X-RAY EMISSION OF ACTIVE GALACTIC NUCLEI WITH CIRCUMNUCLEAR STAR-FORMING RINGS: NGC 1097 AND NGC 7469

摘要

ROSAT high-resolution imager (HRI) observations of the active galaxies NGC 1097 (Liner/Seyfert) and NGC 7469 (Seyfert 1), known to have a circumnuclear star-forming ring, are presented. An extended X-ray emission associated with ring of NGC 1097 is detected in the HRI image of this galaxy. The X-ray luminosity emitted by the ring is L_x = 3.6 x 10~(40) ergs s~(-1) and accounts for about 20% of the integrated X-ray luminosiy of NGC 1097, L_x = 2 x 10~(41) ergs s~(-1). Circumnuclear star-forming rings in active galaxies like NGC 1097 and NGC 1068 have X-rays to radio and Hα luminosities ratios similar to those of pure starburst galaxies like M82, NGC 253, and NGC 1808. On average, starbursts are characterized by L_x/L_(5 GHz) ～ 400, and L_x/L_(Hα) ～ 0.14. The HRI image of NGC 7469 shows no structure, and the X-ray emission emitted by the circumnuclear ring cannot be disentangled from that of the nucleus. However, assuming the star-forming ring in NGC 7469 radiates with the average L_X/L_(5 GHz) = 400 of starbursts, a X-ray luminosity L_x = 5.6 x 10~(41) ergs s~(-1) is inferred. The ring contributes therefore to only about 4% of the integrated X-ray luminosity, L_x = 1.1 x 10~(43) ergs s~(-1). Pure active galactic nuclei (AGNs) are well differentiated from pure starbursts in the L_x/L_(Hα) ratio, independent of their absolute luminosity and activity level. For a given Hα luminosity, AGNs produce 100 times more X-rays than pure starbursts do. The weak X-ray sources detected in some high-luminosity and ultraluminous infrared galaxies are characterized by X-ray to Hα and to radio luminosity ratios L_x/L_(Hα) ～ 0.05 and L_X/L_(5 GHz) ～ 20, respectively. These values are consistent with the ratios measured in nearly edge-on starbursts but about 100 times smaller than those of composite AGNs plus circumnuclear star-forming galaxies like NGC 1097 and NGC 7469.