An Overview of Uncovered and Suspected Large-Scale Structures behind the Milky Way
Renée C. Kraan-Korteweg , Patrick A. Woudt, PASA, 16 (1), in press.

Next Section: Theoretical Reconstructions Title/Abstract Page: An Overview of Uncovered Previous Section: Introduction

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Observational Surveys in the Zone of Avoidance

Optical Surveys

Systematic optical galaxy catalogs are generally limited to the largest galaxies (typically with diameters D $\mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}1\hbox{$^\prime$}$, e.g., Lauberts 1982). These catalogs become, however, increasingly incomplete as the dust thickens, creating a ``Zone of Avoidance'' in the distribution of galaxies of roughly 25% of the sky. Systematically deeper searches for partially obscured galaxies - down to fainter magnitudes and smaller dimensions compared to existing catalogs - were performed with the aim of reducing this ZOA. These surveys are not biased with respect to any particular morphological type.

Most of the ZOA has meanwhile been surveyed (cf. Fig. 1, in Kraan-Korteweg 1998), revealing many galaxy overdensities uncorrelated with the patchy, optical extinction distribution. Analysing the galaxy density as a function of the galaxy size, magnitude and/or morphology in combination with the foreground extinction has led to the identification of various important large-scale structures and their approximate distances. Although the ZOA has been considerably reduced in this way (to about 10% of the sky), this method does not find galaxies in the thickest extinction layers of the Milky Way, i.e., where the optical extinction exceeds 4 - 5 magnitudes, respectively at Galactic latitudes below $b \mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$<$}}}\pm 5{^\circ}$.

Redshift follow-ups of well-defined samples are important in mapping the large-scale structures in redshift space. So far, this has been pursued extensively in the Perseus-Pisces (PP) supercluster area and in large parts of the southern ZOA. The prominant new galaxy structures revealed in this way are summarised below. Their approximate positions (ordered in Galactic latitude) are given as ($\ell, b, v$), with v in units of kms^-1:

$\bullet$ Behind the Galactic Bulge at ( $0\hbox{$.\!\!^\circ$}5, 9\hbox{$.\!\!^\circ$}5, 8500$), Wakamatsu et al. (1994) identified the rich Ophiuchus cluster (or supercluster) with some evidence of it being linked to the adjacent slightly more distant Hercules cluster.

$\bullet$ At ($\ell, b$)$\sim$ ( $33{^\circ}, 5{^\circ}-15{^\circ}$), Marzke et al. (1996) and Roman et al. (1998) found evidence for a nearby cluster close to the Local Void at 1500 kms^-1, as well as a prominent cluster behind the Local Void at 7500 kms^-1. The nearby cluster is independently supported by data from the blind ZOA HI-survey (Henning et al., this volume).

$\bullet$ The connection of the Perseus-Pisces supercluster across the ZOA to the cluster A569, suspected by Focardi et al. in 1984, was confirmed by Chamaraux et al. (1990) and Pantoja et al. (1997). The Perseus-Pisces chain folds back into the ZOA at higher redshifts at ( $195{^\circ}, -10{^\circ}, 7500$), Marzke et al. (1996), Pantoja et al. (1997).

$\bullet$ In 1992, Kraan-Korteweg & Huchtmeier uncovered a nearby cluster in Puppis ( $245{^\circ}, 0{^\circ}, 1500$) which was later shown by Lahav et al. (1993) to contribute a not insignificant component to the peculiar z-motion of the Local Group.

$\bullet$ Kraan-Korteweg et al. (1994) presented evidence for a continous filamentary structure extending over $30{^\circ}$ on the sky from the Hydra and Antlia clusters across the ZOA, intersecting the Galactic Plane at ( $280{^\circ}, 0{^\circ}, 3000$). At the same longitudes, they noted significant clustering at $\sim$ 15000 kms^-1, indicative of a connection between the Horologium and Shapley clusters a hundred degrees apart in the sky.

$\bullet$ Kraan-Korteweg & Woudt (1993) uncovered a shallow but extended supercluster in Vela at ( $285{^\circ}, 6{^\circ}, 6000$).

$\bullet$ Next to the massive cluster A3627 at the core of the Great Attractor (clustering in the Great Attractor region is discussed in the next section), Woudt (1998) discovered a cluster at ( $306{^\circ}, 6{^\circ},6200$) called the Cen-Crux cluster, and a more distant cluster, the Ara cluster at ( $329{^\circ}, -9{^\circ}, 15000$). The latter might be connected to the Triangulum-Australis cluster.

Clustering within the Great Attractor Region?

Recent consensus is that the Great Attractor (GA) is probably an extended region ( $\sim 40{^\circ}{\rm x} 40{^\circ}$) of moderately enhanced galaxy density (Lynden-Bell 1991, Hudson 1994) centered behind the Galactic Plane at ($\ell, b, v$) $\sim$ ( $320{^\circ}, 0{^\circ}, 4500$) (Kolatt et al. 1995).

Based on a deep optical galaxy search and subsequent redshift follow-ups, Kraan-Korteweg et al. (1996) and Woudt (1998) have clearly shown that the Norma cluster, A3627, at ( $325\hbox{$.\!\!^\circ$}3, -7\hbox{$.\!\!^\circ$}2$, 4882) is the most massive galaxy cluster in the GA region known to date and probably marks the previously unidentified but predicted density-peak at the bottom of the potential well of the GA overdensity. The prominence of this cluster has independently been confirmed by ROSAT observations: the Norma cluster ranks as the 6^th brightest X-ray cluster in the sky (Böhringer et al. 1996). It is comparable in size, richness and mass to the well-known Coma cluster. Redshift-independent distance determinations (R$_{\rm C}$ and I$_{\rm C}$ band Tully-Fisher analysis) of the Norma cluster have shown it to be at rest with respect to the rest frame of the Cosmic Microwave Background (Woudt 1998).

One cannot, however, exclude the possibility that other unknown rich clusters reside in the GA region. Finding a hitherto uncharted, rich cluster of galaxies at the heart of the GA would have serious implications for our current understanding of this massive overdensity in the local Universe. Woudt (1998) found various indications that PKS1343-601, the second brightest extragalactic radio source in the southern sky (f_20cm = 79 Jy, McAdam 1991, and references therein) might form the center of yet another highly obscured rich cluster.

At ( $\ell, b) \sim (309\hbox{$.\!\!^\circ$}7, 1\hbox{$.\!\!^\circ$}7$), this radio galaxy lies behind an obscuration layer of about 12 magnitudes of extinction in the B-band, as estimated from the DIRBE/IRAS extinction maps (Schlegel et al. 1998). Its observed diameter of 28 arcsec in the Gunn-z filter (West & Tarenghi 1989) translates into an extinction-corrected diameter of 232 arcsec (following Cameron 1990). With a recession velocity of v = 3872 kms^-1 (West & Tarenghi 1989) this galaxy can be identified with a giant elliptical galaxy. PKS1343-601 has recently been observed in the X-ray band with the ASCA satellite (Tashiro et al. 1998). This source is not detected in the ROSAT All Sky Survey due to the large foreground extinction, i.e. the soft X-ray emission is totally absorbed. However, extended diffuse hard X-ray emission at the position of PKS1343-601 has been detected with ASCA. The excess flux, kT = 3.9 keV, is far too large for it being associated with a galactic halo surrounding the host galaxy, hence it might be due to the Inverse Compton process - or indicative of emission from a cluster. As this prospective cluster is so heavily obscured, little data are available to substantiate the existence of this cluster. In Figure 1, a comparison of the A3627 cluster at ( $325\hbox{$.\!\!^\circ$}3, -7\hbox{$.\!\!^\circ$}2, 4882$) and a mean extinction in the blue of $1\hbox{$.\!\!^{\rm m}$}5$ is compared to the prospective PKS1343 cluster at ( $309\hbox{$.\!\!^\circ$}7, +1\hbox{$.\!\!^\circ$}7, 3872$) with an extinction of 12$^{\rm m}$. The top panel shows both sky distributions. One can clearly see that at the low Galactic latitude of the suspected cluster PKS1343, the optical galaxy survey could not retrieve the underlying galaxy distribution, especially not within the Abell radius (the inner circle in the top right panel of Figure 1) of the suspected cluster. If PKS1343-601 marks the dynamical center of the cluster, then the Abell radius, defined as 1 $\hbox{$.\mkern-4mu^\prime$}$7/z where z is the redshift, corresponds to 2.2${^\circ}$ on the sky at the redshift-distance of PKS1343-601.

Interestingly enough, the shallow blind ZOA-Multibeam HI survey (Henning et al., this volume) picks up a number of prospective cluster members even though the shallow survey is sensitive only to the most HI-rich galaxies at the cluster velocity: over 60% of the galaxies in the shallow survey with velocities from 3000 to 5000 kms^-1 lie in the cluster area, i.e., within 13% of the area covered by the shallow survey.

Figure: A comparison of the rich A3627 cluster (A $_{\rm B}\sim 1\hbox{$.\!\!^{\rm m}$}5$) and the suspected PKS1343 cluster (A $_{\rm B}\sim 12^{\rm m}$) in the GA region. Small dots are optically identified galaxies, large dots galaxies detected in the shallow Multibeam ZOA survey. The top panels display the sky distribution,where the inner circle marks the Abell radius R$_{\rm A}$ = 3 h₅₀^-1 Mpc, the bottom panel the redshift distribution as a function of distance to the central radio source.

The velocity distribution as a function of distance from the cluster center for the PKS1343-601 region (bottom panel) provides further evidence for the existence of this cluster. All measured velocities lie within a narrow range of the central radio source, showing a similar distribution as in the Norma cluster. One of the first data cubes from the full sensitivity Multibeam ZOA survey that has been finished covers the prospective PKS1343 cluster area. A quick inspection gives further support for this prospective cluster: between 3500 < v < 4000 kms^-1 a statistically significant peak is evident in the velocity distribution (Juraszek et al., 1998, priv. commun.).

We will image the prospective cluster within its Abell radius in the near infrared (Woudt et al. in progress). These observations will allow us to determine whether or not PKS1343-601 is embedded in a centrally condensed overdensity of galaxies, comparable to the rich and massive Norma cluster.

IRAS Surveys

The IRAS Point-Source Catalog (PSC) has been exploited in the last decade to identify galaxy candidates behind the ZOA. However, confusion with Galactic sources at low Galactic latitudes still leave a considerable ``ZOA'' of over 10%. Moreover, bright spiral and starburst galaxies dominate these samples, and cluster cores are hardly visible.

The advantage of using the IRAS survey for LSS studies are the homogeneous sky coverage (all data from one instrument), and the various systematic redshift follow-ups, complete to given flux limits, i.e., 2658 galaxies to f $_{60\mu m} = 1.9$ Jy (Strauss et al. 1992), 5321 galaxies to f $_{60\mu m} = 1.2$ Jy (Fisher et al. 1995), and $\sim$ 15000 galaxies to f $_{60\mu m} = 0.6$ Jy (Saunders et al., in prep.).

Considerable improvement towards filling the ZOA has been made through the confirmation of about 1000 IRAS galaxy candidates in the ZOA from K-band snapshots (Saunders et al. in prep.).

Using the IRAS survey, dedicated searches for large-scale clustering within the whole ZOA ( $\vert b\vert \le 15{^\circ}$) have been made by Japanese groups (cf. Takata et al. 1996, for a summary). They used IRAS color criteria to select galaxy candidates which were subsequently verified through visual examination on sky surveys such as the Palomar Observatory Sky Survey (POSS) of the northern hemisphere and the ESO/SRC (United Kingdom Science Research Council) Southern Sky Atlas. Because of their verification procedure, this data-set suffers the same limitations in highly obscured regions as optical surveys.

Based on redshift follow-ups of this ZOA IRAS galaxy sample, they established various filamentary features and connections across the ZOA. Most coincide with the structures described in section 2.1. Both crossings of the Perseus-Pisces arms into the ZOA are very prominent - considerably stronger in IRAS compared to optical data - and the Puppis, Hydra, Centaurus and A3627 connections are clearly visible. They furthermore identified a new structure: the Cygnus-Lyra filament at ( $60{^\circ}-90{^\circ}, 0{^\circ}, 4000$).

HI Surveys

In the regions of the highest obscuration and infrared confusion, the Galaxy is fully transparent to the 21-cm line radiation of neutral hydrogen. HI-rich galaxies can readily be found at lowest latitudes through the detection of their redshifted 21-cm emission. Early-type galaxies generally are very gas-poor and will not be uncovered in HI surveys. Furthermore, low-velocity extragalactic sources (blue- and red-shifted) within the strong Galactic HI emission will be missed, and - because of baseline ripple - galaxies close to radio continuum sources may also be missed.

As demonstrated by the first results from systematic HI-surveys (cf. Henning et al., Rivers et al., and Juraszek et al. in this volume), these surveys clearly are very powerful in tracing spiral and HI-rich dwarf galaxies through the deepest extinction layer of the Milky Way. In particular, the results from the deep Multibeam ZOA survey will be very exciting as they will trace the galaxy distribution across the ZOA to a depth of $\sim$ 10000 kms^-1 (cf. Fig. 2 in Kraan-Korteweg et al., 1998).

Near Infrared Surveys

Near infrared (NIR) surveys are sensitive to early-type galaxies, tracers of massive groups and clusters missed in IRAS and HI surveys, and have little confusion with Galactic objects. Moreover, they are less affected by absorption than optical surveys. Here, the recent NIR surveys such as 2MASS (Skrutskie et al. 1997) and DENIS (Epchtein 1997) provide a new tool to probe the ZOA.

First results from DENIS data are very promising (cf. Schröder et al., this volume). They are complementary to other surveys in the sense that they finally uncover early-type galaxies at low Galactic latitudes ( $\vert b\vert \mathrel{\hbox{\rlap{\hbox{\lower4pt\hbox{$\sim$}}}\hbox{$>$}}}1 - 1\hbox{$.\!\!^\circ$}5$). Furthermore, a fair fraction ($\sim$ 65%) of the heavily obscured spiral galaxies detected in blind HI surveys can be reidentified on DENIS images. The combination of HI data with NIR data allows the study of the peculiar velocity field via the NIR Tully-Fisher relation ``in the ZOA'' compared to earlier interpolations of data ``adjacent to the ZOA'' and this will, for instance, provide important new input for density field reconstructions in the ZOA (cf. section 3).

X-ray Surveys

The Milky Way is transparent to the hard X-ray emission, i.e., above 0.5-2.0 keV. Rich clusters generally are strong X-ray emitters. Hence, the X-ray surveys such as HEAO-1 and the ROSAT All Sky Survey provide an optimal tool to search for clusters of galaxies at low Galactic latitude. So far, this possibility has not yet been pursued in any systematic way, even though a large number of X-ray bright clusters (e.g., PKS0745-191) are located at low Galactic latitude (cf. Fabian 1994). This tool is of particular interest because it can unveil cores of clusters as, for instance, the suspected cluster surrounding PKS1343-601. These are dominated by early-type galaxies and therefore difficult to trace in other wavelengths.

Next Section: Theoretical Reconstructions Title/Abstract Page: An Overview of Uncovered Previous Section: Introduction

Contents Page: Volume 16, Number 1