The curious case of offset bars : markers for a baby galaxy disk or signposts of an interaction with dark matter sub halos?

Abstract

We have used the Spitzer Survey of Stellar Structure in Galaxies (S⁴G) as a representative sample of the local universe (total of 2352 galaxies in S⁴G) to make a catalog of offset disk barred galaxies. Using the combined variation of the position angle and the ellipticity (provided by ellipse fit) and also through visual inspection, we have been able to identify all offset structures in S⁴G. While primary bars are present in 2=3 of the disk galaxies in the visible universe, offset bars have a much lower fraction. Of the ̴ 1500 (3.6µm images) disk galaxies available in S⁴G, we classified only 49 as offset barred disk galaxies. We have determined basic properties (bar to total luminosity ratio, bar length, disk scale-length and bars of offset bars shape) using GALFIT, a widely used galaxy decomposition software package. Our main conclusion is that all the offset bars are boxy, independent of their offset from the galaxy center, or the mass of the host galaxy. Additionally we find that, the early type offset bars seem to be more boxy than the late types. The comparison of our offset sample with two other samples, respectively, low mass and high mass normal barred galaxies ("normal" for bars located at the photometric center of the host galaxy), reveals them to be at an intermediate position between the two normal samples. The bar length, disk scale-length and bar to total luminosity ratio are on average larger than the low mass normal and smaller than high mass normal barred galaxies. We have found, overall, a tighter correlation between the disk and bar properties for offset bars in comparison to the two normal samples. Our explanation is that, although the offset has no visible impact on the global shape of the bars, the process responsible for these disturbances seems to affect the star formation rate such that their disk and bars are on average more active than the normal barred galaxies in the same mass range, but not enough to surpass normal barred galaxies with much higher mass.