The Nature of Blue Cores in Spheroids: a Possible Connection with Agn and Star Formation

We investigate the physical nature of blue cores in early-type galaxies through the first multi-wavelength analysis of a serendipitously discovered field blue-nucleated spheroid in the background of the deep ACS/WFC griz multicolor observations of the cluster Abell 1689. The resolved g− r, r− i and i− z color maps reveal a prominent blue core identifying this galaxy as a “typical” case study, exhibiting variations of 0.5−1.0 mag in color between the center and the outer regions, opposite to the expectations of reddened metallicity induced gradients in passively evolved ellipticals. From a Magellan-Clay spectrum we secure the galaxy redshift at z = 0.624. We find a strong X-ray source coincident with the spheroid galaxy. Spectral features and a high Xray luminosity indicate the presence of an AGN in the galaxy. However, a comparison of the X-ray luminosity to a sample derived from the Chandra Deep Field South displays LX to be comparable to Type I/QSO galaxies while the optical flux is consistent with a normal star-forming galaxy. We conclude that the galaxy’s nonthermal component dominates at high-energy wavelengths while we associate the spheroid blue light with the stellar spectrum of normal star-forming galaxies. We argue about a probable association between the presence of blue cores in spheroids and AGN activity. Subject headings: galaxies: elliptical and lenticular, cD — galaxies: active — X-rays: galaxies