Stars and gas in the most metal-poor galaxies I: SBS 0335-052E

<p>Understanding the properties of stars and gas at low metallicity is of large relevance for a variety of fields in astrophysics, since it relates to multiple topical questions which range from understanding the properties of galaxies that contributed to cosmic reionization to the evolution of metal-poor massive stars that give rise to the formation of heavy binary black holes. Crucial are observational constraints for the theoretical predictions. These can be obtained from rest-frame ultraviolet and optical spectra of the most metal-poor galaxies known. Among these, SBS 0335-052E (z=0.0135, 12+log(O/H)=7.3) is one of the most well- studied. For this galaxy, we present Hubble Space Telescope (HST) / Cosmic Origins Spectrograph (COS) detections of the C IV 1549, 1551, He II 1640, O III 1661, 1666, [C III] 1907, and C III] 1909 UV emission lines; and a Very Large Telescope (VLT) / Multi Unit Spectroscopic Explorer (MUSE) spectrum covering from 4600 to 9400 Ang, which is co-spatial with the UV data and integrated over the same area. Using these datasets we test: a) the latest Charlot & Bruzal spectral synthesis models with very massive (300 Msun) single non-rotating stars; b) the performance of the spectral analysis tool, BayEsian Analysis of GaLaxy sEds (BEAGLE); and c) the extent to which physical properties of the gas and dust derived independently from the UV and optical with BEAGLE are constrained.</p

Near-infrared emission line diagnostics for AGN from the local Universe to z ∼ 3

Optical rest-frame spectroscopic diagnostics are usually employed to distinguish between star formation and active galactic nucleus (AGN) powered emission. However, this method is biased against dusty sources, hampering a complete census of the AGN population across cosmic epochs. To mitigate this effect, it is crucial to observe at longer wavelengths in the rest-frame near-infrared (near-IR), which is less affected by dust attenuation and can thus provide a better description of the intrinsic properties of galaxies. AGN diagnostics in this regime have not been fully exploited so far, due to the scarcity of near-IR observations of both AGN and star-forming galaxies, especially at redshifts higher than 0.5. Using Cloudy photoionization models, we identified new AGN star formation diagnostics based on the ratio of bright near-IR emission lines, namely [SIII] 9530 Å, [CI] 9850 Å, [PII] 1:188 μ m, [FeII] 1:257 μ m, and [FeII] 1:64 μ m to Paschen lines (either Pa or Paß), providing simple, analytical classification criteria. We applied these diagnostics to a sample of 64 star-forming galaxies and AGN at 0 = z = 1, and 65 sources at 1 = z = 3 recently observed with JWST-NIRSpec in CEERS. We find that the classification inferred from the near-IR is broadly consistent with the optical one based on the BPT and the [SII]/Ha ratio. However, in the near-IR, we find ∼60% more AGN than in the optical (13 instead of eight), with five sources classified as "hidden" AGN, showing a larger AGN contribution at longer wavelengths, possibly due to the presence of optically thick dust. The diagnostics we present provide a promising tool to find and characterize AGN from z = 0 to z 3 with low- and medium-resolution near-IR spectrographs in future surveys.</p