The z = 6.6 Lyman-α emitter 'CR7' has been claimed to have a Population IIIlike stellar population, or alternatively, be a candidate Direct Collapse Black Hole (DCBH). In this paper we investigate the evidence for these exotic scenarios using recently available, deeper, optical, near-infrared and mid-infrared imaging. We find strong Spitzer /IRAC detections for the main component of CR7 at 3.6µm and 4.5µm, and show that it has a blue colour ([3.6]-[4.5] = −1.2 ± 0.3). This colour cannot be reproduced by current Pop. III or pristine DCBH models. Instead, the results suggest that the [3.6] band is contaminated by the [OIII]λ 4959, 5007 emission line with an implied rest-frame equivalent width of EW 0 (Hβ + [OIII]) 2000Å. Furthermore, we find that new near-infrared data from the UltraVISTA survey supports a weaker HeII λ 1640 emission line than previously measured, with EW 0 = 40 ± 30Å. For the fainter components of CR7 visible in Hubble Space Telescope imaging, we find no evidence that they are particularly red as previously claimed, and show that the derived masses and ages are considerably uncertain. In light of the likely detection of strong [OIII] emission in CR7 we discuss other more standard interpretations of the system that are consistent with the data. We find that a low-mass, narrow-line AGN can reproduce the observed features of CR7, including the lack of radio and X-ray detections. Alternatively, a young, low-metallicity (∼ 1/200 Z ) star-burst, modelled including binary stellar pathways, can reproduce the inferred strength of the HeII line and simultaneously the strength of the observed [OIII] emission, but only if the gas shows super-solar α-element abundances (O/Fe 5 (O/Fe) ).
We now know that a large number of stars are born in multiple systems. Additionally, more than 70% of massive stars are found in close binary systems, meaning that they will interact over the course of their lifetime (Sana et al., 2012). This has strong implications for their evolution as well as the transients (e.g supernovae) and the potential gravitational wave progenitors they produce. Therefore, in order to understand and correctly interpret astronomical observations of stellar populations, we must use theoretical models able to account for the effects of binary stars. This is the case of the Binary Population and Spectral Synthesis code (BPASS) (Eldridge et al., 2017;Stanway & Eldridge, 2018), which has been a staple of the field for over 10 years (Eldridge, Izzard, & Tout, 2008;Eldridge & Stanway, 2009). As is the case for most other theoretical models, the data products of BPASS are large, varied and complex. As a result, their use requires a level of expertise that is not immediately accessible to a wider community that may hold key observational data. The goal of hoki is to bridge the gap between observation and theory, by providing a set of tools to make BPASS data easily accessible and facilitate analysis. The use of Python is deliberate as it is a ubiquitous language within Astronomy. This allows BPASS results to be used naturally within the pre-existing workflow of most astronomers.
The observable characteristics and subsequent evolution of young stellar populations is dominated by their massive stars. As our understanding of those massive stars and the factors affecting their evolution improves, so our interpretation of distant, unresolved stellar systems can also advance. As observations increasingly probe the distant Universe, and the rare low-metallicity starbursts nearby, so the opportunity arises for these two fields to complement one another and leads to an improved conception of both stars and galaxies. Here, we review the current state of the art in modeling of massive star–dominated stellar populations and discuss their applications and implications for interpreting the distant Universe. Our principal findings include the following: ▪ Binary evolutionary pathways must be included to understand the stellar populations in early galaxies. ▪ Observations constraining the extreme ultraviolet spectrum of early galaxies are showing that current models are incomplete. The best current guess is that some form of accretion onto compact remnants is required. ▪ The evolution and fates of very massive stars, on the order of 100 M⊙ and above, may be key to fully understand aspects of early galaxies.
Comparing Galactic chemical evolution models to the observed elemental abundances in the Milky Way, we show that neutron star mergers can be a leading r-process site only if such mergers have very short delay times and/or beneficial masses of the compact objects at low metallicities. Namely, black hole-neutron star mergers, depending on the black-hole spins, can play an important role in the early chemical enrichment of the Milky Way. We also show that none of the binary population synthesis models used in this paper, i.e., COMPAS, StarTrack, Brussels, ComBinE, and BPASS, can currently reproduce the elemental abundance observations. The predictions are problematic not only for neutron star mergers, but also for Type Ia supernovae, which may point to shortcomings in binary evolution models.
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