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

Abstract

Deep spectroscopy of galaxies in the reionization era has revealed intense C III] and C IV line emission (EW >15 − 20 Å). In order to interpret the nebular emission emerging at z > 6, we have begun targeting rest-frame UV emission lines in galaxies with large specific star formation rates (sSFRs) at 1.3 < z < 3.7. We find that C III] reaches the EWs seen at z > 6 only in large sSFR galaxies with [O III]+Hβ EW >1500 Å. In contrast to previous studies, we find that many galaxies with intense [O III] have weak C III] emission (EW =5 − 8 Å), suggesting that the radiation field associated with young stellar populations is not sufficient to power strong C III]. Photoionization models demonstrate that the spread in C III] among systems with large sSFRs ([O III]+Hβ EW >1500 Å) is driven by variations in metallicity, a result of the extreme sensitivity of C III] to electron temperature. We find that the strong C III] emission seen at z > 6 (EW >15 Å) requires metal poor gas (≃ 0.1 Z⊙) whereas the weaker C III] emission in our sample tends to be found at moderate metallicities (≃ 0.3 Z⊙). The luminosity distribution of the C III] emitters in our z ≃ 1 − 3 sample presents a consistent picture, with stronger emission generally linked to low luminosity systems (MUV > −19.5) where low metallicities are more likely. We quantify the fraction of strong C III] and C IV emitters at z ≃ 1 − 3, providing a baseline for comparison against z > 6 samples. We suggest that the first UV line detections at z > 6 can be explained if a significant fraction of the early galaxy population is found at large sSFR (>200 Gyr−1) and low metallicity (<0.1 Z⊙).

Ultraviolet spectra of extreme nearby star-forming regions: Evidence for an overabundance of very massive stars

“…While C iii and O iii have similar ionization potentials and our systems power extremely prominent [O iii] emission, the C iii] doublet is far more sensitive to electron temperature than the optical [O iii] lines (e.g. Jaskot & Ravindranath 2016;Tang et al 2020). As a result, C iii] is suppressed more strongly by the drop in temperatures associated with the increased efficiency of metal-line cooling at these high gasphase metal abundances, producing the relatively sharp evolution with metallicity we observe.…”

Recovery and analysis of rest-frame UV emission lines in 2052 galaxies observed with MUSE at 1.5 <z< 6.4

“…In the right panel we see that the full sample of PIM-PDFs predicts systems (of star-forming models) that span a range in metallicity from roughly 5 × 10 −3 to 3 × 10 −2 (corresponding to 0.32 Z/Z 1.97) with a mean value of approximately solar at Z = 0.01524 indicated by the vertical gray dashed line. As mentioned earlier, Tang et al (2021a) argue that EW(C iii) > 10 Å mostly requires log 10 (U) −2.5 and metallicity of 0.2 Z or lower. If we add the results from the PIM-PDFs based on the emission line fluxes from the collection of UV line emitters from the literature, the distribution of the best-fit log 10 (U) stays roughly the same, whereas the Z distribution shifts to slightly lower values with a mean of subsolar metallicity at Z ≈ 10 −2 (Z/Z ≈ 0.7).…”

“…In line with these findings, multiple studies including Maseda et al ( 2017 Jaskot & Ravindranath (2016) stress that both EW 0 ([O iii]λ 5007 + Hβ) and EW 0 (C iii) reach their largest values for young, high ionization parameter models, but confirm that C iii emission is more sensitive to metallicity due to its temperature dependence. Tang et al (2021a) show that EW 0 (C iii) depends strongly on the metallicity of the emitting system and that you generally need low metallicity to obtain large EW 0 (C iii). They find that EW 0 (C iii) increases by a factor three when the metallicity changes from 0.3 to 0.1 of solar metallicity, whereas [O iii]λ5007 + Hβ varies little with metallicity in their models.…”

“…They find that EW 0 (C iii) increases by a factor three when the metallicity changes from 0.3 to 0.1 of solar metallicity, whereas [O iii]λ5007 + Hβ varies little with metallicity in their models. Hence, Tang et al (2021a) argue that to obtain EW 0 (C iii) > 10 Å requires an ionization parameter log 10 (U) above −2.5 and a metallicity of 0.2 solar or lower. The fact that we see several such systems at redshift three (lower right panel in Fig.…”

The Interstellar Medium of Dwarf Galaxies

“…A detailed comparison of compact galaxies at low redshift as analogues of high redshift star-forming galaxies has been carried out by [208]. 25,000 compact star-forming z < 1.0 galaxies from the Sloan Digital Sky Survey (SDSS) have been selected and their properties were compared with those of galaxies at z > 1.5, deduced by various authors (e.g., [209][210][211][212]). Those low redshift galaxies with Hβ equivalent widths > ∼ 100 Å and being located in the star-forming region of the BPT diagram (Baldwin et al, 1981) show remarkable similarities with their higher redshift cousins.…”