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

ROSAT high-resolution imager (HRI) observations of the active galaxies NGC 1097 (Liner/Seyfert) and NGC 7469 (Seyfert 1), known to have a ci rcumnuclear star-forming ring, are presented. An extended X-ray emissi on associated with ring of NGC 1097 is detected in the HRI image of th is 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 lum inosiy of NGC 1097, L(X)= 2 x 10(41) ergs s(-1).Circumnuclear star-for ming rings in active galaxies like NGC 1097 and NGC 1068 have X-rays t o radio and H alpha luminosities ratios similar to those of pure starb urst galaxies like M82, NGC 253, and NGC 1808. On average, starbursts are characterized by L(X)/L(5 GHz) similar to 400, and L(X)/L(H alpha) similar to 0.14. The HRI image of NGC 7469 shows no structure, and th e X-ray emission emitted by the circumnuclear ring cannot be disentang led 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 starburst s, 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 lu minosity, 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(H al pha) ratio, independent of their absolute luminosity and activity leve l. For a given H alpha 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 H alpha and to radio luminosity ratios L(X)/L(H alpha) similar to 0.05 and L(X)/L(5 GHz) similar to 20, respectively. These values ar e consistent with the ratios measured in nearly edge-on starbursts but about 100 times smaller than those of composite AGNs plus circumnucle ar star-forming galaxies like NGC 1097 and NGC 7469.