Abstract
Maps of the Sgr B2 region covering up to 4' have been made in 5 metastable (J = K) and 4 non-metastable (J > K) NH3 inversion transitions with a 40'' resolution. Toward Sgr B2(M) and (N), spectra of 20 additional inversion transitions have been taken arising from energy levels up to E/k=1340 K above ground. From the NH3 data, we obtained rotational and kinetic temperatures (T(kin)), cloud sizes, virial masses, NH3 column densities, spatial densities (n(H-2)), and NH3 abundances relative to H-2 (X(NH3)).
In addition to a cool (T(kin) approximately 20 K) component detected at -40 km s-1 (presumably a foreground cloud associated with the 3 kpc arm), seven regions containing gas with T(kin) greater-than-or-equal-to 100 K are identified. All are part of the Sgr B2 molecular complex. (1) A low density (n(H-2) approximately 10(4) cm-3) molecular cloud at v(LSR)=50 to 80 km s-1 is shown to be extended over several arcmin in metastable transitions up to at least the (6,6) line. A second cooler component (T(rot) approximately 20 K) appears to coexist with the warm gas. Together, their area filling factor is almost one. Of all mapped lines from other molecular species, only HC3N emission from the vibrational ground state may arise from the same two gas components. (2) Another warm low density feature, at v(LSR) approximately 85 km s-1, is detected in the metastable lines toward Sgr B2 (N) and becomes increasingly prominent as J rises. (3) Toward the peak positions Sgr B2 (M) and (N) the metastable lines with J < 5 show an absorption featUre at v(LSR) approximately 0 km s-1. (4) A region with v(LSR) approximately 65 km s-1 has n(H-2) approximately 10(5) cm-3 and is observed in non-metastable NH3 emission lines. It is centered along a ridge 10'' west of the main continuum sources. (5) Three Ori-KL like hot cores are identified, one at v(LSR) approximately 65 km s-1 toward Sgr B2 (N) and two, at 55 and 65 km s-1 toward Sgr B2 (M). Provided that the NH3 inversion lines are thermalized and that the clouds are approximately in virial equilibrium, these are the three most massive hot cores known to exist in the galaxy. With NH3 column densities of order 10(21) cm-2, the 55 km s-1 Sgr B2 (M) and the 65 km s-1 Sgr B2 (N) cores are particularly outstanding. X(NH3) > 10(-5), suggesting that a substantial fraction of the nitrogen is in ammonia. The large number of ultracompact HII (UC HII) regions (greater-than-or-equal-to 19) indicates that the lifetime of a hot core is only approximately 10% of the lifetime of an UC HII region.