Galaxies Detected by the Dwingeloo Obscured Galaxies Survey

Publ. Astron. Soc. Aust., 1999, 16, 48–52.

A. J. Rivers1, P. A. Henning1 and R. C. Kraan-Korteweg2

1 Department of Physics and Astronomy, University of New Mexico, 800 Yale NE, Albuquerque, NM 87131, USA [email protected], [email protected] 2 Dept. de Astronomia, Universidad de Guanajuato, Mexico [email protected] Received 1998 November 9, accepted 1999 February 1

Abstract: The Dwingeloo Obscured Galaxies Survey (DOGS) is a 21-cm blind survey for galaxies hidden in the northern ‘Zone of Avoidance’ (ZOA): the portion of the optical extragalactic sky which is obscured by dust in the Milky Way. Like the Parkes southern hemisphere ZOA survey, the DOGS project is designed to reveal hidden dynamically important nearby galaxies and to help ‘ll in the blanks’ in the local large scale structure. To date, 36 galaxies have been detected by the Dwingeloo survey; 23 of these were previously unknown [no corresponding sources recorded in the NASA Extragalactic Database (NED)]. Among the interesting detections are three nearby galaxies in the vicinity of NGC 6946 and 11 detections in the Supergalactic plane crossing region. VLA follow-up observations have been conducted for several of the DOGS detections.

Keywords: surveys—galaxies: general

1 Introduction 2 Telescope Parameters and Search Strategy Approximately 25% of the optical extragalactic sky The 25-m Dwingeloo telescope operating at 21-cm is obscured by the dust and high stellar density has a half-power-beamwidth (HPBW) of 0 6 which of the Milky Way (Figure 1). Although diligent may be thought of as the survey resolution. A optical and infrared searches for galaxies narrow this DAS-1000 channel autocorrelator spectrometer is ‘Zone of Avoidance’, in the most heavily obscured utilised in the telescope backend; the rms noise regions near the Galactic plane, only radio surveys per channel is typically ch =40mJyfora1hr consistently reveal hidden galaxies. Radio galaxy integration. Each DOGS observation consists of a searches near the Galactic plane can help to complete sequence of ve contiguous pointings at constant our knowledge of the nearby large scale structure Galactic latitude (Figure 2). Five On–O pairs and explore the connectivity of superclusters and created from the sequence ensure that a real galaxy voids across the ZOA. will appear twice, once as a positive signal, and again A full survey of the northern ZOA has been recently as a negative one, referenced against two independent completed using the Dwingeloo 25-m radiotelescope scans. Overlapping the constant latitude grids to and the Westerbork array in total power mode. form a honeycombed coverage of the sky allows The survey region (30 ` 220 ; |b|5.25; for detection of galaxies in adjacent pointings and 1 0 VLSR 4000 km s ) was initially covered in facilitates a more accurate determination of their a shallow search by the 25-m (5 min per pointing; positions. A similar observing strategy was employed rms noise per channel ch = 175 mJy) for nearby, during the Westerbork portion of the survey; beam massive galaxies (Henning et al. 1998). Recently, size and velocity coverage were identical and a similar a deeper survey sensitive to nearby dwarfs and survey sensitivity achieved. A 15 pointing grid was to normal spirals at the survey limit has been used for eciency reasons, but the core On–O completed by the 25-m and the Westerbork array in pair strategy remained the same. The complete total power mode. Results from the portion of the survey incorporates approximately 15,000 partially deep survey completed by the 25-m are reported overlapping pointings. here. Interesting regions covered by DOGS include a portion of the Local Void, the nearby IC342/Maei 3 Results group of galaxies and the region of sky where the Approximately 60% of the DOGS survey was Local Supercluster crosses the Galactic plane. completed using the Dwingeloo 25-m telescope.

᭧Astronomical Society of Australia 1999 1323-3580/99/010048$05.00

Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.14, on 24 Sep 2021 at 18:04:00, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1071/AS99048 Dwingeloo Obscured Galaxies Survey 49

Figure 1—A combined Galactic longitude versus latitude plot of approximately 30 000 galaxies collected from the MCG, UGC and ESO optical catalogues shows the ‘Zone of Avoidance’ caused by dust in the Galactic plane. The spatial region covered by the Dwingeloo survey is indicated. The hatched region represents the spatial overlap with the Parkes ZOA survey.

Figure 2—Distribution of survey grid points. Galactic longitude spacing is ` =0.4 while the successive rows of constant Galactic latitude are separated by b =0 35. Each grid point is marked with a + surrounded by a circle indicating the HPBW of 0.6.

In this portion of the survey, 36 galaxies were Five of the 36 sources were originally identied by conrmed, 23 of which were previously unknown the shallow survey including Dwingeloo 1 and Maei (no NED counterpart). The number of galaxies 2, both members of the nearby Maei/IC 342 group detected is consistent with calculations based on of galaxies (Kraan-Korteweg et al. 1994). During an assumed HI mass function (Zwaan, Briggs & the deep survey another known group member, Sprayberry 1997) and the survey sensitivity which MB 1 (McCall & Buta 1995), was identied and predict between 50 and 100 detections within the two additional members still await conrmation survey range. Recent Westerbork observations observations. completed the survey and galaxies discovered will The most signicant nearby, previously unknown soon be incorporated into the Dwingeloo sample galaxy identied by DOGS was Dwingeloo 1. Given (cf. Figure 3 for the location of detected galaxies). the 80% coverage of the survey region by the shallow

Figure 3—Spatial distribution of DOGS sources (indicated by ?) combined with Lyon–Meudon Extragalactic Database 1 (LEDA) galaxies out to V LSR 4000 km s (). The 60% of the survey covered by the 25-m telescope and analysed in this paper is shown by the dashed line.

Figure 4—Spectra from the Dwingeloo 25-m telescope of three galaxies found in the vicinity of NGC 6946. Pictured are, from left to right, IRAS 21189+4503, Dw0928+05 and Dw0950+10.

Table 1. Possible members of the NGC 6946 group HI masses are calculated based on the assumption that all galaxies lie at the same distance as NGC 6946 (6 Mpc, Sharina, Karachentsev & Tikhonov 1997). The HI mass of NGC 6946 is based on the mean of the six ux entries in LEDA which are corrected for the beam-lling eect. The uncertainty in the masses of the three Dwingeloo detected sources is primarily due to the uncertainty in the positions of the galaxies

Galaxy name ` bVLSR log(M HI) Reference 1 (deg.) (deg.) (km s )(M) HKK L150 9603 1235 132 71 Huchtmeier et al. (1997) UGC 11583 9563 1231 127 82 Huchtmeier et al. (1997) HKK L149 9557 1222 126 81 Huchtmeier et al. (1997) NGC 6946 9572 1167 48 101 LEDA Cepheus 1 9438 801 65 91 Burton et al. (1999) Dw0950+109505 116 142 86: Henning et al. (1998) Dw0928+059282 053 230 78: IRAS 21189+4503 8885 320 228 83: Nakanishi et al. (1997)

survey (Henning et al. 1998), chances are low that 3.1 Supergalactic Plane Crossing Region and Local a massive nearby spiral was missed, since nearby Void galaxies appear in many adjacent pointings, all of Although the Dwingeloo survey is limited in survey 1 which would have to be missed for the galaxy to depth (VLSR 4000 km s ), two primary large escape detection. Thus, it is fairly unlikely that there scale structures fall partially within the survey range: is another previously unidentied massive galaxy the Local Void and the Local Supercluster. Eleven whose gravitational inuence signicantly impacts galaxies were discovered in the survey coverage of the Local Group peculiar motion or internal dynamics Local Supercluster crossing region (` 140; |b| in the area covered by the survey. 5.25); six of these sources are noted in NED. Known

Figure 5—A synthesis map of DOGS detection Dw073242 from the VLA. A galaxy group is revealed in a sample velocity slice (VLSR = 3219 km s 1,v'10 km s 1).

structures appear continuous and well dened across Dw0928+05. Assuminga6Mpcdistance yields 8 8 the Galactic plane with a narrow bridge of galaxies MHI ' 210 M and 110 M respectively. The 1 visible at ` 142 and VLSR 1400 km s . estimated neutral hydrogen masses are typical of Near the Local Void (` 33, b 15), Marzke, dwarf galaxies, consistent with the gaussian shape Huchra & Geller (1996) and Roman et al. (1998) of the 21-cm proles (Figure 4). With the recent found evidence for a nearby cluster at 1500 km s1. discovery of the LSB galaxy Cepheus 1 (Burton et Two previously unknown galaxies were detected in al. 1999) seven galaxies with recessional velocites 1 this region, adding support for the hypothesised VLSR 250 km s have been identied within overdensity. These galaxies, Dw030624(`= 15 of NGC 6946 (Table 1). If these galaxies do 1 3060, b = 248, VLSR = 1480 km s ) and in fact signify a new nearby group, this group Dw030143(`=3009, b = 435, VLSR = would lie some 40 from the Supergalactic plane, 1528 km s1), were independently identied by the considerably more than any other known group in Parkes ZOA survey (Henning et al. 1999, present the local universe. The potential exists for the issue p. 35). discovery of additional galaxies in this group since the data recently collected by the Westerbork array 3.2 NGC 6946 Group for +1 05 b +5.25 have not yet been analysed. Three dwarf galaxies were detected in the region of NGC 6946 (` =9572, b =1167) suggesting 3.3 Results from Synthesis Follow-ups of DOGS the possibility of a new nearby group. Of these, Sources Dw0950+10 was originally recorded as a compact Synthesis observations of Dwingeloo galaxies were high velocity cloud (HVC) (Wakker 1990), but conducted with the VLA and WSRT in 1997. present data suggest it is in fact a nearby dwarf Snapshot WSRT observations yielded positions of 1 galaxy. With a velocity of VGSR = 368 km s ,it DOGS sources though sensitivities were not adequate has the highest redshift of any HVC in the Wakker for detailed mapping. VLA follow-ups yielded a few catalog and the 50% velocity width of V50 = 100 km interesting results. A galaxy group unresolved by the s1 seen in the Dwingeloo spectrum is signicantly Dwingeloo 0.6 beam was resolved into ve galaxies by broader than the 20–30 km s1 velocity dispersion follow-up VLA observations (Figure 5). Since early generally observed in HVCs. Naively assuming synthesis follow-ups to Parkes ZOA detections also this galaxy is at the same distance as NGC 6946 have revealed interacting galaxy groups (Staveley- (6 Mpc, Sharina, Karachentsev & Tikhonov 1997) Smith et al. 1998), it will be interesting to see if this 8 yields MHI ' 410 M. Also detected in the area is commonly seen in HI selected samples. The total 9 2 were IRAS 21189+4503 (Nakanishi et al. 1997) and HI mass measured for this group was 2 10 Mh0

1 with a mean velocity of VLSR = 3222 km s and Burton for helpful discussions. We would also like a calculated group diameter of approximately 150 to thank J. M. van der Hulst and R. Braun for their 1 h0 kpc. No optical counterparts were identied on help in instigating and supporting the Westerbork POSS E plates under signicant extinction (AB ' portion of the DOGS project. The Dwingeloo 25-m 45 mag from DIRBE reddening maps, Schlegel, radio telescope is supported by the Netherlands Finkbeiner & Davis 1998). A previously unknown Foundation for Scientic Research (NWO). This low mass companion to IRAS 05596+1451 was also research has made use of the NASA/IPAC Extra- discovered in a separate VLA synthesis map. galactic Database (NED) and the Lyon–Meudon Extragalactic Database (LEDA). The research of 4 Conclusions and Future Directions P. A. Henning is supported by NSF Faculty Early A wide eld ux limited survey for HI luminous Career Development (CAREER) Program award galaxies hidden in the northern ZOA has been AST 95–02268. completed. With approximately 60% of the data analysed we have detected 36 galaxies, 23 of which were previously unknown. Given the number found References to date, in the full survey we expect to detect Burton, W. B., Braun, R., Walterbos, R. A. M., & Hoopes, around 60 sources, which falls within our survey C. G. 1999, AJ, in press Henning, P. A., Kraan-Korteweg, R. C., Rivers, A. J., Loan, predictions of 50–100 galaxies. A. J., Lahav, O., & Burton, W. B. 1998, AJ, 115, 584 Since the DOGS slice covers a wide range of the Henning, P. A., Staveley-Smith, L., Kraan-Korteweg, R. C., extragalactic sky including portions of the Local & Sadler, E. M. 1999, PASA, 16, 35 Supercluster and the Local Void, the collected sample Huchtmeier, W. K., Karachentsev, I. D., & Karachentseva, will be well suited for a comparison study of galaxies V. E. 1997, A&A, 322, 375 Kraan-Korteweg, R. C., Loan, A. J., Burton, W. B., Lahav, in dierent environments. O., Ferguson, H. C., Henning, P. A., & Lynden-Bell, D. Eorts are also underway to quantify the Galactic 1994, Nature, 372, 77 extinction, a poorly known quantity near the McCall, M. L., & Buta, R. J. 1995, AJ, 109, 2460 Galactic plane. Although the recently released Marzke, R. O., Huchra, J. P., & Geller, M. J. 1996, AJ, DIRBE reddening maps (Schlegel, Finkbeiner & 112, 1803 Nakanishi, K., Takata, T., Yamada, T., Takeuchi, T., Shiroya, Davis 1998) prove useful, they remain uncalibrated R., Miyazawa, M., Watanabe, S., & Saito, M. 1997, ApJS, at low latitude (|b|10 ). We are exploring the 112, 245 possibility of using near-infrared colours of galaxies Roman, A. T., Takeuchi, T., Nakanishi, K., & Saito, M. from 2MASS data to independently determine the 1998, PASJ, 50, 47 extinction and to thereby determine calibration Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, 500, 525 constants and uncertainties for the DIRBE maps at Sharina, M. E., Karachentsev, I. D., & Tikhonov, N. A. low latitude. 1997, Astron. Lett., 23, 373 Staveley-Smith, L., Juraszek, S., Koribalski, B. S., Ekers, Acknowledgments R. D., Green, A. J., Haynes, R. F., Henning, P. A., We are grateful to A. Foley and D. Moorrees for Kesteven, M. J., Kraan-Korteweg, R. C., Price, R. M., Sadler, E. M. 1998, AJ, 116, 2717 their supervision of the 25-m day-to-day telescope Wakker, B. P. 1990, PhD thesis, Groningen Univ. operations, to T. Hess for the development of data Zwaan, M., Briggs, F., & Sprayberry, D. 1997, PASA, 14, translation routines, and to O. Lahav and W. B. 126