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We present Spitzer Space Telescope IRAC and MIPS observations toward a sample of nine high-mass star forming regions at a distance of around 2 kpc. Based on IRAC and MIPS 24 m photometric results and 2MASS JHK s data, we carry out a census of young stellar objects (YSOs) in a 5 0 ; 5 0 field toward each region. Toward seven out of the nine regions, we detect parsec-sized clusters with around 20 YSOs surrounded by a more extended and sparse distribution of young stars and protostars. For the other two regions, IRAS 20126+4104 and IRAS 22172+5549, the former has the lowest number of YSOs in the sample and shows no obvious cluster, and the latter appears to be part of a larger, potentially more evolved cluster. The deep IRAC imaging reveals at least 12 outflows in eight out of the nine regions, with nine outflows prominent in the 4.5 m band most probably attributed to shocked H 2 emission, two outflows dominated by scattered light in the 3.6 and 4.5 m bands, and one outflow standing out from its hydrocarbon emission in the 8.0 m band. In comparison with previous ground-based observations, our IRAC observations reveal new outflow structures in five regions. The dramatically different morphologies of detected outflows can be tentatively interpreted in terms of possible evolution of massive outflows. The driving sources of these outflows are deeply embedded in dense dusty cores revealed by previous millimeter interferometric observations. We detect infrared counterparts of these dusty cores in the IRAC or MIPS 24 m bands. Reflection nebulae dominated by the emission from UV-heated hydrocarbons in the 8 m band can be found in most regions and they may imply the presence of young B stars.

We report ALMA observations with resolution ≈ 0.5 at 3 mm of the extended Sgr B2 cloud in the Central Molecular Zone (CMZ). We detect 271 compact sources, most of which are smaller than 5000 AU. By ruling out alternative possibilities, we conclude that these sources consist of a mix of hypercompact H ii regions and young stellar objects (YSOs). Most of the newly-detected sources are YSOs with gas envelopes which, based on their luminosities, must contain objects with stellar masses M * 8 M . Their spatial distribution spread over a ∼ 12 × 3 pc region demonstrates that Sgr B2 is experiencing an extended star formation event, not just an isolated 'starburst' within the protocluster regions. Using this new sample, we examine star formation thresholds and surface density relations in Sgr B2. While all of the YSOs reside in regions of high column density (N (H 2 ) 2 × 10 23 cm −2 ), not all regions of high column density contain YSOs. The observed column density threshold for star formation is substantially higher than that in solar vicinity clouds, implying either that high-mass star formation requires a higher column density or that any star formation threshold in the CMZ must be higher than in nearby clouds. The relation between the surface density of gas and stars is incompatible with extrapolations from local clouds, and instead stellar densities in Sgr B2