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

¹ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monteporzio Catone, Italy
e-mail: aayush.saxena@inaf.it
² Dipartimento di Fisica, Sapienza Università di Roma, Piazzale Aldo Moro 5, 00185 Roma, Italy
³ Geneva Observatory, University of Geneva, Ch. des Maillettes 51, 1290 Versoix, Switzerland
⁴ INFN, Sezione di Roma I, Piazzale Aldo Moro 2, 00185 Roma, Italy
⁵ SUPA (Scottish Universities Physics Alliance), Institute for Astronomy, University of Edinburgh, Royal Observatory, EH9 3HJ Edinburgh, UK
⁶ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán, 1305 La Serena, Chile
⁷ Departamento de Física y Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
⁸ INAF – OAS Bologna, Via P. Gobetti 93/3, 40129 Bologna, Italy
⁹ Cosmic DAWN Center, Niels Bohr Institute, University of Copenhagen, Juliane Maries Vej 30, 2100 Copenhagen Ø, Denmark
¹⁰ INAF – IASF Milano, Via Bassini 15, 20133 Milano, Italy
¹¹ Núcleo de Astronomía, Facultad de Ingeniería, Universidad Diego Portales, Av. Ejército 441, Santiago, Chile
¹² Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹³ Department of Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA
¹⁴ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX 77843-4242, USA

Received: 22 November 2019
Accepted: 5 March 2020

Abstract

Aims. Strong He II emission is produced by low-metallicity stellar populations. Here, we aim to identify and study a sample of He IIλ1640-emitting galaxies at redshifts of z ∼ 2.5 − 5 in the deep VANDELS spectroscopic survey.

Methods. We identified a total of 33 Bright He II emitters (S/N >  2.5) and 17 Faint emitters (S/N <  2.5) in the VANDELS survey and used the available deep multi-wavelength data to study their physical properties. After identifying seven potential AGNs in our sample and discarding them from further analysis, we divided 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 created stacks of Faint, Narrow, and Broad emitters and measured other rest-frame UV lines such as O III] and C III] in both individual galaxies and stacks. We then compared 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 are needed, such as X-ray binaries or stripped stars. The models favour subsolar metallicities (∼0.1 Z_⊙) and young stellar ages (10⁶ − 10⁷ 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.