The two z ∼ 13 galaxy candidates HD1 and HD2 are likely not lensed

Abstract

ABSTRACT The discovery of two ultraviolet (UV)-bright galaxy candidates at z ∼ 13, HD1 and HD2, laid the foundation for a new race to study the early Universe. Previous investigations suggested that they are powered either by a supermassive black hole or by an extreme, transient burst of star formation. Given their uncertain nature, we investigate whether these sources could be lensed by a hitherto undetected, faint foreground galaxy. We find that at the current limiting magnitude with which HD1 and HD2 were imaged, there is only a $7.39{{\ \rm per\ cent}}$ probability they are strongly lensed by spherical deflectors and that the hypothetical lensing galaxy was too faint to be detected. Meanwhile, with the limiting magnitudes of the Hubble Space Telescope (HST) and JWST, the theoretical probability would drop precipitously to 0.058 and $0.0012{{\ \rm per\ cent}}$, respectively. We further find it unlikely that the luminosities of both sources can be accounted for by lensing that produces a single, resolved image with sufficiently high magnification. Alternatively, in the unlikely event that their brightness results from lensing by an elliptical isothermal galaxy, there is a $30.9 {{\ \rm per\ cent}}$ probability that the lensing galaxy is too faint to be observable at the current limiting magnitude. Future HST (JWST) imaging will drop this probability to $0.245 {{\ \rm per\ cent}}$ ($0.0025 {{\ \rm per\ cent}}$). In summary, while deep imaging with HST and JWST is required to discard the lensing hypothesis entirely, it is unlikely that the exceptional luminosity of the two z ∼ 13 sources can be accounted for by gravitational lensing.