Bibcode
Papaderos, P.; Gomes, J. M.; Vílchez, J. M.; Kehrig, C.; Lehnert, M. D.; Ziegler, B.; Sánchez, S. F.; Husemann, B.; Monreal-Ibero, A.; García-Benito, R.; Bland-Hawthorn, J.; Cortijo-Ferrero, C.; de Lorenzo-Cáceres, A.; del Olmo, A.; Falcón-Barroso, J.; Galbany, L.; Iglesias-Páramo, J.; López-Sánchez, Á. R.; Marquez, I.; Mollá, M.; Mast, D.; van de Ven, G.; Wisotzki, L.
Referencia bibliográfica
Astronomy and Astrophysics, Volume 555, id.L1, 5 pp.
Advertised on:
7
2013
Revista
Número de citas
97
Número de citas referidas
92
Descripción
We use deep integral field spectroscopy data from the CALIFA survey to
study the warm interstellar medium (wim) over the entire extent and
optical spectral range of 32 nearby early-type galaxies (ETGs). We find
that faint nebular emission is extended in all cases, and its surface
brightness decreases roughly as ∝ r-α. The large
standard deviation in the derived α (1.09 ± 0.67) argues
against a universal power-law index for the radial drop-off of nebular
emission in ETGs. Judging from the properties of their extranuclear
component, our sample ETGs span a broad, continuous sequence with
respect to their α, Hα equivalent width (EW) and Lyman
continuum (Lyc) photon leakage fraction (plf). We propose a
tentative subdivision into two groups: Type i ETGs are characterized by
rather steep Hα profiles (α ≃ 1.4), comparatively large
(≳1 Å), nearly radially constant EWs, and plf ≃ 0.
Photoionization by post-AGB stars appears to be the main driver of
extended nebular emission in these systems, with nonthermal sources
being potentially important only in their nuclei. Typical properties of
type ii ETGs are shallower Hα profiles (α ≃ 0.8), very
low (≲0.5 Å) EWs with positive radial gradients, and a mean
plf ≳ 0.7, rising to ≳0.9 in their centers. Such properties
point to a low, and inwardly decreasing wim density and/or volume
filling factor. We argue that, because of extensive Lyc
photon leakage, emission-line luminosities and EWs are reduced in type
ii ETG nuclei by at least one order of magnitude. Consequently, the line
weakness of these ETGs is by itself no compelling evidence for their
containing merely "weak"(sub-Eddington accreting) active galactic nuclei
(AGN). In fact, Lyc photon escape, which has heretofore not
been considered, may constitute a key element in understanding why many
ETGs with prominent signatures of AGN activity in radio continuum and/or
X-ray wavelengths show only faint emission lines and weak signatures of
AGN activity in their optical spectra. The Lyc photon escape,
in conjunction with dilution of nuclear EWs by line-of-sight integration
through a triaxial stellar host, can systematically impede detection of
AGN in gas-poor galaxy spheroids through optical emission-line
spectroscopy, thereby leading to an observational bias. We further find
that type i&ii ETGs differ little (≲0.4 dex) in their mean BPT
line ratios, which in both cases are characteristic of LINERs and are,
within their uncertainties, almost radius-independent. This potentially
hints at a degeneracy of the projected, luminosity-weighted BPT ratios
in the LINER regime, for the specific 3D properties of the wim and the
ionizing photon field in ETGs.
Based on observations collected at the Centro Astronómico Hispano
Alemán (CAHA) at Calar Alto, operated jointly by the
Max-Planck-Institut für Astronomie (MPIA) and the Instituto de
Astrofísica de Andalucía (CSIC).
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