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: Acknowledgements Title/Abstract Page: An Overview of Uncovered Previous Section: Observational Surveys in the

Contents Page: Volume 16, Number 1

Theoretical Reconstructions

Various mathematical methods exist to reconstruct the galaxy distribution in the ZOA without having access to direct observations.

One possibility is the expansion of galaxy distributions adjacent to the ZOA into spherical harmonics to recover the structures in the ZOA, either with 2-dimensional catalogs (sky positions) or 3-dimensional data sets (redshift catalogs).

A statistical method to reconstruct structures behind the Milky Way is the Wiener Filter (WF), developed explicitly for reconstructions of corrupt or incomplete data (cf. Lahav 1994, Hoffman 1994). Using the WF in combination with linear theory allows the determination of the real-space density of galaxies, as well as their velocity and potential fields.

The POTENT analysis developed by Bertschinger & Dekel (1989) can reconstruct the potential field (mass distribution) from peculiar velocity fields in the ZOA (Kolatt et al. 1995). The reconstruction of the potential fields versus density fields have the advantage that they can locate hidden overdensities (their signature) even if ``unseen''.

Because of the sparsity of data and the heavy smoothing applied in all these methods, only structures on large scales (superclusters) can be mapped. Individual (massive) nearby galaxies that can perturb the dynamics of the Universe quite locally (the vicinity of the Local Group or its barycenter) will not be uncovered in this manner. But even if theoretical methods can outline LSS accurately, the observational efforts do not become superfluous. The comparison of the real galaxy distribution $\delta_{g}$ (r), from e.g., complete redshift surveys, with the peculiar velocity field v(r) will lead to an estimate of the density and biasing parameter ( $\Omega^{0.6}/{b}$) through the equation

$$\nabla \cdot {\bf v (r)} = - {{\Omega^{0.6}}\over{b}} \, \, \delta_g ({\bf r}),$$ (1)

cf. Strauss & Willick (1995) for a detailed review.

Early Predictions

Early reconstructions on relatively sparse data galaxy catalogs have been performed within volumes out to $v \le$ 5000 kms^-1. Despite heavy smoothing, they have been quite successful in pinpointing a number of important features:

$\bullet$ Scharf et al. (1992) applied spherical harmonics to the 2-dimensional IRAS PSC and noted a prominent cluster behind the ZOA in Puppis ( $\ell \sim 245{^\circ}$) which was simultanously discovered as a nearby cluster through HI-observations of obscured galaxies in that region by Kraan-Korteweg & Huchtmeier (1992).

$\bullet$ Hoffman (1994) predicted the Vela supercluster at ( $280{^\circ}, 6{^\circ}, 6000$) using 3-dimensional WF reconstructions on the IRAS 1.9 Jy redshift catalogue (Strauss et al. 1992), which was observationally discovered just a bit earlier by Kraan-Korteweg & Woudt (1993).

$\bullet$ Using POTENT analysis, Kolatt et al. (1995) predicted the center of the Great Attractor overdensity - its density peak - to lie behind the ZOA at ( $320{^\circ}, 0{^\circ}, 4500$). Shortly thereafter, Kraan-Korteweg et al. (1996) unveiled the cluster A3627 as being very rich and massive and at the correct distance. It hence is the most likely candidate for the central density peak of the GA.

Deeper Reconstructions

Recent reconstructions have been applied to denser galaxy samples covering larger volumes (8-10000 kms^-1) with smoothing scales of the order of 500 kms^-1 (compared to 1200 kms^-1). It therefore seemed of interest to see whether these reconstructions find evidence for unknown major galaxy structures at higher redshifts.

The currently most densely-sampled, well-defined galaxy redshift catalog is the Optical Redshift Survey (Santiago et al. 1995). However, this catalog is limited to $\vert b\vert \ge 20{^\circ}$ and the reconstructions (cf. Baker et al. 1998) within the ZOA are strongly influenced by 1.2 Jy IRAS Redshift Survey data and a mock galaxy distribution in the inner ZOA. We will therefore concentrate on reconstructions based on the 1.2 Jy IRAS Redshift Survey only. In the following, the structures identified in the ZOA by (a) Webster et al. (1997) using WF plus spherical harmonics and linear theory and (b) Bistolas (1998) who applied a WF plus linear theory and non-constrained realizations on the 1.2 Jy IRAS Redshift Survey will be discussed and compared to observational data. Fig. 2 in Webster et al. displays the reconstructed density fields on shells of 2000, 4000, 6000 and 8000 kms^-1; Fig. 5.2 in Bistolas displays the density fields in the ZOA from 1500 to 8000 kms^-1 in steps of 500 kms^-1.

The WLF reconstructions clearly find the recently identified nearby cluster at ( $33{^\circ}, 5{^\circ}-15{^\circ}$, 1500), whereas Bistolas reveals no clustering in the region of the Local Void out to 4000 kms^-1. At the same longitudes, the clustering at 7500 kms^-1 is seen by Bistolas, but not by Webster et al.. The Perseus-Pisces chain is strong in both reconstructions, and the 2nd Perseus-Pisces arm - which folds back at $\ell\sim 195{^\circ}$ - is clearly confirmed. Both reconstructions find the Perseus-Pisces complex to be very extended in space, i.e., from 3500 kms^-1 out to 9000 kms^-1. Whereas the GA region is more prominent compared to Perseus-Pisces in the Webster et al. reconstructions, the signal of the Perseus-Pisces complex is considerably stronger than the GA in Bistolas where it does not even reveal a well-defined central density peak. Both reconstructions find no evidence for the suspected PKS1343 cluster but its signal could be hidden in the central (A3627) density peak due to the smoothing. While the Cygnus-Lyra complex ( $60{^\circ}-90{^\circ}, 0{^\circ}, 4000$) discovered by Takata et al. (1996) stands out clearly in Bistolas, it is not evident in Webster et al.. Both reconstructions find a strong signal for the Vela SCL ( $285{^\circ}, 6{^\circ}, 6000$), labelled as HYD in WLF. The Cen-Crux cluster identified by Woudt (1998) is evident in Bistolas though less distinct in Webster et al.. A suspected connection at ( $\ell,v) \sim (345{^\circ}, 6000$) - cf. Fig. 2 in Kraan-Korteweg et al. (1998) - is supported by both methods. The Ophiuchus cluster just becomes visible in the most distant reconstruction shells (8000 kms^-1).

Conclusions

Not all reconstructions find the same features, and when they do, the prominence of the density peaks as well as their locations in space do vary considerable. At velocities of $\sim 4000$ kms^-1 most of the dominant structures lie close to or within the ZOA while at larger distances, clusters and voids seem to be more homogeneously distributed over the whole sky. Out to 8000 kms^-1 none of the reconstructions predict any major structures which are not mapped or suggested from observational data. So no major surprises seem to remain hidden in the ZOA. The various multi-wavelength explorations of the Milky Way will soon be able to verify this. Still, the combination of both the reconstructed potential fields and the observationally mapped galaxy distribution will lead to estimates of the cosmological parameters $\Omega$ and b.

Next Section: Acknowledgements Title/Abstract Page: An Overview of Uncovered Previous Section: Observational Surveys in the

Contents Page: Volume 16, Number 1