CS4G Disc Galaxy Extension

Following the original strategy of the survey, we produce masks, surface brightness profiles, and curves of growth using reduced and masked 3.6um and i-band images. From these profiles, we derive the integrated quantities: total magnitude, stellar mass, concentration parameter, and galaxy size. We derive a recipe for the conversion between optical i-band and 3.6um.

We present new morphological revised T-types and we showcase mid-IR scaling relations for the stellar mass, galaxy size, concentration index, and morphological type. We improve the masking procedure and increase the number of pixels masked out by a factor of five improving the masking of fainter regions over previous S4G data. Our photometric parameters from i-band imaging yield consistent measurements with the original sample (S4G) and its ETG extension in the 3.6um band.

The new Disc Galaxy (DG) extension consists of galaxies with a wide morphological range (-5<T_HL_<10 according to HyperLeda) and a mass range of 6<log(M*/M_{sun}_)<11. The galaxies in the DG sample have an average mass of log(M*/M_{sun}_)=9.21, an average galaxy isophotal radius at 25.5mag/arcsec^2^ of R_25.5_=7.1kpc, and an average concentration index of C82=2.92. We complete the S4G sample by incorporating 422 galaxies into the original dataset.

Our revised classification based on 3.6um and i-band imaging suggests that many galaxies in the DG sample are ellipticals (T<-4 for 29 galaxies). The new galaxies constitutes 15% of the total previous sample (S4G+ETG), but in the lower-mass bin (M*<10^9^M_{sun}_), the disc galaxy extension increases the sample by 30%. The CS4G includes at least 99.94% of the complete sample of nearby galaxies.

We make the images and surface brightness profiles available to the community together with the conjunct catalogue of the whole CS4G dataset with consistent photometric measurements for 3239 galaxies. The CS4G will enable a wide set of investigations into galaxy structure and evolution, and complement optical, near-infrared and MIR imaging to be obtained in coming years with Euclid, Rubin, Roman and other research projects.