Bar properties as a function of wavelength: A local baseline with S4G for high-redshift studies

Abstract

The redshift evolution of bars is an important signpost of the dynamic maturity of disc galaxies. To characterize the intrinsic evolution safe from band-shifting effects, it is necessary to gauge how bar properties vary locally as a function of wavelength. We investigate bar properties in 16 nearby galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) at ultraviolet, optical, and mid-infrared wavebands. Based on the ellipticity and position angle profiles from fitting elliptical isophotes to the two-dimensional light distribution, we find that both bar length and ellipticity-the latter often used as a proxy for bar strength-increase at bluer wavebands. Bars are 9 per cent longer in the B band than at 3.6 μm. Their ellipticity increases typically by 8 per cent in the B band, with a significant fraction (>40 per cent) displaying an increase up to 35 per cent. We attribute the increase in bar length to the presence of star-forming knots at the end of bars: These regions are brighter in bluer bands, stretching the bar signature further out. The increase in bar ellipticity could be driven by the apparent bulge size: The bulge is less prominent at bluer bands, allowing for thinner ellipses within the bar region. Alternatively, it could be due to younger stellar populations associated with the bar. The resulting effect is that bars appear longer and thinner at bluer wavebands. This indicates that band-shifting effects are significant and need to be corrected for high-redshift studies to reliably gauge any intrinsic evolution of the bar properties with redshift.