3.5.3. The Void in Bootes

Identification of very large scale voids can be an ambiguous process. In 1981, Kirshner et al. claimed to have detected a void of unprecedented proportion. This claim was based on the results of 3 pencil beam redshift surveys that were each separated by 35 degrees. In all three fields there was a gap in the redshift distribution at velocities 15,000 km s^-1. A followup redshift survey by Kirshner et al. (1987) of 240 bright galaxies in the region between these three fields revealed that none had a redshift between 12,000 - 19,000 km s^-1. A uniform distribution of galaxies would predict that 31 out of 240 should have redshifts in this range. This region of space is in the constellation Bootes and the redshift gap is known as the Bootes void. The overall size scale of this, the first discovered large void, was 7500 km s^-1, significantly larger than the typical void size seen in the Slice surveys. The question then became, is the Bootes void the largest void there is?

The announcement of a large hole in space issued a challenge to many observers to work extra hard to fill it in. This challenge was met by a variety of workers who probed the void with objective prism surveys to search for emission line galaxies. This region of space was also well surveyed by the IRAS satellite. IRAS measures the integrated dust emission, which is heated by the ambient interstellar radiation field, of galaxies as it peaks in the 20 - 100 micron range of the electromagnetic spectrum. The total amount of energy contained in this emission can be quite large (and in some galaxies it comprises more than 90% of the total energy released) and thus IRAS can detect galaxies out to relatively large distances. Greg Aldering and collaborators have extensively analyzed the IRAS data that is available in the Bootes void area. After many years of investigation, Aldering et al. (1997) have now discovered 53 galaxies within the boundaries of this void as defined by Kirshner et al. (1987). With these data, Aldering et al. have redefined the density profile in the Bootes void and have now shown that the overall characteristics of the void are quite typical of those seen in the Slice surveys. Hence, if larger scale voids than those seen in the CFA Slice exist, they have not been detected yet. This is consistent with the most recent results of the Las Campanas Redshift Survey (LCRS) of the southern survey. The LCRS goes deeper than the CFA surveys to date and the largest void seen in that survey has a diameter of 6000 km s^-1 (Da Costa et al. 1996). Figure 3-12 shows some of the results of the LCRS and the by now familiar pattern of walls and voids.

Figure 3-12: Slice diagrams of a section of the Las Campanas Redshift Survey initiated by Steve Shectman and collaborators (see Da Costa etal 1996). The void filled Universe is quite apparent in this slice and numerous thin walls structures are also evident.