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

TL;DR

This study uses high-resolution UVIT observations to analyze the diffuse ultraviolet emission in Holmberg II, revealing its significant contribution, correlations with infrared emission, and insights into dust scattering and other emission mechanisms.

Contribution

First high-resolution UVIT measurements of Holmberg II's diffuse UV emission, analyzing dust scattering and UV-IR correlations in detail.

Findings

Diffuse emission accounts for over 70% of UV light.

Strong UV-IR correlation at 70 micron in high HI regions.

Dust scattering explains UV emission in HI regions, but not in cavities.

Abstract

We present measurements of diffuse ultraviolet emission in the dwarf irregular galaxy Holmberg II obtained with the UltraViolet Imaging Telescope (UVIT) onboard AstroSat. With a spatial resolution of 1.2 to 1.6 arcsec, these are the highest resolution UV observations of the galaxy to date. We find that diffuse emission accounts for 70.6 % (58.1 %) of the total FUV(NUV) emission, respectively. We perform a UV-IR correlation study of the diffuse emission in this galaxy using infrared observations from 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 micron for high HI density locations, indicating that warm dust grains dominate the IR emission, in agreement with earlier studies, while NUV is better correlated with 160 micron emission associated with cold dust grains. Low HI density regions or cavities, do not show any significant UV-IR correlation except at 160 micron, 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 HI density regions, estimated using a single scattering model with foreground dust clouds with LMC reddening, gives best-fit albedo and asymmetry factor values of 0.2 and 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 HI density regions, dust scattering can be one the sources of the observed diffuse UV emission, apart from possible contributions from molecular hydrogen fluorescence, However, the diffuse UV component in HI cavities can only be explained via other mechanisms, such as two photon emission.