF, which is then supplied to the 1 2 GHz bandpass of the standard OVRO back end. The cross-correlations were performed with a 1 GHz wide bandwidth analog correlator for which there is an added level of fast 180 phase-switching to reduce any offsets. Details of the correlator and phaseswitching are given in Padin (1995). The fluxes of the quasars used as gain calibrators (amplitude and phase) were determined relative to Mars. The brightness temperature of Mars for each observation was estimated using a thermal-radiative model that has an estimated uncertainty of 4% when uncertainties in the values of the electrical properties of the surface of Mars are included (Rudy 1987). This 4% dominated the uncertainty in the derived fluxes of the calibrators. The gain calibrator for CL was , and its flux was determined to be 0.63 H 0.03 Jy. For Abell 773, was used, and its flux was 1.92 H 0.08 Jy. For Abell 1704, was used, and its flux was 0.17 H 0.01 Jy. The uncertainties quoted for the calibrators are those for the absolute flux of Mars added in quadrature with those for one calibration relative to Mars. There was no evidence for time variation in the flux of these gain calibrators reflected in the derived gain of the interferometer or in the daily comparison of quasar and planet observations. The absolute flux scales for the cluster observations presented here are therefore accurate to 6% or better. The data were obtained from three different configurations of the five-element array, providing projected baselines ranging from the 10.4 m shadowing limit to 75 m. The MMA software package (Scoville et al. 1993) was used to calibrate the visibility data and then write it in FITS format. The image processing was done using DIFMAP (Shepherd, Pearson, & Taylor 1994). The DIFMAP package allows easy inspection and flagging of data. Flagging large fractions (up to 50%) of data only increased the noise level and did not significantly change the results, demonstrating that the data quality is high and the results are robust. False correlations were detected only on baselines for which one of the telescopes shadowed the other. We omitted all data from baselines that included a telescope that was within 10 cm of being shadowed by any telescope within the array. 3. RESULTS Three clusters were targeted for observations: CL , Abell 773, and Abell Their selection was based on a combination of strong X-ray emission, small angular size, previously reported S-Z decrements, and position. Abell 773 and Abell 1704 are at similar distances, z and , respectively, while CL is much more distant at z (Elvis et al. 1992; Huchra et al. 1990; Stocke et al. 1991). The S-Z effect toward CL is believed to be strong (Uson 1987; Birkinshaw 1991). The array is well suited to observations of CL since it allows reasonable u, v coverage at declination 16, and the 11 synthesized beam is better matched to more distant clusters. The S-Z effect toward Abell 773 was observed with the Ryle telescope with a resulting signal-to-noise ratio (S/N) of 15 (Grainge et al. 1993). We observed it to test our system and to provide a confirmation of their 15 GHz interferometric result. Searches for the S-Z effect toward Abell 1704 have been made by Birkinshaw and collaborators; there has not been a significant detection (see Birkinshaw 1991). We now present our imaging results for each cluster CL The distant cluster CL was observed with three configurations of the array for a combined total of 13 transits. A decrement and the associated sidelobes were clearly detected in an image made with no taper applied to the u, v data, even though the beam 3 was rather small, 49"5 27"7. To search for point sources that may contaminate the S-Z effect, we made images with no taper applied to the u, v data and with a2k inner u, v cutoff to decrease the sensitivity to the S-Z effect. The resulting images have a 41"8 21"4 beam and a rms of 98 Jy beam 1. No point sources were detected. A survey of point sources toward the cluster was made with the VLA at 1.4, 4.9, and 14.9 GHz by Moffet & Birkinshaw (1989, hereafter MB). They found 24 sources, although only one (source 14) lies within the half-power response of our primary beam. This source was only 2.7 mjy at 1.4 GHz and undetected ( 3 mjy) at 4.9 and 14.9 GHz. It is unlikely to be a source of contamination at 28.7 GHz; at the location of this source, the flux level in our 41"8 21"4 resolution image is 180 Jy, consistent with the noise level (1.8 ). For an additional 11 transits, we interleaved observations of five positions toward CL : the central position and fields offset by 1#5 to the north, east, west, and south. The rms noise in these images is 250 Jy. We detected only one (source 15) of the MB sources in this extended area. This source appears in our southern field as a 1.2 mjy peak located 4#14 south of the field center. According to our holographic measurements of the beam patterns, the source should be attenuated by approximately 15 db, indicating its true flux may be as high as 40 mjy at 28.7 GHz. It is not surprising that we do not detect the source in the central field, since it is located 5#64 from the beam center where the attenuation is at least 24 db and at most azimuths 30 db or greater. We therefore conclude that sidelobes due to this source do not contaminate the S-Z decrement observed near the field center. For maximum sensitivity to the S-Z effect (i.e., best surface 3 In this Letter, beam refers to the FWHM of an elliptical Gaussian fit to the central peak of the point source response pattern. The Gaussian is estimated from the second moment of the sampling in the u, v plane, which gives the curvature of the point source response pattern at the origin.

3 No. 2, 1996 INTERFEROMETRIC IMAGING OF S-Z EFFECT L77 TABLE 1 FITTED CLUSTER POSITIONS Cluster a Epoch CL : Decrement: this Letter h 18 m 33! H5 J(2000) X-ray: Hughes et al H4 J(2000) Abell 773: Decrement: this Letter H8 B(1950) Decrement: Grainge et al H15 B(1950) X-ray: Einstein centroid H5 B(1950) a Uncertainties for the interferometric results are estimated by the beam FWHM (S/N) 1. brightness sensitivity), we applied a Gaussian taper to the u, v data with a half-power radius of 1.2 k, which results in a 71"7 65"7 beam and a rms of 90 Jy beam 1, which corresponds to a Rayleigh-Jeans (R-J) brightness sensitivity of 28 K. The deconvolved image (CLEANed) is shown in Figure 1a (Plate L10). The peak is 1350 Jy beam 1, which corresponds to T RJ 426 K and a S/N of 115. The flux integrated over a 4 4 box centered on the decrement is 3.0 mjy. The data quality allows us to make higher resolution images with sufficient S/N to check for substructure. In Figures 1b and 1c we show images made with a 2.0 k Gaussian u, v taper and with no taper. The resulting beams are 56"5 51"2 and 49"5 27"7, respectively. The decrement is clearly resolved. The most noticeable structure is the elongation at p.a In Figure 2 (Plate L11) we show our 56"5 51"2 resolution S-Z image overlaid on an X-ray image taken with the ROSAT PSPC, which has a resolution of 130 (Hughes, Birkinshaw, & Huchra 1995). The bright X-ray emission to the north of the cluster is from an active galactic nucleus (AGN) at z (Margon, Downes, & Spinrad 1983). The overall similarity of the two images is striking; the positions agree within the uncertainties, and both images show resolved structure elongated at a position angle of 150. Table 1 lists the position of the peak of the S-Z effect and of the X-ray emission. While the images give an impression of the data quality, quantitative information is best obtained by directly fitting models to the visibility data. We fitted the visibility data with a standard isothermal model for the cluster gas T T 0 1 1/ 2 3 /2 2, C where C is the gas core radius. With the typical value of 2/3 for, the above expression has an analytical transform, and it is straightforward to fit the measured visibilities directly for position, central decrement T 0, and a major and minor core radius and position angle. This functionality has been added to the DIFMAP package. The best fit gave 4 T K, a core radius of 75, an ellipticity of 0.1, and a position angle of 60. These parameters are not well constrained by the fits, however. Acceptable fits (H1 ) were found for core radii of and ellipticities of The central decrement was found to be relatively insensitive to the core radius: for core radii spanning , T 0 spans 700 K to 870 K witha50 core radius giving the smallest decrement. A 130 core radius leads to 4 A factor of is included in T 0 to correct for the Rayleigh-Jeans approximation. T K. Hughes et al. (1995) fitted the X-ray data and derived 0.74, a 42 core radius with an ellipticity of 0.17 at a position angle of 51. Using the X-ray derived parameters, but with 2/3, gave an acceptable fit to our S-Z data with T H 65 K, which corresponds to a Compton y- value of The uncertainty was estimated by combining the uncertainty of the absolute flux scale and the uncertainty set by the S/N in quadrature. The central decrement derived here is in good agreement with the values reported by Uson (1987) and Birkinshaw (1991) Abell 773 Abell 773 was observed with two configurations of the array for a combined total of nine transits. A decrement and the associated sidelobes were clearly detected in an image made with no taper applied to the u, vv data, even though the beam was rather small, 30"7 23"0. As for CL , we made images with no taper applied to the u, v data and with a2k inner u, v cutoff to search for point sources. The resulting images have a 25"7 19"3 beam and a rms of 92 Jy beam 1. No point sources were detected. To improve the sensitivity to the S-Z effect, we applied a Gaussian taper to the u, v data with a half-power radius of 1.7 k, resulting in a 61"9 59"6 beam and a rms of 95 Jy beam 1, corresponding to a R-J brightness sensitivity of 40 K. The deconvolved image is shown in Figure 3 (Plate L12). The peak is 775 Jy beam 1, corresponding to T RJ 312 K and a S/N of 18. The flux integrated over a 4 4 box centered on the decrement is 1.2 mjy. Table 1 lists the position of the peak as well as other relevant positions for Abell 773; the positions agree well. The best fit to the isothermal gas model (see 3.1) with zero ellipticity gave a core radius of 40 and T K. However, the core radius was constrained (H1 ) only from 5 to 120. The smallest T 0 is found for a core radius of 50 ; T 0 spans 575 K to 760 K for values of the core radius ranging from 20 to 100, corresponding to Compton y-values of Our Compton y-value is 27% smaller than the value derived from data taken with the Ryle telescope at 15 GHz for the same core radius of 50 and 0.65 (Grainge et al. 1993). Their formal measurement uncertainties are 20%, and ours are 13%; when uncertainties in determining the central decrement are included, the difference between the two measurements is not significant. In addition, source contamination may also be a factor, especially at lower observing frequencies, where emission from contaminating synchrotron sources is stronger; Grainge et al. removed a contaminating flux of 491 Jy contributed by two weak radio point sources before

4 L78 CARLSTROM, JOY, & GREGO determining their net decrement of 590 Jy beam 1. Our higher frequency observations are much less sensitive to contamination from nonthermal sources since the flux of the S-Z decrement is stronger and nonthermal emission from point sources should be much weaker Abell 1704 Abell 1704 was observed with two configurations of the array for a combined total of 10 transits. The rms noise in an image made with no u, v taper was 100 Jy in a 33"7 31"3 beam corresponding to a R-J brightness sensitivity of 140 K. No significant emission from point sources and no significant decrement was detected. To search only for point sources, we useda2k inner u, v cutoff to decrease the sensitivity to large-scale features. The resulting image had a 26"4 24"3 beam and a rms of 130 Jy beam 1. No point sources were detected. To improve the brightness sensitivity, we applied a Gaussian taper to the u, v data with a half-power radius of 1.7 k, resulting in a 63"4 56"6 beam and a rms of 135 Jy beam 1, corresponding to a R-J brightness sensitivity of 56 K. No significant decrement was found, indicating either that (1) the S-Z effect in Abell 1704 is smoothly extended on angular scales larger than those sampled by our observations or (2) the magnitude of the effect is below our sensitivity limit. We can use the Abell 1704 data to check for systematic offsets introduced by our instrument, as well as offsets due to the sidelobe response of point sources outside our field of view. The 63"4 56"6 resolution image is shown in Figure 4 (Plate L13); it has not been CLEANed. There is a small decrement at the center of the image, but it is not statistically significant. The mean pixel amplitude within the primary field of view is about 1 Jy, and within 20 of the field center we find that the largest negative and positive features are consistent with the noise at 3.6 and 3.0, respectively. Therefore, any systematic bias in our maps is insignificant compared to the thermal noise. 4. CONCLUSIONS We have imaged with high S/N the S-Z effect toward the clusters CL and Abell 773 at 28.7 GHz. A null result is reported for Abell The results of fitting isothermal models for the cluster gas with core radii spanning to the visibility data suggest the central decrement T 0 lies within the range 700 to 820 K for CL and 575 to 760 K for Abell 773, corresponding to Compton y-values of and , respectively. A comparison of the image of the S-Z effect toward CL with a ROSAT image of the X-ray emission shows good correspondence; the positions agree within the uncertainties, and both images show resolved structure elongated at a position angle of 150. Using the core radius and ellipticity derived from the X-ray data to constrain the model parameters resulted in a decrement for CL of T H 65 K. Measurements of the S-Z effect can lead to a determination of the Hubble constant and other cosmological parameters. However, a sensitive, unbiased imaging survey of galaxy clusters at both microwave and X-ray wavelengths will be necessary to achieve a reliable determination of these parameters. The results presented here demonstrate the power of using the OVRO millimeter array equipped with centimeter receivers to obtain high S/N images of the S-Z effect toward distant clusters. With the goal of conducting a survey of clusters, we have made several improvements to increase the sensitivity and imaging speed of the instrument: a sixth receiver and additional telescope stations were built to make full use of the six-element array, the receiver noise temperatures have been improved by exploiting the newest developments in InP HEMT devices, and the correlation bandwidth has been expanded to 2 GHz by using the additional 1 GHz bandwidth analog correlator recently added to the array back end (the bandwidth of the receiver IF is 10 GHz). These improvements have resulted in over a factor of 3 improvement in the point-source sensitivity of the system, increasing the imaging speed by a factor of 19. We thank M. Pospieszalski for the construction of the HEMT amplifiers; R. Padman, S. Padin, and D. Woody for their help in the design of the instrument; M. Bergmann, J. Ozbolt, P. Whitehouse, R. Zellar, and the MSFC fabrication division for their help in building the instrument; the Princeton Gravity Group and A. Readhead for the loan of HEMT amplifiers; the OVRO staff, especially C. Giovanine, R. Lawrence, S. Padin, S. Scott, and D. Woody, for their help in bringing the instrument online; J. Hughes for use of the X-ray data prior to publication; T. Pearson and M. Shepherd for their help with the DIFMAP package and model fitting; and S. Myers and the referee, J. Uson, for helpful comments. J. C. gratefully acknowledges support from a NSF-Young Investigator Award and the David and Lucile Packard Foundation. Radio astronomy with the OVRO millimeter array is supported by NSF grant AST The funds for the additional hardware built for the S-Z effect observations are from a NASA CDDF grant, a NSF-YI Award, and the David and Lucile Packard Foundation. Birkinshaw, M. 1991, in Physical Cosmology, ed. J. Tran Thanh Yan (Gif sur Yvette: Editions Frontières), 177 Grainge, K., Jones, M., Pooley, G., Saunders, R., & Edge, A. 1993, MNRAS, 265, L57 Elvis, M., Plummer, D., Schachter, J., & Fabbiano, G. 1992, ApJS, 80, 257 Huchra, J. P., Henry, J. P., Postman, M. K., & Geller, M. J. 1990, ApJ, 365, 66 Hughes, J. P., Birkinshaw, M., & Huchra, J. P. 1995, ApJ, 448, L93. Jones, M. E. 1990, in ASP Conf. Proc. 19, Radio Interferometry; Theory, Techniques and Applications, ed. T. J. Cornwell & R. A. Perley (San Francisco: ASP), 395 Jones, M., et al. 1993, Nature, 365, 320 Margon, B., Downes, R. A., & Spinrad, H. 1983, Nature, 301, 221 Moffet, A. T., & Birkinshaw, M. 1989, AJ, 98, 1148 (MB) Myers, S. T., Baker, J. E., Readhead, A. C. S., Leitch, E. M., & Herbig, T. 1996, in preparation Padin, S. 1995, IEEE Trans., Instr. & Meas., 43, 782 REFERENCES Rudy, D. J. 1987, Ph.D. thesis, California Inst. of Technology Scoville, N. Z., Carlstrom, J. E., Chandler, C. J., Phillips, J. A., Scott, S. L., Tilanus, R. P. J., & Wang, Z. 1993, PASP, 105, 1482 Shepherd, M. C., Pearson, T. J., & Taylor, G. B. 1994, BAAS, 26, 987 Stocke, J. T., Morris, S. L., Gioia, I. M., Maccacaro, T., Schild, R., Wolter, A., Fleming, A., & Henry, J. P. 1991, ApJS, 76, 813 Sunyaev, R. A., & Zeldovich, Ya. B. 1970, Comments Astrophys. Space Phys., 2, , Comments Astrophys. Space Phys., 4, , ARA&A, 18, 537 Uson, J. 1987, in Radio Continuum Processes in Clusters of Galaxies, Proc. NRAO Greenbank Workshop 16, ed. C. O Dea & J. Uson (Green Bank, WV: NRAO), 255 Wilbanks, T. M., Ade, P. A. R., Fischer, M. L., Holzapfel, W. L., & Lange, A. E. 1994, ApJ, 427, L75

5 PLATE L10 FIG. 1. Deconvolved interferometric images of the Sunyaev-Zeldovich effect at 28.7 GHz toward the distant cluster CL The pointing and phase center of the observations was h 15 m 58!40, "0. (a) The resolution is 71"7 65"7 at p.a. 5, resulting from a Gaussian taper applied to the u, v data with a half-power radius of 1.2 k. Contours are multiples of 150 Jy beam 1 (1.7 ), which corresponds to a R-J brightness of 47.3 K. The rms is 90 Jy beam 1, corresponding to a R-J brightness sensitivity of 28 K. The peak is 1350 Jy beam 1, corresponding to T RJ 426 K and a S/N of 115. (b) The resolution is 56"2 51"2 at p.a. 13, resulting from a 2.0 k taper. Contours are multiples of 120 Jy beam 1 (1.5 ). (c) The resolution is 49"5 27"7 at p.a. 7 resulting from no u, v taper. Contours are multiples of 120 Jy beam 1 (1.7 ). Note that the images are shown over a region much larger than the 4 FWHM of the primary beam; no correction has been made for the primary beam response, so the noise level should be uniform over the entire image. CARLSTROM, JOY, & GREGO (see 456, L77)

6 PLATE L11 FIG. 2. Overlay of our 56"2 51"2 resolution image of the Sunyaev-Zeldovich effect toward CL (contours) on an X-ray image (gray scale) taken with the ROSAT PSPC, which has a resolution of 130 (Hughes, Birkinshaw, & Huchra 1995). The bright X-ray emission to the north of the cluster is from an AGN at z (Margon, Downes, & Spinrad 1983). CARLSTROM, JOY, & GREGO (see 456, L77)

7 PLATE L12 FIG. 3. Deconvolved image of the Sunyaev-Zeldovich effect toward Abell 773 at 28.7 GHz. The pointing and phase center of the observations was h 14 m 25!00, "0. The contours are multiples of 150 Jy beam 1 (1.6 ), which corresponds to a R-J brightness of 60.5 K. A Gaussian taper with a half-power radius of 1.7 k was applied to the u, v, data resulting in a 61"9 59"6 at p.a. 70 beam and a rms of 95 Jy beam 1, corresponding to a R-J brightness sensitivity of 40 K. The peak is 775 Jy beam 1, corresponding to T RJ 312 K and a S/N of 18. CARLSTROM, JOY, & GREGO (see 456, L77)

8 PLATE L13 FIG. 4. Image toward the cluster Abell 1704 at 28.7 GHz. The pointing and phase center of the observation was h 12 m 36!80, "0. The image has not been deconvolved. A Gaussian fit to the center of the synthesized beam gives a FWHM of 63"4 56"6 at p.a. 87 as shown in the bottom left-hand corner. The contours are multiples of 200 Jy beam 1 (1.5 ). The small decrement at the center is not statistically significant. CARLSTROM, JOY, & GREGO (see 456, L77)

9 THE ASTROPHYSICAL JOURNAL, 461 : L59, 1996 April The American Astronomical Society. All rights reserved. Printed in U.S.A. ERRATA In the Letter Revealing the Effects of Orientation in Composite Quasar Spectra by Joanne C. Baker and Richard W. Hunstead (ApJ, 452, L95 [1995]), a correction should be made on page L95. In accordance with the definition of compact, steep-spectrum sources in the third paragraph of the Introduction, the second sentence should state the condition 15 kpc rather than 15 kpc. In the Letter Facts and Artifacts in Interstellar Diamond Spectra by H. Mutschke, J. Dorschner, Th. Henning, C. Jäger, and U. Ott (ApJ, 454, L157 [1995]), there exists a series of misprints: in all chemical formulae, single bonds have been replaced by double bonds. The only case in which the double bonds are correct is CAO. In the Letter Interferometric Imaging of the Sunyaev-Zeldovich Effect at 30 GHz by John E. Carlstrom, Marshall Joy, and Laura Grego (ApJ, 456, L75 [1996]), the printer included the wrong figures (Plates L10 L14). The correct Figures 1 4 with captions are reproduced here as Plates L11 L14. L59

10 PLATE L11 FIG. 1. Deconvolved interferometric images of the Sunyaev-Zeldovich effect at 28.7 GHz toward the distant cluster CL The pointing and phase center of the observations was h 15 m 58!40, "0. (a) The resolution is 71"7 65"7 at p.a. 5, resulting from a Gaussian taper applied to the u, v data with a half-power radius of 1.2 k. Contours are multiples of 150 Jy beam 1 (1.7 ), which corresponds to a R-J brightness of 47.3 K. The rms is 90 Jy beam 1, corresponding to a R-J brightness sensitivity of 28 K. The peak is 1350 Jy beam 1, corresponding to T RJ 426 K and a SNR of 115. (b) The resolution is 56"2 51"2 at p.a. 13, resulting from a 2.0 k taper. Contours are multiples of 120 Jy beam 1 (1.5 ). (c) The resolution is 49"5 27"7 at p.a. 7 resulting from no u, v taper. Contours are multiples of 120 Jy beam 1 (1.7 ). Note that the images are shown over a region much larger than the 4 FWHM of the primary beam; no correction has been made for the primary beam response, so the noise level should be uniform over the entire image. CARLSTROM, JOY, & GREGO (see 461, L59)

11 PLATE L12 FIG. 2. Overlay of our 56"2 51"2 resolution image of the Sunyaev-Zeldovich effect toward CL (contours) on an X-ray image ( gray scale) taken with the ROSAT PSPC, which has a resolution of 130 (Hughes, Birkinshaw, & Huchra 1995). The bright X-ray emission to the north of the cluster is from an AGN at z (Margon, Downes, & Spinrad 1983). CARLSTROM, JOY, & GREGO (see 461, L59)

12 PLATE L13 FIG. 3. Deconvolved image of the Sunyaev-Zeldovich effect toward Abell 773 at 28.7 GHz. The pointing and phase center of the observations was h 14 m 25!00, "0. The contours are multiples of 150 Jy beam 1 (1.6 ), which corresponds to a R-J brightness of 60.5 K. A Gaussian taper with a half-power radius of 1.7 k was applied to the u, v data, resulting in a 61"9 59"6 at p.a. 70 beam and a rms of 95 Jy beam 1, corresponding to a R-J brightness sensitivity of 40 K. The peak is 775 Jy beam 1, corresponding to T RJ 312 K and a SNR of 18. CARLSTROM, JOY, & GREGO (see 461, L59)

13 PLATE L14 FIG. 4. Image toward the cluster Abell 1704 at 28.7 GHz. The pointing and phase center of the observations was h 12 m 36!80, "0. The image has not been deconvolved. A Gaussian fit to the center of the synthesized beam gives a FWHM of 63"4 56"6 at p.a. 87 as shown in the bottom left-hand corner. The contours are multiples of 200 Jy beam 1 (1.5 ). The small decrement at the center is not statistically significant. CARLSTROM, JOY, & GREGO (see 461, L59)