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

TL;DR

This study analyzes star-forming complexes in NGC 2090's extended outer disk using UV, optical, and infrared data, revealing ongoing massive star formation in low-density, metal-poor outskirts consistent with a top-heavy IMF.

Contribution

It provides a detailed characterization of star-forming complexes beyond the optical disk, highlighting their properties and implications for star formation and IMF in low-density environments.

Findings

Outer disk SFCs are smaller and have narrower SFR surface density distribution.

PAH emission correlates with active star formation regions.

Outer disk shows evidence of ongoing massive star formation with a top-heavy IMF.

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}$), estimating their physical sizes and star formation rates (SFRs). The outer-disk SFCs are generally smaller in area and show a narrower distribution of SFR surface density ($\Sigma_{\mathrm{SFR}}$) compared to the inner-disk SFCs. We investigate the properties of the inner disk using mid-infrared data from the James Webb Space Telescope (JWST), and find that the polycyclic aromatic hydrocarbon (PAH) emission is strongly correlated with regions of active star formation. The specific SFR (sSFR) increases with radius, consistent with a scenario of inside-out disk growth. The observed number of SFCs and their H$\alpha$-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.