Spectroscopic aperture biases in inside-out evolving early-type galaxies from CALIFA^⋆

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Centro de Astrofísica da Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: jean@astro.up.pt
² Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n Aptdo. 3004, 18080 - Granada, Spain
³ Estación Experimental de Zonas Aridas (CSIC), Ctra. de Sacramento s.n., La Cañada, 04120 Almería, Spain
⁴ Institut d’Astrophysique de Paris, UMR 7095, CNRS, Université Pierre et Marie Curie, 98 bis boulevard Arago, 75014 Paris, France
⁵ Instituto de Astronomía,Universidad Nacional Autonóma de Mexico, A.P. 70-264, 04510, México, D.F., Mexico
⁶ University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria
⁷ Sydney Institute for Astronomy, University of Sydney, NSW 2006, Australia
⁸ Millennium Institute of Astrophysics, Chile
⁹ Departamento de Astronomía, Universidad de Chile, Casilla 36-D, 1058 Santiago, Chile
¹⁰ Astronomical Institute of the Ruhr-University Bochum, Universitätsstr. 150, 44580 Bochum, Germany
¹¹ RUB Research Department “Plasmas with Complex Interactions”, Universitätsstr. 150, 44580 Bochum, Germany
¹² Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis E. Erro 1, 72840 Tonantzintla, Puebla, Mexico
¹³ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
¹⁴ CIEMAT, Avda. Complutense 40, 28040 Madrid, Spain
¹⁵ CEI Campus Moncloa, UCM-UPM, Departamento de Astrofísica y CC. de la Atmósfera, Facultad de CC. Físicas, Universidad Complutense de Madrid, Avda. Complutense s/n, 28040 Madrid, Spain
¹⁶ Department of Physics, Institute for Astronomy, ETH Zürich, 8093 Zürich, Switzerland
¹⁷ Departamento de Astrofísica y CC. de la Atmósfera, Universidad Complutense de Madrid, 28040 Madrid, Spain
¹⁸ Australian Astronomical Observatory, PO Box 915, North Ryde, NSW 1670, Australia
¹⁹ Department of Physics and Astronomy, Macquarie University, NSW 2109, Australia

Received: 5 September 2015
Accepted: 2 November 2015

Abstract

Integral field spectroscopy (IFS) studies based on CALIFA survey data have recently revealed ongoing low-level star formation (SF) in the periphery of a small fraction (~10%) of local early-type galaxies (ETGs), witnessing a still ongoing inside-out galaxy growth process. A distinctive property of the nebular component in these ETGs, classified i+, is a structure with two radial zones, the inner of which displays LINER emission with a Hα equivalent width EW(Hα) ≃ 1 Å, the outer (3 Å <EW(Hα) ≲ 20 Å) Hii-region characteristics. Using CALIFA IFS data, we empirically demonstrate that the confinement of nebular emission to the galaxy periphery leads to a strong aperture (or, correspondingly, redshift) bias in spectroscopic single-fiber studies of type i+ ETGs: at low redshift (z ≲ 0.45), SDSS spectroscopy is restricted to the inner (SF-devoid LINER) zone, which causes the galaxies to be erroneously classified as “retired”, that is, systems entirely lacking SF, and whose faint nebular emissionis solely powered by the post-AGB stellar component. The SDSS aperture progressively encompasses the outer SF zone only at higher z, at which the galaxies are unambiguously classified as “composite SF/LINER”. We also empirically demonstrate that the principal effect of a decreasing spectroscopic aperture on the classification of i+ ETGs through standard [Nii]/Hα vs. [Oiii]/Hβ emission-line (BPT) ratios consists of a monotonic shift upward and to the right precisely along the upper right wing of the “seagull” distribution on the BPT plane, that is, along the pathway connecting composite SF/Hii galaxies with AGN/LINERs. Motivated by these observational insights, we also investigate theoretically observational biases in aperture-limited studies of inside-out growing galaxies as a function of z. To this end, we devise a simple 1D model that involves an outward-propagating exponentially decreasing SF process since z ~ 10 and reproduces the radial extent and two-zone EW(Hα) distribution of local i+ ETGs. By simulating the 3′′ spectroscopic SDSS aperture in this model, we find that SDSS studies at z ≲ 1 are progressively restricted to the inner (SF-devoid LINER) zone and miss an increasingly larger portion of the Hα-emitting periphery. This leads to the incorrect spectroscopic classification of these inside-out assembling galaxies as retired ETG/LINERs and also to a severe underestimation of their total star formation rate (SFR) in a manner inversely related to z. More specifically, the SFR inferred from the Hα luminosity registered within the SDSS fiber is reduced by 50% at z ~ 0.86, reaching only 0.1% of its integral value at z = 0.1. We argue that the aperture-driven biases described above pertain to any morphological analog of i+ ETGs (e.g., SF-quiescent bulges within star-forming disks), regardless of whether it is viewed from the perspective of inside-out growth or inside-out SF quenching, and might be of considerable relevance to galaxy taxonomy and studies of the cosmic SFR density as a function of z.