Homogenization of the Stellar Population along Late-Type Spiral Galaxies

Abstract

We present a study of the broadband UBV color profiles for 257 Sbc barred and nonbarred galaxies, using photoelectric aperture photometry data from the literature. Using robust statistical methods, we have estimated the color gradients of the galaxies, as well as the total and bulge mean colors. A comparative photometric study using CCD images was done. In our sample, the color gradients are negative (reddish inward) in approximately 59% of the objects, are almost null in 27%, and are positive in 14%, considering only the face-on galaxies, which represent approximately 51% of the sample. The results do not change, essentially, when we include the edge-on galaxies. As a consequence of this study we have also found that barred galaxies are overrepresented among the objects having null or positive gradients, indicating that bars act as a mechanism of homogenization of the stellar population. This effect is more evident in the U-B color index, although it can also be detected in the B-V color. A correlation between the total and bulge colors was found that is a consequence of an underlying correlation between the colors of bulges and disks found by other authors. Moreover, the mean total color is the same irrespective of the gradient regime, while bulges are bluer in galaxies with null or positive gradients, which indicates an increase of the star formation rate in the central regions of these objects. We have also made a quantitative evaluation of the amount of extinction in the center of these galaxies. This was done using the Wide Field Planetary Camera 2 (WFPC2) and the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) Hubble Space Telescope (HST) archival data, as well as CCD B, V, and I images. We show that although the extinction in the V-band can reach values up to 2 mag in the central region, it is unlikely that dust plays a fundamental role in global color gradients. We found no correlation between color and O/H abundance gradients. This result could suggest that the color gradients are more sensitive to the age rather than to the metallicity of the stellar population. However, the absence of this correlation may be caused by dust extinction. We discuss this result by considering a picture in which bars are a relatively fast, recurrent phenomenon. These results are not compatible with a pure classical monolithic scenario for bulge and disk formation. On the contrary, they favor a scenario in which both these components are evolving in a correlated process in which stellar bars play a crucial role.