CONSTRAINTS ON THE ABUNDANCE OF HIGHLY IONIZED PROTOCLUSTER REGIONS FROM THE ABSENCE OF LARGE VOIDS IN THE Lyα FOREST

摘要

Energetic feedback processes during the formation of galaxy clusters may have heated and ionized the majority of the intergalactic gas in protocluster regions. When such a highly ionized superbubble falls along the sight line to a background quasar, it would be seen as a large void with little or no absorption in the Lyα forest. We examine the spectra of 137 quasars in the Sloan Digital Sky Survey to search for such voids and find no clear evidence of their existence. The size distribution of voids in the range 5 A (approx ＜ ) Δλ (approx ＜ ) 70 A (corresponding to physical sizes of 3 h~(1) (approx ＜ ) L (approx ＜ ) 35 h~(-1) comoving Mpc) is consistent with the standard model for the Lyα forest without additional ionized bubbles. We adapt a physical model for H Ⅱ bubble growth during cosmological reionization to describe the expected size distribution of ionized superbubbles at z ～ 3. This model incorporates the conjoining of bubbles around individual neighboring galaxies. Using the nondetection of voids, we find that models in which the volume filling factor of ionized bubbles exceeds ～20% at z ～ 3 can be ruled out, primarily because they overproduce the number of large (40-50 A) voids. We conclude that any preheating mechanism that explains galaxy cluster observations must avoid heating the low-density gas in the protocluster regions, either by operating relatively recently (z (approx ＜ ) 3) or by increasing entropy primarily in high-density regions.