Initial phases of massive star formation in high infrared extinction clouds I. Physical parameters

Aims. The earliest phases of massive star formation are found in cold and dense infrared dark clouds (IRDCs). Since the detection method of IRDCs is very sensitive to the local properties of the background emission, we present here an alternative method to search for high column density in the Galactic plane by using infrared extinction maps. Using this method we find clouds between 1 and 5 kpc, of which many were missed by previous surveys. By studying the physical conditions of a subsample of these clouds, we aim at a better understanding of the initial conditions of massive star formation. Methods. We have made extinction maps of the Galactic plane based on the 3.6-4.5 mu m color excess between the two shortest wavelength Spitzer IRAC bands, reaching to visual extinctions of similar to 100 mag and column densities of 9 x 1022 cm(-2). From this we compiled a new sample of cold and compact high extinction clouds. We used the MAMBO array at the IRAM 30 m telescope to study the morphology, masses and densities of the clouds and the dense clumps within them. The latter were followed up by pointed ammonia observations with the 100 m Effelsberg telescope, to determine rotational temperatures and kinematic distances. Results. Extinction maps of the Galactic plane trace large scale structures such as the spiral arms. The extinction method probes lower column densities, N-H2 similar to 4 x 10(22) cm(-2), than the 1.2 mm continuum, which reaches up to N-H2 similar to 3 x 10(23) cm(-2) but is less sensitive to large scale structures. The 1.2 mm emission maps reveal that the high extinction clouds contain extended cold dust emission, from filamentary structures to still diffuse clouds. Most of the clouds are dark in 24 mu m, but several show already signs of star formation via maser emission or bright infrared sources, suggesting that the high extinction clouds contain a variety of evolutionary stages. The observations suggest an evolutionary scheme from dark, cold and diffuse clouds, to clouds with a stronger 1.2 mm peak and to finally clouds with many strong 1.2 mm peaks, which are also warmer, more turbulent and already have some star formation signposts.