Predicting the observability of population III stars with ELT-HARMONI via the helium 1640 Å emission line

Abstract

Population III (Pop. III) stars, as of yet, have not been detected, however as we move into the era of extremely large telescopes this is likely to change. One likely tracer for Pop. III stars is the He iiλ1640 emission line, which will be detectable by the HARMONI spectrograph on the European Extremely Large Telescope (ELT) over a broad range of redshifts (2 ≤ z ≤ 14). By post-processing galaxies from the cosmological, AMR-hydrodynamical simulation newhorizon with theoretical spectral energy distributions (SED) for Pop. III stars and radiative transfer (i.e. the Yggdrasil Models and cloudy look-up tables, respectively) we are able to compute the flux of He iiλ1640 for individual galaxies. From mock 10 h observations of these galaxies we show that HARMONI will be able to detect Pop. III stars in galaxies up to z ∼ 10 provided Pop. III stars have a top heavy initial mass function (IMF). Furthermore, we find that should Pop. III stars instead have an IMF similar to those of the Pop. I stars, the He iiλ1640 line would only be observable for galaxies with Pop. III stellar masses in excess of $10^{7}\, {\rm M}_\odot$, average stellar age $\lt 1\, {\rm Myr}$ at z = 4. Finally, we are able to determine the minimal intrinsic flux required for HARMONI to detect Pop. III stars in a galaxy up to z = 10.

EMPRESS. VII. Ionizing Spectrum Shapes of Extremely Metal-Poor Galaxies: Uncovering the Origins of Strong HeII and the Impact on Cosmic Reionization

“…However, the convex downward spectra of J1631+4426 and I Zw 18 NW suggest that contributions from stars to hard photons around 35.1 to 54.4 eV might not be as significant as expected in BPASS binary models. Stellar models including fast-rotating stars (e.g., Kubátová et al 2019) and population III stars (e.g., Grisdale et al 2021) also have convex upward spectra around 13.6 − 54.4 eV, unlike our results. These mismatches suggest that hard radiation from stellar sources cannot solely explain He ii ionizing photons for the two of our galaxies with convex downward spectrum shapes.…”

Conditions for detecting lensed Population III galaxies in blind surveys with the James Webb Space Telescope, the Roman Space Telescope and Euclid

“…Photometrically detected objects showing indicative Pop III color signatures may in principle be targeted for follow-up spectroscopyrendering the He II 𝜆1640 emission line detectable for sufficiently deep exposures. As shown in Grisdale et al (2021), Pop III galaxies with total stellar masses of ≥ 10 4 𝑀 , a top-heavy IMF and a compact stellar distribution would have a detectable He II 𝜆1640 emission line for redshifts 𝑧 = 4 − 10 with the HARMONI spectrograph on the ELT.…”

“…We emphasize that this does not mean that these are the only viable options for studying Pop III signatures in the early Universe. For example, Grisdale et al (2021) recently demonstrated the potential for detecting He II 𝜆1640 emission line from Pop III galaxies using the ground based ELT/HARMONI spectrograph. However, due to the small field of view of this instrument, the success of such an approach hinges on knowing where to point the ELT, as we have no a priori information on the location of Pop III galaxies on the sky 2 .…”

“…Detecting Pop III stars remain one of the milestone observations needed to piece together the puzzle of early structure formation, but individual Pop III stars are expected to be too faint, by a large ★ E-mail: anton.vikaeus@physics.uu.se margin, to be directly observed with present or nascent facilities, unless subject to extreme gravitational lensing (Rydberg et al 2013;Windhorst et al 2018). The exception would be the rare case of a supermassive (𝑀 ∼ 10 5 𝑀 ) Population III star (Haemmerlé et al 2018). These gargantuan stars, considered already in the 1960s as direct sources of the, by then, newly discovered quasi-stellar objects (although argued as dynamically unstable by e.g., Chandrasekhar 1964) now also offer a venue of direct detection of Pop III stars, their supernovae or their resulting direct collapse black holes with only mild magnifications or even without the help of gravitational lensing (Surace et al 2018(Surace et al , 2019Whalen et al 2020).…”

Revealing the formation histories of the first stars with the cosmic near-infrared background

“…The association of the He 𝜆1640 line with Pop III stars has been pursued in the context of both targeted observations (e.g., Nagao et al 2005;Cai et al 2011;) and statistical measurements via the line-intensity mapping technique (e.g., Visbal et al 2015). While possible identifications have been made for objects such as "CR7" (Sobral et al 2015), the measurements are controversial and a solid He 𝜆1640 detection of Pop III stars may not be possible until the operation of next-generation ground-based telescopes such as the E-ELT (Grisdale et al 2021). A number of alternative (and often complementary) probes of Pop III stars have therefore been proposed, including long gamma-ray bursts (GRBs) associated with the explosive death of massive Pop III stars (Mészáros & Rees 2010;Toma et al 2011), caustic transits behind lensing clusters (Windhorst et al 2018), the cosmic near-infrared background (NIRB, Santos et al 2002;Kashlinsky et al 2004;Fernandez & Zaroubi 2013;Yang et al 2015;Helgason et al 2016;Kashlinsky et al 2018), and spectral signatures in the global 21-cm signal (Thomas & Zaroubi 2008;Fialkov et al 2014;Mirocha et al 2018;Mebane et al 2020) and 21-cm power spectrum (Fialkov et al 2013(Fialkov et al , 2014Qin et al 2021).…”