Nebular emission and the Lyman continuum photon escape fraction in CALIFA ETGs (Papaderos e

Vol. 555
In section 1. Letters

Nebular emission and the Lyman continuum photon escape fraction in CALIFA early-type galaxies

Why do many early-type galaxies (ETGs) with prominent signatures of AGN activity in radio continuum and/or X-rays only show weak signatures of AGN activity in their optical spectra? A solution may be in the Lyman continuum (Lyc) photon escape, in conjunction with dilution of nuclear Halpha equivalent width (EW) by line-of-sight integration through a triaxial stellar host. Through deep field spectroscopy (CALIFA survey data), the authors studied 32 ETGs and find two types of objects according to their radial decrease in Halpha (in r^-alpha), their Halpha EW and Lyc photon leakage fraction (plf). The first type of ETG has alpha=1.4, large EW, and plf =0, while the second type has alpha =0.8, low EW, and plf = 0.7-0.9, implying a low density of the warm medium in their center. Both types have the same excitation diagnostics ([OIII]/Hbeta and N[II]/Halpha), which are characteristics of LINERs. The authors propose that, because of extensive Lyc photon leakage, emission-line luminosities and EWs in the second type of ETG nuclei are reduced by at least one order of magnitude. Consequently, the line weakness of these ETGs is insufficient evidence that they contain “weak” (or sub-Eddington accreting) active galactic nuclei.