Imaging and polarimetry of the Galactic Centre in the near-infrared

Abstract

Infrared images of the Galactic Centre with 2.25 arcsec resolution have been obtained at wavelengths of 1.64 μm (H), 2.2 μm (K), 3.8 μm (L') and 4.8 μm (M). From these we have measured positions, magnitudes and colours for 35 sources. The observed sources can be divided into two classes: those whose colours are typical of reddened stars and those with much redder colours which dominate at 3.8 and 4.8 μm whose infrared radiation is due to dust emission. Several of the latter sources are detected at K and have H–K colours much redder than those of the stars, indicating the presence of hot dust (T ∼ 1000 K). If these sources are heated by embedded stars, the stars are too faint to be observed directly in the near-infrared. Polarization measurements for a number of the brighter sources have been made at J, H, and K. Most of the sources with stellar colours show wavelength dependence typical of interstellar polarization with polarizations at K of about 6 per cent. IRS 16 is included in this group and shows no evidence for any intrinsic linear or circular polarization. IRS 1 shows significant intrinsic polarization at K which we attribute to emission from aligned grains. IRS 7 shows unusual wavelength dependence indicating significant intrinsic polarization which may arise from dust scattering in a circumstellar shell. IRS 7 has a small circular polarization which can be explained as due to the action of interstellar birefringence on its intrinsic polarization.