Mid‐Infrared High Spatial Resolution Observations of NGC 1569: Detection of Embedded Embryos of Star Formation

Abstract

We present high spatial resolution mid-infrared (MIR) imaging and spectroscopic observations of the dwarf galaxy NGC 1569 with COMICS on the Subaru telescope. The [S IV] 10.5 μm image clearly shows four pointlike sources and one extended source together with the diffuse emission. The brightest MIR source (MIR1) coincides with the intense H II region, which does not have a corresponding optical source and should thus be a source embedded in dust clouds. The infrared luminosity suggests that there are about 40 O7 stars associated with MIR1. The second brightest source located ~15 south of MIR1 (MIR2) is detected for the first time. This source is not seen in the radio continuum. These characteristics can be accounted for if ionizing regions in the source are compact and optically thick in the radio. The large infrared luminosity (~2 × 108 L☉) suggests that there are about 950 O7 stars in MIR2 and significant contributions from sources other than O-type stars, such as less massive stars and/or pre-main-sequence stars. If this interpretation is correct, MIR2 could be in a very early phase of massive star formation. The unidentified infrared (UIR) band at 11.2 μm is also detected in the spectrum of MIR2. The strength of the UIR band decreases with the [S IV] emission, suggesting that the UIR band weakens and the band carriers are destroyed in strong radiation fields. Super star cluster (SSC) A is also detected in the N-band images. The mid-infrared characteristics support the difference in the ages between two components of SSC A.