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Centro de Astrofísica da Universidade do Porto

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

P. Papaderos, J. M. Gomes, J. M. Vílchez, C. Kehrig, M. D. Lehnert, B. Ziegler, S. F. Sánchez, B. Husemann, A. Monreal Ibero, R. García-Benito, J. Bland-Hawthorn, C. Cortijo-Ferrero, A. de Lorenzo-Cáceres, A. del Olmo, J. Falcón-Barroso, L. Galbany, J. Iglésias-Páramo, Á. R. López-Sánchez, I. Márquez, M. Mollá, D. Mast, G. van de Ven, L. Wisotzki, the COROT Team

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. 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 (plƒ). 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 plƒ ≃ 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 plƒ≃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 emissionline 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.

Palavras chave
galaxies: elliptical and lenticular, cD - galaxies: nuclei - galaxies: ISM

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).

Astronomy and Astrophysics
Volume 555, Página L1_1
julho 2013

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Instituto de Astrofísica e Ciências do Espaço

O Instituto de Astrofísica e Ciências do Espaço (IA) é uma nova, mas muito aguardada, estrutura de investigação com uma dimensão nacional. Ele concretiza uma visão ousada, mas realizável para o desenvolvimento da Astronomia, Astrofísica e Ciências Espaciais em Portugal, aproveitando ao máximo e realizando plenamente o potencial criado pela participação nacional na Agência Espacial Europeia (ESA) e no Observatório Europeu do Sul (ESO). O IA é o resultado da fusão entre as duas unidades de investigação mais proeminentes no campo em Portugal: o Centro de Astrofísica da Universidade do Porto (CAUP) e o Centro de Astronomia e Astrofísica da Universidade de Lisboa (CAAUL). Atualmente, engloba mais de dois terços de todos os investigadores ativos em Ciências Espaciais em Portugal, e é responsável por uma fração ainda maior da produtividade nacional em revistas internacionais ISI na área de Ciências Espaciais. Esta é a área científica com maior fator de impacto relativo (1,65 vezes acima da média internacional) e o campo com o maior número médio de citações por artigo para Portugal.

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