Lyman ‘bump’ galaxies - II. A possible signature of massive extremely metal-poor or metal-free stars in z= 3.1 Lyα emitters★

Abstract

(Abridged) Deep NB359 imaging with Subaru by Iwata et al. have detected\nsurprisingly strong Lyman continuum (LyC; ~900A in the rest-frame) from some\nLAEs at z=3.1. However, the redshifts might be misidentified due to a narrow\nwavelength coverage in previous spectroscopy. We here present new deep\nspectroscopy covering the observed 4,000-7,000A with VLT/VIMOS and Subaru/FOCAS\nof 8 LAEs detected in NB359. All the 8 objects have only one detectable\nemission line around 4,970A which is most likely to be Ly-A at z=3.1, and thus,\nthe objects are certainly LAEs at the redshift. However, 5 of them show a\n~0.''8 spatial offset between the Ly-A emission and the source detected in\nNB359. No indications of the redshifts of the NB359 sources are found although\nit is statistically difficult that all the 5 LAEs have a foreground object\naccounting for the NB359 flux. The rest 3 LAEs show no significant offset from\nthe NB359 position. Therefore, they are truly LyC emitting LAEs at z=3.1. We\nalso examine the stellar population which simultaneously accounts for the\nstrength of the LyC and the spectral slope of non-ionizing ultraviolet of the\nLAEs. We consider the latest statistics of Lyman limit systems to estimate the\nLyC optical depth in the IGM and an additional contribution of the bound-free\nLyC from photo-ionized nebulae to the LyC emissivity. As a result, we find that\nstellar populations with metallicity Z>=1/50Z_sun can explain the observed LyC\nstrength only with a very top-heavy initial mass function (IMF; <m>~50 M_sun).\nHowever, the critical metallicity for such an IMF is expected to be much lower.\nA very young (~1 Myr) and massive (~100 M_sun) extremely metal-poor\n(Z<=5e-4Z_sun) or metal-free (so-called Population III) stellar population can\nreproduce the observed LyC strength. The required mass fraction of such\n`primordial' stellar population is ~1--10% in total stellar mass of the LAEs.\n