COLLISIONS OF ELLIPTICALS AND THE ONSET OF FANAROFF-RILEY TYPE-I RADIO-SOURCES

This paper presents the first detailed quantitative study on the morph ological characterization of Fanaroff-Riley type I radio source (FR I) host galaxies. The study is based on a two-dimensional isophote analy sis of the largest sample (44) available so far of FR I host galaxies. FR I host galaxies are luminous ellipticals following the same mu(e) - r(e) relation as nonradio ellipticals. However, a large fraction (60 %) of FR I host galaxies in the sample show at least two of the follow ing morphological peculiarities: (1) isophote twists which are larger than 15 degrees, (2) isophote displacements which are larger than 3.5% , (3) excesses over a de Vaucouleurs law which exceeds 0.10 mag arcsec (-2) at radii beyond r(e), and (4) one companion galaxy at a distance of less than 50 kpc. These morphological peculiarities are the signatu re of a recent strong gravitational collision. The collision, involvin g a pair of ellipticals, is characterized by (1) a median mass ratio ( companion to FR I host galaxy) of similar to 0.2, (2) a median project ed distance of similar to 23 kpc, (3) a median relative velocity of si milar to 492 km s(-1), (4) a median age of similar to 4.0 x 10(7) year s, and (5) a median interaction strength parameter of 0.19. Collisions between ellipticals which share the above mentioned mean properties a ppear to be very efficient in generating a FR I radio source. Therefor e, the scenario of the onset of an active nucleus by galaxy collisions , well established for very luminous starbursts and radio galaxies, ca n also be extended to FR I radio sources. In a broader scenario, the c onclusions of this paper also reinforce the idea that collisions betwe en different type of galaxies give rise to different type of activity in their nuclei: collisions involving two spirals produce luminous cir cumnuclear starbursts, collisions involving one elliptical and one spi ral generate powerful Fanaroff-Riley type II sources (FR II), which co llisions between twoellipticals ignite FR I radio sources.