The naked nuclei of low ionization nuclear emission line regions

¹ Dipartimento di Fisica e Astronomia, Università di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino (Firenze), Italy
e-mail: balmaverd@arcetri.inaf.it
² INAF–Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi 5, 50125 Firenze, Italy
³ INAF–Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy

Received: 8 May 2015
Accepted: 1 July 2015

Abstract

We analyze HST spectra and Chandra observations of a sample of 21 low ionization nuclear emission line regions (LINERs), at least 18 of which genuine active galactic nuclei. We find a correlation between the X-rays and emission lines luminosities, extending over three orders of magnitude and with a dispersion of 0.36 dex; no differences emerge between LINERs with and without broad lines, or between radio-loud and radio-quiet sources. The presence of such a strong correlation is remarkable considering that for half of the sample the X-ray luminosity can not be corrected for local absorption. This connection is readily understood since the X-ray light is associated with the same source producing the ionizing photons at the origin of the line emission. This implies that we have a direct view of the LINERs nuclei in the X-rays: the circumnuclear, high column density structure (the torus) is absent in these sources. Such a conclusion is also supported by mid-infrared data. We suggest that this is due to the general paucity of gas and dust in their nuclear regions that causes also their low rate of accretion and low bolometric luminosity.