Observations of the cosmic microwave background and galactic foregrounds at 12-17 GHz with the COSMOSOMAS experiment

11 pages, 21 figures. Submitted to MNRAS. For paper with figures at full resolution, see http://www.iac.es/project/cmb/cosmosomas/(Abridged) We present the analysis of the first 18 months of data obtained with the COSMOSOMAS experiment at the Teide Observatory (Tenerife). Three maps have been obtained at 12.7, 14.7 and 16.3 GHz covering 9000 square degrees each with a resolution of ~1 degree and with sensitivities 49, 59 and 115 muK per beam respectively. These data in conjuction with the WMAP first year maps have revealed that the Cosmic Microwave Background (CMB) is the dominant astronomical signal at high galatic latitude in the three COSMOSOMAS channels with an average amplitude of 29.7+/- 1.0 \muK (68% c.l. not including calibration errors). This value is in agreement with the predicted CMB signal in the COSMOSOMAS maps using the best fit Lambda-CDM model to the WMAP power spectrum. Cross-correlation of COSMOSOMAS data with the DIRBE map at 100 \mu m shows the existence of a common signal with amplitude 7.4+/- 1.1, 7.5+/- 1.1, and 6.5+/-2.3 muK in the 12.7, 14.7 and 16.3 GHz COSMOSOMAS maps at |b|>30^\deg. Using the WMAP data we find this DIRBE correlated signal rises from high to low frequencies flattening below ~20 GHz. At higher galactic latitudes the average amplitude of the correlated signal with the DIRBE maps decreases slightly. The frequency behaviour of the COSMOSOMAS/WMAP correlated signal with DIRBE is not compatible with the expected tendency for thermal dust. A study of the H-alpha emission maps do not support free-free as a major contributor to that signal. Our results provide evidence of a new galactic foreground with properties compatible with those predicted by the spinning dust models