Tracing the outer disk of NGC 300: an ultraviolet view

Abstract

We present an ultra-violet (UV) study of the galaxy NGC 300 using GALEX far-UV (FUV) and near-UV (NUV) observations. We studied the nature of UV emission in the galaxy and correlated it with optical, H I and mid-infrared (3.6 μm) wavelengths. Our study identified extended features in the outer disk, with the UV disk extending up to a radius of 12 kpc (>2R25). We estimated the FUV and NUV disk scale-length as 3.05 ± 0.27 kpc and 2.66 ± 0.20 kpc respectively. The scale-length in FUV is 2.3 times larger than that at 3.6 μm, and we also find the disk to gradually become flatter from longer to shorter wavelengths. We performed a statistical source subtraction to eliminate the background contaminants and identified 261 unresolved UV sources between the radii 5.3 kpc and 10 kpc (1 ∼ 2R25). The identified UV sources show an age range between 1–300 Myr with a peak at 25 Myr and a mass range between 103M to 106M , estimated using Starburst99 models. The north-eastern spiral arm is found to be populated by young low mass sources suggesting that the star formation in this spiral arm is a recent phenomenon. The UV emission beyond the R25 radius has contribution from these low mass sources and is extended up to ∼2R25 radius. We conclude that NGC 300 has an extended UV disk, mainly populated by young low mass sources. The star formation rate is measured to be ∼0.46 M /yr which is comparable to its near optical twin M33.