Spitzer Space Telescope observations of the Carina nebula: the steady march of feedback-driven star formation

We report the first results of imaging the Carina nebula (NGC 3372) with the Infrared Array Camera (IRAC) onboard the Spitzer Space Telescope, providing a photometry catalogue of over 44 000 point sources as well as a catalogue of over 900 candidate young stellar objects (YSOs) based on tits to their spectral energy distributions (SEDs). We discuss several aspects of the extended emission, including the structure of dozens of dust pillars that result when a clumpy molecular cloud is shredded by feedback from massive stars. There are surprisingly few of the 'extended green objects' (EGOs) that are normally taken as signposts of outflow activity in Spitzer data, and not one of the dozens of Herbig-Haro jets detected optically are seen as EGOs. EGOs are apparently poor tracers of outflow activity in strongly irradiated environments, due to the effects of massive star feedback. A population of 'extended red objects' tends to be found around late O-type and early B-type stars, some with clear bow-shock morphology. These are dusty shocks where stellar winds collide with photoevaporative flows off nearby clouds. Finally, the relative distributions of O-type stars, small star clusters and subclusters of YSOs as compared to the dust pillars show that while some YSOs are located within dust pillars, many more stars and YSOs reside just outside pillar heads. We suggest that pillars are transient phenomena, part of a continuous outwardly propagating wave of star formation driven by feedback from massive stars. As the pillars are destroyed, they leave newly formed stars in their wake, and these are then subsumed into the young OB association. The YSOs are found predominantly in the cavity between pillars and massive stars, arguing that their formation was in fact triggered. Altogether, the current generation of YSOs shows no strong deviation from a normal initial mass function (IMF). The number of YSOs is consistent with a roughly constant star-formation rate over the past similar to 3 Myr, implying that propagating star formation in pillars constitutes an important mechanism to construct unbound OB associations. These accelerated pillars may give birth to massive O-type stars that, after several million years, could appear to have formed in isolation.