arXiv subjects

M. Dickinson

Publications and source records attributed to M. Dickinson.

At least 19 recordsLinked to original sources

arXiv2026

The gradual decline of Ly$\alpha$ visibility in the CANDELS fields: evidence for the combined effects of galaxy evolution and reionization

We investigate the evolution of Ly$\alpha$ visibility and the physical properties of Ly$\alpha$ emitters (LAEs) across the five CANDELS fields using publicly available JWST/NIRSpec PRISM spectroscopy. Our catalog comprises 3446 spectroscopically confirmed sources at 4 $\leq $ z < 14.2, including 3361 star-forming galaxies (SFGs), of which 539 are robust (S/N > 3) LAEs. We measure the fraction of LAEs with EW$_0$ > 25A (X$_{\mathrm{Ly\alpha}}$) and trace its redshift evolution in two UV luminosity bins, namely -20.25 < M$_{\mathrm{UV}}$ < -18.75 and -21.75 < M$_{\mathrm{UV}}$ < -20.25. Within the fainter-UV range, X$_{\mathrm{Ly\alpha}}$ increases from z = 5 to z = 6 at 3$\sigma$ significance and subsequently declines toward higher redshifts with a significant monotonic trend at z > 6. We also investigate the physical properties of both LAEs and the full SFG population. We find that the stellar mass, UV slope $\beta$, stellar reddening, SFR, metallicity, sSFR, and burstiness of LAEs remain approximately stable with redshift. The only exception is the mass-weighted age which decreases with increased redshift. Conversely, the properties of the full SFG population evolve significantly, progressively approaching the region of galaxy-property space occupied by LAEs as redshift increases. This suggests that galaxy evolution may enhance the intrinsic production and escape of Ly$\alpha$ photons toward earlier epochs. We argue that this effect should be accounted when inferring the evolution of the neutral hydrogen content of the IGM from the observed visibility of Ly$\alpha$ emission. To this end, we employ a physically motivated framework based on the Attenuation-Free Model, jointly accounting for galaxy evolution and IGM attenuation. Our observations favor reionization histories that proceed gradually over scenarios characterized by a rapid increase in the cosmic neutral hydrogen fraction.

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arXiv2026

Stardust Galaxies at z>9: A Dust-Origin Transition Behind the Excess of UV-Bright Galaxies

Recent JWST observations suggest that galaxies at z > 9 may be dominated by low-opacity SNe-produced dust before efficient ISM grain growth is established. This transition in dust origin and opacity could explain both the prevalence of galaxies with extremely low dust attenuation and the excess of UV-bright galaxies relative to most pre-JWST predictions. We investigate whether this transition, combined with evolving star-formation efficiency, can reproduce these observed properties. We develop a physically motivated attenuation framework combining (i) extinction laws for reverse-shock-processed SNe dust, (ii) metallicity- and dust-to-metal-dependent opacity scalings, and (iii) porous radiative-transfer geometries allowing partial UV-photon leakage. Unlike outflow-driven scenarios requiring large-scale gas evacuation, our approach preserves gas reservoirs while reducing effective UV opacity through dust composition and geometry. We introduce extinction-based, gas-based, and hybrid attenuation prescriptions linking SNe-dominated and ISM grain-growth dust regimes. We find that the observed A_FUV-M_star relation at z > 9 is best reproduced for an intrinsic FUV dust opacity kappa_UV(dust)=10^3 - 10^4 cm2/g, characteristic of low-opacity SNe dust, naturally producing very low attenuation even in gas-rich galaxies. This regime reproduces galaxies with extremely low dust attenuation (GELDAs), which dominate observed samples at z > 9. Applied to intrinsic UV luminosity function models, our SNe-dominated and hybrid prescriptions mainly suppress the brightest galaxies, bringing predictions into agreement with JWST measurements without requiring extreme star-formation efficiencies or dust-free interstellar media. Our results suggest that the UV-bright galaxy excess at z > 9 reflects a transition in dust origin and opacity during the earliest phases of galaxy evolution.

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arXiv2026

A search for the first galaxies across $>0.6$ deg$^2$ of JWST imaging: new evidence for a rapid decline in star-formation activity at $z>12$

We present a new determination of the evolving galaxy UV luminosity function (LF) over the extreme redshift range $12.5 $0.6 deg$^2$ of JWST NIRCam imaging containing $>150$ independent sight-lines. We find evidence for an accelerated decline in the UV LF, and hence inferred star-formation rate density ($\rho_{\rm SFR}$), over the $\simeq100\rm{Myr}$ cosmic time interval between $z=11$ and $z=13.5$. Moreover, based on a notable lack of galaxy candidates at $z>14.5$, we find evidence for an even more rapid descent in star-formation activity towards earlier times, with our new measurement of $\rho_{\rm SFR}$ at $z\simeq15.5$ lying significantly below an extrapolation of the log-linear $\rho_{\rm SFR}(\rm z)$ relation inferred from early JWST LF studies. Instead, we find that the evolution in $\rho_{\rm SFR}(\rm z)$ at these very early times is better described by a piece-wise log-linear relation, in which the decline in $\rho_{\rm SFR} (\rm z)$ at $z>12$ is $\simeq4$ times steeper than at redshifts $z < 12$. Our observational results are consistent with a number of theoretical models of galaxy evolution which have incorporated a range of treatments in an attempt to explain the prevalence of UV-bright galaxies at least out to $z \simeq 12$ (e.g., increased star-formation efficiency, stochastic star-formation histories, an evolving stellar initial mass function and/or a shift towards attenuation-free stellar populations). However, our results are also entirely consistent with a relatively simple galaxy evolution model with no such adjustments, in which the rapid evolution of the dark-matter halo mass function at early times is for a while partially masked by progressively younger stellar ages, with the inferred epoch of first galaxy formation lying at $z\simeq15$.

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arXiv2026

The Collective Voice of Ly$\alpha$ Emitters: Insights from JWST Stacked Spectroscopy

We present a spatially resolved stacked analysis of 287 LAEs at $z>4$ observed with JWST/NIRSpec prism spectroscopy. By constructing a two-dimensional stack from public surveys (CAPERS, CEERS, JADES, and RUBIES), we probe the average internal structure of typical LAEs on sub-kiloparsec scales. We find a clear radial decoupling between resonant and non-resonant emission: while EW(H$\beta$) and other optical lines decline with radius, EW(Ly$\alpha$) increases toward the outskirts, and the Ly$\alpha$ escape fraction rises from $\sim16\%$ in the center to $\gtrsim24\%$ at larger radii. This behavior suggests that resonant scattering redistributes Ly$\alpha$ photons into lower-density outer regions, where escape becomes more efficient. Optical diagnostics and $T_e$ measurements reveal low metallicities ($12+\log(\rm O/H)\simeq7.7\pm0.2$), high ionization parameters, negligible dust attenuation, and systematically elevated N/O ratios ($\log({\rm N/O})\sim-0.4$). The latter place typical LAEs among the growing population of nitrogen-enhanced high-redshift galaxies, pointing to rapid and possibly feedback-driven chemical enrichment. The inferred ionizing photon production efficiency, $\log(\xi_{\rm ion}/{\rm Hz\,erg^{-1}})\simeq25.1-25.2$, together with the high Ly$\alpha$ escape fractions, suggests that these systems are efficient, though not extreme, contributors to the ionizing photon budget. Comparison with SPICE radiation-hydrodynamic simulations shows that bursty supernova feedback models naturally reproduce the observed radial trends in Ly$\alpha$ escape, UV slope, and emission-line equivalent widths, linking the spatial redistribution of Ly$\alpha$ to stochastic star formation and feedback-driven gas flows. Our results demonstrate that Ly$\alpha$ emission, chemical enrichment, and feedback are tightly connected in typical $z>4$ LAEs. (Shortened version for arXiv)

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arXiv2026

Investigating the role of mergers in galaxy assembly in the early Universe (z > 5)

Galaxy mergers play a crucial role in shaping the morphology, the star formation, and the mass growth of galaxies across cosmic time. While mergers have been extensively investigated in the local Universe, the evolution of their frequency and physical properties in the early Universe has yet to be fully understood. We investigate the role of mergers in a large spectroscopic sample of 1233 galaxies in the range 5 3) in JWST imaging, covering six different extragalactic fields. We identify mergers from rest-frame optical disturbances in F444W, using a combination of Gini, M-20, and Asymmetry parameters. We find a morphological merger fraction f_m that does not strongly evolve with redshift from z=0 to z ~ 8. The average f_m of our primary major merger condition (Gini+0.14xM-20 > 0.33, A>0.35) is ~ 5 %, which increases to ~13 % for major+minor merger tracers. Accounting for the evolving observability timescale of each tracer, we find that the merger rate is strongly increasing from z=1 to 7 by more than 1 dex, averaging ~ 2 merger/galaxy/Gyr at 5 5 have a significant impact, although significantly lower than at z<1, on the SFR of galaxies. When averaged over 10 Myr (comparable to the observability timescale of morphological disturbances), their SFRs are a factor of 1.7 higher than a mass and redshift matched sample of non-mergers, suggesting that mergers trigger new star-formation through short-lived powerful bursty episodes. Despite this, mergers contribute only by 5% - 10% to the mass build-up of galaxies in the redshift range explored.

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arXiv2025

The AGN nature of strong CIII emitters in the Early Universe with JWST

The semi-forbidden CIII] $\lambda\lambda$1907,1909 doublet is a key tracer of high-ionization emission in the early universe. We present a study of CIII] emission in galaxies at z=5-7, using publicly available JWST/NIRSpec prism data from programs including CEERS, JADES, RUBIES and CAPERS. We built a sample of 61 CIII]-emitting galaxies, and we classified them as star-forming or active galactic nuclei (AGN) host galaxies using (1) rest-frame UV and optical emission-line diagnostic diagrams, and (2) the presence/absence of broad Balmer emission lines. The UV diagnostics are based on the combination of the rest-frame equivalent width (EW) of CIII] versus CIII]/HeII $\lambda$1640, and the EW of CIV versus CIV/HeII $\lambda$1640. For optical diagnostics, we employ the OHNO diagram, which combines [OIII] $\lambda$5007, H$\beta$, [NeIII] $\lambda$3869, and [OII] $\lambda\lambda$3727,3729- and we find it has a low efficiency on separating AGN from SFG. We find that half of the sources in our sample (29 out of 61 galaxies) exhibit at least one secure indication of AGN activity while 13 are potential AGNs based on the CIII] diagnostic. Physical properties, including stellar mass and star formation rate, are derived through spectral energy distribution modeling with Bagpipes. Our analysis reveals that JWST is uncovering a population of strong CIII] emitters at high redshift (5<z<7) with a median EW of 22.8 A. This EW is higher than that of a control sample of CIII] emitters at redshift 3<z<4 with a median EW of 4.7 A. We find that for the same range of Muv, the CIII] EW increases by $\sim$0.67 dex from 3<z<4 to 5<z<7, indicating strong redshift evolution in the line's strength. Finally, we identify five sources in our sample as Little Red Dots (LRDs); while four of these have already been identified as LRD in the literature, one is presented here for the first time.

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arXiv2025

HELM's deep: Highly Extincted Low-Mass galaxies seen by JWST

The dust content of star-forming galaxies is generally positively correlated with their stellar mass. However, some recent JWST studies have shown the existence of a population of dwarf galaxies with an unexpectedly large dust attenuation. Using the Cosmic Evolution Early Release Science Survey (CEERS) data, we identified a sample of 1361 highly extincted low-mass (HELM) galaxies, defined as dwarf galaxies ($M_*<10^{8.5}$) with Av>1mag or more massive galaxies with an exceptionally high dust attenuation given their stellar mass (i.e., $Av>1.6log_{10}(M_*/Mo)-12.6$). The selection is performed using the multiparameter distribution obtained through a comprehensive spectral energy distribution fitting analysis, based on optical to near-infrared data. After excluding possible contaminants, like brown dwarfs, little red dots, high-z (z>8.5) and ultra-high-z (z>15) galaxies, the sample mainly includes sources at z<1, with a tail extending up to z=7.2. The sample has a median stellar mass of $10^7$ Mo and a median dust attenuation of Av=2mag. We analysed the morphology, environment and star-formation rate of these sources to investigate the reason of their large dust attenuation. In particular, HELM sources have sizes (effective radii, Re) similar to non-dusty dwarf galaxies and no correlation is visible between the axis ratios (b/a) and the dust attenuation. This findings indicate that it is unlikely that the large dust attenuation is due to projection effects, but a prolate or a disk-on oblate geometry are still possible, at least for a subsample of the sources. We have found that the distribution of HELM sources is slightly skewed toward more clustered environments than non-dusty dwarfs and tend to be slightly less star forming. This finding, if confirmed by spectroscopic follow-up, indicates that HELM sources could be going through some environmental processes, such as galaxy interactions.

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arXiv2025

Investigating ionising sources and the complex interstellar medium of GHZ2 at $z=12.3$

An accurate characterisation of the physical properties of galaxies at cosmic dawn is key to understanding the origin of the high abundance of UV-bright galaxies at z$\gtrsim$10. We exploit deep NIRSpec PRISM observations of GHZ2 to constrain the sources of ionising radiation and the properties of the ISM in this bright, compact, and highly ionising galaxy at z=12.3. We measure with high significance the prominent N IV, C IV, He II, O III, C III, O II, and Ne III emission features previously detected in shallower observations, and confirm the detection of the N III] $\lambda 1750$ multiplet, yielding tight constraints on the N/O ratio, which is found to be $\simeq$2 times the solar value. We also detect the Mg II $\lambda 2800$, [Fe IV] $\lambda 2833$ and Si II $\lambda 1812$ doublets, the H8+HeI $\lambda\lambda 3889$ blend, and the Si IV+O IV] $\lambda\lambda 1400$ absorption complex. The O III $\lambda 3133$ fluorescence line is only detected in the first observing epoch, implying variability on a rest-frame time span of 19 days, strongly suggesting the presence of an active nucleus. Combining the NIRSpec dataset with available optical and far-infrared constraints from MIRI and ALMA, we show that the emission spectrum of GHZ2 cannot be reproduced by single-density spectro-photometric models. Multi-zone photoionisation modelling performed with the HOMERUN code demonstrates that star formation must be occurring in a strongly stratified ISM, where both low-/intermediate-density gas and high-density regions (log($n_e$/cm$^{-3}) \gtrsim 4$) coexist. The GHZ2 emission landscape is consistent with either a composite star-formation plus AGN scenario, or with star formation occurring in a combination of radiation- and matter-bounded regions. Purely radiation-bounded stellar models fail to reproduce the observed He II emission, making an additional hard ionising component unavoidable.

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arXiv2025

Extreme equivalent width-selected low-mass starbursts at $z=4-9$: insights into their role in cosmic reionization

We investigate the properties of extreme emission line galaxies (EELGs) at $z=4-9$ and their role in reionization. Compact, low-mass galaxies with intense optical emission lines are linked to elevated specific star formation rates (sSFRs) and recent bursts of star formation. Feedback in these systems may enable the leakage of ionizing radiation into the intergalactic medium. Using JWST/NIRSpec spectroscopy from the CAPERS, CEERS, and RUBIES surveys, we compile 160 NIRCam-selected EELGs in the EGS field. These galaxies show extreme rest-frame equivalent widths (EWs), with a median EW([O III]+H${\beta}$)=1616\r{A} and EW(H${\alpha}$)=763\r{A}. They are low-mass (median log(M$_{\star}$/M$_{\odot}$)=8.26) with high sSFRs (median 43 Gyr$^{-1}$), above the $z\sim6$ main sequence. UV slopes are diverse, with a mean $\beta=-2.0$, and only 7% have extremely blue continua ($\beta<-2.6$). Emission-line diagnostics suggest stellar populations as the primary ionizing source, although an AGN fraction of 14% is cannot be entirely ruled out. These galaxies are efficient ionizing photon producers, with mean log($\xi_{\rm ion}$ [Hz erg$^{-1}$])=25.37, exceeding typical values at similar redshifts. Escape fractions, however, are heterogeneous: 16% of EELGs at $z<7$ show escape fractions $>$5% for both Ly${\alpha}$ and LyC photons, while 82% lack detectable Ly${\alpha}$ emission. The median inferred LyC escape fraction is modest (5%) but enhanced in compact super-Eddington systems with sSFR >25 Gyr$^{-1}$. These results indicate that EELGs contribute approximately 16-40% of the total ionizing emissivity required to sustain hydrogen reionization. EELGs are extremely compact, with a median effective radius of 0.49 kpc, and exhibit a recent star-formation burst. Our analysis indicates that sSFR and star-formation rate surface density are the primary drivers of their extreme emission line strengths.

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arXiv2025

Ly$\alpha$ visibility from z = 4.5 to 11 in the UDS field: Evidence for a high neutral hydrogen fraction and small ionized bubbles at z $\sim$ 7

The resonant scattering nature of Ly$\alpha$ photons interacting with neutral hydrogen makes Ly$\alpha$ emitters (LAEs) robust tracers of the intergalactic neutral hydrogen fraction, and thus sensitive probes of cosmic reionization. We present an extensive study of the Ly$\alpha$ evolution from galaxies at 4.5 $\leq$ z $\leq$ 11 in the UDS field, observed as part of the CAPERS survey, and complemented with spectra from the DAWN JWST Archive. The combined sample includes 651 spectroscopically confirmed Ly$\alpha$-break galaxies, among which we find 73 S/N>3 LAEs in JWST-NIRSpec PRISM spectra. We trace the redshift evolution of the LAE fraction with EW$_0$ >25 A (X$_{\mathrm{Ly\alpha}}$) between z = 5 and z = 9, extending such an analysis to the UDS field for the first time. At z = 5 and 6, the UDS results agree with the average JWST X$_{\mathrm{Ly\alpha}}$ values from multiple fields. However, JWST measurements are consistently lower than ground-based results. To investigate this, we compare JWST observations to a population of star-forming galaxies at z$\sim$6 observed with VLT-FORS2. We find that a Ly$\alpha$ slit-loss of 35 $\pm$ 10% in JWST spectra accounts for the offset, as the resonant Ly$\alpha$ emission is more spatially extended than the stellar continuum. From z = 6 to 7, the UDS field shows a significant drop in Ly$\alpha$ visibility, from which we infer a neutral hydrogen fraction of X$_{\mathrm{HI}}$ = 0.7--0.9. Finally, we identify two robust ionized bubbles at z = 7.29 and 7.77, with radii of $R_{\mathrm{ion}}$ = 0.6 and 0.5 physical Mpc and photometric overdensities of N/$\langle$N$\rangle$ = 3 and 4, based on candidate counts down to the photometric completeness limit. Compared to the large ionized region at z$\sim$7 in the EGS field, these results indicate significant field-to-field variation, supporting a patchy, inhomogeneous reionization process.

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arXiv2025

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

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.

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arXiv2025

Ultra High-Redshift or Closer-by, Dust-Obscured Galaxies? Deciphering the Nature of Faint, Previously Missed F200W-Dropouts in CEERS

The James Webb Space Telescope (JWST) is revolutionizing our understanding of the Universe by unveiling faint, near-infrared dropouts previously beyond our reach, ranging from exceptionally dusty sources to galaxies up to redshift $z \sim 14$. In this paper, we identify F200W-dropout objects in the Cosmic Evolution Early Release Science (CEERS) survey which are absent from existing catalogs. Our selection method can effectively identify obscured low-mass ($\log \text{M}_* \leq 9$) objects at $z \leq 6$, massive dust-rich sources up to $z \sim 12$, and ultra-high-redshift ($z > 15$) candidates. Primarily relying on NIRCam photometry from the latest CEERS data release and supplementing with Mid-Infrared/(sub-)mm data when available, our analysis pipeline combines multiple SED-fitting codes, star formation histories, and CosMix - a novel tool for astronomical stacking. Our work highlights three $2 15$, with best-fit masses compatible with $\Lambda$CDM and a standard baryons-to-star conversion efficiency. Their bi-modal redshift probability distributions suggest they could also be $z<1.5$ dwarf galaxies with extreme dust extinction. We also identify a strong line emitter galaxy at $z \sim 5$ mimicking the near-infrared emission of a $z \sim 13$ galaxy. Our sample holds promising candidates for future follow-ups. Confirming ultra high-redshift galaxies or lower-z dusty dwarfs will offer valuable insights into early galaxy formation, evolution with their central black holes and the nature of dark matter, and/or cosmic dust production mechanisms in low-mass galaxies, and will help us to understand degeneracies and contamination in high-z object searches.

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arXiv2025

Little impact of mergers and galaxy morphology on the production and escape of ionizing photons in the early Universe

Compact, star-forming galaxies with high star formation rate surface densities ($\Sigma_{\text{SFR}}$) are often efficient Lyman continuum (LyC) emitters at $z\leq 4.5$, likely as intense stellar feedback creates low-density channels that allow photons to escape. Irregular or disturbed morphologies, such as those resulting from mergers, can also facilitate LyC escape by creating anisotropic gas distributions. We investigate the influence of galaxy morphology on LyC production and escape at redshifts $5 \leq z \leq 7$ using observations from various \textit{James Webb Space Telescope} (JWST) surveys. Our sample consists of 436 sources, which are predominantly low-mass ($\sim 10^{8.15} M_\odot$), star-forming galaxies with ionizing photon efficiency ($\xi_{\rm ion}$) values consistent with canonical expectations. Since direct measurements of $f_{\rm esc}$ are not possible during the Epoch of Reionization (EoR), we predict $f_{\rm esc}$ for high-redshift galaxies by applying survival analysis to a subsample of LyC emitters from the Low-Redshift Lyman Continuum Survey (LzLCS), selected to be direct analogs of reionization-era galaxies. We find that these galaxies exhibit on average modest predicted escape fractions ($\sim 0.04$). Additionally, we assess the correlation between morphological features and LyC emission. Our findings indicate that neither $\xi_{\rm ion}$ nor the predicted $f_{\rm esc}$ values show a significant correlation with the presence of merger signatures. This suggests that in low-mass galaxies at $z \geq 5$, strong morphological disturbances are not the primary mechanism driving LyC emission and leakage. Instead, compactness and star formation activity likely play a more pivotal role in regulating LyC escape.

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arXiv2024

Evolution of the UV slope of galaxies at cosmic morning (z > 4): the properties of extremely blue galaxies

We present an analysis of the UV continuum slope, beta, using a sample of 733 galaxies selected from a mixture of JWST ERS/GTO/GO observational programs and with z > 4. We consider spectroscopic data obtained with the low resolution PRISM/CLEAR NIRSpec configuration. Studying the correlation of beta with M_UV we find a decreasing trend of beta = (-0.056 +- 0.017) M_UV - (3.01 +- 0.34), consistent with brighter galaxies having redder beta as found in previous works. However, analysing the trend in separate redshift bins, we find that at high redshift the relation becomes much flatter, consistent with a flat slope. Furthermore, we find that beta decreases with redshift with an evolution as beta = (-0.075 +- 0.010) z - (1.496 +- 0.056), consistent with most previous results that show a steepening of the spectra going at higher z. We then select a sample of galaxies with extremely blue slopes (beta < -2.6): such slopes are steeper than what is predicted by stellar evolution models, even for dust free, young, metal poor populations, when the contribution of nebular emission is included. We select 51 extremely blue galaxies (XBGs) and we investigate the possible physical origin of their steep slopes, comparing them to a sub-sample of redder galaxies (matched in redshift and M_UV). We find that XBGs have younger stellar populations, stronger ionization fields, lower dust attenuation, and lower but not pristine metallicity (~ 10% solar) compared to red galaxies. However, these properties alone cannot explain the extreme beta values. By using indirect inference of Lyman continuum escape, using the most recent models, we estimate escape fractions f_esc > 10% in at least 25% of XBGs, while all the red sources have smaller f_esc. A reduced nebular continuum contribution as due to either a high escape fraction or to a bursty star-formation history is likely the origin of the extremely blue slopes.

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arXiv2024

Seven wonders of Cosmic Dawn: JWST confirms a high abundance of galaxies and AGNs at z $\simeq$ 9-11 in the GLASS field

We present JWST/NIRSpec PRISM follow-up of candidate galaxies at z=9-11 selected from deep JWST/NIRCam photometry in GLASS-JWST Early Release Science data. We spectroscopically confirm six sources with secure redshifts at z = 9.52-10.43, each showing multiple emission lines. An additional object is likely at z = 10.66, based on its Lya-break and a single emission feature, while one source is a lower redshift interloper. The sample includes the first JWST-detected candidate at z=10, GHZ1/GLASS-z10, which we confirm at z = 9.875, and the X-ray detected AGN GHZ9 confirmed at z = 10.145. Three objects in our sample, including GHZ9, have EW(CIII])>20A and occupy a region compatible with AGN emission in the EW(CIII]) vs CIV/CIII] diagram. The spectroscopic sample confirms a high abundance of galaxies at z > 9. We measure a number density of z=10 galaxies in the GLASS-JWST ERS field that is a factor of >3 higher than other JWST-based estimates at demagnified rest-frame magnitudes of -21 < Muv < -19. We find that the positions of these galaxies in redshift and angular space are not consistent with all of them being part of a unique progenitor of present-day galaxy clusters. The high density of objects in the GLASS region can be explained either by clustering on large scales or by a superposition of different forming structures of which we observe only the brightest members. By considering all the spectroscopic z=10 sources in the Abell-2744 field, we identify two potential galaxy proto-clusters centered around GHZ9 and JD1, with relative separations between their members of 1-2 pMpc. The potential AGN nature of three of the sources in our sample lends support to a scenario in which the high abundance of bright sources determined by JWST surveys at cosmic dawn may be affected by AGN contribution to their UV luminosity.

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arXiv2024

ASTRODEEP-JWST: NIRCam-HST multiband photometry and redshifts for half a million sources in six extragalactic deep fields

We present a set of photometric catalogs primarily aimed at providing the community with a comprehensive database for the study of galaxy populations in the high redshift Universe. The set gathers data from eight JWST NIRCam observational programs, targeting the Abell 2744 (GLASS-JWST, UNCOVER, DDT2756 and GO3990), EGS (CEERS), COSMOS and UDS (PRIMER), and GOODS North and South (JADES and NGDEEP) deep fields, for a total area of $\sim$0.2 sq. degrees. Photometric estimates are obtained by means of well-established techniques, including tailored improvements designed to enhance the performance on the specific dataset. We also include new measurements from HST archival data, thus collecting 16 bands spanning from 0.44 to 4.44 $\mu$m. A grand total of $\sim$530 thousand sources is detected on stacks of NIRCam 3.56 and 4.44 $\mu$m mosaics. We assess the photometric accuracy by comparing fluxes and colors against archival catalogs. We also provide photometric redshift estimates, statistically validated against a large set of robust spectroscopic data. The catalogs are publicly available on the Astrodeep website.

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arXiv2024

Physical properties of strong 1 < z < 3 Balmer and Paschen lines emitters observed with JWST

The ultraviolet continuum traces young stars while the near-infrared unveils older stellar populations and dust-obscured regions. Balmer emission lines provide insights on gas properties and young stellar objects but are highly affected by dust attenuation. The near-infrared Paschen lines suffer less dust attenuation and can be used to measure star formation rates (SFRs) in star-forming regions obscured by dust clouds. We select 13 sources between redshifts 1 and 3 observed with HST, JWST/NIRCam and NIRSpec based on the availability of at least one Balmer and one Paschen line with S/N > 5. With a newly-developed version of CIGALE, we fit their hydrogen line equivalent widths (EWs) and photometric data. We assess the impacts of the removal of spectroscopic data by comparing the quality of the fits of the spectro-photometric data to those with photometric data only. We compare the single (BC03) vs binary (BPASS) stellar populations models in the fitting process of spectro-photometric data. We derive the differential attenuation and explore different attenuation recipes by fitting spectro-photometric data with BC03. For each stellar model and for each input dataset (with and without EWs), we quantify the deviation on the SFRs and stellar masses from the "standard" choice. On average, the SFRs are overestimated and the stellar masses are underestimated when EWs are not included as input data. We find a major contribution of the H$α$ emission line to the broadband photometric measurements of our sources, and a trend of increasing contribution with specific SFR. Using the BPASS models has a significant impact on the derived SFRs and stellar masses. We show that a flexible attenuation recipe provides more accurate estimates of the dust attenuation parameters, especially the differential attenuation which agrees with the original value of Charlot & Fall (2000).

astro-ph.GA↗

arXiv2024

Anatomy of a z=6 Lyman-{\alpha} emitter down to parsec scales: extreme UV slopes, metal-poor regions and possibly leaking star clusters

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy ($\rm \mu=17-21$) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint ($\rm M_{UV}=-17.8$) Lyman-$\alpha$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\rm H\alpha$, $\rm H\beta$ and [OIII]$\lambda\lambda4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\rm H\beta$+[OIII])$\gtrsim1000$ \AA, and elevated ionising photon production efficiencies $\rm log(\xi_{ion}/erg^{-1}Hz)=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of $\rm R_{eff}=3-8~pc$ in size and $\rm M\sim2\cdot10^5-5\cdot10^{6}~M_\odot$ in mass. These clusters are characterised by steep UV slopes, $\rm\beta_{UV}\lesssim-2.5$, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-$\rm \alpha$ emission peaking at $\rm \sim100~km~s^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim0.1~Z_\odot$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\rm Z\lesssim2\%~Z_\odot$, barely detected in NIRCam imaging; this region is displaced by $\rm >200~pc$ from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

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