Central free-free-dominated 880 $\micron$ emission in II Zw 40

TL;DR

This study observes the central star-forming region of II Zw 40 at 880 microns, revealing that most of the emission is free-free radiation from ionized gas, with minimal dust contribution, and explores implications for the radio-FIR relation.

Contribution

The paper provides the first high-resolution 880 micron observations of II Zw 40's core, quantifies free-free dominance, and models the impact of free-free absorption and dust extent on the radio-FIR relation.

Findings

880 micron flux is ~75% free-free emission

Dust contribution to flux is at most 4±2.5 mJy

Free-free absorption influences the radio-FIR relation

Abstract

The central star-forming region in a blue compact dwarf galaxy, II Zw 40, was observed in the 340 GHz ($880 \micron$) band at $\sim 5$ arcsec (250 pc) resolution with the Submillimetre Array (SMA). A source associated with the central star-forming complex was detected with a flux of $13.6\pm 2.0$ mJy. The structure is more extended than the beam in the east-west direction. The SMA 880 $\micron$ flux is analyzed by using theoretical models of radio spectral energy distribution along with centimetre interferometric measurements in the literature. We find (i) that the SMA 880 $\micron$ flux is dominated ($\sim 75$ per cent) by free-free emission from the central compact star-forming region, and (ii) that the contribution from dust emission to the SMA 880 $\micron$ flux is at most $4\pm 2.5$ mJy. We also utilize our models to derive the radio--FIR relation of the II Zw 40 centre, suggesting that free-free absorption at low frequencies ($\nu\la$ several GHz; $\lambda\ga$ several cm) and spatial extent of dust affect the radio-FIR relation.