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Session 11 - QSOs and Radio Galaxies.
Display session, Monday, June 08
Atlas Ballroom,
Mrk 231, a Luminous Infrared Galaxy with log L_fir = 12.35(L_ødot) is generally considered to be a dust-enshrouded quasar. Mrk 231 shares many characteristics with quasars including a broad, Sy1 optical emission spectrum, optical and radio variability, and broad-absorption-line (BAL) spectrum. In common with other Luminous IR Galaxies, Mrk 231 is a molecular-gas-rich system showing evidence for a merger or other dynamical disturbance in the host galaxy, which is itself a luminous system with evidence for rapid star formation.
We have obtained VLBI imaging observations in the 18cm continuum and 1667MHz OH maser line. The continuum shows a strong, unresolved nuclear core with an extended, limb-brightened lobe extending about 30mas (26pc) to the south, and a much fainter lobe 25mas (21pc) to the north. The compact radio morphology places it among the Compact Symmetric Objects (CSOs) in which radio emission originates from hot spots or lobes symmetrically placed on sub-kpc scales about a central compact source. It has been suggested that these sources are young, \tau << 10^6 yr, with the hot spots representing the working surface of a relativistic jet upon the ambient medium. If the southern (primary) lobe/hot-spot in Mrk 231 is confined by ram pressure, we estimate a lobe advance speed, v_a 10^-4c and an age for the jet/compact source, \tau < 10^6 yr.
The OH 1667MHz emission is extended over a region of angular dimension approximately 100 x 50 mas (85 x 43pc) and is probably the inner region of the disk which is seen in CO emission (Bryant amp; Scoville 1996, Ap.J., 457, 678) and HI absorption (Carilli, Wrobel amp; Ulvestad 1998, A.J., 115, 928). The radio continuum emission associated with this gaseous disk is likely to be produced by a circumnuclear starburst; if this starburst has a normal FIR-to-radio ratio, q = 2.34, then up to 2/3 of the far-infrared continuum in Mrk 231 may come from star-formation. We suggest that Mrk 231 is a nascent quasar (\tau < 10^6 yr) emerging from its dust-enshrouded starburst phase along the lines of the evolutionary scenario suggested by Sanders et al.(1988, Ap.J., 325, 74).
