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1. CLEAR: High-ionization [Ne v] λ3426 Emission-line Galaxies at 1.4 < z < 2.3

   https://doi.org/10.3847/1538-4357/acc1e6  

   Cleri, Nikko J. ; Yang, Guang ; Papovich, Casey ; Trump, Jonathan R. ; Backhaus, Bren E. ; Estrada-Carpenter, Vicente ; Finkelstein, Steven L. ; Giavalisco, Mauro ; Hutchison, Taylor A. ; Ji, Zhiyuan ; et al ( May 2023 , The Astrophysical Journal)

   We analyze a sample of 25 [Nev] (λ3426) 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 LyαEmission 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]λλ3726, 3729, [Neiii]λ3869, Hβ, [Oiii]λλ4959, 5007, Hα+[Nii]λλ6548, 6583, [Sii]λλ6716, 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”more »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.

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2. Using [Ne v]/[Ne iii] to Understand the Nature of Extreme-ionization Galaxies

   https://doi.org/10.3847/1538-4357/acde55  

   Cleri, Nikko J. ; Olivier, Grace M. ; Hutchison, Taylor A. ; Papovich, Casey ; Trump, Jonathan R. ; Amorín, Ricardo O. ; Backhaus, Bren E. ; Berg, Danielle A. ; Fernández, Vital ; Finkelstein, Steven L. ; et al ( July 2023 , The Astrophysical Journal)

   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 Heiirecombination lines. Spectroscopic studies of EIGs can probe exotic stellar populations or accretion onto intermediate-mass black holes (∼10²–10⁵M_⊙), which are the possibly key contributors to the reionization of the Universe. To facilitate the use of EIGs as probes of high-ionization systems, we focus on ratios constructed from several rest-frame UV/optical emission lines: [Oiii]λ5008, Hβ, [Neiii]λ3870, [Oii]λλ3727, 3729, and [Nev]λ3427. These lines probe the relative intensity at energies of 35.12, 13.62, 40.96, 13.62, and 97.12 eV, respectively, covering a wider range of ionization than traced by other common rest-frame UV/optical techniques. We use the ratios of these lines ([Nev]/[Neiii] ≡ Ne53, [Oiii]/Hβ, and [Neiii]/[Oii]), which are nearby in wavelength, mitigating the effects of dust attenuation and uncertainties in flux calibration. We make predictions from photoionization models constructed fromCloudythat use a broad range of stellar populations and black hole accretion models to explore the sensitivity of these line ratios to changes in the ionizing spectrum. We compare our models to observations from the Hubble Spacemore »Telescope and JWST of galaxies with strong high-ionization emission lines atz∼ 0,z∼ 2, and 5 <z< 8.5. We show that the Ne53 ratio can separate galaxies with ionization from “normal” stellar populations from those with active galactic nuclei and even “exotic” Population III models. We introduce new selection methods to identify galaxies with photoionization driven by Population III stars or intermediate-mass black hole accretion disks that could be identified in upcoming high-redshift spectroscopic surveys.

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3. CLEAR: Emission-line Ratios at Cosmic High Noon

   https://doi.org/10.3847/1538-4357/ac3919  

   Backhaus, Bren E. ; Trump, Jonathan R. ; Cleri, Nikko J. ; Simons, Raymond ; Momcheva, Ivelina ; Papovich, Casey ; Estrada-Carpenter, Vicente ; Finkelstein, Steven L. ; Matharu, Jasleen ; Ji, Zhiyuan ; et al ( February 2022 , The Astrophysical Journal)

   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 atz∼ 1.5 in the CANDELS LyαEmission at Reionization survey. We compare [Oiii]/Hβversus [Sii]/(Hα+ [Nii]) as an “unVO87” diagram for 461 galaxies and [Oiii]/Hβversus [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]/Hβline ratios are significantly anticorrelated with stellar mass and significantly correlated with$\log (L_{\mathrm{H}\beta })$, while [Sii]/(Hα+ [Nii]) is significantly anticorrelated with$\log (L_{\mathrm{H}\beta })$. 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]/Hβversus [Neiii]/[Oii] willmore »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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4. A closer look at NGC 7314 nuclear region: a multiwavelength analysis of the Seyfert nucleus and its surroundings

   https://doi.org/10.1093/mnras/stac3454  

   da Silva, Patrícia ; Menezes, R. B. ; Díaz, Y. ; Rodríguez-Ardila, A. ; López-Navas, E. ; Arévalo, P. ; Hernández-García, L. ( November 2022 , Monthly Notices of the Royal Astronomical Society)

   The central regions of galaxies harbouring active galactic nuclei (AGNs) can be quite complex, especially at high activity, presenting, besides variability, a variety of phenomena related, e.g. to ionization/excitation mechanisms. A detailed study is necessary in order to understand better those objects. For that reason, we performed a multiwavelength analysis of the nuclear region of the nearby Seyfert galaxy NGC 7314, using an optical data cube obtained with the Integral Field Unit from the Gemini Multi-Object Spectrograph, together with Hubble Space Telescope images, X-ray data from the XMM–Newton and the Nuclear Spectroscopic Telescope Array and radio data from Atacama Large Millimeter/Submillimeter Array. The goals were to study the nuclear and circumnuclear emission, the emission of the AGN and the gas kinematics. The optical spectrum shows the emission of a Seyfert nucleus, with broad components in the H α and H β emission lines, characterising a type 1 AGN, with a spectrum rich in coronal emission lines. The spatial morphology of the [O iii] λ5007 suggests the presence of an ionization cone, west of the nucleus, meanwhile the east cone seems to be obscured by dust. An extended [Fe vii] λ6087 emission was also detected, which could be possibly explained by a scenario involving photoionization + shocks mechanisms.more »X-rays analyses showed that there are variations in the flux; however, we did not detect any variations in the column density along the line of sight. Its variability may be a consequence of changes in the AGN accretion rate.

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5. A Near-infrared Look at AGN Feedback in Bulgeless Galaxies

   https://doi.org/10.3847/1538-4357/ac6870  

   Bohn, Thomas ; Canalizo, Gabriela ; Satyapal, Shobita ; Sales, Laura V. ( May 2022 , The Astrophysical Journal)

   While it is generally believed that supermassive black holes (SMBHs) lie in most galaxies with bulges, few SMBHs have been confirmed in bulgeless galaxies. Identifying such a population could provide important insights to the BH seed population and secular BH growth. To this end, we obtained near-infrared (NIR) spectroscopic observations of a sample of low-redshift bulgeless galaxies with mid-infrared colors suggestive of active galactic nuclei (AGNs). We find additional evidence of AGN activity (such as coronal lines and broad permitted lines) in 69% (9/13) of the sample, demonstrating that mid-infrared selection is a powerful tool to detect AGNs. More than half of the galaxies with confirmed AGN activity show fast outflows in [Oiii] in the optical and/or [Sivi] in the NIR, with the latter generally having much faster velocities that are also correlated to their spatial extent. We are also able to obtain virial BH masses for some targets and find they fall within the scatter of other late-type galaxies in theM_BH–M_stellarrelation. The fact that they lack a significant bulge component indicates that secular processes, likely independent of major mergers, grew these BHs to supermassive sizes. Finally, we analyze the rotational gas kinematics and find two notable exceptions: twomore »AGN hosts with outflows that appear to be rotating faster than expected. There is an indication that these two galaxies have stellar masses significantly lower than expected from their dark matter halo masses. This, combined with the observed AGN activity and strong gas outflows, may be evidence of the effects of AGN feedback.

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