AstroSat/UVIT Study of the Diffuse Ultraviolet Radiation in the Dwarf Galaxy Holmberg II

Abstract

We present measurements of diffuse ultraviolet (UV) emission in the dwarf irregular galaxy Holmberg II obtained with the Ultra Violet Imaging Telescope (UVIT) instrument onboard AstroSat, India’s first multiwavelength space mission. With a spatial resolution of 1.″2–1.″6, these are the highest resolution UV observations of the galaxy to date. We find that diffuse emission accounts for ∼70.6% of the total far-ultraviolet (FUV) and for ~58.1% of the total near-ultraviolet (NUV) emission. In the FUV, this is reasonably close to the fraction reported for the SMC bar. We perform a UV–IR correlation study of the diffuse emission in this galaxy using infrared (IR) observations from the Spitzer Space Telescope and Herschel Space Observatory for selected locations, free of detectable bright point sources. The strongest positive correlation between FUV and IR is observed at 70 μm for high H i density (N(H i) > 1 × 1021 cm−2) locations, indicating that warm dust grains dominate the IR emission, in agreement with earlier studies, while NUV is better correlated with 160 μm emission associated with cold dust grains. Low H i density regions (N(H i) < 1 × 1021 cm−2), or cavities, do not show any significant UV–IR correlation except at 160 μm, implying either the presence of colder dust grains in cavities being irradiated by the general radiation field, or insufficient amount of dust. The dust scattering contribution in high H i density regions, estimated using a single scattering model with foreground dust clouds with LMC reddening, gives best-fit albedo and asymmetry factor values of α = 0.2 and g = 0.5, respectively, in reasonable agreement with the theoretical predictions for LMC dust. Our model-derived scattering optical depths in the FUV range from 0.02 to 0.12, implying the medium is optically thin. Therefore, in high H i density regions, dust scattering can be one of the sources of the observed diffuse UV emission, apart from possible contributions from H2 fluorescence. However, the diffuse UV component in H i cavities can only be explained via other mechanisms, such as two-photon emission.