Annu. Rev. Astron. Astrophys. 1988. 26: 245-294 Copyright © 1988 by Annual Reviews. All rights reserved

2.1.2. MATERIAL CONTENT OF VOIDS

A recent review by Oemler (126b) contains information in addition to that provided below.

The following evidence supports the hypothesis that the Coma and Hercules voids are actually empty of matter: (a) Bothun et al. (30) studied the 4624 galaxies with known redshifts listed in the Nilson UGC catalog (123a). The sky was subdivided into cells of width 10° in declination and length 4^h. For each of these cells, a histogram of Doppler velocity for galaxies within each of three surface brightness categories was constructed. The three velocity histograms had similar shapes. More specifically, there was no tendency for redshift voids to be filled in by low-surface-brightness galaxies (see Section 3.1.2 for additional statistical evidence). (b) Thuan et al. (193) obtained a nearly complete sample of redshifts for 58 low-surface-brightness galaxies (the majority with m_p > 15.5) that are classified as dwarf, irregular, or Magellanic irregular in the UGC and that are located in the region of the sky for which the CfA redshift survey of galaxies with m_p 15.5 is complete (Figure 4a). Thuan et al. found that the low-surface-brightness dwarf galaxies lie on the structures delineated by the normal high-surface-brightness galaxies. They do not fill in the voids. (c) The Coma void is evident in the three-dimensional distribution of IRAS infrared galaxies (A. Yahil, private communication, 1987). (d) With the Green Bank 300-ft telescope, Krumm & Brosch (109) searched for neutral hydrogen clouds in roughly 7% of the Perseus-Pisces void and 19% of the Hercules void. No clouds were detected, and the detection threshold was sufficient for Krumm & Brosch to conclude that voids do not contain a cosmologically significant population of HI protogalaxies. Independent confirmation of this result, at about three times lower sensitivity, was recently obtained from observations with the Dwingeloo telescope by Hulsbosch (94a). Some suspected signals worthy of follow-up checks were identified in the latter study.

Although measured redshifts of the 22 surveyed IRAS galaxies within a region covering one-sixth of the solid angle of the Boötes void indicate that not one lies within the void (206a), the following results indicate that the Boötes void is not empty. (a) Superseding anticipatory results of Balzano & Weedman (20), Moody et al. (120a) discovered several emission-line galaxies (which typically, but not always, spectroscopically resemble H II regions with narrow lines and moderately weak stellar continua) in the Boötes void, bringing the total number of known emission-line galaxies in this void to eight. (b) Brosch & Gondhalekar (32) report the detection of absorption lines in the ultraviolet spectra of background quasars at the redshifts of the voids in Boötes and Perseus-Pisces; they state that the detection of metal lines suggests chemical enrichment of gas in the voids in support of the explosion-dominated model of formation of galaxies (98, 133). Additional discussions and interpretations of these observations are found in (33, 35, 136 - 138).