High-mass X-ray binaries in nearby metal-poor galaxies: on the contribution to nebular He ii emission

Abstract

Despite significant progress both observationally and theoretically, the origin of highionization nebular He ii emission in galaxies dominated by stellar photoionization remains unclear. Accretion-powered radiation from high-mass X-ray binaries (HMXBs) is still one of the leading proposed explanations for the missing He + -ionizing photons, but this scenario has yet to be conclusively tested. In this paper, we present nebular line predictions from a grid of photoionization models with input SEDs containing the joint contribution of both stellar atmospheres and a multi-color disk model for HMXBs. This grid demonstrates that HMXBs are inefficient producers of the photons necessary to power He ii, and can only boost this line substantially in galaxies with HMXB populations large enough to power X-ray luminosities of 10 42 erg/s per unit star formation rate (SFR). To test this, we assemble a sample of eleven low-redshift star-forming galaxies with high-quality constraints on both X-ray emission from Chandra and He ii emission from deep optical spectra, including new observations with the MMT. These data reveal that the HMXB populations of these nearby systems are insufficient to account for the observed He ii strengths, with typical X-ray luminosities or upper limits thereon of only 10 40 -10 41 erg/s per SFR. This indicates that HMXBs are not the dominant source of He + ionization in these metal-poor star-forming galaxies. We suggest that the solution may instead reside in revisions to stellar wind predictions, softer X-ray sources, or very hot products of binary evolution at low metallicity.

X-ray properties of He ii λ 1640 emitting galaxies in VANDELS

“…Our findings are comparable to what was reported by Senchyna et al (2020), who found no strong correlation between L X /SFR and He II /Hβ ratios for a small sample of nearby galaxies either, leading them to conclude that high-mass XRBs are not the dominant sources of He II ionizing photon production. However, our results appear to be inconsistent with the findings of Lebouteiller et al (2017), Schaerer et al (2019), andHeap, Hubeny &Lanz (2019), who reported that contribution from XRBs in the well-studied metal-poor galaxy in the local Universe, I Zw 18, can account for the nebular He II (λ4686) seen in its spectrum.…”

“…In the local Universe (z ∼ 0), the He II λ4868 line is often seen in the spectra of low-mass star-forming galaxies and almost all of them are metal-poor (Garnett et al 1991;Guseva, Izotov & Thuan 2000;Izotov & Thuan 2004;Shirazi & Brinchmann 2012;Kehrig et al 2015Kehrig et al , 2018Berg et al 2016;Senchyna et al 2017). Rest-frame UV observations of some of these z ∼ 0 metal-poor galaxies have revealed the presence of both He II λ1640 as well as C IV λ1540 emission lines (Berg et al 2019;Senchyna et al 2020), reinforcing the idea of high ionization due to massive, metal-poor stars. The samples of He II emitting galaxies at high redshifts have increased too, primarily using lensing (Patrício et al 2016;Berg et al 2018) and large-area spectroscopic surveys (Cassata et al 2013;Nanayakkara et al 2019;Saxena et al 2020), leading to detections of He II emitting galaxies out to z ∼ 4.…”

“…Other physical mechanisms, such as strong shocks (Dopita & Sutherland 1996;Thuan & Izotov 2005;Izotov, Thuan & Privon 2012), stellar rotation mixing leading to higher effective temperatures (Szécsi et al 2015), 'stellar stripping' that results in the rejuvenation of old stars that provide extra He II ionizing photons (Götberg et al 2018(Götberg et al , 2019, presence of metal-free Pop III stars (Schaerer 2003;Cassata et al 2013;Visbal, Bryan & Haiman 2017), low-level active galactic nuclei (AGNs) activity (e.g. Mignoli et al 2019), and contribution from X-ray binaries (XRBs; Garnett et al 1991;Kehrig et al 2015;Stasińska et al 2015;Schaerer, Fragos & Izotov 2019;Senchyna et al 2020) have been proposed as possible explanations to account for the missing He II ionizing photons seen in star-forming galaxies.…”

Recovery and analysis of rest-frame UV emission lines in 2052 galaxies observed with MUSE at 1.5 <z< 6.4

“…Binaries (for example, as implemented in the BPASS models) have in some cases relieved part of the tension (Steidel et al 2016), but they are often not enough. It has therefore been proposed and debated that the inclusion of X-ray binaries (Schaerer et al 2019;Senchyna et al 2020) or energetic shocks (Allen et al 2008;Senchyna et al 2017;Plat et al 2019) could provide means for reproducing the observed He ii fluxes and other lines produced by high-energy ionizing photons like N v and C iv within photoionization model frameworks.…”

A meeting at z ∼ 3: Young massive galaxies and an AGN within 30 kpc of the luminous QSO LBQS 0302–0019

“…Examples are local low-metallicity dwarf galaxies (e.g. Kehrig et al 2015Kehrig et al , 2018Senchyna et al 2020) Mechtley et al 2016;Villforth et al 2017;Marian et al 2019), it is still a potential channel for coordinated galaxy and black hole growth as AGN luminosity, star formation, and gas content seem to be linked (e.g. Rosario et al 2013;Husemann et al 2017;Zhuang et al 2021).…”