The warm interstellar medium in early-type galaxies from the CALIFA Survey

J. M. Gomes, P. Papaderos, C. Kehrig, J. M. Vílchez

Abstract.
The first studies of early-type galaxies (ETGs) have suggested that these systems consist of ancient, metal-rich stellar populations, formed almost coevally about 15 Gyr ago, and are devoid of dust and gas. However, our understanding of ETGs underwent significant development in the past three decades, as subsequent work has revealed non-circularly-symmetric features (e.g. embedded spiral or bar-like patterns) and faint nebular emission in many of them. The origin and the excitation mechanism(s) of the warm ISM in ETGs is a subject of controversy. Are the LINER spectral characteristics frequently observed in the nuclear regions of ETGs due to, e.g., the ionizing field from a central low-luminosity AGN and large-scale shocks, or can it be accounted for by post-AGB stars? This question has motivated our systematic study of ETGs in the nearby (~100 Mpc) Universe using integral field unit (IFU) data from the CALIFA Legacy Survey. The CALIFA data permit a spatially resolved analysis of stellar populations and BPT diagnostics over the entire optical extent of the sample galaxies, offering a key advantage for this study. This is because they enable us to investigate whether the warm ISM in ETGs is centrally confined and has spectral characteristics being indicative of point-source illumination by an AGN, or whether it is spatially extended and shows a lower excitation, in better agreement with the post-AGB hypothesis.
In this pilot study, we have applied a pipeline developed at Centro de Astrofísica da Universidade do Porto which permits automated spectral fitting of IFU data, and the post-processing and convenient storage of relevant output quantities (e.g. emission-line fluxes and kinematics, luminosity- and mass-weighted stellar age and metallicity). Our pipeline allows, through subtraction of the best-fitting stellar model at each spatial element, to detect even faint (equivalent width ~2 Å) nebular emission over a large portion of ETGs. Additionally, an innovative aspect of it is that it permits computation of the ratio of predicted to observed Balmer line fluxes (T-ratio) for the ionizing output from post-AGB stars, thereby providing a means for checking the consistency between spectral synthesis models and observations, and for quantifying the contribution of post-AGB stars for the excitation of the warm ISM.
We will present our conclusions from a recent study of two ETGs (Kehrig et al. 2012), and a preliminary discussion of the results obtained for a few other ETGs from CALIFA. All ETGs in our sample show extended nebular emission out to ~2 effective radii, with line ratios falling into the LINER region of BPT diagrams. Outside the nuclear region of ETGs, the T-ratio in generally close to unity suggesting that the ionizing photon budget is mainly provided by post-AGB stars.

Galaxy surveys using Integral Field Spectroscopy: Achievements and Opportunities
9th Potsdam Thinkshop
Potsdam, Germany
September 2012

Type: Oral comunication