Photon leaking or very hard ionizing radiation? Unveiling the nature of He II-emitters using the softness diagram

Abstract

Aims. Star-forming galaxies with nebular He II emission contain very energetic ionizing sources of radiation, which can be considered as analogs to the major contributors of the reionization of the Universe in early epochs. It is therefore of great importance to provide a reliable absolute scale for the equivalent effective temperature (T*) for these sources. Methods. We study a sample of local (z < 0.2) star-forming galaxies showing optical nebular He II emission using the so-called softness diagrams, involving emission lines of two elements in two consecutive stages of ionization (e.g., [S II]/[S III] vs. [O II]/[O III]). We use for the first time the He I/He II ratio in these diagrams in order to explore the higher range of T* expected in these objects, and to investigate the role of possible mechanisms driving the distribution of galaxy points in these diagrams. We build grids of photoionization models covering different black-body temperatures, model cluster atmospheres, and density-bounded geometries to explain the conditions observed in the sample. Results. We verified that the use of the softness diagrams including the emission-line ratio He I/He II combined with black-body photoionization models can provide an absolute scale of T* for these objects. The application of a Bayesian-like code indicates T* in the range 50−80 kK for the sample of galaxies, with a mean value higher than 60 kK. The average of these high temperature values can only be reproduced using cluster model populations with nearly metal-free stars, although such ionizing sources cannot explain either the highest T* values, beyond 1σ, or the dispersion observed in the softness diagrams. According to our photoionization models, most sample galaxies could be affected to some extent by ionizing photon leaking, presenting a mean photon absorption fraction of 26% or higher depending on the metallicity assumed for the ionizing cluster. The entire range of He I/He II, [S II]/[S III], and [O II]/[O III] ratios for these HeII-emitting galaxies is reproduced with our models, combining nearly metal-free ionizing clusters and photon leaking under different density-bounded conditions.

J-PLUS: Uncovering a large population of extreme [OIII] emitters in the local Universe

“…The error bars represent the maximum and minimum value without outliers, which are plotted as dots (and defined as the values that lie beyond 1.5 times the inter quartilic range). We plot in this figure results from the present work and several samples from the literature (Yang et al 2017;Cardamone et al 2009;Amorín et al 2015;Onodera et al 2020;Hsyu et al 2018;Guseva et al 2017;Pérez-Montero et al 2020;Matthee et al 2021;Izotov et al 2021), as well as the KISS survey (Wegner et al 2003;Gronwall et al 2004;Jangren et al 2005;Salzer et al 2005) and the Hα dots class (Kellar et al 2012;Salzer et al 2020). We have selected the subsample with extremely low metallicity in Hsyu et al ( 2018) (H18_XMP), as well as those galaxies with EW(Hβ)>100 Å in both Hsyu et al (2018) XMPs (H18_XMP Hβ>100 Å) and the KISS survey (KISS Hβ>100 Å).…”

Hardness of ionizing radiation fields in MaNGA star-forming galaxies

“…However, we find in Section 3.1 that 𝜂 and 𝜂 are correlated with strong line ratios such that N 2 and O 3 N 2 which are not only sensitive to abundance but also to the ionization parameter. Hence, ionization parameter might be related to radiation hardness as well, as indicated by other works (see e.g Pérez-Montero et al 2020)…”

“…The solid black curve represents the maximum starburst line taken from Shirazi & Brinchmann (2012), and was devised to separate the star-forming galaxies from the AGNs or composite objects. Pérez-Montero et al 2020). The He 𝜆4686 line can only be produced by the sources of hard ionizing radiation because the ionization potential of He is very high (i.e.…”

“…However, only a few works have also explored the use of near-infrared (NIR) CELs such as [S ] 𝜆𝜆9069, 9532 (see e.g. Vilchez & Pagel 1988;Vilchez & Esteban 1996;Díaz & Pérez-Montero 2000;Stasińska 2006;Pérez-Montero & Díaz 2005;Pérez-Montero & Vílchez 2009;Fernández et al 2018;Mingozzi et al 2020;Pérez-Montero et al 2020) mainly due to the following two reasons. First, only a few spectrographs used in galaxy surveys include useful wavelengths above 9000Å.…”

“…The 𝜂 parameter has been used to study the ionization structure and the relative hardness of the ionizing sources in the H regions within the Milky Way and Magellanic Clouds (Martín-Hernández et al 2002;Morisset et al 2004), in the star-forming galaxies (Hägele et al 2006;Kehrig et al 2006;Hägele et al 2008;Pérez-Montero et al 2020) and the radial variation of the hardness of the ionizing radiation of H regions in the discs of spiral galaxies (Pérez-Montero & Vílchez 2009;Pérez-Montero et al 2019). However, it is important to consider other variables while interpreting 𝜂 as radiation hardness.…”