COBE FIRAS Products

1999 FIRAS Line Emission Products

Fixsen et al. (1999) revisited the 1994 COBE/FIRAS survey of spectral line emission from the Galaxy. The 1999 analysis provides an improvement of approximately a factor of 2 in signal-to-noise ratio (SNR) and fills in some sky coverage gaps. The improved SNR enabled the detection of additional CO lines and the 115.8 um CH line. The 269 um H₂O line was detected in absorption toward the Galactic center.

Provided are Galactic plane spectral line intensities for 19 lines, computed within bins that are 1 deg wide in latitude about the Galactic plane and with a 5 deg spacing in Galactic longitude. These data are stored in a FITS binary table format, with data for each line in its own extension. For each line, the extension contains the line id, Galactic longitudes, the line flux at those positions, and the associated uncertainty. The table below lists the line id and its corresponding FITS extension number. The ordering presents the list of the original 18 lines from the 1994 analysis first, then adds the CO 7-8 transition at the end. Not all of the listed lines are detections: see Fixsen et al. for details.

In a separate FITS file, all-sky maps pixelized in quad-cube format are provided for two high-signal lines: N+ 205 μm and C+ 158 μm. Pixel numbers in the Galactic projection, pixel center coordinates, line fluxes, and 1-sigma noise and gain uncertainties are provided in the first binary table extension for both lines. See both Fixsen et al. 1999 and the FIRAS Explanatory Supplement for details.

1997 Line Emission Maps

The Line Emission Maps cover the entire sky and give the intensity and the associated uncertainty in each of 18 emission lines: CO (115.27, 230.54, 345.80,461.04, 576.27, and 691.47 GHz), O₂ (424.75 GHz), [C I] (492.23 and 809.44 GHz), H₂O (556.89, 1113.3, and 1716.6 GHz), [N II] (1461.1 and 2459.4 GHz), [C II] (1900.5GHz), [O I] (2060.1 GHz), [Si I] (2311.7 GHz), and CH (2589.6 GHz).

Of the selected 18 spectral lines, the following 9 were detected (Bennett et al. 1994; included as Appendix D of the FIRAS Explanatory Supplement): the 158 micron ground state transition of C+, an important neutral interstellar gas coolant; the N+ 122 micron and 205 micron transitions, which trace extended low-density ionized gas; the 370 micron and 609 micron lines of neutral carbon, which arise in photodissociation regions; and the CO J=2-1, 3-2, 4-3, and 5-4 lines, which trace the molecular gas. As one would expect, the line emission is strongest in the Galactic plane and concentrated in the inner Galaxy.

There are three Line Emission Map FITS files. Two of these are at low spectral resolution: FIRAS_LINE_EMISSION_MAP_LOWF.FITS covers frequencies < 630 GHz; FIRAS_LINE_EMISSION_MAP_HIGH.FITS covers higher frequencies and also includes the parameters of an interstellar dust model (see below). FIRAS_LINE_EMISSION_MAP_HRES.FITS contains high spectral resolution data from 30 to 660 GHz. Synthetic spectral line profiles (FIRAS_LINE_PROFILES_*.FITS) are provided for each of the channel and scan modes or merged modes.

The synthetic profiles can be used in conjunction with a FIRAS spectrum to calculate line intensities.

References:

• Fixsen, D. J., Bennett, C. L. and Mather, J. C. 1999 ApJ 526, 207
• Bennett, C. L., et al. 1994 ApJ 434, 587