arXiv subjects

Nicholas S. Martis

Publications and source records attributed to Nicholas S. Martis.

At least 37 records · Page 2Linked to original sources

arXiv2025

Star Formation under a Cosmic Microscope: Highly magnified z = 11 galaxy behind the Bullet Cluster

We present measurements of stellar population properties of a newly discovered spectroscopically confirmed $z=11.10^{+0.11}_{-0.26}$, gravitationally lensed galaxy, using JWST NIRSpec PRISM spectroscopy and NIRCam imaging. The arc is highly magnified by the Bullet Cluster (magnification factor $μ=14.0^{+6.2}_{-0.3}$. It contains three star-forming components of which one is barely resolved and two are unresolved, giving intrinsic sizes of $\lesssim 10pc$. The clumps also contain ~50% of the total stellar mass. The galaxy formed the majority of its stars ~150Myr ago (by z~14). The spectrum shows a pronounced damping wing, typical for galaxies deep in the reionisation era and indicating a neutral IGM at this line of sight. The intrinsic luminosity of the galaxy is $0.086^{+0.008}_{-0.030} L^*$ (with $L^*$ being the characteristic luminosity for this redshift), making it the lowest luminosity spectroscopically confirmed galaxy at $z>10$ discovered to date.

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arXiv2025

Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

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arXiv2025

The Stellar Mass and Age Distributions of Star-Forming Clumps at $0.5 < z < 5$ in JWST CANUCS: Implications for Clump Formation and Destruction

We investigate the resolved properties of star-forming clumps and their host galaxies at $0.5<z<5$ in the JWST CANUCS fields. We find that the fraction of clumpy galaxies peaks near $z\sim2$ for galaxies with masses of $\log(M_{g,*}/M_\odot)\geq10$, while galaxies with masses of $8.5 \leq \log(M_{g,*}/M_\odot) < 10$ show lower clumpy fractions with little redshift evolution. We identify and measure individual clump masses, finding that the aggregated clump stellar mass function (cSMF) follows a power-law slope of $α= -2$ across all redshift bins, broadly consistent with \textit{in-situ} clump formation. However, when split by galaxy masses, the cSMF is found to be flatter ($α\sim-1.6$) for massive galaxies and steeper ($α\sim-2.3$) for lower mass galaxies, with little redshift evolution in both cases. We explore how different formation mechanisms and disruptive processes affect the shape of the clump mass function. In particular, we find that the cSMF slope is flatter with increasing gas fractions in younger clump populations ($<300$ Myr old), suggesting that higher gas availability leads to more massive clumps forming at the time of formation. Alternatively, many high-redshift galaxies in the sample have disturbed morphologies and simulations show that clumps of \textit{ex-situ} origins can flatten the cSMF slope. We also investigate the evolution of clump populations, where we find the cSMF slope become flatter as clumps evolve and age. We interpret this as an indication of the long-term survivability of massive clumps, with feedback mechanisms preferentially disrupting low-mass clumps. Overall, the galaxy-mass dependent cSMF and age distribution point to a complex history for clumps, involving different and competing mechanisms for their formation and destruction.

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arXiv2025

Resolved mass assembly and star formation in Milky Way Progenitors since $z = 5$ from JWST/CANUCS: From clumps and mergers to well-ordered disks

We present a resolved study of 877 progenitors of Milky Way Analogs (MWAs) at $0.3<z<5$ selected with abundance matching in the ten fields of the Canadian NIRISS Unbiased Cluster Survey (CANUCS). Utilizing 18-21 bands of deep NIRCam, NIRISS, and HST photometry, we create resolved stellar mass maps and star formation rate maps via spectral energy distribution fitting with Dense Basis. We examine their resolved stellar mass and specific star formation rate (sSFR) profiles as a function of galactocentric radius, and find clear evidence for inside-out mass assembly. The total $\mstar$ of the inner 2 kpc regions of the progenitors remain roughly constant ($10^{9.3-9.4}\msun$) at $2<z<5$, while the total $\mstar$ of the regions beyond 2 kpc increases by 0.8 dex, from $10^{7.5}\msun$ to $10^{8.3}\msun$. Additionally, the sSFR of the outer regions increase with decreasing redshift, until $z\sim 2$. The median Sérsic index of the MWA progenitors stays nearly constant at $n \sim 1$ at $2<z<5$, while the half-mass radii of their stellar mass profiles double. We perform additional morphological measurements on the stellar mass maps via the Gini-M20 plane and asymmetry parameters. They show that the rate of double-peak mergers and disturbances to galaxy structure also increase with redshift, with $\sim50\%$ of galaxies at $4<z<5$ classified as disturbed, and $\sim20\%$ classified as ongoing mergers. Overall, the early evolution of MWAs is revealed as chaotic, with significant mergers and high SFRs. Mass growth is primarily inside-out and galaxies become more disk-like after $z=3$.

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arXiv2025

JWST Photometry of Globular Cluster Populations in MACS0417.5-1154

Deep JWST imaging of the massive galaxy cluster MACS0417.5-1154, at redshift z=0.443, reveals a huge population of globular clusters (GCs) and Ultra-Compact Dwarfs (UCDs) primarily distributed around its single central giant galaxy (BCG). We present NIRCam/SWC photometry of the GC system in four bands (F090W, F115W, F150W, F200W). The spatial distribution of the system matches well in radial and ellipticity profile with the high elongation (b/a = 0.5) of the BCG halo light. The total GC population within MACS0417 is estimated to be near 1.5 x 10^5, similar to the systems in Abell 2744, Coma, and other galaxy clusters with comparable masses. With similar results for GC photometry in hand from other lensing clusters at a range of redshifts, it is now possible to trace on purely observational grounds the luminosity evolution of GC systems over many Gigayears of lookback time, as seen through their color-magnitude diagrams. We show this sequence for five systems reaching to lookback times of more than 7 Gyr. A systematic change in the GC/UCD sequence with lookback time is clearly visible, near what is expected for age-fading of a simple stellar population with time. Lastly, we evaluate the effectiveness of the various JWST NIRCam filters for broadband photometry of GC systems as a function of redshift, as an aid to planning further studies.

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arXiv2025

Metal-Poor Star-Forming Clumps in Cosmic Noon Galaxies: Evidence for Gas Inflow and Chemical Dilution Using JWST NIRISS

The formation and evolution of galaxies are intricately linked to the baryon cycle, which fuels star formation while shaping chemical abundances within galaxies. Investigating the relationship between star formation and metallicity for large samples of galaxies requires expensive IFU surveys or sophisticated tools to analyze grism data. Here we analyze JWST NIRISS slitless grism data using Sleuth, a tool that forward models and infers spatially resolved physical properties from grism data, including observations from JWST NIRISS/NIRCam and future grism data like that from the Roman Space Telescope. Sleuth enables extraction of high-quality emission line maps from slitless spectra, overcoming contamination and spatially varying stellar populations, which previously limited such studies. Utilizing Sleuth with data from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), we investigated the relationship between metallicity and star formation in the star-forming clumps of galaxies at 0.6 < z < 1.35. We analyzed a sample of 20 galaxies, extracted high-quality emission line maps with Sleuth, and analyzed, in detail, the spatially resolved properties of star-forming clumps. Using $Hα$, [SII], and [SIII] emission line maps, we examined the spatially resolved metallicities, ionization, and star formation rates of our sample. Our findings reveal that these star-forming clumps show lower metallicities ($\sim$ 0.1 dex) than their surrounding galactic environments, indicating a metallicity dilution of 20 $\%$ within the clumps' gas. Our analysis indicates that these clumps exhibit intensified star formation and reduced metallicity, likely due to the inflow of metal-poor gas. These clumps illustrate the dynamic relationship between star formation and chemical enrichment within galaxies.

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arXiv2025

MINERVA: A NIRCam Medium Band and MIRI Imaging Survey to Unlock the Hidden Gems of the Distant Universe

We present an overview of the MINERVA survey, a 259.8 hour (prime) and 127 hour (parallel) Cycle 4 treasury program on the James Webb Space Telescope (JWST). MINERVA is obtaining 8 filter NIRCam medium band imaging (F140M, F162M, F182M, F210M, F250M, F300M, F360M, F460M) and 2 filter MIRI imaging (F1280W, F1500W) in four of the five CANDELS Extragalactic fields: UDS, COSMOS, AEGIS and GOODS-N. These fields were previously observed in Cycle 1 with 7 - 9 NIRCam filters by the PRIMER, CEERS and JADES programs. MINERVA reaches a 5$σ$ depth of 28.1 mag in F300M and covers $\sim$ 542 arcmin$^2$, increasing the area of existing JWST medium-band coverage in at least 8 bands by $\sim$ 7$\times$. The MIRI imaging reaches a 5$σ$ depth of 23.9 mag in F1280W and covers $\sim$ 275 arcmin$^2$ in at least 2 MIRI filters. When combined with existing imaging, these data will provide a photometric catalog with 20-26 JWST filters (depending on field) and 26-35 filters total, including HST. This paper presents a detailed breakdown of the filter coverage, exposure times, and field layout relative to previous observations, as well as an overview of the primary science goals of the project. These include uncovering the physics of enigmatic sources hiding in current broadband catalogs, improving systematics on stellar mass functions and number densities by factors of $\gtrsim$ 3, and resolved mapping of stellar mass and star formation at 1 $< z <$ 6. When complete, MINERVA will become an integral part of the treasury deep field imaging datasets, significantly improving population studies with well-understood completeness, robust photometric redshifts, stellar masses, and sizes, and facilitating spectroscopic follow up for decades to come.

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arXiv2025

Possible environmental quenching in an interacting little red dot pair at $z\sim7$

We report the discovery of a $z\sim7$ group of galaxies that contains two little red dots (LRDs) just 3.3 kpc apart, along with three potential satellite galaxies, as part of the Canadian NIRISS Unbiased Cluster Survey (CANUCS). The spectral energy distributions (SEDs) of this LRD pair show evidence of a Balmer break, consistent with a recent ($\sim 100$ Myr) quenching of star formation. In contrast, the satellites are compatible with a recent-onset ($\sim 100$ Myr), ongoing burst of star formation. LRD1's SED is consistent with a dust-free active galactic nucleus (AGN) being the source of the UV excess in the galaxy. The optical continuum would be powered by the emission from an obscured post-starburst and the AGN at a subdominant level. LRD2's SED is more ambiguous, but it could also be indicative of a dust-free AGN. In this scenario, these LRDs would be massive ($M_\star\sim10^{10}\,M_\odot$) and dusty (A(V) $>1$ mag) and the three satellites would be lower-mass objects ($M_\star\sim10^{8-9}\,M_\odot$) subject to low dust attenuations. The proximity of the two LRDs suggests that their interaction is responsible for their recent star formation histories, which can be interpreted as environmental bursting and quenching in the epoch of reionization.

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arXiv2025

In Search of the First Stars: An Ultra-Compact and Very Low Metallicity Lyman-$α$ Emitter Deep Within the Epoch of Reionization

We present {\it JWST} observations of a gravitationally-lensed, extremely metal-poor galaxy at redshift $z=8.203\pm 0.001$ from the CANUCS survey. Based on the low oxygen to Balmer line ratios we infer a gas-phase metallicity of $12+{\rm log(O/H)}=6.85$ (1.4\% solar), making CANUCS-A370-z8-LAE one of the most metal-poor galaxies known at $z>7$. With a high H$β$ equivalent width of $225\pm50$\,Å and a half-light radius of only $r_{\rm hl} = 38 ^{+3}_{-19} $\,pc, the galaxy has a high star-formation-rate density of $50 - 100\,M_{\odot}$\,yr$^{-1}$\,kpc$^{-2}$. The galaxy shows high equivalent width Lyman-$α$ emission with an inferred Lyman-$α$ escape fraction of $0.21 \pm 0.05$. The high escape fraction of Lyman-$α$ is likely due to the compact starbursting nature of the galaxy combined with its location in an overdensity traced by at least two other galaxies spectroscopically confirmed to lie within $δz = 0.01$ that have helped to reionize the environment. The low metallicity of CANUCS-A370-z8-LAE is best explained by a model where infalling metal-poor gas dilutes the interstellar medium, rather than being a young galaxy forming its first stellar populations.

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arXiv2025

CANUCS/Technicolor Data Release 1: Imaging, Photometry, Slit Spectroscopy, and Stellar Population Parameters

We present the first data release of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), a JWST Cycle 1 GTO program targeting 5 lensing clusters and flanking fields in parallel (Abell 370, MACS0416, MACS0417, MACS1149, MACS1423; survey area \tilda100 arcmin$^{2}$), with NIRCam imaging, NIRISS slitless spectroscopy, and NIRSpec prism multi-object spectroscopy. Fields centered on cluster cores include imaging in 8 bands from 0.9-4.4$μ$m, alongside continuous NIRISS coverage from 1.15-2$μ$m, while the NIRCam flanking fields provide 5 wide and 9 medium band filters for exceptional spectral sampling, all to \tilda29 mag$_{AB}$. We also present JWST in Technicolor, a Cycle 2 follow-up GO program targeting 3 CANUCS clusters (Abell 370, MACS0416, MACS1149). The Technicolor program adds NIRISS slitless spectroscopy in F090W to the cluster fields while adding 8 wide, medium, and narrow band filters to the flanking fields. This provides NIRCam imaging in all wide and medium band filters over \tilda30 arcmin$^{2}$. This paper describes our data reduction and photometry methodology. We release NIRCam, NIRISS, and HST imaging, PSFs, PSF-matched imaging, photometric catalogs, and photometric and spectroscopic redshifts. We provide lens models and stellar population parameters in up to 19 filters for \tilda53,000 galaxies in the cluster fields, and \tilda44,000 galaxies in up to 29 filters in the flanking fields. We further present 733 NIRSpec spectra and redshift measurements up to $z=10.8$. Comparing against our photometric redshifts, we find catastrophic outlier rates of only 4-7\% and scatter of $σ_{\rm NMAD}$ of 0.01-0.03.

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arXiv2025

CANUCS: Constraining the MACS J0416.1-2403 Strong Lensing Model with JWST NIRISS, NIRSpec and NIRCam

(Abridged) The strong lensing models of galaxy clusters rely on the number and quality of multiple images with spectroscopic redshifts. We present an updated strong lensing model of MACS J0416.1-2403 with the largest sample of multiple images with spectroscopic redshifts in a galaxy cluster field to date. Furthermore, we aim to demonstrate the effectiveness of JWST, particularly its NIRISS camera, for strong lensing studies. We used NIRCam imaging and NIRSpec and NIRISS spectroscopy from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The cluster mass model was constrained using Lenstool software. Our new dataset, which we used for constraining the lens model, comprises 303 secure multiple images with spectroscopic redshifts from 111 background sources and includes 95 systems with previously known MUSE redshift and 16 systems (with 46 multiple images) for which we obtained spectroscopic redshift for the first time using NIRISS and NIRSpec spectroscopy. Three of the spectroscopic systems were not identified by previous JWST studies. The total number of secure spectroscopic systems is more than 20% higher than in the previous strong lensing studies of this cluster. The model can reproduce multiple images with the root-mean-square distance of ~ 0.52''. We also provide a full catalogue with 415 multiple images, including less reliable candidates. In total, we provide 15 new multiple-image system candidates (with 38 multiple images) not reported in previous studies. Furthermore, we demonstrate the effectiveness of JWST, particularly NIRISS, for obtaining spectroscopic redshifts of multiple images. As NIRISS F115W, F150W, and F200W grism spectroscopy captures at least two of the [OII], [OIII] and H$α$ lines at 1 < z < 3 (a redshift range particularly relevant for strong lensing studies) without target pre-selection, it complements MUSE and NIRSpec observations extremely well.

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arXiv2025

Improving photometric redshifts of Epoch of Reionization galaxies: a new empirical transmission curve with neutral hydrogen damping wing Ly$α$ absorption

We present a new analytical model for the attenuation to Epoch of Reionization (EoR) galaxies by proximate neutral hydrogen gas. Many galaxy spectra in the EoR taken by JWST have shown a flux deficit at wavelengths just redward of the Lyman break, and this has been regarded as resulting from Ly$α$ damping wing absorption by the increasing amount of neutral hydrogen in the line-of-sight. However, previous attenuation models for the intergalactic medium (IGM) commonly used in photometric redshift template-fitting codes assume that the Lyman break is rather sharp, which leads to systematic overestimation of photometric redshifts at $z>7$. In this letter, we build and empirically calibrate a new attenuation model that takes the increased Ly$α$ damping wing absorption into account. Our model consists of the canonical IGM absorption and an additional absorption component due to dense neutral hydrogen gas clouds proximate to the galaxy, and we derive the redshift evolution of HI column density of the proximate clouds by calibrating the model using CANUCS JWST observations. The resulting total transmission curve resolves the photometric redshift bias at $z>7$, an improvement that is robust to choice of template-fitting code, template set, and photometric catalog used. The new attenuation model can be easily implemented in existing template-fitting codes, and significantly improves the photometric redshift performance in the EoR.

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arXiv2024

Detailed Study of Stars and Gas in a z = 8.3 Massive Merger with Extreme Dust Conditions

We present galaxy MACS0416-Y1 at z$_{\rm{spec}} = 8.312$ as observed by the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). MACS0416-Y1 has been shown to have extreme dust properties, thus, we study the physical properties and star formation histories of its resolved components. Overall, we find that MACS0416-Y1 is undergoing a star formation burst in three resolved clumps. The central clump is less massive compared to the other clumps and possibly formed in the merging process of the two larger clumps. Although the star formation history indicates an ongoing star formation burst, this gas-rich galaxy shows comparable star formation efficiency to cosmic noon galaxies. Using NIRSpec prism spectroscopy, we measure metallicity, $12 +\log\rm{(O/H)} = 7.76\pm0.03$ , ionisation parameter, $\log U = -2.48\pm0.03$, and electron temperature $\rm{T}_e = 18000\pm 4000 K $. The emission line ratios of the galaxy indicate an evolved Interstellar medium (ISM) similar to $z\sim2$ star-forming galaxies. Further, we find possible presence of ionisation from an active galactic nuclei (AGN) using emission line diagnostics, however, we do not detect broad line component in H$β$ emission line. As this gas-rich galaxy is undergoing a major merger, we hypothesise that the high dust temperature in MACS0416-Y1 is caused by the star formation burst or a possible narrow-line AGN.

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arXiv2024

CANUCS: UV and Ionising Properties of Dwarf Star Forming Galaxies at z = 5 to 7

The epoch of reionisation progressed through the emission of ionising photons from galaxies to their local intergalactic medium. In this work, we characterise the dwarf star-forming galaxies as candidates for the source of ionising photons that drove EoR. We investigate the ionising properties and star formation histories of star-forming dwarf galaxies at the last stages of EoR at $4.8<\rm{z}<7$ using observations from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS). The magnification due to gravitational lensing allows us to probe large dynamic ranges in stellar mass ($2\times 10^{6}\leq\rm{M}_*/\rm{M}_\odot\leq5\times 10^{9}$) and UV magnitudes ($-22.68\leq$M$_{UV}\leq=-15.95$).We find a median UV slope \buv of $-2. 56\pm0.23$ and the production efficiency of ionising photons $\log$ \xiion $=25.39\pm0.6$ for the full sample ($4.8<\rm{z}<7$) with a median stellar mass of $6.3\pm0.5\times10^{7} \rm{M}_\odot$. We find both \buv and \xiion are marginally correlated with the stellar mass of the galaxy, indicating a possible greater contribution of dwarf galaxies to the reionisation of the Universe. We find that on average, galaxies in our sample are experiencing a recent rise/burst of star formation which translates to a higher scatter in \xiion and a large scatter in H$α$ equivalent widths. Finally, we investigate the trends of H$α$ and [OIII]+H$β$ EWs with UV magnitude and find M$_{UV}$ is correlated between H$α$ but not with [OIII]+H$β$ EWs indicating low metallicities and recent burst in the UV faint galaxies.

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arXiv2024

When, Where, and How Star Formation Happens in a Galaxy Pair at Cosmic Noon Using CANUCS JWST/NIRISS Grism Spectroscopy

Spatially resolved studies are key to understanding when, where, and how stars form within galaxies. Using slitless grism spectra and broadband imaging from the CAnadian NIRISS Unbiased Cluster Survey (CANUCS) we study the spatially resolved properties of a strongly lensed ($μ$ = 5.4$\pm$1.8) z = 0.8718 galaxy pair consisting of a blue face-on galaxy (10.2 $\pm$ 0.2 log($M/M_\odot$)) with multiple star-forming clumps and a dusty red edge-on galaxy (9.9 $\pm$ 0.3 log($M/M_\odot$)). We produce accurate H$α$ maps from JWST/NIRISS grism data using a new methodology that accurately models spatially varying continuum and emission line strengths. With spatially resolved indicators, we probe star formation on timescales of $\sim$ 10 Myr (NIRISS H$α$ emission line maps) and $\sim$ 100 Myr (UV imaging and broadband SED fits). Taking the ratio of the H$α$ to UV flux ($η$), we measure spatially resolved star formation burstiness. We find that in the face-on galaxy both H$α$ and broadband star formation rates (SFRs) drop at large galactocentric radii by a factor of $\sim$ 4.7 and 3.8 respectively, while SFR over the last $\sim$ 100 Myrs has increased by a factor of 1.6. Additionally, of the 20 clumps identified in the galaxy pair we find that 7 are experiencing bursty star formation, while 10 clumps are quenching and 3 are in equilibrium (either being in a state of steady star formation or post-burst). Our analysis reveals that the blue face-on galaxy disk is predominantly in a quenching or equilibrium phase. However, the most intense quenching within the galaxy is seen in the quenching clumps. This pilot study demonstrates what JWST/NIRISS data can reveal about spatially varying star formation in galaxies at Cosmic Noon.

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arXiv2024

Bulge+disc decomposition of HFF and CANDELS galaxies: UVJ diagrams and stellar mass-size relations of galaxy components at $0.2 \leq z \leq 1.5$

Using deep imaging from the CANDELS and HFF surveys, we present bulge+disc decompositions with GalfitM for $\sim$17,000 galaxies over $0.2 \leq z\leq 1.5$. We use various model parameters to select reliable samples of discs and bulges, and derive their stellar masses using an empirically calibrated relation between mass-to-light ratio and colour. Across our entire redshift range, we show that discs follow stellar mass-size relations that are consistent with those of star-forming galaxies, suggesting that discs primarily evolve via star formation. In contrast, the stellar mass-size relations of bulges are mass-independent. Our novel dataset further enables us to separate components into star-forming and quiescent based on their specific star formation rates. We find that both star-forming discs and star-forming bulges lie on stellar mass-size relations that are similar to those of star-forming galaxies, while quiescent discs are typically smaller than star-forming discs and lie on steeper relations, implying distinct evolutionary mechanisms. Similar to quiescent galaxies, quiescent bulges show a flattening in the stellar mass-size relation at $\sim$10$^{10}$M$_\odot$, below which they show little mass dependence. However, their best-fitting relations have lower normalisations, indicating that at a given mass, bulges are smaller than quiescent galaxies. Finally, we obtain rest-frame colours for individual components, showing that bulges typically have redder colours than discs, as expected. We visually derive UVJ criteria to separate star-forming and quiescent components and show that this separation agrees well with component colour. HFF bulge+disc decomposition catalogues used for these analyses are publicly released with this paper.

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arXiv2024

$Λ$CDM not dead yet: massive high-z Balmer break galaxies are less common than previously reported

Early JWST observations that targeted so-called double-break sources (attributed to Lyman and Balmer breaks at $z>7$), reported a previously unknown population of very massive, evolved high-redshift galaxies. This surprising discovery led to a flurry of attempts to explain these objects' unexpected existence including invoking alternatives to the standard $Λ$CDM cosmological paradigm. To test these early results, we adopted the same double-break candidate galaxy selection criteria to search for such objects in the JWST images of the CAnadian NIRISS Unbiased Cluster Survey (CANUCS), and found a sample of 19 sources over five independent CANUCS fields that cover a total effective area of $\sim60\,$arcmin$^2$ at $z\sim8$. However, (1) our SED fits do not yield exceptionally high stellar masses for our candidates, while (2) spectroscopy of five of the candidates shows that while all five are at high redshifts, their red colours are due to high-EW emission lines in star-forming galaxies rather than Balmer breaks in massive, evolved systems. Additionally, (3) field-to-field variance leads to differences of $\sim 1.5$ dex in the maximum stellar masses measured in the different fields, suggesting that the early single-field JWST observations may have suffered from cosmic variance and/or sample bias. Finally, (4) we show that the presence of even a single massive outlier can dominate conclusions from small samples such as those in early JWST observations. In conclusion, we find that the double-break sources in CANUCS are not sufficiently massive or numerous to warrant questioning the standard $Λ$CDM paradigm.

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arXiv2024

A Steep Decline in the Galaxy Space Density Beyond Redshift 9 in the CANUCS UV Luminosity Function

We present a new sample of 158 galaxies at redshift $z>7.5$ selected from deep \jwst\ NIRCam imaging of five widely-separated sightlines in the CANUCS survey. Two-thirds of the pointings and 80\% of the galaxies are covered by 12 to 14 NIRCam filters, including seven to nine medium bands, providing accurate photometric redshifts and robustness against low redshift interlopers. A sample of 28 galaxies at $z>7.5$ with spectroscopic redshifts shows a low systematic offset and scatter in the difference between photometric and spectroscopic redshifts. We derive the galaxy UV luminosity function at redshifts 8 to 12, finding a slightly higher normalization than previously seen with \hst\ at redshifts 8 to 10. We observe a steeper decline in the galaxy space density from $z=8$ to $12$ than found by most \jwst\ Cycle 1 studies. In particular, we find only eight galaxies at $z>10$ and none at $z>12.5$, with no $z>10$ galaxies brighter than F277W AB=28 or $M_{\rm UV}=-20$ in our unmasked, delensed survey area of 53.4 square arcminutes. We attribute the lack of bright $z>10$ galaxies in CANUCS compared to GLASS and CEERS to intrinsic variance in the galaxy density along different sightlines. The evolution in the CANUCS luminosity function between $z=8$ and $12$ is comparable to that predicted by simulations that assume a standard star formation efficiency, without invoking any special adjustments.

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