Herschel observations of the Sagittarius B2 cores: Hydrides, warm CO, and cold dust

Context. Sagittarius B2 is one of the most massive and luminous star-forming regions in the Galaxy and shows a very rich chemistry and physical conditions similar to those in much more distant extragalactic starbursts. Aims. We present large-scale far-infrared/submillimeter photometric images and broadband spectroscopic maps taken with the PACS and SPIRE instruments onboard Herschel. Methods. High angular resolution dust images (complemented with Spitzer MIPS 24 mu m images) as well as atomic and molecular spectral maps were made and analyzed in order to constrain the dust properties, the gas physical conditions, and the chemical content of this unique region. Results. The spectra towards the Sagittarius B2 star-forming cores, B2(M) and B2(N), are characterized by strong CO line emission (from J = 4 to 16), emission lines from high-density tracers (HCN, HCO+, and H2S), [Nii] 205 mu m emission from ionized gas, and a large number of absorption lines from light hydride molecules (OH+, H2O+, H2O, CH+, CH, SH+, HF, NH, NH2, and NH3). The rotational population diagrams of CO suggest the presence of two different gas temperature components: an extended warm component with T-rot similar to 50-100 K, which is associated with the extended envelope, and a hotter component at T-rot similar to 200 K and T-rot similar to 300 K, which is only seen towards the B2(M) and B2(N) cores, respectively. As observed in other Galactic center clouds, such gas temperatures are significantly higher than the dust temperatures inferred from photometric images (T-d similar or equal to 20-30 K). We determined far-IR luminosities (L-FIR(M) similar to 5 x 106 L similar to and L-FIR(N) similar to 1.1 x 106 L-circle dot) and total dust masses (M-d(M) similar to 2300 M-circle dot and M-d(N) similar to 2500 M-circle dot) in the cores. Non-local thermodynamic equilibrium models of the CO excitation were used to constrain the averaged gas density (n(H-2) similar to 106 cm(-3)) in the cores (i.e., similar or lower than the critical densities for collisional thermalization of midand high-J CO levels). A uniform luminosity ratio, L(CO)/L-FIR similar to (1-3) x 10(-4), is measured along the extended envelope, suggesting that the same mechanism dominates the heating of the molecular gas at large scales. Conclusions. Sgr B2 shows extended emission from warm CO gas and cold dust, whereas only the cores show a hotter CO component. The detection of high-density molecular tracers and of strong [Nii] 205 mu m line emission towards the cores suggests that their morphology must be clumpy to allow UV radiation to escape from the inner Hii regions. Together with shocks, the strong UV radiation field is likely responsible for the heating of the hot CO component. At larger scales, photodissociation regions models can explain both the observed CO line ratios and the uniform L(CO)/L(FIR)luminosity ratios.