Overview of Uncovered and Suspected Large-Scale Structures Behind the Milky Way

Publ. Astron. Soc. Aust., 1999, 16, 53–9.

Overview of Uncovered and Suspected Large-scale Structures behind the Milky Way

Renee C. Kraan-Korteweg1 and Patrick A. Woudt2

1 Depto. de Astronoma, Univ. de Guanajuato, Apartado Postal 144, Guanajuato, GTO 36000, Mexico [email protected] 2 ESO, Karl-Schwarzschildstr. 2, D-85748 Garching bei Munchen, Germany [email protected] Received 1998 November 2, accepted 1999 February 1

Abstract: Various dynamically important extragalactic large-scale structures in the local Universe lie behind the Milky Way. Most of these structures (predicted and unexpected) have only recently been made ‘visible’ through dedicated deep surveys at various wavelengths. The wide range of observational searches (optical, near infrared, far infrared, radio and X-ray) for galaxies in the Zone of Avoidance (ZOA) will be reviewed and the uncovered and suspected large-scale structures summarised. Particular emphasis is given to the Great Attractor region where the existence of yet another cluster is suspected (Woudt 1998). Predictions from reconstructions of the density eld in the ZOA are discussed and compared with observational evidence. Although no major structures are predicted out to about v < 10, 000 km s 1 for which no observational evidence exists, the comparison between reconstructed density elds and the observed galaxy distribution remain important as they allow derivations of the density and biasing parameters.

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

1 Introduction 2 Observational Surveys in Zone of Avoidance In the last few years, much progress has been 2.1 Optical Surveys achieved in uncovering the galaxy distribution behind the Milky Way through various multi-wavelength Systematic optical galaxy catalogues are generally approaches. These sometimes quite tedious tasks limited to the largest galaxies (typically with 0 are necessary in order to understand the dynamics diameters D > 1 , e.g. Lauberts 1982). These in the nearby Universe and answer the question catalogues become, however, increasingly incomplete whether the perturbations on the smooth Hubble as the dust thickens, creating a ‘Zone of Avoidance’ in expansion, such as the dipole in the Cosmic Microwave the distribution of galaxies of roughly 25% of the sky. Background and other velocity ow elds, can be fully Systematically deeper searches for partially obscured explained by the irregular galaxy distribution, i.e. galaxies—down to fainter magnitudes and smaller mass distribution if galaxies are fair tracers of mass. dimensions compared to existing catalogues—were In Section 2, the various observational methods performed with the aim of reducing this ZOA. These are described and the large-scale structures (LSS) surveys are not biased with respect to any particular uncovered to date summarised. With respect to HI morphological type. and near infrared surveys, only their characteristics— Most of the ZOA has meanwhile been surveyed advantages, limitations and selection eects—in (cf. Figure 1 in Kraan-Korteweg 1998), revealing context to other approaches are discussed as specic many galaxy overdensities uncorrelated with the surveys and results are presented in detail elsewhere patchy, optical extinction distribution. Analysing in this volume (cf. Henning et al. 1999; Rivers et the galaxy density as a function of the galaxy size, al. 1999; Juraszek 1999; Schroder et al. 1999). In magnitude and/or morphology in combination with the last section, theoretical predictions applied to the foreground extinction has led to the identication the recent deeper sampled galaxy surveys covering of various important large-scale structures and their volumes out to v 10000 s1 are reviewed, to see approximate distances. Although the ZOA has been whether they give new indications of unknown or considerably reduced in this way (to about 10% unsuspected prominent structures in the ZOA. of the sky), this method does not nd galaxies in

᭧Astronomical Society of Australia 1999 1323-3580/99/010053$05.00

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m Figure 1—A comparison of the rich A3627 cluster (AB 1 .5) and the suspected PKS1343 m cluster (AB 12 ) in the GA region. Small dots are optically identied galaxies and large dots galaxies detected in the shallow Multibeam ZOA survey. The top panels display the sky 1 distribution, where the inner circle marks the Abell radius RA =3h50 Mpc, the bottom panel the redshift distribution as a function of distance to the central radio source.

the thickest extinction layers of the Milky Way, i.e. The connection of the Perseus–Pisces superclus- where the optical extinction exceeds 4–5 magnitudes ter across the ZOA to the cluster A569, suspected at Galactic latitudes below b < 5 . by Focardi, Marano & Vettolani in 1984, was con- Redshift follow-ups of well-dened samples are rmed by Chamaraux et al. (1990) and Pantoja et important in mapping the large-scale structures al. (1997). The Perseus–Pisces chain folds back into in redshift space. So far, this has been pursued the ZOA at higher redshifts at (195, 10, 7500), extensively in the Perseus-Pisces (PP) supercluster see Marzke, Huchra & Geller (1996) and Pantoja area and in large parts of the southern ZOA. The et al. (1997). prominent new galaxy structures revealed in this In 1992, Kraan-Korteweg & Huchtmeier un- way are summarised below. Their approximate covered a nearby cluster in Puppis (245, 0, 1500) positions (ordered in Galactic latitude) are given as which was later shown by Lahav et al. (1993) to (`, b, v), with v in units of km s1: contribute a not insignicant component to the Behind the Galactic Bulge at (0.5, 9.5, 8500), peculiar z-motion of the Local Group. Wakamatsu et al. (1994) identied the rich Ophiuchus Kraan-Korteweg et al. (1994) presented evidence cluster (or supercluster) with some evidence of it for a continous lamentary structure extending over being linked to the adjacent slightly more distant 30 on the sky from the Hydra and Antlia clusters Hercules cluster. across the ZOA, intersecting the Galactic Plane at At (`, b) (33, 5 15), Marzke, Huchra (280, 0, 3000). At the same longitudes, they noted & Geller (1996) and Roman et al. (1998) found signicant clustering at 15,000 s1, indicative of evidence for a nearby cluster close to the Local Void a connection between the Horologium and Shapley at 1500 s1, as well as a prominent cluster behind clusters a hundred degrees apart in the sky. the Local Void at 7500 km s1. The nearby cluster Kraan-Korteweg & Woudt (1993) uncovered is independently supported by data from the blind a shallow but extended supercluster in Vela at ZOA HI survey (Henning et al. 1999). (285 , 6 , 6000).

Next to the massive cluster A3627 at the core of i.e. the soft X-ray emission is totally absorbed. the Great Attractor (clustering in the Great Attractor However, extended diuse hard X-ray emission at region is discussed in the next section), Woudt (1998) the position of PKS1343601 has been detected discovered a cluster at (306, 6, 6200) called the with ASCA. The excess ux, kT =39 keV is far Cen–Crux cluster, and a more distant cluster, the too large for it to be associated with a galactic halo Ara cluster at (329, 9, 15000). The latter might surrounding the host galaxy, hence it might be due be connected to the Triangulum–Australis cluster. to the Inverse Compton process—or indicative of emission from a cluster. 2.1.1 Clustering within the Great Attractor Region As this prospective cluster is so heavily obscured, Recent consensus is that the Great Attractor (GA) little data are available to substantiate the existence is probably an extended region ( 40 40 )of of this cluster. In Figure 1, a comparison of the moderately enhanced galaxy density (Lynden-Bell A3627 cluster at (325.3, 7.2, 4882) and a mean 1991; Hudson 1994) centred behind the Galactic extinction in the blue of 1m.5 is compared to the Plane at (`, b, v) (320 , 0 , 4500) (Kolatt, Dekel prospective PKS1343 cluster at (309.7, +1.7, 3872) & Lahav 1995). with an extinction of 12m. The top panel shows both Based on a deep optical galaxy search and sky distributions. One can clearly see that at the low subsequent redshift follow-ups, Kraan-Korteweg et Galactic latitude of the suspected cluster PKS1343, al. (1996) and Woudt (1998) have clearly shown the optical galaxy survey could not retrieve the that the Norma cluster, A3627, at (325.3, 7.2, underlying galaxy distribution, especially not within 4882) is the most massive galaxy cluster in the the Abell radius (the inner circle in the top right GA region known to date and probably marks the panel of Figure 1) of the suspected cluster. If previously unidentied but predicted density-peak at PKS1343601 marks the dynamical centre of the the bottom of the potential well of the GA overdensity. cluster, then the Abell radius, dened as 1.7/0 z, The prominence of this cluster has independently where z is the redshift, corresponds to 2.2 on the been conrmed by ROSAT observations: the Norma sky at the redshift-distance of PKS1343601. cluster ranks as the sixth brightest X-ray cluster in the Interestingly enough, the shallow blind ZOA- sky (Bohringer et al. 1996). It is comparable in size, Multibeam HI survey (Henning et al. 1999) picks richness and mass to the well-known Coma cluster. up a number of prospective cluster members even Redshift-independent distance determinations (RC though the shallow survey is sensitive only to the and IC band Tully–Fisher analysis) of the Norma most HI-rich galaxies at the cluster velocity: over cluster have shown it to be at rest with respect to 60% of the galaxies in the shallow survey with the rest frame of the Cosmic Microwave Background velocities from 3000 to 5000 km s1 lie in the (Woudt 1998). cluster area, i.e. within 13% of the area covered by One cannot, however, exclude the possibility the shallow survey. that other unknown rich clusters reside in the GA The velocity distribution as a function of distance region. Finding a hitherto uncharted, rich cluster of from the cluster centre for the PKS1343601 region galaxies at the heart of the GA would have serious (bottom panel) provides further evidence for the implications for our current understanding of this existence of this cluster. All measured velocities lie massive overdensity in the local Universe. Woudt within a narrow range of the central radio source, (1998) found various indications that PKS1343601, showing a similar distribution as in the Norma the second brightest extragalactic radio source in cluster. One of the rst data cubes from the full the southern sky (f20 cm = 79 Jy; McAdam 1991, sensitivity Multibeam ZOA survey that has been and references therein) might form the centre of yet nished covers the prospective PKS1343 cluster area. another highly obscured rich cluster. A quick inspection gives further support for this At (`, b) (309.7, 1.7), this radio galaxy lies prospective cluster: between 3500

candidates behind the ZOA. However, confusion spiral and HI-rich dwarf galaxies through the deepest with Galactic sources at low Galactic latitudes still extinction layer of the Milky Way. In particular, leaves a considerable ‘ZOA’ of over 10%. Moreover, the results from the deep Multibeam ZOA survey bright spiral and starburst galaxies dominate these will be very exciting as they will trace the galaxy samples, and cluster cores are hardly visible. distribution across the ZOA to a depth of 10,000 The advantage of using the IRAS survey for LSS km s1 (cf. Fig. 2 in Kraan-Korteweg et al. 1998). studies is the homogeneous sky coverage (all data from one instrument), and the various systematic 2.4 Near Infrared Surveys redshift follow-ups, complete to given ux limits, Near infrared (NIR) surveys are sensitive to early- i.e. 2658 galaxies to f60m =1 9 Jy (Strauss et al. type galaxies, tracers of massive groups and clusters 1992), 5321 galaxies to f =12 Jy (Fisher et 60m missed in IRAS and HI surveys, and have little al. 1995), and 15000 galaxies to f60m =0 6Jy confusion with Galactic objects. Moreover, they (Saunders et al. 1999 in prep.). are less aected by absorption than optical surveys. Considerable improvement towards lling the ZOA Here, the recent NIR surveys such as 2MASS has been made through the conrmation of about (Skrutskie et al. 1997) and DENIS (Epchtein 1997) 1000 IRAS galaxy candidates in the ZOA from provide a new tool to probe the ZOA. K-band snapshots (Saunders et al. 1999 in prep.). First results from DENIS data are very promising Using the IRAS survey, dedicated searches for (cf. Schroder et al. 1999, this issue p. 42). They large-scale clustering within the whole ZOA (|b| are complementary to other surveys in the sense 15 ) have been made by Japanese groups (cf. Takata, that they nally uncover early-type galaxies at low Yamada & Saito 1996, for a summary). They used Galactic latitudes (|b| > 1 1.5). Furthermore, a IRAS colour criteria to select galaxy candidates fair fraction (65%) of the heavily obscured spiral which were subsequently veried through visual galaxies detected in blind HI surveys can be re- examination on sky surveys such as the Palomar identied on DENIS images. The combination of HI Observatory Sky Survey (POSS) of the northern data with NIR data allows the study of the peculiar hemisphere and the ESO/SRC (United Kingdom velocity eld via the NIR Tully–Fisher relation Science Research Council) Southern Sky Atlas. ‘in the ZOA’ compared to earlier interpolations Because of their verication procedure, this data- of data ‘adjacent to the ZOA’ and this will, for set suers the same limitations in highly obscured instance, provide important new input for density regions as optical surveys. eld reconstructions in the ZOA (cf. Section 3). Based on redshift follow-ups of this ZOA IRAS galaxy sample, they established various lamentary 2.5 X-ray Surveys features and connections across the ZOA. Most coincide with the structures described in Section The Milky Way is transparent to the hard X-ray 2.1. Both crossings of the Perseus–Pisces arms into emission, i.e. above 0 5–2 0 keV. Rich clusters the ZOA are very prominent—considerably stronger generally are strong X-ray emitters. Hence, the in IRAS compared to optical data—and the Puppis, X-ray surveys such as HEAO-1 and the ROSAT Hydra, Centaurus and A3627 connections are clearly All Sky Survey provide an optimal tool to search visible. They furthermore identied a new structure: for clusters of galaxies at low Galactic latitude. So the Cygnus–Lyra lament at (60 90, 0, 4000). far, this possibility has not yet been pursued in any systematic way, even though a large number of X-ray 2.3 HI Surveys bright clusters (e.g. PKS0745191) are located at In the regions of the highest obscuration and low Galactic latitude (cf. Fabian 1994). This tool infrared confusion, the Galaxy is fully transparent is of particular interest because it can unveil cores to the 21-cm line radiation of neutral hydrogen. of clusters as, for instance, the suspected cluster surrounding PKS1343601. These are dominated HI-rich galaxies can readily be found at lowest latitudes through the detection of their redshifted by early-type galaxies and therefore dicult to trace 21-cm emission. Early-type galaxies generally are in other wavelengths. very gas-poor and will not be uncovered in HI surveys. Furthermore, low-velocity extragalactic 3 Theoretical Reconstructions sources (blue- and red-shifted) within the strong Various mathematical methods exist to reconstruct Galactic HI emission will be missed and—because of the galaxy distribution in the ZOA without having baseline ripple—galaxies close to radio continuum access to direct observations. One possibility is sources may also be missed. the expansion of galaxy distributions adjacent to As demonstrated by the rst results from system- the ZOA into spherical harmonics to recover the atic HI surveys (cf. Henning et al. 1999, Rivers et structures in the ZOA, either with 2-dimensional al. 1999 and Juraszek 1999—see pp. 35, 38 and 48), catalogues (sky positions) or 3-dimensional data sets these surveys clearly are very powerful in tracing (redshift catalogues).

A statistical method to reconstruct structures et al. (1996) unveiled the cluster A3627 as being behind the Milky Way is the Wiener Filter (WF), very rich and massive and at the correct distance. developed explicitly for reconstructions of corrupt It hence is the most likely candidate for the central or incomplete data (cf. Lahav 1994; Homan 1994). density peak of the GA. Using the WF in combination with linear theory allows the determination of the real-space density 3.2 Deeper Reconstructions of galaxies, as well as their velocity and potential Recent reconstructions have been applied to denser elds. galaxy samples covering larger volumes (8–10,000 The POTENT analysis developed by Bertschinger km s1) with smoothing scales of the order of 500 km & Dekel (1989) can reconstruct the potential eld s1 (compared to 1200 km s1). It therefore seemed (mass distribution) from peculiar velocity elds in of interest to see whether these reconstructions nd the ZOA (Kolatt, Dekel & Lahav 1995). The evidence for unknown major galaxy structures at reconstruction of the potential elds versus density higher redshifts. elds have the advantage that they can locate hidden The currently most densely-sampled, well-dened overdensities (their signature) even if ‘unseen’. galaxy redshift catalogue is the Optical Redshift Because of the sparsity of data and the heavy Survey (Santiago et al. 1995). However, this catalogue smoothing applied in all these methods, only is limited to |b|20 and the reconstructions (cf. structures on large scales (superclusters) can be Baker et al. 1998) within the ZOA are strongly mapped. Individual (massive) nearby galaxies that inuenced by 12 Jy IRAS Redshift Survey data and can perturb the dynamics of the Universe quite locally a mock galaxy distribution in the inner ZOA. We (the vicinity of the Local Group or its barycentre) will therefore concentrate on reconstructions based will not be uncovered in this manner. But even on the 12 Jy IRAS Redshift Survey only. In the if theoretical methods can outline LSS accurately, following, the structures identied in the ZOA by the observational eorts do not become superuous. (a) Webster, Lahav & Fisher (1997, WLT) using The comparison of the real galaxy distribution WF plus spherical harmonics and linear theory and g(r), from e.g. complete redshift surveys, with the (b) Bistolas (1998) who applied a WF plus linear peculiar velocity eld v(r) will lead to an estimate of theory and non-constrained realisations on the 12 the density and biasing parameter (0 6/b) through Jy IRAS Redshift Survey will be discussed and the equation compared to observational data. Fig. 2 in Webster et al. displays the reconstructed density elds on 06 shells of 2000, 4000, 6000 and 8000 km s1; Fig. ∇v(r)= g(r), b 52 in Bistolas displays the density elds in the ZOA from 1500 to 8000 km s1 in steps of 500 km cf. Strauss & Willick (1995) for a detailed review. s1. The WLF reconstructions clearly nd the recently 3.1 Early Predictions identied nearby cluster at (33, 5 15, 1500), Early reconstructions on relatively sparse data galaxy whereas Bistolas reveals no clustering in the region catalogues have been performed within volumes out of the Local Void out to 4000 km s1. At the same to v 5000 km s1. Despite heavy smoothing, longitudes, the clustering at 7500 km s1 is seen by they have been quite successful in pinpointing a Bistolas, but not by Webster et al. The Perseus– number of important features: Pisces chain is strong in both reconstructions, and Scharf et al. (1992) applied spherical harmonics the second Perseus–Pisces arm—which folds back at to the 2-dimensional IRAS PSC and noted a ` 195—is clearly conrmed. Both reconstructions prominent cluster behind the ZOA in Puppis nd the Perseus–Pisces complex to be very extended (` 245) which was simultaneously discovered in space, i.e. from 3500 km s1 out to 9000 km 1 as a nearby cluster through HI-observations of s . Whereas the GA region is more prominent obscured galaxies in that region by Kraan-Korteweg compared to Perseus–Pisces in the Webster et al. & Huchtmeier (1992). reconstructions, the signal of the Perseus–Pisces Homan (1994) predicted the Vela supercluster complex is considerably stronger than the GA in at (280, 6, 6000) using 3-dimensional WF recon- Bistolas where it does not even reveal a well-dened structions on the IRAS 19 Jy redshift catalogue central density peak. Both reconstructions nd no (Strauss et al. 1992), which was observationally evidence for the suspected PKS1343 cluster but its discovered just a bit earlier by Kraan-Korteweg & signal could be hidden in the central (A3627) density Woudt (1993). peak due to the smoothing. While the Cygnus–Lyra Using POTENT analysis, Kolatt et al. (1995) complex (60 90, 0, 4000) discovered by Takata, predicted the centre of the Great Attractor overdensity— Yamada & Saito (1996) stands out clearly in Bistolas, its density peak—to lie behind the ZOA at it is not evident in Webster et al. Both reconstructions (320, 0, 4500). Shortly thereafter, Kraan-Korteweg nd a strong signal for the Vela SCL (285, 6, 6000),

labelled as HYD in WLF. The Cen–Crux cluster Korteweg, ASP Conf. Ser. 67 (San Franciso: ASP), identied by Woudt (1998) is evident in Bistolas p. 76 though less distinct in Webster et al. A suspected Fisher, K. B., Davis, M., Strauss, M. A., et al. 1995, ApJS, 100, 69 connection at (`, v) (345 , 6000)—cf. Fig. 2 in Focardi, P., Marano, B., & Vettolani, G. 1984, A&A, 136, Kraan-Korteweg, Koribalski & Juraszek (1998)— 178 is supported by both methods. The Ophiuchus Henning, P. A., Staveley-Smith, L., Kraan-Korteweg, R. C., cluster just becomes visible in the most distant & Sadler, E. M. 1999, PASA, 16, 35 reconstruction shells (8000 km s1). Homan, Y. 1994, in Unveiling Large-Scale Structures behind the Milky Way, ed. C. Balkowski & R. C. Kraan-Korteweg, 3.3 Conclusions ASP Conf. 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