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 $\mu$m) wavelengths. Our study identified extended features in the outer disk, with the UV disk extending up to a radius of 12 kpc (${>}2R_{25}$). We estimated the FUV and NUV disk scale-length as $3.05\pm 0.27$ kpc and $2.66\pm 0.20$ kpc respectively. The scale-length in FUV is 2.3 times larger than that at 3.6 $\mu$m, 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 ($1 \sim 2 R_{25}$). The identified UV sources show an age range between 1–300 Myr with a peak at 25 Myr and a mass range between $10^3 M_{\odot }$ to $10^6 M_{\odot }$, 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 $R_{25}$ radius has contribution from these low mass sources and is extended up to ${\sim }2R_{25}$ 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 ${\sim }0.46\,M_{\odot }$/yr which is comparable to its near optical twin M33.