The Contribution of
Galactic Free-Free Emission
to Anisotropies in the
Cosmic Microwave Background Radiation
Measured by MSAM-I

(A poster presented at the 188th meeting of the AAS, June 1996)

John H. Simonetti, Gregory A. Topasna, Brian Dennison
Martin Observatory, Institute for Particle Physics and Astrophysics,
and Department of Physics,
Virginia Tech

The first phase of the Medium Scale Anisotropy Measurement experiment (MSAM-I) has detected anisotropies in the cosmic microwave background radiation with an rms between about 50 micro-Kelvins and 100 micro-Kelvins. MSAM-I uses double-difference and single-difference demodulation signals from a chopped, 30-arcminute beam at a number of frequencies between 170 and 680 GHz. We observed the region covered by MSAM-I using the Virginia Tech Spectral Line Imaging Camera (SLIC), a wide-field camera sensitive to faint interstellar H-alpha emission. We duplicated the MSAM-I chopping and demodulation procedure using samples from our H-alpha image (after the subtraction of a continuum image and smoothing to a 30-arcminute beam). The rms values of our double-difference and single-difference demodulation signals are 4.4 and 3.0 Rayleighs, respectively. The implied rms for the microwave brightness temperature of the interstellar plasma is less than 1 micro-Kelvin at 170 GHz. If an obvious contaminatino of the result by one star is removed, the rms signals are 1.6 and 1.0 Rayleighs, and the implied rms for the microwave brightness temperature is less than 0.3 micro-Kelvins at 170 GHz. Thus the MSAM-I anisotropies are not significantly contaminated by foreground Galactic emission irregularities.

This research was supported by NSF grant AST-9319670 and a grant from the Horton Foundation to Virginia Tech.

For a bibliography of MSAM results see MSAM/TopHat Bibliography .

Figure 1: H-alpha Image of the NCP. This is a 110-minute integration centered on the North Celestial Pole, using the H-alpha filter. The image is 30 degrees in diameter; approximate Right Ascension coordinates are marked. Lines of constant galactic latitude run approximately horizontally through the image. The galactic plane is about 12 degrees below the bottom edge of the image. Polaris is the bright star near the center.

Figure 2: H-alpha Image minus a Continuum image. This image was produced by subtracting a properly scaled and registered red continuum image from the original H-alpha image, then smoothing the result to a 30-arcminute beam (the beamwidth of the MSAM-I experiment). Visible localized irregularities are due to incompletely subtracted stars. The range of pixel values displayed is smaller than in Figure 1. The approximate path along which the MSAM-I balloon-borne telescope sampled the sky is an arc of Declination approximately equal to 82 degrees and Right Ascension running between approximately 14.5 to 20.5 hours.

Simply put, the MSAM-I telescope samples the sky during a chopping motion along a tangent to the arc of constant declination (82 degrees), with a chopping amplitude of 40 arcminutes. Then the samples are combined in a "single-difference" and a "double-difference" demodulation scheme. The result is a signal produced by an observing beam with a cross-section (along the line tangent to the circle of constant Declination) plotted as a solid curve in a each panel in Figure 3, below.

Figure 3: MSAM-I Results. These are plots of the MSAM signal for the (a) double-difference demodulation scheme, and the (b) single-difference scheme. Diamonds represent data from a 1994 flight of the experiment; crosses represent data from a 1992 flight. The small solid curve in each panel shows a cross section of the double-difference and single-difference beam pattern. The figure is from Cheng et al. 1996, Astrophysical Journal, L71 (the HTML version of the paper is MSAM1-94 Paper at the Astrophysical Journal web site).

Figure 4: Implied Signal from H-alpha Sky. These plots are analogous to those from MSAM-I, but here the signal is derived by mimicking the MSAM-I sampling and demodulation scheme on the H-alpha image of Figure 2.

Sampling the H-alpha intensity produces a demodulation signal in units of Rayleighs (intensity). Assuming an optically thin situation, there is a simple relation between the H-alpha intensity of ionized hydrogen gas, and its microwave brightness temperature at any specific frequency (see explanation below). The inferred microwave brightness temperature is proportional to the inverse square of the frequency; we have computed the implied brightness temperature at the smallest frequency in the MSAM-I experiment --- yielding the largest brightness temperature variations.

Note that the brightness temperature scale on this figure is two orders of magnitude smaller than on the MSAM-I figure. It appears that the contamination in the MSAM-I results by foreground galactic microwave emission is very small.

The H-alpha Intensity and Microwave Brightness Temperature
of Interstellar Ionized Hydrogen Gas

The free-free brightness temperature (in micro-Kelvins) of optically thin, ionized hydrogen gas is

T = ( 5.43 / f10^2 sqrt(T4) ) g E

where f10 is the frequency in units of 10 GHz, T4 is the gas temperature in units of 10^4 K, E is the emission measure in units of pc cm^{-6}, and g is the free-free Gaunt factor (Osterbrock 1989, Astrophysics of Gaseous Nebulae and Active Galactic Nuclei). The same gas produces an H-alpha surface brightness (in Rayleighs) of

I = ( 0.36 E / sqrt(T4) ) ( 1 - 0.37 ln(T4) )

(Bennett et al. 1992, ApJ, 396, L7), assuming no extinction. Therefore, we have

T = 14.9 ( g I / f10^2 ( 1 - 0.37 ln(T4) ) ).

The brightness temperature T is not strongly dependent upon the gas temperature; we assume T4 = 1.

As stated above, the third equation assumes any observed H-alpha emission is unaffected by interstellar extinction. If extinction is significant, the microwave brightness temperature is higher than given in the third equation. The 100-micron intensities observed by IRAS within 10 degrees of the north celestial pole are less than about 10 MJy/sr (Wheelock et al. 1994, IRAS Sky Survey Atlas Explanatory Supplement), implying less than about 0.6 magnitudes of visual extinction (Beichman, 1987, ARA&A, 25, 521). Therefore, for this field, the brightness temperature calculated by the third equation should be increased by 60%, at most. The full 60% adjustment would apply only if the dust detected by IRAS lies entirely in front of any H-alpha-emitting gas. A less than 60% increase in brightness temperature does not affect our final conclusions.