The Anisotropic Radiation Field in NGC 3516

Extract

We present new narrow-band images of the Extended Emission-Line Region (EELR) in NGC 3516 in light of [O III] λ 4959, Hα + [N II] λλ 6548, 84, [O I] λ 6364, He is/c λ 6678 and [Fe VII] + [Ca V] λ 6087. The observations were carried with the 2-m reflector of the Bulgarian National Astronomical Observatory and the Focal Reducer of the Max-Plank-Institut for Aeronomy. Our [O III] and Hα + [N II] images confirm previously reported EELR features. In contrast, the image in the high-excitation [Fe VII] + [Ca V] line shows a different structure. We identify a biconical morphology over a kiloparsec scale with peak intensities 5.9 × 10⁻¹⁶ ergs cm⁻² s⁻¹ arcsec⁻² and 3.5 × 10⁻¹⁶ ergs cm⁻² s⁻¹ arcsec⁻² to north and south of the nucleus, respectively. The total flux of the [Fe VII] + [Ca V] emission in 5″ and 24″ circular apertures centered at the nucleus is (9.97 ± 0.38) × 10⁻¹⁴ ergs cm⁻² s⁻¹ and (1.53 ± 0.15) × 10⁻¹³ ergs cm⁻² s⁻¹, respectively, which is in good agreement with measurements of Boksenberg & Netzer (1977) through the 5″ aperture. The cone axis lies at PA ∼ −10°. The continuum images (Veilleux et al., 1993, Miyaji et al., 1992) indicate a “bar” aligned along PA ∼ −10°. The velocity extrema regions revealed by Veilleux et al. (1993) are coincident with the peak intensities in our [Fe VII] + [Ca V]. We suppose that our image in [Fe VII] + [Ca V] outlines a Coronal-Line Region (CLR) of NGC 3516, which extends far beyond the classical NLR of the galaxy. Korista & Ferland (1989) have recently shown theoretically that the CLR in Seyferts may be a result of a low-density interstellar medium exposed to and photoionized by a “bare” active nucleus. A typical ISM with N_e ∼ 1–5 cm⁻³ may produce such an extended CLR as that observed by us.