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Pushing JWST to the extremes: search and scrutiny of bright galaxy candidates at z$\simeq$15-30

M. Castellano, A. Fontana, E. Merlin, P. Santini, L. Napolitano, N. Menci, P. G. Pérez-González, A. Calabrò, D. Paris, L. Pentericci, J. Zavala, M. Dickinson, S. L. Finkelstein, T. Treu, R. O. Amorin, P. Arrabal Haro, P. Bergamini, L. Bisigello, M. Catone, E. Daddi, P. Dayal, A. Dekel, A. Ferrara, F. Fortuni, G. Gandolfi, M. Giavalisco, C. Grillo, S. T. Guida, N. P. Hathi, B. W. Holwerda, A. M. Koekemoer, V. Kokorev, Z. Li, M. Llerena, R. A. Lucas, S. Mascia, B. Metha, T. Morishita, T. Nanayakkara, F. Pacucci, G. Roberts-Borsani, G. Rodighiero, P. Rosati, V. Salazar, R. Schneider, R. S. Somerville, A. Taylor, M. Trenti, A. Trinca, X. Wang, P. J. Watson, L. Yang, L. Y. A. Yung

TL;DR

This study targets galaxies at extreme redshifts ($15\leq z \leq 28$) with JWST by applying customized Lyman-break selections to the ASTRODEEP-JWST catalogs across ~0.2 deg$^2$. It demonstrates that five $z\sim15-20$ candidates emerge under strict color cuts, yet spectroscopic follow-up reveals all five are likely low-$z$ interlopers, including dusty star-forming and quiescent systems, highlighting substantial contamination uncertainties at these epochs. By computing the UV luminosity function under different contamination assumptions, the work shows that even a partial presence of genuine $z>15$ galaxies would imply a mild evolution of the bright end inconsistent with many theoretical models, while a null result would require surveys covering much larger areas to detect rarer $M_{UV}\lesssim-20$ galaxies at $z>15$. The paper further argues for deeper observations in the F150W/F200W bands and spectroscopic characterization to design robust future surveys capable of breaking the $z=15$ barrier.

Abstract

We designed customized Lyman-break color selection techniques to identify galaxy candidates in the redshift ranges $15 \leq z \leq 20$ and $20 \leq z \leq 28$. The selection was performed on the ASTRODEEP-JWST multi-band catalogs of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of $\sim0.2$ sq. deg. We identify five candidates at $15 \leq z \leq 20$, while no objects are found based on the $z\gtrsim20$ color selection criteria. Despite exhibiting a $>$1.5 mag break, all the objects display multimodal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z$\sim$3-7. This conclusion is supported by the analysis of five F200W-dropout objects that we find to be interlopers on the basis of NIRSpec PRISM spectra: four dusty star-forming galaxies at z$\sim$2.2-6.6, and a passive galaxy at z=4.91 with log$(M_{\rm star}/{\rm M}_{\odot}) \lesssim$ 9. We measured the UV luminosity function under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates is indeed at $z\gtrsim15$, the resulting UV LF points to a very mild evolution compared to estimates at $z<15$, implying a significant tension with existing theoretical models. In particular, confirming our bright ($M_{\text{UV}}<-21$) candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates will be confirmed to be interlopers, we conclude that future surveys may need ten times wider areas to select $M_{\text{UV}}\lesssim-20$ galaxies at $z>15$. Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands are also required to mitigate contamination from rare red objects at z$\lesssim$8.

Pushing JWST to the extremes: search and scrutiny of bright galaxy candidates at z$\simeq$15-30

TL;DR

This study targets galaxies at extreme redshifts () with JWST by applying customized Lyman-break selections to the ASTRODEEP-JWST catalogs across ~0.2 deg. It demonstrates that five candidates emerge under strict color cuts, yet spectroscopic follow-up reveals all five are likely low- interlopers, including dusty star-forming and quiescent systems, highlighting substantial contamination uncertainties at these epochs. By computing the UV luminosity function under different contamination assumptions, the work shows that even a partial presence of genuine galaxies would imply a mild evolution of the bright end inconsistent with many theoretical models, while a null result would require surveys covering much larger areas to detect rarer galaxies at . The paper further argues for deeper observations in the F150W/F200W bands and spectroscopic characterization to design robust future surveys capable of breaking the barrier.

Abstract

We designed customized Lyman-break color selection techniques to identify galaxy candidates in the redshift ranges and . The selection was performed on the ASTRODEEP-JWST multi-band catalogs of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of sq. deg. We identify five candidates at , while no objects are found based on the color selection criteria. Despite exhibiting a 1.5 mag break, all the objects display multimodal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z3-7. This conclusion is supported by the analysis of five F200W-dropout objects that we find to be interlopers on the basis of NIRSpec PRISM spectra: four dusty star-forming galaxies at z2.2-6.6, and a passive galaxy at z=4.91 with log 9. We measured the UV luminosity function under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates is indeed at , the resulting UV LF points to a very mild evolution compared to estimates at , implying a significant tension with existing theoretical models. In particular, confirming our bright () candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates will be confirmed to be interlopers, we conclude that future surveys may need ten times wider areas to select galaxies at . Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands are also required to mitigate contamination from rare red objects at z8.

Paper Structure

This paper contains 24 sections, 2 equations, 16 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Observations and data analysis
  3. Lyman-break selection at z$\simeq$15-30
  4. Color selection criteria
  5. Selected sources over $\sim$0.2 deg$^2$
  6. A closer scrutiny of the selected candidates
  7. The photometric redshift probability distribution
  8. The nature of potential interlopers
  9. The confirmed interlopers
  10. COSMOS_31168 at $z_{spec}$=2.29
  11. COSMOS_35731 at $z_{spec}$=3.66
  12. CEERS_15937 at $z_{spec}$=4.91
  13. COSMOS_76919 at $z_{spec}$=5.18
  14. UDS_56824 at $z_{spec}$=6.56
  15. Critical assessment
  16. ...and 9 more sections

Figures (16)

  • Figure 1: Color selection diagrams (left panels) for the selection of galaxies at z$\sim$15-20 (top) and z$\sim$20-30 (bottom). The cyan solid lines enclose the regions in which the reference sample to estimate the luminosity functions are selected. The cyan dashed lines enclose regions where the additional extended samples are selected. The relevant redshift distributions of the selected reference (extended) samples are shown in the right panels as continuous (dashed) histograms. The points color-coded according to the relevant redshift show objects from a mock generated over an area of 0.12 sq. deg, with low-redshift populations generated through the EGG software Schreiber2017EGG. Black stars show the position of brown dwarf models from Marley2021. All fluxes have been perturbed with realistic noise properties to reproduce the typical depth of the JADES-GS field. Similar diagrams have been analysed for all fields using both the EGG- and JAGUAR-based simulations described in Sect. \ref{['fig_LBGSIM']}.
  • Figure 2: Observed color selection diagrams for LBGs at z$\sim$15-20 (left) and z$\sim$20-30 (right). The black continuous lines enclose the region where reference samples are selected. Large, filled markers show the position of the objects selected from the various fields, while the seven selected interlopers are shown as open symbols: CEERS-93316 ArrabalHaro2023b, the transient A2744_27713, and the five objects observed with NIRSpec PRISM by the CAPERS survey (Sect. \ref{['subsec:CAPERS_interloper']}). Small markers show the position of candidates in the extended samples selected within the color region enclosed by dashed lines. Objects detected at SNR$>$10 at any redshift in the JADES-GS field are shown as black points to highlight the parameter space where the bulk of ASTRODEEP-JWST objects are found. The colored tracks mark the expected colors of stellar plus nebular BC03 templates at the different redshifts indicated by the relevant labels: high-redshift star-forming galaxies at z$\geq$10 with Age=20 Myr, Z=0.02 Z$_{\odot}$, E(B-V)=0 (blue); passively evolving galaxies at 0$\leq z\leq$10 with Z=0.2 Z$_{\odot}$ formed with an instantaneous burst at z=15 (dark red); dusty objects at 0$\leq z\leq$10 with Age=100 Myr, Z=0.2 Z$_{\odot}$, E(B-V)=0.8 (dark green).
  • Figure 3: Spectral energy distributions, $P(z)$ and NIRCam thumbnails of the five F200W dropout candidates. For each object the best-fit templates at high- and low-redshift from the zphot run are shown in blue and red, respectively. The relevant predicted magnitudes are indicated by blue empty squares and red empty hexagons, respectively. The photometric measurements are from M24, with black (magenta) circles and error-bars indicating JWST (HST) bands. The 2$\sigma$ upper limits are shown as triangles. The $P(z)$ from zphot are shown as orange lines, the ones from EAzY adopting standard (standard plus Larson) templates are shown as continuous (dashed) light blue lines, the $P(z)$ from BAGPIPES are shown in green with continuous, dashed and dotted lines respectively assuming a delayed, double power-law and exponential SFH, and the $P(z)$ from CIGALE using a star-forming (star-forming+AGN) fit are shown as continuous (dashed) magenta lines. All curves are normalized to have $P(z)$=1 at the peak. The 1.2 $\times$ 1.2 arcsec thumbnails, from left to right, respectively show the objects in the F090W (where available), F115W, F150W, F200W, F277W, F356W, F410M and F444W bands used for the ASTRODEEP-JWST measurements.
  • Figure 4: The average redshift probability distribution functions $P(z)$ for objects in the extended samples of F200W dropouts (left) and F277W dropouts (right) computed with zphot, EAzY, BAGPIPES, and CIGALE (same color conventions as in Fig. \ref{['fig_SEDs_ALL']}). The curves are normalized to have $P(z)$=1 at the peak.
  • Figure 5: Top: the position in the $E(B-V)$ vs. $M_{star}$ plane of galaxy templates (points colour-coded according to the redshift) that provide an acceptable fit with probability $P(z)>0.5$ to the F200W dropout candidates. The regions occupied by the 90% to 10%, at 10% steps, of the aforementioned templates are enclosed by dotted curves. The continuous curves enclose the regions occupied by the 90% to 10%, at 10% steps, of the objects in the same redshift range from the JADES-GS field. The black square and error-bars mark the positions of the confirmed interlopers. Bottom: same as top panel for low-redshift solutions at $2\leq z \leq 8$ in the $sSFR$ vs. $E(B-V)$ plane, colour-coded according to the stellar age.
  • ...and 11 more figures