Abstract
We present the highest sensitivity and angular resolution study at 0.32 GHz of the dwarf irregular galaxy IC\,10, observed using the Giant Metrewave Radio Telescope, probing $\sim45$ pc spatial scales. We find the galaxy-averaged radio continuum spectrum to be relatively flat, with a spectral index $\alpha = -0.34\pm0.01$ ($S_\nu \propto \nu^\alpha$), mainly due to a high contribution from free--free emission. At 0.32 GHz, some of the H{\sc ii} regions show evidence of free--free absorption as they become optically thick below $\sim0.41$ GHz with corresponding free electron densities of $\sim11-22~\rm cm^{-3}$. After removing the free--free emission, we studied the radio--infrared relations on 55, 110 and 165 pc spatial scales. We find that on all scales the non-thermal emission at 0.32 and 6.2 GHz correlates better with far-infrared (FIR) emission at $70\,\mu$m than mid-infrared emission at $24\,\mu$m. The dispersion of the radio--FIR relation arises due to variations in both magnetic field and dust temperature, and decreases systematically with increasing spatial scale. The effect of cosmic ray transport is negligible as cosmic ray electrons were only injected $\lesssim5$ Myr ago. The average magnetic field strength ($B$) of $12~\mu$G in the disc is comparable to that of large star-forming galaxies. The local magnetic field is strongly correlated with local star formation rate ($\mathrm{SFR}$) as $B \propto \mathrm{SFR}^{0.35\pm0.03}$, indicating a star-burst driven fluctuation dynamo to be efficient ($\sim10$ per cent) in amplifying the field in IC\,10. The high spatial resolution observations presented here suggest that the high efficiency of magnetic field amplification and strong coupling with SFR likely sets up the radio--FIR correlation in cosmologically young galaxies.
Highlights
According to models of hierarchical structure formation, low mass and low luminosity dwarf irregular galaxies are thought to be the analogues of the first galaxies that formed in the early Universe which evolved into larger systems like the normal star-forming spirals found in the Local Volume
In Section 4.2.1 though, we show that the Hii regions showing 100 per cent thermal emission agree with our estimated thermal flux
The brighter radio continuum emission regions in IC 10 are coincident with high star formation rate (SFR) regions and giant molecular clouds (GMCs) detected in the survey of CO(J = 1 → 0) emission by Leroy et al (2006)
Summary
According to models of hierarchical structure formation, low mass and low luminosity dwarf irregular galaxies are thought to be the analogues of the first galaxies that formed in the early Universe which evolved into larger systems like the normal star-forming spirals found in the Local Volume. These galaxies have low stellar mass because of their small sizes, but can have large gas-to-stellar mass ratio compared to that in spiral galaxies (Begum et al 2005; Ott et al 2012; Hunter et al 2012; McNichols et al 2016). Dwarf galaxies are fundamentally different in terms of the physical nature of the ISM as compared to that of large star-forming galaxies. Studies of these objects may provide important clues linked to the cosmic evolution of ISM properties in normal galaxies
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