We analyse a suite of 29 high-resolution zoom-in cosmological hydrodynamic simulations of massive galaxies with stellar masses $M_{\rm star} \gt 10^{10.9} \, \mathrm{M}_\odot$, with the goal of better understanding merger activity among active galactic nuclei (AGN), AGN activity in merging systems, SMBH growth during mergers, and the role of gas content in triggering AGN. Using the radiative transfer code Powderday, we generate HST-WFC3 F160W mock observations of central galaxies at redshift 0.5 < z < 3; convolve each image with a CANDELS-like point spread function; stitch each image into a real CANDELS image; and identify mergers within the synthetic images using commonly adopted non-parametric statistics. We study the connection between mergers and AGN activity in both the simulations and synthetic images and find reasonable agreement with observations from CANDELS. We find that AGN activity is not primarily driven by major mergers (stellar mass ratio > 1:4) except in a select few cases of gas-rich mergers at low redshifts (0.5 < z < 0.9). We also find that major mergers do not significantly grow the central SMBHs, indicating major mergers do not sustain long-term accretion. Moreover, the most luminous AGN in our simulations (Lbol > 1045 erg s−1) are no more likely than inactive galaxies (Lbol < 1043 erg s−1) to be found in merging systems. We conclude that mergers are not the primary drivers of AGN activity in the simulated massive galaxies studied here.
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ABSTRACT -
Abstract 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
ii recombination 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 ]λ 5008, Hβ , [Neiii ]λ 3870, [Oii ]λ λ 3727, 3729, and [Nev ]λ 3427. 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 fromCloudy that 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 Space 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. -
Abstract We report the discovery of an accreting supermassive black hole at
z = 8.679. This galaxy, denoted here as CEERS_1019, was previously discovered as a Lyα -break galaxy by Hubble with a Lyα redshift from Keck. As part of the Cosmic Evolution Early Release Science (CEERS) survey, we have observed this source with JWST/NIRSpec, MIRI, NIRCam, and NIRCam/WFSS and uncovered a plethora of emission lines. The Hβ line is best fit by a narrow plus a broad component, where the latter is measured at 2.5σ with an FWHM ∼1200 km s−1. We conclude this originates in the broadline region of an active galactic nucleus (AGN). This is supported by the presence of weak high-ionization lines (N V, N IV], and C III]), as well as a spatial point-source component. The implied mass of the black hole (BH) is log (M BH/M ⊙) = 6.95 ± 0.37, and we estimate that it is accreting at 1.2 ± 0.5 times the Eddington limit. The 1–8μ m photometric spectral energy distribution shows a continuum dominated by starlight and constrains the host galaxy to be massive (log M/M⊙∼9.5) and highly star-forming (star formation rate, or SFR ∼ 30 M⊙yr−1; log sSFR ∼ − 7.9 yr−1). The line ratios show that the gas is metal-poor (Z /Z ⊙∼ 0.1), dense (n e ∼ 103cm−3), and highly ionized (logU ∼ − 2.1). We use this present highest-redshift AGN discovery to place constraints on BH seeding models and find that a combination of either super-Eddington accretion from stellar seeds or Eddington accretion from very massive BH seeds is required to form this object. -
Abstract We present a search for extremely red, dust-obscured,
z > 7 galaxies with JWST/NIRCam+MIRI imaging over the first 20 arcmin2of publicly available Cycle 1 data from the COSMOS-Web, CEERS, and PRIMER surveys. Based on their red color in F277W−F444W (∼2.5 mag) and detection in MIRI/F770W (∼25 mag), we identify two galaxies, COS-z8M1 and CEERS-z7M1, that have best-fit photometric redshifts of and , respectively. We perform spectral energy distribution fitting with a variety of codes (includingbagpipes ,prospector ,beagle , andcigale ) and find a >95% probability that these indeed lie atz > 7. Both sources are compact (R eff≲ 200 pc) and highly obscured (A V ∼ 1.5–2.5) and, at our best-fit redshift estimates, likely have strong [Oiii ]+Hβ emission contributing to their 4.4μ m photometry. We estimate stellar masses of ∼1010M ⊙for both sources; by virtue of detection in MIRI at 7.7μ m, these measurements are robust to the inclusion of bright emission lines, for example, from an active galactic nucleus. We identify a marginal (2.9σ ) Atacama Large Millimeter/submillimeter Array detection at 2 mm within 0.″5 of COS-z8M1, which, if real, would suggest a remarkably high IR luminosity of ∼1012L ⊙. These two galaxies, if confirmed atz ∼ 8, would be extreme in their stellar and dust masses and may be representative of a substantial population of highly dust-obscured galaxies at cosmic dawn. -
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.