Volume 586, February 2016
|Number of page(s)||6|
|Published online||21 January 2016|
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
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
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.
Key words: galaxies: elliptical and lenticular, cD / galaxies: nuclei / galaxies: ISM / galaxies: star formation
© ESO, 2016
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