Analysis of the spatially-resolved $V$-3.6$\mu$m colors and dust extinction in 257 nearby NGC and IC galaxies

Abstract

We present and analyze spatially-resolved maps for the observed $V$- and $g$-band to 3.6$\mu$m flux ratios and the inferred dust extinction values, $A_V$, for a sample of 257 nearby NGC and IC galaxies. Flux ratio maps are constructed using PSF-matched mosaics of SDSS $g$- and $r$-band images and Spitzer/IRAC 3.6$\mu$m mosaics, with all pixels contaminated by foreground stars or background objects masked out. By applying the $\beta_V$ method (Tamura et al. 2009, 2010), which was recently calibrated as a function of redshift and morphological type by Kim, Jansen, & Windhorst (2017), dust extinction maps were created for each galaxy. The typical 1-$\sigma$ scatter in $\beta_V$ around the average, both within a galaxy and in each morphological type bin, is $\sim$20%. Combined, these result in a $\sim$0.4 mag scatter in $A_V$. $\beta_V$ becomes insensitive to small-scale variations in stellar populations once resolution elements subtend an angle larger than that of a typical giant molecular cloud ($\sim$200pc). We find noticeably redder $V$$-$3.6$\mu$m colors in the center of star-forming galaxies and galaxies with a weak AGN. The derived intrinsic $V$$-$3.6$\mu$m colors for each Hubble type are generally consistent with the model predictions of Kim et al. (2017). Finally, we discuss the applicability of the $\beta_V$ dust-correction method to more distant galaxies, for which well-matched HST rest-frame visible and JWST rest-frame $\sim$3.5$\mu$m images will become available in the near-future.