2 Millimeter Dust around NGC 7538 — IRS 1, 2, and 3: I. Unfilled Structure of Dust Cloud

A 2-mm continuum mapping observation around NGC 7538 — IRS 1, 2, and 3 was made by the Nobeyama Bolometer Array (NOBA) with a spatial resolution of 12$$^{\prime\prime}$$. An extended source of $$\sim {32{}^{\mathrm {\prime \prime }}} \times {44{}^{\mathrm {\prime \prime }}}$$ ($$\Delta\alpha \times \Delta\delta$$), having a hardly resolved central core structure, was observed. The core gives a flux density of $$3.1 \,\mathrm{Jy} \pm 1 \,\mathrm{Jy}$$, and is clearly identified with the free-free emission from the compact H II regions. A smooth and extended source gives an integrated flux density of $$5 \,\mathrm{Jy} \pm 1.5 \,\mathrm{Jy}$$, and an intensity spectrum of $$\nu^{3}$$–$$\nu^{3.5}$$ in the millimeter wave region. The fitting of a gray body function to the observational data shows that an absorption index, $$\beta$$, of 1.8–2.0, a dust filling factor, $$\eta$$, of 0.3–0.5, and a dust temperature, $$T_\mathrm{d}$$, of 34 K–45 K are reasonable parameters for this 2-mm extended dust. Astronomical silicate grains were examined for a dust model. The total dust mass and luminosity were obtained as $$M_\mathrm{d} \simeq 0.85 \times 10^{2} \,{{{M}_{\odot}}}$$ and $$L_\mathrm{d} \simeq 2.2 \times 10^{5} \,{{{L}_{\odot}}}$$, respectively. The extent of 2-mm dust emission partly coincides with those of the 100-$$/mu$$ m map, of the 57-$$/mu$$ m map, and of the total intensity of the wing parts of the CO ($$J = 1-0$$) line emission of the region. The absorbing silicate dust layer must be distinct from the extended 2-mm dust, and must arise on smaller scales than the extended 2-mm dust.