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

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Marino, Raffaella Anna and Catalán Torrecilla, Cristina and ..., otros. (2016) Spectroscopic aperture biases in inside-out evolving early-type galaxies from CALIFA. Astronomy & Astrophysics, 586 . ISSN 1432-0746

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Official URL: http://dx.doi.org/10.1051/0004-6361/201527312




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.


Item Type:Article
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© ESO, 2016.
Artículo firmado por 23 autores.
This paper is based on data from the Calar Alto Legacy Integral Field Area Survey, CALIFA (http://califa.caha.es), funded by the Spanish Ministery of Science under grant ICTS-2009-10, and the Centro Astronómico Hispano-Alemán. JMG acknowledges support by Fundacao para a Ciencia e a Tecnologia (FCT) through the Fellowship SFRH/BPD/66958/2009 and POPH/FSE (EC) by FEDER funding through the program Programa Operacional de Factores de Competitividade (COMPETE). P.P. is supported by FCT through the Investigador FCT Contract No. IF/01220/2013 and POPH/FSE (EC) by FEDER funding through the program COMPETE. J.M.G., P.P., I.B. and S.N.d.R. acknowledge support by FCT under project FCOMP-01-0124-FEDER-029170 (Reference FCT PTDC/FIS-AST/3214/2012), funded by FCT-MEC (PIDDAC) and FEDER (COMPETE). They also acknowledge support by the exchange programme "Study of Emission-Line Galaxies with Integral-Field Spectroscopy" (SELGIFS, FP7-PEOPLE-2013-IRSES-612701), funded by the EU through the IRSES scheme. SFS acknowledges support from CONACyT-180125 and PAPIIT-IA100815 grants. Support for LG is provided by the Ministry of Economy, Development, and Tourism's Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. L.G. acknowledges support by CONICYT through FONDECYT grant 3140566. C.J.W. acknowledges support through the Marie Curie Career Integration Grant 303912. I.M. acknowledges financial support by the Junta de Andalucía through project TIC114, and the Spanish Ministry of Economy and Competitiveness (MINECO) through projects AYA2010-15169 and AYA2013-42227-P. R.A. Marino is funded by the Spanish program of International Campus of Excellence Moncloa (CEI). This research made use of the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

Uncontrolled Keywords:Digital sky survey; Integral-field spectroscopy; Star-forming galaxies; Evolutionary sequences; Stellar models; Radiative opacities; Sauron project; Lenticular galaxies; Elliptic galaxies; Retired galaxies
Subjects:Sciences > Physics > Astrophysics
Sciences > Physics > Astronomy
ID Code:37335
Deposited On:26 Apr 2016 12:32
Last Modified:10 Dec 2018 15:05

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