RELICS: spectroscopy of gravitationally lensed z ≃ 2 reionization-era analogues and implications for C iii] detections at z > 6

Abstract

Recent observations have revealed the presence of strong CIII] emission (EW CIII] > 20Å) in z > 6 galaxies, the origin of which remains unclear. In an effort to understand the nature of these line emitters, we have initiated a survey targeting CIII] emission in gravitationally-lensed reionization era analogs identified in HST imaging of clusters from the RELICS survey. Here we report initial results on four galaxies selected to have low stellar masses (2-8×10 7 M ) and J 125 -band flux excesses indicative of intense [OIII]+Hβ emission (EW [OIII]+Hβ =500-2000Å), similar to what has been observed at z > 6. We detect CIII] emission in three of the four sources, with the CIII] EW reaching values seen in the reionization era (EW CIII] 17 − 22Å) in the two sources with the strongest optical line emission (EW [OIII]+Hβ 2000Å). We have obtained a Magellan/FIRE near-infrared spectrum of the strongest CIII] emitter in our sample, revealing gas that is both metal poor and highly ionized. Using photoionization models, we are able to simultaneously reproduce the intense CIII] and optical line emission for extremely young (2-3 Myr) and metal poor (0.06-0.08 Z ) stellar populations, as would be expected after a substantial upturn in the SFR of a low mass galaxy. The sources in this survey are among the first for which CIII] has been used as the primary means of redshift confirmation. We suggest that it should be possible to extend this approach to z > 6 with current facilities, using CIII] to measure redshifts of objects with IRAC excesses indicating EW [OIII]+Hβ 2000Å, providing a method of spectroscopic confirmation independent of Lyα.

The Mass–Metallicity Relation at z ≃ 8: Direct-method Metallicity Constraints and Near-future Prospects

“…Nonetheless, higher densities are possible (e.g., n e >1000 cm −3 in extremẽ z 3 starbursts; Zhang et al 2018) in which case true metallicities would be above the nominal bounds. e.g., Labbé et al 2013;Laporte et al 2014;Smit et al 2014Smit et al , 2015Huang et al 2016;Roberts-Borsani et al 2016), confirmed in lower-redshift analogs (e.g., Mainali et al 2020 O is very large, but the expected effect is of order ∼1% for the conditions described above (e.g., Izotov et al 2006;Guseva et al 2012 O has been underestimated, offsetting the overall effect. Hence, we consider the ICF from + O 3 and higher ionization states to be negligible.…”

Rest-frame UV spectroscopy of extreme [O iii] emitters at 1.3 < z < 3.7: toward a high-redshift UV reference sample for JWST

“…The rest-frame UV spectra presented in this subsection build on a large spectroscopic effort to obtain rest-frame optical spectra of extreme [O III] emitters at z = 1.3 − 3.7 in the CANDELS fields (T19; Tang et al in prep) and the recent rest-frame UV spectroscopic surveys presented in Du et al (2020) and Mainali et al (2020). We direct the reader to T19 for the full description of the sample selection and the follow-up near-infrared (rest-frame optical) spectroscopic observations.…”

“…Our goals are twofold. First we aim to build a large enough sample of z 1 − 3 galaxies with [O III]+Hβ EW = 1000 − 2000 Å such that we can assess whether the C III] intensities seen at z > 6 (i.e., C III] EW > 20 Å) commonly arise in galaxies with extremely young stellar populations, as suggested by the single galaxy identified in Mainali et al (2020). And secondly, we seek to quantify the rate at which very metal poor stellar populations (< 0.1 Z ) appear among low mass extreme line emitting galaxies at z 1 − 3 via measurement of the fraction of nebular C IV emitters (with line ratios indicating a stellar origin) in our spectroscopic sample.…”

Probing Cosmic Dawn : Ages and Star Formation Histories of Candidate $z\geq$9 Galaxies

“…HST upper limits are showed at 1𝜎, IRAC limits are at 2𝜎 expected to be significantly diminished via resonant scattering by neutral hydrogen. For this reason, various efforts have been made to target metallic UV lines such as [C III]1909 Å and C IV1548 Å with limited success (Laporte et al 2017;Mainali et al 2020). Surprisingly, for 11 galaxies with spectroscopic redshifts above 𝑧 = 7.5, Lyman-𝛼 was detected in 7 with additional UV metallic lines in only 4 cases.…”