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

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) H-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 [N]/Hα vs.[O]/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/H 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.

The mass–metallicity relation revisited with CALIFA

“…While in retired galaxies (or areas within galaxies) this ionized gas is most probably dominated by post-AGB ionization (e.g. Sarzi et al 2010;Singh et al 2013;Gomes et al 2016), in star-forming galaxies the photon leaking from H ii regions may represent a large contribution to the integrated Hα luminosity (e.g. Relaño et al 2012;Morisset et al 2016), and the SFR estimation.…”

“…They applied aperture corrections to the SFR (Brinchmann et al 2004), due to the strong aperture effects of the SDSS single fiber spectroscopic information (e.g. Iglesias-Páramo et al 2013;Gomes et al 2016). However, they did not apply any aperture correction to oxygen abundance indicators, what could be substantially important (e.g.…”

Emission-line diagnostics of nearby H ii regions including interacting binary populations

“…In the absence of integral field spectroscopy, which allows estimates of aperture losses on the parameters of entire galaxies (e.g. Duarte Puertas et al 2017;Iglesias-Páramo et al 2013Gomes et al 2016), a limiting factor is the combination of a spectral aperture with the seeing at the time of observation. In Table 1 we provide an estimate of the projected physical aperture size of the 2 wide slit at the distance of each galaxy.…”

The EDGE-CALIFA Survey: Evidence for Pervasive Extraplanar Diffuse Ionized Gas in Nearby Edge-on Galaxies

“…López-Cobá et al (2019) studied ionization by shocks in the context of galactic outflows using CALIFA. HOLMES are commonly found in retired galaxies (galaxies which lie below the star-forming main sequence) and in the bulges and halos of disk galaxies and produce line ratios similar to low-ionization nuclear emission-line regions (LINERs) (Sarzi et al 2010;Belfiore et al 2016;Gomes et al 2016;Lacerda et al 2018). Recent ionization models of hot, young stars in the midplane of galaxies, however, can reproduce observed line ratios in the DIG indicating that additional ionization mechanisms may not be necessary (Weber et al 2019).…”