BMW

The Brera Multi-Wavelet Survey is a project (headquartered at the Osservatorio di Brera outside Milan, Italy) that has analyzed thousands of ROSAT HRI exposures and measured the brightness and sizes of tens of thousands of sources. From these, we have selected a sample of extended sources for optical follow-up. The first level of follow-up is the digitized Palomar sky survey, a (nearly) all sky survey using photographic plates. Individual nearby galaxies and clusters are detected in this manner, leaving only the distant cluster sample, composed of some 250 cluster candidates. We are observing these using both imaging and spectroscopic facilities, with the goal of measuring the evolution of the x-ray luminosity function--that is, how the number of X-ray luminous clusters of galaxies varies over time. This is a diagnostic of the matter density of the Universe.

This is a color image of one of the lower redshift cluster candidates from the BMW survey. The cluster galaxies (which appear yellow in this color mapping) are concentrated near the center of the image. The central (cD) galaxy at the center is surrounded by tangentially stretched blue galaxies--gravitational lensing arcs! This cluster has a spectroscopically confirmed redshift (z=0.34). The field of view of this image is approximately 5 arcminutes by 5 arcminutes, which corresponds to roughly1/h Mpc at the redshift of the cluster. This image was made by combining R and I band images taken at the TNG telescope on La Palma.

The Brera Multi-Wavelet survey (BMW)

Astronomical observations can often tell us much more than what they were planned to study. In particular, when imaging a part of the sky to study one object, you obtain information about other objects at positions in the sky near the original science. This opens up the possibility of serendipitous science. Serendipitous observations have some obvious limitations. Because you do not get to choose the sightlines, you cannot plan beforehand which objects to observe, and so it is not very useful for studying a preselected list of objects. The great power of the serendipitous observations comes from their ability to tell you about the statistics, or populations, of objects. In effect, you are taking a random sampling of the sky (as long as you are careful about which observations you choose). So quantities such as the abundance or the spatial correlation of sources can be measured very accurately. This is particularly important for X-ray astronomy, because access to X-ray observatories has always lagged behind optical or radio astronomy (this is because X-rays are absorbed by the atmosphere, and so the observatory has to be in space). Past serendipitous X-ray studies have mostly used the PSPC detector on the ROSAT satellite. This is because the field of view of the PSPC and its sensitivity were the best available until the very recent advent of the Chandra and XMM-Newton missions. However, the majority of the observations taken by ROSAT were taken with another instrument, the HRI (High-Resolution Imager). The HRI has a lower sensitivity and a smaller field of view than the PSPC, so at first glance it would seem inferior for a serendipitous search. However, there are many more HRI fields available, and especially, there are many more extremely long observations in the HRI archive. That, and the superior angular resolution of the HRI, make it a fruitful source for serendipitous searches.

Right is the grayscale image of one of the cluster candidates taken with the TNG telescope on La Palma (the color version is below). Superimposed on it are the X-ray contours from the Rosat HRI imager. The white squares are spectroscopically confirmed cluster galaxies.

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