4 research outputs found
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JWST CEERS probes the role of stellar mass and morphology in obscuring galaxies
In recent years, observations have uncovered a population of massive galaxies that are invisible or very faint in deep optical/near-infrared (near-IR) surveys but brighter at longer wavelengths. However, the nature of these optically dark or faint galaxies (OFGs; one of several names given to these objects) is highly uncertain. In this work, we investigate the drivers of dust attenuation in the JWST era. In particular, we study the role of stellar mass, size, and orientation in obscuring star-forming galaxies (SFGs) at 3'., '., 1, typically) SFGs, of which OFGs are a specific case. For this subset, we find that the key distinctive feature is their compact size (for massive systems with log(M∗/M™ ) > 10); OFGs exhibit a 30% smaller effective radius than the average SFG at the same stellar mass and redshift. On the contrary, OFGs do not exhibit a preference for low axis ratios (i.e., edge-on disks). The results in this work show that stellar mass is the primary proxy for dust attenuation and compact stellar light profiles behind the thick dust columns obscuring typical massive SFGs.</p
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A z = 1.85 galaxy group in CEERS: evolved, dustless, massive intra-halo light and a brightest group galaxy in the making
We present CEERS JWST/NIRCam imaging of a massive galaxy group at z = 1.85, to explore the early JWST view on massive group formation in the distant Universe. The group contains ≲16 members (including six spectroscopic confirmations) down to log10(M∗/M·) = 8.5, including the brightest group galaxy (BGG) in the process of actively assembling at this redshift. The BGG is comprised of multiple merging components extending ∼3.6 (30 kpc) across the sky. The BGG contributes 69% of the group' s total galactic stellar mass, with one of the merging components containing 76% of the total mass of the BGG and a star formation rate > 1810 M· yr-1. Most importantly, we detected intra-halo light (IHL) in several HST and JWST/NIRCam bands, allowing us to construct a state-of-the-art rest-frame UV-NIR spectral energy distribution of the IHL for the first time at this high redshift. This allows stellar population characterisation of both the IHL and member galaxies, as well as the morphology distribution of group galaxies versus their star formation activity when coupled with Herschel data. We created a stacked image of the IHL, giving us a sensitivity to extended emission of 28.5 mag arcsec-2 at rest-frame 1 μm. We find that the IHL is extremely dust poor (Av ∼ 0), containing an evolved stellar population of log10(t50/yr) = 8.8, corresponding to a formation epoch for 50% of the stellar material 0.63 Gyr before z = 1.85. There is no evidence of ongoing star formation in the IHL. The IHL in this group at z = 1.85 contributes ∼10% of the total stellar mass, comparable with what is observed in local clusters. This suggests that the evolution of the IHL fraction is more self-similar with redshift than predicted by some models, challenging our understanding of IHL formation during the assembly of high-redshift clusters. JWST is unveiling a new side of group formation at this redshift, which will evolve into Virgo-like structures in the local Universe.</p
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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
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Dusty starbursts masquerading as ultra-high redshift galaxies in JWST CEERS observations
Lyman-break galaxy (LBG) candidates at z ? 10 are rapidly being identified in James Webb Space Telescope (JWST)/NIRCam observations. Due to the (redshifted) break produced by neutral hydrogen absorption of rest-frame UV photons, these sources are expected to drop out in the bluer filters while being well detected in redder filters. However, here we show that dust-enshrouded star-forming galaxies at lower redshifts (z ? 7) may also mimic the near-infrared (near-IR) colors of z > 10 LBGs, representing potential contaminants in LBG candidate samples. First, we analyze CEERS-DSFG-1, a NIRCam dropout undetected in the F115W and F150W filters but detected at longer wavelengths. Combining the JWST data with (sub)millimeter constraints, including deep NOEMA interferometric observations, we show that this source is a dusty star-forming galaxy (DSFG) at z ˜ 5.1. We also present a tentative 2.6s SCUBA-2 detection at 850 µm around a recently identified z ˜ 16 LBG candidate in the same field and show that, if the emission is real and associated with this candidate, the available photometry is consistent with a z ~ 5 dusty galaxy with strong nebular emission lines despite its blue near-IR colors. Further observations on this candidate are imperative to mitigate the low confidence of this tentative submillimeter emission and its positional uncertainty. Our analysis shows that robust (sub)millimeter detections of NIRCam dropout galaxies likely imply z ~ 4-6 redshift solutions, where the observed near-IR break would be the result of a strong rest-frame optical Balmer break combined with high dust attenuation and strong nebular line emission, rather than the rest-frame UV Lyman break. This provides evidence that DSFGs may contaminate searches for ultra-high redshift LBG candidates from JWST observations.</p