The extensive and high quality absolute photometric data from the COBE DIRBE and FIRAS instruments have permitted the first definitive measurements of the cosmic infrared background radiation in both the near and far infrared and submillimeter spectral ranges. The robustness of these determinations is still modest, limited by the systematic uncertainties associated with discriminating strong foreground radiations from the solar system and Galaxy. In particular, the uncertainty in the amount of scattered and emitted radiation from interplanetary dust is a major remaining source of uncertainty from 1-100 µm wavelength. Significant reduction in that uncertainty may require measurements with instruments in deep space, either out of the ecliptic plane or at several AU from the Sun. The spectral energy distribution of the CIB sugggests a dominant contribution from starlight, with redshifted UV-optical rest-frame emission of about 45 nW m-2 sr-1 observed in the 0.3-3.5 µm range, and dust-absorbed and re-emitted energy of about 16 nW m-2 sr-1 observed at 125-2000 µm. Present limits allow the possibility of significant additional energy in the 3.5-125 µm range (< 60 nW m-2 sr-1). Assuming that star formation is the major source of the observed background, the large amount of energy in the long wavelength background suggests that a substantial amount of star formation activity at early times was embedded in dust, and that a significant fraction of cosmic baryons has been processed in stars. The present observations indicate that the energy density in the 0.3-2000 µm background is about 10% of that in the cosmic microwave background.
Acknowledgments. The author gratefully acknowledges the assistance of Eli Dwek in the preparation of this paper. This work was partially supported by NASA grant NAG5-3899 and by NASA contract NAS 5-26555 to the Association of Universities for Research in Astronomy, Inc.
Michael Werner: Could you comment on the suggestion of an increased contribution to the galactic FIR background from the ionized medium proposed earlier by Boulanger?
Michael Hauser: 1. The analysis of Fixsen et al. (1998) included a method which fitted the data to a component of neutral gas (represented by HI and a quadratic term in HI) and a component tracing ionized gas (represented by the FIRAS CII 158µm map). Their extracted IR background from this method was consistent with that from several other methods. 2. The DIRBE team analysis of the 140 and 240µm backgrounds was done on dark fields where strong limits could be placed on IR emission from ionized gas.
Floyd Stecker: I would point out that the best upper limit we have in
the mid-infrared at
~ 20 µm is ~ 4nWm-2sr-1, which is
obtained from the TeV
-ray data
(Stecker & De Jager, 1997
- see Stecker's article, this volume, for the reference).
Hauser: I am aware of
-ray limits, but chose not
to address them in my limited time since the subject will be addressed
by subsequent speakers.