Annu. Rev. Astron. Astrophys. 1994. 32: 531-590 Copyright © 1994 by Annual Reviews. All rights reserved

8. THE BLACK HOLE SCENARIO

One of the most important signatures of the black hole scenario would be the infrared/submillimeter background generated by the stellar precursors. In Section 5.1 we discussed a general constraint on _*(M), which depended only on the fact that the background light must appear in some waveband. Here we discuss more precise constraints for VMOs, exploiting the fact that we can predict the waveband in this case very exactly. The calculation can be extended to cover the mass range below 100 M, but that range may be excluded by nucleosynthetic constraints anyway. We also consider the generation of gravitational radiation by VMO or SMO black holes.

8.1 Background Light Observations

The detection of cosmological background radiation in the IR and submillimeter bands is difficult because of foregrounds from scattered zodiacal light (ZL), interplanetary dust (IPD), and interstellar dust (ISD). Estimates of these competing backgrounds are shown in Figure 6, where the background light intensity I() has been expressed in critical density units by defining a quantity _R() 4 I() / c³ _crit. One sees that there are minima at around 4 µ, 100 µ, and 400 µ, so these are the best "windows" in which to search for an extragalactic background. Although positive detections have been claimed in all of these windows, none has been subsequently confirmed, so only upper limits on _R() are currently available; we begin by summarizing these. For comparison, the CMB peaks at _peak = 1400 µ with a density _R = 2 × 10^-5h^-2.

Figure 6. Comparison of the observational constraints on the extragalactic background radiation density from DIRBE, FIRAS, IRAS, Nagoya-Berkeley (NB), Malsumoto et al (MAM), and Noda et al with the background expected in the VMO scenario for different dust abundances. Also shown are the CMB, the local foregrounds, and the background from galaxies, accreting 106 M halo black holes, and 0.08 M halo BDs.

The FIRAS results (Mather et al 1990, 1994) imply that the CMB is so well fit by a black-body spectrum that any extra background must have an intensity less than 0.03% of the CMB density over the range 500-5000 µ. This implies _R() < 6 × 10^-9 h^-2( / _peak)^-1. The DIRBE results at the south ecliptic pole (Hauser et al 1991) give upper limits in the J, K, L, M, 12 µ, 25 µ, 60 µ, 100 µ, 120-200 µ and 200-300 µ bands. However, the limits indicated in Figure 6 are very conservative since they do not include any subtraction for the foreground backgrounds from interstellar and interplanetry dust. Careful modeling of these foreground contributions may improve the limits. Figure 6 includes the limits derived by Oliver et al (1992) by using IRAS and DIRBE data in conjunction with detailed dust models. It also shows the limits obtained from an analysis (Lange et al 1991) of the Nagoya-Berkeley rocket data (Matsumoto et al 1988a). At one stage IRAS data seemed to indicate a 100 µ background with _R(100 µ) = 3 × 10^-6h^-2 (Rowan-Robinson 1986) but this is inconsistent with the DIRBE results.

The DIRBE and IRAS limits are very weak around 12 µ and 25 µ because the interplanetary dust emission is so large. In the near-IR, a Japanese rocket experiment (Matsumoto et al 1988b) gave a limit _R(1-5 µ) < 3 × 10^-5 h^-2 with the possible detection of a "line" at 2.2 µ with _R(2.2 µ) = 3 × 10^-6h^-2. However, this claim was always controversial because of the problem of subtracting starlight and rocket exhaust. Recent observations by Noda et al (1992) give _R(1.6-4.7 µ) < 3 × 10^-6h^-2, which seems to exclude such a line.