Star formation beyond the optical disk

The low-density outskirts of NGC 2090

¹ Instituto de Astrofísica de Canarias, Vía Láctea s/n, E-38205 La Laguna, Spain
² Departamento de Astrofísica, Universidad de La Laguna, E-38206 La Laguna, Spain
³ Indian Institute of Astrophysics, Koramangala II Block, Bangalore 560034, India
⁴ Oxford Astrophysics, Denys Wilkinson Building, University of Oxford, Keble Road, Oxford OX1 3RH, UK
⁵ School of Sciences, European University Cyprus, Diogenes street, Engomi 1516, Nicosia, Cyprus

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 February 2026
Accepted: 24 March 2026

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, and compare the FUV emission with that from the optical and infrared bands. NGC 2090 exhibits prominent star formation in its extended outer disk, with FUV emission traced out to ∼30 kpc, far beyond the truncation of the old stellar disk at ∼5 kpc. It is classified as an extended UV (XUV) disk galaxy. We identified and characterized the SFCs both within and beyond the optical radius (R₂₅), 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_SFR) compared to the inner-disk SFCs. We investigated the properties of the inner disk using mid-infrared data from the James Webb Space Telescope and find that the polycyclic aromatic hydrocarbon emission is strongly correlated with regions of active star formation. The specific SFR increases with radius, consistent with a scenario of inside-out disk growth. The observed number of SFCs and their Hα-to-FUV flux ratios in the outer disk of NGC 2090 indicate ongoing massive star formation and are consistent with a top-heavy initial mass function (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 densities and metallicities.