(Sub)millimetre emission from NGC 1569: An abundance of very small grains

¹ IRAM, Avendida Divina Pastora 7, N.C., 18012 Granada, Spain
² Sterrewacht Leiden, Postbus 9513, 2300 RA Leiden, The Netherlands
³ Physics Department, Queen's University, Kingston ON K7L 4P1, Canada

Corresponding author: U. Lisenfeld, ute@iram.es

Received: 18 May 2001
Accepted: 5 December 2001

Abstract

We present new data of the dwarf galaxy NGC 1569 at 450 μm, 850 μm and 1200 μm taken with SCUBA at the JCMT and the bolometer array at the IRAM 30 m telescope. After including data from IRAS at 12, 25, 60 and 100 μm, we have successfully fitted the dust grain population model of Désert et al. (1990) to the observed midinfrared-to-millimetre spectrum. The fit requires a combination of both large and very small grains exposed to a strong radiation field as well as an enhancement of the number of very small grains relative to the number of large grains. We interpret this as the consequence of large grain destruction due to shocks in the turbulent interstellar medium of NGC 1569. The contribution of polyaromatic hydrocarbons (PAH's) is found to be negligible. Comparison of the dust emission maps with an HI map of similar resolution shows that both dust and molecular gas distributions peak close to the radio continuum maximum and at a minimum in the HI distribution. From a comparison of these three maps and assuming that the gas-to-dust mass ratio is the same everywhere, we estimate the ratio of molecular hydrogen column density to integrated CO intensity to be about 25–30 times the local Galactic value. The gas-to-dust ratio is 1500–2900, about an order of magnitude higher than in the Solar Neighbourhood.