We analyze a sample of 25 [Ne
Spectroscopic studies of extreme-ionization galaxies (EIGs) are critical to our understanding of exotic systems throughout cosmic time. These EIGs exhibit spectral features requiring >54.42 eV photons: the energy needed to ionize helium into He2+fully and emit He
- NSF-PAR ID:
- 10435989
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal
- Volume:
- 953
- Issue:
- 1
- ISSN:
- 0004-637X
- Format(s):
- Medium: X Size: Article No. 10
- Size(s):
- ["Article No. 10"]
- Sponsoring Org:
- National Science Foundation
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Abstract v ] (λ 3426) emission-line galaxies at 1.4 <z < 2.3 using Hubble Space Telescope/Wide Field Camera 3 G102 and G141 grism observations from the CANDELS Lyα Emission at Reionization (CLEAR) survey. [Nev ] emission probes extremely energetic photoionization (creation potential of 97.11 eV) and is often attributed to energetic radiation from active galactic nuclei (AGNs), shocks from supernovae, or an otherwise very hard ionizing spectrum from the stellar continuum. In this work, we use [Nev ] in conjunction with other rest-frame UV/optical emission lines ([Oii ]λ λ 3726, 3729, [Neiii ]λ 3869, Hβ , [Oiii ]λ λ 4959, 5007, Hα +[Nii ]λ λ 6548, 6583, [Sii ]λ λ 6716, 6731), deep (2–7 Ms) X-ray observations (from Chandra), and mid-infrared imaging (from Spitzer) to study the origin of this emission and to place constraints on the nature of the ionizing engine. The majority of the [Nev ]-detected galaxies have properties consistent with ionization from AGNs. However, for our [Nev ]-selected sample, the X-ray luminosities are consistent with local (z ≲ 0.1) X-ray-selected Seyferts, but the [Nev ] luminosities are more consistent with those fromz ∼ 1 X-ray-selected QSOs. The excess [Nev ] emission requires either reduced hard X-rays or a ∼0.1 keV excess. We discuss possible origins of the apparent [Nev ] excess, which could be related to the “soft (X-ray) excess” observed in some QSOs and Seyferts and/or be a consequence of a complex/anisotropic geometry for the narrow-line region, combined with absorption from a warm, relativistic wind ejected from the accretion disk. We also consider implications for future studies of extreme high-ionization systems in the epoch of reionization (z ≳ 6) with the James Webb Space Telescope. -
Abstract We present rest-frame optical emission-line flux ratio measurements for five
z > 5 galaxies observed by the James Webb Space Telescope Near-Infared Spectrograph (NIRSpec) in the SMACS 0723 Early Release Observations. We add several quality-control and post-processing steps to the NIRSpec pipeline reduction products in order to ensure reliablerelative flux calibration of emission lines that are closely separated in wavelength, despite the uncertainabsolute spectrophotometry of the current version of the reductions. Compared toz ∼ 3 galaxies in the literature, thez > 5 galaxies have similar [Oiii ]λ 5008/Hβ ratios, similar [Oiii ]λ 4364/Hγ ratios, and higher (∼0.5 dex) [NeIII ]λ 3870/[OII ]λ 3728 ratios. We compare the observations to MAPPINGS V photoionization models and find that the measured [NeIII ]λ 3870/[OII ]λ 3728, [Oiii ]λ 4364/Hγ , and [Oiii ]λ 5008/Hβ emission-line ratios are consistent with an interstellar medium (ISM) that has very high ionization ( , units of cm s−1), low metallicity (Z /Z ⊙≲ 0.2), and very high pressure ( , units of cm−3). The combination of [Oiii ]λ 4364/Hγ and [Oiii ]λ (4960 + 5008)/Hβ line ratios indicate very high electron temperatures of , further implying metallicities ofZ /Z ⊙≲ 0.2 with the application of low-redshift calibrations for “T e -based” metallicities. These observations represent a tantalizing new view of the physical conditions of the ISM in galaxies at cosmic dawn. -
Abstract We use Hubble Space Telescope Wide Field Camera 3 G102 and G141 grism spectroscopy to measure rest-frame optical emission-line ratios of 533 galaxies at
z ∼ 1.5 in the CANDELS Lyα Emission at Reionization survey. We compare [Oiii ]/Hβ versus [Sii ]/(Hα + [Nii ]) as an “unVO87” diagram for 461 galaxies and [Oiii ]/Hβ versus [Neiii ]/[Oii ] as an “OHNO” diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and [Nev ] sources from star-forming galaxies, indicating that the unVO87 properties of star-forming galaxies evolve with redshift and overlap with AGN emission-line signatures atz > 1. The OHNO diagram does effectively separate X-ray AGN and [Nev ]-emitting galaxies from the rest of the population. We find that the [Oiii ]/Hβ line ratios are significantly anticorrelated with stellar mass and significantly correlated with , while [Sii ]/(Hα + [Nii ]) is significantly anticorrelated with . Comparison with MAPPINGS V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-star formation rate (SFR) galaxies. We do not find evidence for redshift evolution of the emission-line ratios outside of the correlations with mass and SFR. Our results suggest that the OHNO diagram of [Oiii ]/Hβ versus [Neiii ]/[Oii ] will be a useful indicator of AGN content and gas conditions in very high-redshift galaxies to be observed by the James Webb Space Telescope. -
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