Mapping the Hidden Universe: the Galaxy Distribution in the Zone of Avoidance

Publ. Astron. Soc. Aust., 2000, 17, 6–12.

Mapping the Hidden Universe: The Galaxy Distribution in the Zone of Avoidance

Renee C. Kraan-Korteweg1 and Sebastian Juraszek2,3

1Departamento de Astronoma, Universidad de Guanajuato, Apartado Postal 144, Guanajuato GTO 36000, Mexico [email protected] 2School of Physics, University of Sydney, NSW 2006, Australia 3ATNF, CSIRO, PO Box 76, Epping, NSW 2121, Australia [email protected] Received 1999 August 26, accepted 1999 October 26

Abstract: Due to the foreground extinction of the Milky Way, galaxies become increasingly faint as they approach the Galactic Equator creating a ‘zone of avoidance’ (ZOA) in the distribution of optically visible galaxies of about 25%. A ‘whole-sky’ map of galaxies is essential, however, for understanding the dynamics in our local Universe, in particular the peculiar velocity of the Local Group with respect to the Cosmic Microwave Background and velocity ow elds such as in the Great Attractor (GA) region. The current status of deep optical galaxy searches behind the Milky Way and their completeness as a function of foreground extinction will be reviewed. It has been shown that these surveys—which in the mean time cover the whole ZOA (Figure 2)—result in a considerable reduction of the ZOA from extinction levels of m m AB =10 (Figure 1) to AB =30 (Figure 3). In the remaining, optically opaque ZOA, systematic HI surveys are powerful in uncovering galaxies, as is demonstrated for the GA region with data from the full sensitivity Parkes Multibeam HI survey (300 ` 332, |b|55, Figure 4).

Keywords: zone of avoidance — surveys — ISM: dust, extinction — large-scale structure of universe

1 Introduction presented in an Aito equal-area projection centred on the Galactic plane. The same corrections as In 1988, Lynden-Bell & Lahav for the rst time advocated in Lahav (1987) have been applied to prepared a ‘whole-sky’ distribution of the extra- homogenise the data of the three dierent galaxy galactic light to map the density enhancements in catalogs, i.e. D25 =115 DUGC, D25 =096 DESO the local Universe, to compare them with cosmic and D25 =129 DMCG. A cut-o at D = 13was ow elds and to determine the gravity eld on imposed—the completeness limit of the respective the Local Group. Assuming that light traces mass, catalogs according to Hudson & Lynden-Bell (1991). this could be realised through a diameter-coded Figure 1 clearly displays the irregularity in the 0 distribution of galaxies larger than D > 10 taken distribution of galaxies in the nearby Universe with its from the following galaxy catalogs: the Uppsala dynamically important density enhancements such General Catalog UGC (Nilson 1973) for the north as the Local Supercluster visible as a circle (the ( 25), the ESO Uppsala Catalog (Lauberts Supergalactic Plane) centred on the Virgo cluster at 1982) for the south ( 175), while the missing ` = 284,b=74, the Perseus–Pisces chain bending strip (175 <<25) was lled with data into the ZOA at ` =95 and ` = 165, the general from the Morphological Catalog of Galaxies MCG overdensity in the Cosmic Microwave Background (Vorontsov-Velyaminov & Archipova 1963–74). dipole direction (` = 280,b =27, Kogut et al. Because these optical galaxy catalogs are limited 1993) and the Great Attractor region centred on to the largest galaxies they become increasingly ` = 320,b=0 (Kolatt, Dekel & Lahav 1995) with incomplete close to the Galactic equator where the the Hydra (270, 27), Antlia (273, 19), Centaurus dust thickens, reducing the apparent diameters of (302, 22) and Pavo (332, 24) clusters. the galaxies. Added to this are the growing number Most conspicuous in this distribution is, however, of foreground stars which fully or partially block the the very broad, nearly empty band of about 20: view of galaxy images. This is clearly demonstrated the Zone of Avoidance. Comparing this band with in Figure 1, where such a light distribution is the 100 m dust extinction maps of the DIRBE

᭧ Astronomical Society of Australia 2000 10.1071/AS00006 1323-3580/00/010006$05.00

Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.19, on 01 Oct 2021 at 08:30:03, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1071/AS00006 Mapping the Hidden Universe 7 , medium 0 2 < D 3 0 0 as determined from the dust extinction maps of Schlegel, Finkbeiner & Davis m =1 B 3. The galaxies are diameter-coded: small circles represent galaxies with 1 0 1 . The contour marks absorption in the blue of A 0 3 , and large circles D 0 3 < D —Aito equal-area projection in Galactic coordinates of galaxies with D 0 (1998). The displayed contour surrounds the area where the galaxy distribution becomes incomplete (the ZOA) remarkably well. circles 2 Figure 1

Figure 2—An overview of the dierent optical galaxy surveys in the ZOA centred on the Galaxy. The labels identifying the search areas are explained in the text. Note that the surveyed regions entirely cover the ZOA dened by the foreground m extinction level of AB =10 in Figure 1.

experiment (Schlegel, Finkbeiner & Davis 1998) we (2000), the Hydra/Antlia Supercluster region [D2] found that the ZOA—the area where our galaxy by Kraan-Korteweg (2000), the Crux region [D3] counts are severely incomplete—is described almost by Woudt (1998) and Woudt & Kraan-Korteweg m perfectly by the extinction contour AB =10 [where (2000a), the GA region [D4] by Woudt (1998) AB =414 E(B V), Cardelli, Clayton & Mathis and Woudt & Kraan-Korteweg (2000b), and the 1989]. Scorpius region [D5] by Fairall & Kraan-Korteweg (2000)]; 2 Deep Optical Galaxy Searches E: the Ophiuchus Supercluster by Wakamatsu et Systematic deeper searches for ‘partially obscured’ al. (1994) and Hasegawa et al. (1999); galaxies—down to fainter magnitudes and smaller F: the northern GP/SGP crossing by Hau et al. dimensions (D 001) than existing catalogs—have (1996). been performed with the aim to reduce the ZOA. These surveys have uncovered over 50,000 pre- Using existing sky surveys (POSS I and POSS II viously unknown galaxies and are not biased with in the north and the ESO/SERC surveys in the respect to any particular morphological type. Al- south), the whole ZOA has in the mean time been though the various optical surveys are based on visually surveyed for galaxies. Here, examination dierent plate material and were performed by dier- by eye is still the best technique. A separation ent groups, the search techniques overall are similar. of galaxy and star images can as yet not be done All ZOA regions have been searched to diameter by automated measuring machines (e.g. COSMOS limits of at least D > 002. This is considerably deeper or APM) on a viable basis below |b| < 10 15 , than the previously regarded ‘whole-sky’ catalogs 0 though surveys by eye are clearly both very trying with their completeness limits of D > 13. How can and time consuming, and possibly not as objective. these catalogs be merged to arrive at a well-dened The various surveyed regions are displayed in whole-sky galaxy distribution with a reduced ZOA? Figure 2. Details and results on the uncovered galaxy distributions for the various regions are discussed in: 3 Completeness of Optical Galaxy Searches A: the Perseus–Pisces Supercluster by Pantoja In order to merge the various deep optical ZOA surveys et al. (1997); with existing galaxy catalogs, Kraan-Korteweg (2000) B13: the northern Milky Way [B1 by Seeberger et and Woudt (1998) have analysed the completeness al. (1994), Seeberger, Saurer & Weinberger (1996), of their ZOA galaxy catalogs—the Hy/Ant [D2], Lercher, Kerber & Weinberger (1996), Saurer, Crux [D3] and GA [D4] region—as a function of Seeberger & Weinberger (1997) and Seeberger the foreground extinction. & Saurer 1998 from POSS I; B2 by Marchiotti, By studying the apparent diameter distribution Wildauer & Weinberger (1999) from POSS II; B3 as a function of the extinction E(BV) (Schlegel, by Weinberger, Gajdosik & Zanin (1999) from Finkbeiner & Davis 1998) as well as the location POSS II]; of the attening in the slope of the cumulative C13: the Puppis region by Saito et al. (1990, diameter curves log D–log N for various extinction 1991) [C1], the Sagittarius/Galactic region by intervals (cf. Figures 5 and 6 in Kraan-Korteweg Roman, Nakanishi & Saito (1998) [C2], and the 1999), we conclude that our optical ZOA surveys Aquila and Sagittarius region by Roman et al. are complete to an apparent diameter of D = 1400— (1996) [C3]; where the diameters correspond to an isophote 2 D15: the southern Milky Way [the Hydra to of 245 mag/arcsec (Kraan-Korteweg 2000)—for m Puppis region [D1] by Salem & Kraan-Korteweg extinction levels less than AB =30.

Downloaded from https://www.cambridge.org/core. IP address: 170.106.33.19, on 01 Oct 2021 at 08:30:03, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1071/AS00006 Mapping the Hidden Universe 9 3, including galaxies identied in the optical ZOA galaxy searches 0 1 0 (contour). The diameters are coded as in Figure 1. With the exception of the areas for which either the positions of the galaxies or their diameters m 3 B —Aito equal-area distribution of ESO, UGC, MCG galaxies with extinction-corrected diameters D for extinction-levels of A are not yet available (demarcated areas), the ZOA could be reduced considerably compared to Figure 1. Figure 3

How about the intrinsic diameters, i.e. the diame- galaxies which also are based on POSS I survey ters galaxies would have if they were unobscured? A material (D25 =115 DPOSSI). m spiral galaxy seen through an extinction of AB =10 A comparison of Figure 1 with Figure 3 demon- will, for example, be reduced to 80% of its un- strates convincingly how the deep optical galaxy obscured size. Only 22% of a (spiral) galaxy’s searches realise a considerable reduction of the ZOA; original dimension is seen when it is observed we can now trace the large-scale structures in the m m through AB =30. In 1990, Cameron derived nearby Universe to extinction levels of AB =30. analytical descriptions to correct for the obscuration Inspection of Figure 3 reveals that the galaxy den- eects by articially absorbing the intensity proles sity enhancement in the GA region is even more of unobscured galaxies. These corrections depend pronounced and a connection of the Perseus–Pisces quite strongly on morphological type due to the chain across the Milky Way at ` = 165 more likely. dierence in surface brightness proles and mean Hence, these supplemented whole-sky maps certainly surface brightness between early-type galaxies and should improve our understanding of the velocity spiral galaxies. Applying these corrections, we nd ow elds and the total gravitational attraction on m that at AB =30, an obscured spiral or elliptical the Local Group. galaxy at our apparent completeness limit of D = 1400 Redshift follow-ups of well-dened samples of would have an intrinsic diameter of D 6000 or ZOA galaxies will be important in analysing the D 5000, respectively. At extinction levels higher large-scale structures in redshift space. Systematic m 00 than AB =30, an elliptical galaxy with D 60 redshift surveys have been performed for various would appear smaller than the completeness limit ZOA regions and revealed a number of dynamically D=1400 and might have gone unnoticed. The op- important structures such as tical galaxy catalog discussed here should therefore the rich, massive ( 2–51015M) cluster A3627 at 00 be complete to D 60 for galaxies of all morpho- (`, b, v) (325, 7, 4882 km s1) which seems m logical types down to extinction levels of AB 3 0, to constitute the previously unrecognised but with the possible exception of extremely low-surface predicted density peak at the bottom of the brightness galaxies. Only intrinsically very large potential well of the Great Attractor (Kraan- and bright galaxies—particularly galaxies with high Korteweg et al. 1996) surface brightness—will be recovered in deeper ex- the 3C129 cluster at (`, b, v) (160, 0, 5500 km tinction layers. This completeness limit could be s1) connecting Perseus–Pisces and A569 across conrmed by independently analysing the diame- the Galactic Plane (Chamaraux et al. 1990; Pantoja ter versus extinction and the cumulative diameter et al. 1997) diagrams for extinction-corrected diameters. and the Ophiuchus supercluster at (`, b, v) We can thus supplement the ESO, UGC and (0, 8, 8500 km s1) behind the Galactic Centre 0 MCG catalogs—which are complete to D = 13— (Wakamatsu et al. 1994; Hasegawa et al. 1999). with galaxies from optical ZOA galaxy searches that Optical galaxy searches, however, fail in the 0 m have D 1 3 and AB 3 0. As our completeness most opaque part of the Milky Way, the region limit lies well above the ESO, UGC and MCG m encompassed by the AB =30 contour in Figure 3— catalogs, we can assume that the other similarly a suciently large region to hide further dynamically performed optical galaxy searches in the ZOA should 0 important galaxy densities. Here, systematic HI also be complete to D =13 for extinction levels surveys have proven very powerful as the Galaxy is m of AB 3 0. transparent to the 21-cm line radiation of neutral In Figure 3 we have then taken the rst step in hydrogen and HI-rich galaxies can readily be found arriving at an improved whole-sky galaxy distribution through detection of their redshifted 21-cm emission. with a reduced ZOA. In this Aito projection we have plotted all the UGC, ESO, MCG galaxies that have extinction-corrected diameters D 103 (remember 4HI Galaxy Searches in the ZOA that galaxies adjacent to the optical galaxy search In March 1997, a systematic blind HI survey began regions are also aected by absorption though to a in the most opaque part of the southern Milky Way m lesser extent: AB 1 0), and added the galaxies (213 ` 33 ; |b|55) with the Multibeam from the various optical surveys with D =103 (MB) receiver (13 beams in the focal plane array) m and AB 3 0 for which positions and diameters at the 64 m Parkes telescope. The ZOA is being were available. The regions for which these data surveyed along constant Galactic latitudes in 23 are not yet available are marked in Figure 3. As contiguous elds of length ` =8. The ultimate some searches were performed on older generation goal is 25 scans per eld where adjacent strips will POSS I plates, which are less deep compared to be oset in latitude by b =105 for homogeneous the second generation POSS II and ESO/SERC sampling. With an eective integration time of plates, an additional correction was applied to those 25 min/beam we obtain a 3 detection limit of diameters, i.e. the same correction as for the UGC 25 mJy. The survey covers a velocity range of

Figure 4—A sky distribution (left) and redshift cone (right) for galaxies with v<10000 km s 1 in the GA region. Circles mark redshifts from the literature (LEDA), squares mark redshifts from the optical galaxy search in the Hy/Ant-Crux-GA regions (outlined on left panel), and crosses mark detections in the full-sensitivity HI MB-ZOA survey (box).

1200 < v < 12700 km s 1, with a channel spacing overdensity is a ‘great-wall’ like structure starting at of 132kms1 per channel, and will be sensitive the Pavo cluster, having its core at the A3627 cluster to normal spiral galaxies well beyond the Great and then bending over towards shorter longitudes Attractor region. across the ZOA. So far, a shallow survey based on two of the This becomes even clearer in the right panel of foreseen 25 driftscan passages has been analysed Figure 4 where the galaxies are displayed in a redshift (cf. Kraan-Korteweg, Koribalski & Juraszek 1998; cone out to v 10000 km s1 for the longitude Henning et al. 1999). A total of 107 galaxies were range 300 ` 332 analysed so far of the MB catalogued with peak HI ux densities of > 80 mJy full-sensitivity data. The A3627 cluster is clearly (rms = 15 mJy after Hanning smoothing) and their the most massive galaxy cluster uncovered by the detection shows no dependence on Galactic latitude, various surveys in the GA region and therefore the nor the amount of foreground obscuration through most likely candidate for the previously unidentied, which they have been detected. but predicted, density peak at the bottom of the Four cubes centred on the Great Attractor region potential well of the GA overdensity. (300 ` 332, |b|55) of the full-sensitivity Finding a hitherto uncharted further cluster of survey have been analysed (Juraszek et al. 2000) and galaxies at the heart of the GA would have serious uncovered 236 galaxies above the 3 detection level implications for our current understanding of this of 25 mJy. 70% of the detections had no previous massive overdensity in the local Universe. Various identication. indications suggest that PKS1343–601, the second In the left panel of Figure 4, a sky distribution brightest extragalactic radio source in the southern centred on the GA region displays all galaxies with sky (f20 cm = 79 Jy, McAdam 1991), might form redshifts v 10000 km s1. Next to redshifts the centre of yet another highly obscured rich from the literature (circles; LEDA), redshifts from cluster, particularly as it also shows signicant X- the follow-up observations of Kraan-Korteweg and ray emission (cf. Kraan-Korteweg & Woudt 1999 collaborators in the Hy/Ant-Crux-GA ZOA surveys for further details). At (`, b) (310, 2), this (dashed area) are plotted. They clearly reveal radio galaxy lies behind an obscuration layer of the prominence of the cluster A3627 at (`, b, v)= about 12 magnitudes of extinction in the B-band; (325, 7, 4882 km s1, Kraan-Korteweg et al. hence optical surveys are ineective. Still, West & 1996) close to the core of the GA region at Tarenghi (1989) observed this source and identied (`, b, v) = (320, 0, 4500 km s1) as predicted it—with an extinction-corrected diameter of D 40 by Kolatt, Dekel & Lahav (1995). Adding now and a recession velocity of v = 3872 km s1 —as the new detections from the systematic blind HI a giant elliptical galaxy. Giant ellipticals generally MB-ZOA survey (box), for the rst time we can reside at the cores of clusters. trace structures all the way across the Milky Way. Interestingly enough, the HI MB survey does The new picture seems to suggest that the GA uncover a signicant excess of galaxies at this

position in velocity space (cf. Figure 4). However, Kraan-Korteweg, R. C., & Woudt, P. A. 1999, PASA, 16, 53 we do not see a ‘nger of God’, the characteristic Kraan-Korteweg, R. C., Woudt, P. A., Cayatte, V., et al. signature of a cluster in redshift space. Could it be 1996, Nature, 379, 519 Lahav, O. 1987, MNRAS, 225, 213 that too many central cluster galaxies are missed by Lauberts, A. 1982, The ESO/Uppsala Survey of the ESO the HI observations because spiral galaxies generally (B) Atlas (Garching: ESO) avoid the cores of clusters? The existence of this Lercher, G., Kerber, F., & Weinberger, R. 1996, A&ASS, possible cluster still remains a mystery. Meanwhile, 117, 369, [B1] this prospective cluster has been imaged in the near Lynden-Bell, D., & Lahav, O. 1988, in Large-Scale Motions in the Universe, ed. V. C. Rubin & G. V. Coyne (Princeton infrared (Woudt et al. in progress), where extinction Univ. Press), p. 199 eects are less severe compared to the optical, and McAdam, W. B. 1991, PASA, 9, 255 which should uncover early-type cluster members Marchiotti, W., Wildauer, H., & Weinberger, R. 1999, in if they are there. The forthcoming results should progress [B2] then unambiguously settle the question of whether Nilson, P. 1973, Uppsala General Catalog of Galaxies (University of Uppsala) another cluster forms part of the GA supercluster. Pantoja, C. A., Altschuler, D. R., Giovanardi, C., & Giovanelli, R. 1997, AJ, 113, 905 [A] Acknowledgments Roman, A. T., Nakanishi, K., Tomita, A., & Saito, M. 1996, PASJ, 48, 679 [C3] The collaboration with our colleagues in the optical Roman, A. T., Nakanishi, K., & Saito, M. 1998, PASJ, 50, surveys and redshift follow-up observations is greatly 37 [C2] appreciated: C. Balkowski, V. Cayatte, A. P. Fairall, Saito, M., Ohtani, A., Asomuna, A., et al. 1990, PASJ, 42, C. Salem, P. A. Woudt, and the HIPASS ZOA team 603 [C1] Saito, M., Ohtani, A., Baba, A., et al. 1991, PASJ, 43, 449 members R. D. Ekers, A. J. Green, R. F. Haynes, [C1] P. A. Henning, M. J. Kesteven, B. Koribalski, R. Salem, C., & Kraan-Korteweg, R. C. 2000, in preparation M. Price, E. Sadler and L. Staveley-Smith. [D1] Saurer, W., Seeberger, R., & Weinberger, R. 1997, A&ASS, 126, 247, [B1] References Schlegel, D. J., Finkbeiner, D. P., & Davis, M. 1998, ApJ, Cameron, L. M. 1990, A&A, 233, 16 500, 525 Cardelli, J. A., Clayton, G. C., & Mathis, J. S. 1989, ApJ, Seeberger, R., & Saurer, W. 1998, A&ASS, 127, 101 [B1] 345, 245 Seeberger, R., Saurer, W., Weinberger, R., & Lercher, G. Chamaraux, P., Cayatte, V., Balkowski, C., & Fontanelli, 1994, in Unveiling Large-Scale Structures behind the P. 1990, A&A, 229, 340 Milky Way, ed. C. Balkowski & R. C. Kraan-Korteweg, Fairall, A. P., & Kraan-Korteweg, R. C. 2000, in preparation ASP Conf. Ser. 67 (San Francisco), p. 81 [B1] [D5] Seeberger, R., Saurer, W., & Weinberger, R. 1996, A&ASS, Hasegawa, T., Wakamatsu, K., Malkan, M., et al. 1999, 117, 1 [B1] MNRAS, in press [E] Vorontsov-Velyaminov, B., & Archipova, V. 1963–74, Mor- Hau, G. K. T., Ferguson, H. C., Lahav, O., et al. 1996, phological Catalog of Galaxies, Parts 2–5 (Moscow: MNRAS, 277, 125 [F] Moscow University) Henning, P. A., Staveley-Smith, L., Kraan-Korteweg, R. C., Wakamatsu, K., Hasegawa, T., Karoji, H., et al. 1994, in & Sadler, E. M. 1999, PASA, 16, 35 Unveiling Large-Scale Structures behind the Milky Way, Hudson, M., & Lynden-Bell, D. 1991, MNRAS, 252, 219 ed. C. Balkowski & R. C. Kraan-Korteweg, ASP Conf. Juraszek, S., Staveley-Smith, L., Kraan-Korteweg, R. C., Ser. 67 (San Francisco), p. 131 [E] Green, A. J., Ekers, R. D., Henning, P. A., Koribalski, B. Weinberger, R., Gajdosik, M., & Zanin, C. 1999, A&ASS, S., Sadler, E. M., & Schroder, A. C. 2000, in preparation 137, 293 [B5] Kogut, A., Lineweaver, C., Smoot, G. F., et al. 1993, ApJ, West, R. M., & Tarenghi, M. 1989, A&A, 223, 61 419, 1 Woudt, P. A. 1998, PhD thesis, University of Cape Town Kolatt, T., Dekel, A., & Lahav, O. 1995, MNRAS, 275, 797 [D3,D4] Kraan-Korteweg, R. C. 2000, A&ASS, 141, 123 Woudt, P. A., & Kraan-Korteweg, R. C. 2000a, A&ASS, in Kraan-Korteweg, R. C., Koribalski, B., & Juraszek, S. 1998, preparation [D3] in Looking Deep in the Southern Sky, ed. R. Morganti Woudt, P. A., & Kraan-Korteweg, R. C. 2000b, A&ASS, in & W. Couch (New York: Springer), p. 23 preparation [D4]