Star Formation Beyond the Optical Disk : The Low-Density Outskirts of NGC2090

Abstract

We present a far-ultraviolet (FUV) analysis of the star-forming complexes (SFCs) in the nearby spiral galaxy NGC\,2090, based on observations from the Ultraviolet Imaging Telescope (UVIT), and compare it with emission from the optical and infrared bands. NGC\,2090 exhibits prominent star formation in its extended outer disk, with FUV emission traced out to $\sim$30 kpc, far beyond the truncation of the old stellar disk at $\sim$5 kpc. It is classified as an extended UV (XUV) disk galaxy. We identify and characterize the SFCs both within and beyond the optical radius (R$_{25}$), estimating their physical sizes and star formation rates (SFRs). The outer-disk SFCs are generally smaller in area and show a narrower distribution of SFR surface density ($Σ_{\mathrm{SFR}}$) compared to the inner-disk SFCs. We investigate the properties of the inner disk using mid-infrared data from the James Webb Space Telescope (JWST), and find that the polycyclic aromatic hydrocarbon (PAH) emission is strongly correlated with regions of active star formation. The specific SFR (sSFR) increases with radius, consistent with a scenario of inside-out disk growth. The observed number of SFCs and their H$α$-to-FUV flux ratios in the outer disk of NGC\,2090 indicate ongoing massive star formation and are consistent with a top-heavy IMF, implying that the upper end of the IMF is not truncated in the low-density, metal-poor outskirts. These results suggest that XUV disks can host significant massive star formation despite their low stellar density and metallicity.

Figures (10)

• Figure 1: Left and right panels display the FUV-$g$ and Spitzer IRAC CH4-CH2 color maps of NGC,2090, respectively. The FUV–$g$ map highlights the bluer spiral arms, indicative of recent star formation, while the IRAC CH4-CH2 map traces regions of bright dust and PAH emission.
• Figure 2: Identified SFCs in FUV(left) and H$\alpha$(right). Blue and red symbols denote SFCs located inside and outside the optical radius, respectively, while the yellow contour marks the optical radius. The grey boxes highlight the regions shown in the inset, providing a zoomed-in view of the SFCs within those areas.
• Figure 3: The top and bottom panels show the area and $\Sigma_{SFR}$ of SFC's in FUV (blue) and H$\alpha$ (red), respectively.
• Figure 4: The left, middle, and right panels show the observed F335M image, the underlying stellar continuum in F335M, and the isolated PAH emission component at 3.3 $\mu$m, respectively.
• Figure 5: The left, and right panels show the maps of the F335M$_{PAH}$/F770W and F770W/F2100W band ratios, respectively.