The Nature of Optical Features in the Inner Region of the 3C 48 Host Galaxy

The well-known quasar 3C48 is the most powerful compact steep-spectrumradio-loud QSO at low redshifts. It also has two unusual optical featureswithin the radius of the radio jet (~1"): (1) an anomalous, high-velocitynarrow-line component, having several times as much flux as does thenarrow-line component coinciding with the broad-line redshift; and (2) a brightcontinuum peak (3C48A) ~1" northeast of the quasar. Both of these opticalfeatures have been conjectured to be related to the radio jet. Here we explorethese suggestions. We have obtained Gemini North GMOS integral-field-unit (IFU)spectroscopy of the central region around 3C48. We use the unique features ofthe IFU data to remove unresolved emission at the position of the quasar. Theresolved emission at the wavelength of the high-velocity component is peaked<~0.25" north of the quasar, at virtually the same position angle as the baseof the radio jet. These observations appear to confirm that this high-velocitygas is connected with the radio jet. However, most of the emission comes from aregion where the jet is still well collimated, rather than from the regionswhere the radio maps indicate strong interaction with an external medium. Wealso present the results of HST STIS spectroscopy of 3C48A. We show that 3C48Ais dominated by stars with a luminosity-weighted age of ~1.4 X 10^8 years,substantially older than any reasonable estimate for the age of the radiosource. Our IFU data indicate a similar age. Thus, 3C48A almost certainlycannot be attributed to jet-induced star formation. The host galaxy of 3C48 isclearly the result of a merger, and 3C48A seems much more likely to be thedistorted nucleus of the merging partner, in which star formation was inducedduring the previous close passage.Comment: 10 pages, accepted by The Astrophysical Journa