Microjansky source counts and spectral indices at 8.44 GHz

We used the VLA to make deep images of two 7′ × 7′ fields at 8.44 GHz with 10″ resolution. With an rms noise of 3.2 and 5.1 μJy, respectively, in the two fields, we compiled a catalog of 82 sources. From the complete sample of 20 sources with S ≥ 14.5 μJy, the differential 8.44 GHz source count is dN(S)/ dS = (-4.6 ± 0.7) × S^{-2.3 ± 0.2} Jy^-1 sr^-1 in the range 14.5-1000 mJy. Analysis of statistical image fluctuations from weak sources (Fomalont et al. 1993) suggests that this slope remains unchanged at γ = 2.3 ± 0.2 down to ∼4 μJy. The normalized differential 8.44 GHz counts are similar to those at 1.41 and 4.86 GHz. All show a similarly steep submillijansky slope, which is only somewhat flatter than that expected for a nonevolving Euclidean population (γ = 2.5). Microjansky radio sources at 4.86 GHz have been identified with faint blue galaxies (18 ≲ V ≲ 28 mag). We argue that their expected median redshift is ∼0.5-0.75. Hence, cosmological evolution may be needed to explain the steep slope of the microjansky counts. The 8.44 GHz counts must converge with slope γ ≤ 2.0 below S_8.44 ≃ 300 nanojanskys (nJy), or they would exceed the available field galaxy counts down to V ∼ 28 mag, and they must permanently converge below S_8.44 ≃ 20 nJy, or their integrated sky brightness would distort the observed thermal cosmic background radiation spectrum at centimeter wavelengths. Radio spectral information is summarized for these sources between 0.33 and 8.44 GHz. The high-frequency spectral indices (S ∝ v^-α) span the range -2 ≲ α^4.9_8.4, α^1.4_8.4 ≲ + 1.3 with median α_med ≃ 0.35 ± 0.15. About 40% of the sources have angular size Θ ≥ 5″, and the median is Θ_med ≃ 2″.6 ± 1″.4, or ≃ 5-40 kpc at the expected median redshift. The extended steep-spectrum sources suggest synchrotron emission in distant galactic disks, while the extended flat-spectrum sources may indicate thermal bremsstrahlung from large-scale star formation, both occasionally with opaque radio cores. The estimated 31.5 GHz sky brightness from nanojansky to jansky levels is ≲ 36 μK (3 σ). Even if weak radio sources cluster on scales of degrees as faint galaxies do, their anisotropic contribution to the COBE DMR experiment (with 7° FWHM beam) would not exceed ∼ 1.2 μK.