Balmer Decrement and IRX Break in Tracing Dust Attenuation at Scales of Individual Star-forming Regions in NGC 628

Abstract

We investigate the relationships between infrared excess (IRX=$L_{\mathrm{IR}}/L_{\mathrm{UV}}$) and Balmer decrement ($\mathrm{H}\hat{I}\pm /\mathrm{H}{\hat{I}}^2$) as indicators of dust attenuation for 609 $\mathrm{H}\mathrm{II}$ regions at scales of $\sim 50-200$ pc in NGC 628, utilizing data from AstroSat, James Webb Space Telescope (JWST) and Multi Unit Spectroscopic Explorer (MUSE). Our findings indicate that about three fifths of the sample $\mathrm{H}\mathrm{II}$ regions reside within the regime occupied by local star-forming galaxies (SFGs) along the dust attenuation correlation described by their corresponding color excess parameters $E(B-V{)}_{\mathrm{IRX}}=0.51E(B-V{)}_{\mathrm{H}\hat{I}\pm /\mathrm{H}{\hat{I}}^2}$. Nearly 27$\%$ of the sample exhibits $E(B-V{)}_{\mathrm{IRX}}>E(B-V{)}_{\mathrm{H}\hat{I}\pm /\mathrm{H}{\hat{I}}^2}$, while a small fraction ($\sim 13\%$) displays significantly lower $E(B-V{)}_{\mathrm{IRX}}$ compared to $E(B-V{)}_{\mathrm{H}\hat{I}\pm /\mathrm{H}{\hat{I}}^2}$. These results suggest that the correlation between the two dust attenuation indicators no longer holds for spatially resolved $\mathrm{H}\mathrm{II}$ regions. Furthermore, the ratio of $E(B-V{)}_{\mathrm{IRX}}$ to $E(B-V{)}_{\mathrm{H}\hat{I}\pm /\mathrm{H}{\hat{I}}^2}$ remains unaffected by various physical parameters of the $\mathrm{H}\mathrm{II}$ regions, including star formation rate (SFR), SFR surface density, infrared luminosity ($L_{\mathrm{IR}}$), $L_{\mathrm{IR}}$ surface density, stellar mass, gas-phase metallicity, circularized radius, and the distance to galactic center. We argue that the ratio is primarily influenced by the evolution of surrounding interstellar medium (ISM) of the star-forming regions, transitioning from an early dense and thick phase to the late blown-away stage.

20 pages, 20 figures, 1 table; accepted for publication in RAA