The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?

摘要

[ABRIDGED] This is the third in a series of three papers exploring theconnection between the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. We selected 116 AGN candidatesrequiring a radio flux of 1 mJy. We classified the objects with HST images into``core'' and ``power-law'' galaxies, on the basis of the nuclear slope of theirbrightness profiles. We used HST and Chandra data to isolate their nuclearemission to study the multiwavelength behaviour of their nuclei. The propertiesof the nuclei hosted by the 29 core galaxies were presented in Paper II. Coregalaxies invariably host a radio-loud nucleus, with a median radio-loudness ofLog R = 3.6 and an X-ray based radio loudness parameter of Log R,X = -1.3. Herewe discuss the properties of the nuclei of the 22 ``power-law'' galaxies. Theyshow a substantial excess of optical and X-ray emission with respect to coregalaxies at the same level of radio luminosity. Conversely, theirradio-loudness parameters, Log R ~ 1.6 and Log R,X ~ -3.3, are similar to thosemeasured in Seyfert galaxies. Thus the radio-loudness of AGN hosted byearly-type galaxies appears to be univocally related to the host's brightnessprofile: radio-loud AGN are only hosted by core galaxies, while radio-quiet AGNare found only in power-law galaxies. The brightness profile is determined bythe galaxy's evolution, through its merger history; our results suggest thatthe same process sets the AGN flavour.