The properties of He IIλ1640 emitters at z ∼  2.5–5 from the VANDELS survey

Abstract

Aims. Strong He ii emission is produced by low-metallicity stellar populations and in this study, we aim to identify and study a sample of He ii λ1640 emitting galaxies at redshifts of z − 2.5 − 5 in the deep VANDELS spectroscopic survey. Methods. We identify a total of 33 Bright He ii emitters (SNR > 2.5) and 17 Faint emitters (SNR < 2.5) in the VANDELS survey. We use the available deep multiwavelength data to study their physical properties. After identifying 7 potential AGN in our sample and discarding them from further analysis, we divide the sample of Bright emitters into 20 Narrow (FWHM < 1000 km s −1 ) and 6 Broad (FWHM > 1000 km s −1 ) He ii emitters. We create stacks of Faint, Narrow and Broad emitters and measure other rest-frame UV lines such as O iii] and C iii] in both individual galaxies and stacks. We then compare the UV line ratios with the output of stellar population synthesis models to study the ionising properties of He ii emitters. Results. We do not see a significant difference between the stellar masses, star-formation rates and rest-frame UV magnitudes of galaxies with He ii and no He ii emission. The stellar population models reproduce the observed UV line ratios from metals in a consistent manner, however they under-predict the total number of He ii ionising photons, confirming earlier studies and suggesting that additional mechanisms capable of producing He ii, such as X-ray binaries or stripped stars are needed. The models favour subsolar metallicites (∼ 0.1Z ) and young stellar ages (10 6 − 10 7 years) for the He ii emitters. However, the metallicity measured for He ii emitters is comparable to that of non He ii emitters at similar redshifts. We argue that galaxies with He ii emission may have undergone a recent star-formation event, or may be powered by additional sources of He ii ionisation.

The NIRVANDELS Survey: a robust detection of α-enhancement in star-forming galaxies at z ≃ 3.4

“…The fact that the BPASSv2.2 models lie closer to the observed line ratios clearly indicates that increasing the strength of the ionizing continuum at fixed ★ helps to alleviate this problem, and that an insufficiently hard ionizing continuum is likely to be contributing to the discrepancy. Indeed, this conclusion has also frequently been reached with respect to modelling the He emission line in metal-poor starforming galaxies across a large range of redshifts (e.g., Plat et al 2019;Saxena et al 2020). Additionally, the offset may be related to the fact that most photoionization models treat galaxies as single H regions, when in reality the observed spectra are a line luminosityweighted averages of many H regions, each with distinct physical properties.…”

Spectroscopic evolution of massive stars near the main sequence at low metallicity

“…Possible sources for such hard radiation are (in addition to Wolf-Rayet stars themselves) population III stars (Schaerer 2003), massive stars undergoing quasi-chemically homogeneous evolution (Kubátová et al 2019), stripped binary stars (Götberg et al 2017), X-ray binaries (Schaerer et al 2019), and radiative shocks (Allen et al 2008). Saxena et al (2020) report EW values of ∼1−4 Å in a sample of He ii 1640-emitting galaxies at redshift 2.5−5.0 (see also Steidel et al 2016;Patrício et al 2016). Slightly larger values (5 to 30 Å) are given by Nanayakkara et al (2019) at redshifts from 2 to 4, while values lower than 1 Å are also reported by Senchyna et al (2017) in nearby galaxies.…”

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

“…As Saxena et al (2020) noted, there is a slight caveat of the stellar metallicity measurement method from Cullen et al (2019) used in their work. With this method, the template fitting used to determine stellar metallicities from features in the rest-frame UV spectra assumes a constant star-formation history.…”

“…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.…”

“…Some stellar synthesis models including binary stars can reproduce the UV emission line ratios of He II, O III] and C III] of galaxies that show He II λ1640 line observed at high redshifts. However, these models still underpredict the observed equivalent widths (EWs) of the He II line (Nanayakkara et al 2019;Saxena et al 2020). 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.…”