Astronomy and Astrophysics, volume 586A, 22-22 (2016/2-1)
Spectroscopic aperture biases in inside-out evolving early-type galaxies from CALIFA.
GOMES J.M., PAPADEROS P., VILCHEZ J.M., KEHRIG C., IGLESIAS-PARAMO J., BREDA I., LEHNERT M.D., SANCHEZ S.F., ZIEGLER B., DOS REIS S.N., BLAND-HAWTHORN J., GALBANY L., BOMANS D.J., ROSALES-ORTEGA F.F., WALCHER C.J., GARCIA-BENITO R., MARQUEZ I., DEL OLMO A., MOLLA M., MARINO R.A., CATALAN-TORRECILLA C., GONZALEZ DELGADO R.M., LOPEZ-SANCHEZ A.R. (The CALIFA Collaboration)
Abstract (from CDS):
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)≃1Å, the outer (3Å<EW(Hα)≲20Å) 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 emission is 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.
galaxies: elliptical and lenticular, cD - galaxies: nuclei - galaxies: ISM - galaxies: star formation