arXiv subjects

Nor Pirzkal

Publications and source records attributed to Nor Pirzkal.

At least 73 records · Page 4Linked to original sources

arXiv2024

EPOCHS IX. When cosmic dawn breaks: Evidence for evolved stellar populations in $7 < z < 12$ galaxies from PEARLS GTO and public NIRCam imaging

The presence of evolved stars in high-redshift galaxies can place valuable indirect constraints on the onset of star formation in the Universe. Thus we use PEARLS GTO and public NIRCam photometric data to search for Balmer-break candidate galaxies at $7 < z < 12$. We find that our Balmer-break candidates at $z \sim 10.5$ tend to be older (115 Myr), have lower inferred [O III] + H$β$ emission line equivalent widths (120 Å), have lower specific star formation rates (6 Gyr$^{-1}$) and redder UV slopes ($β= -1.8$) than our control sample of galaxies. However, these trends all become less strong at $z \sim 8$, where the F444W filter now probes the strong rest-frame optical emission lines, thus providing additional constraints on the current star formation activity of these galaxies. Indeed, the bursty nature of Epoch of Reionisation galaxies can lead to a disconnect between their current SED profiles and their more extended star-formation histories. We discuss how strong emission lines, the cumulative effect of weak emission lines, dusty continua and AGN can all contribute to the photometric excess seen in the rest-frame optical, thus mimicking the signature of a Balmer break. Additional medium-band imaging will thus be essential to more robustly identify Balmer-break galaxies. However, the Balmer break alone cannot serve as a definitive proxy for the stellar age of galaxies, being complexly dependent on the star-formation history. Ultimately, deep NIRSpec continuum spectroscopy and MIRI imaging will provide the strongest indirect constraints on the formation era of the first galaxies in the Universe, thereby revealing when cosmic dawn breaks.

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arXiv2024

EPOCHS Paper II: The Ultraviolet Luminosity Function from $7.5<z<13.5$ using 180 square arcminutes of deep, blank-fields from the PEARLS Survey and Public JWST data

We present an analysis of the ultraviolet luminosity function (UV LF) and star formation rate density of distant galaxies ($7.5 < z < 13.5$) in the `blank' fields of the Prime Extragalactic Areas for Reionization Science (PEARLS) survey combined with Early Release Science (ERS) data from the CEERS, GLASS, NGDEEP surveys/fields and the first data release of JADES. We use strict quality cuts on EAZY photometric redshifts to obtain a reliable selection and characterisation of high-redshift ($z>6.5$) galaxies from a consistently processed set of deep, near-infrared imaging. Within an area of 180 arcmin$^{2}$, we identify 1046 candidate galaxies at redshifts $z>6.5$ and we use this sample to study the ultraviolet luminosity function (UV LF) in four redshift bins between $7.5<z<13.5$. The measured number density of galaxies at $z=8$ and $z=9$ match those of past observations undertaken by the {\em Hubble Space Telescope} (HST). Our $z=10.5$ measurements lie between early JWST results and past HST results, indicating cosmic variance may be the cause of previous high density measurements. However, number densities of UV luminous galaxies at $z=12.5$ are high compared to predictions from simulations. When examining the star formation rate density of galaxies at this time period, our observations are still largely consistent with a constant star formation efficiency, are slightly lower than previous early estimations using JWST and support galaxy driven reionization at $z\leq8$.

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arXiv2024

NGDEEP Epoch 1: Spatially Resolved H$α$ Observations of Disk and Bulge Growth in Star-Forming Galaxies at $z \sim$ 0.6-2.2 from JWST NIRISS Slitless Spectroscopy

We study the H$α$ equivalent width, EW(H$α$), maps of 19 galaxies at $0.6 < z < 2.2$ in the Hubble Ultra Deep Field (HUDF) derived from NIRISS slitless spectroscopy as part of the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey. Our galaxies mostly lie on the star-formation main sequence with a stellar mass range of $\mathrm{10^9 - 10^{11} M_\odot}$, characterized as "typical" star-forming galaxies at these redshifts. Leveraging deep HST and JWST broad-band images, spanning 0.4-4.8 $μ$m, we perform spatially-resolved fitting of the spectral energy distributions (SEDs) for these galaxies and construct specific star formation rate (sSFR) and stellar-mass-weighted age maps with a spatial resolution of $\sim$1 kpc. The pixel-to-pixel EW(H$α$) increases with increasing sSFR and with decreasing age. The average trends are slightly different from the relations derived from integrated fluxes of galaxies from the literature, suggesting complex evolutionary trends within galaxies. We quantify the radial profiles of EW(H$α$), sSFR, and age. The majority (84%) of galaxies show positive EW(H$α$) gradients in line with the inside-out quenching scenario. A few galaxies (16%) show inverse (and flat) trends possibly due to merging or starbursts. We compare the distributions of EW(H$α$) and sSFR to the star formation history models (SFHs) as a function of galactocentric radius. We argue that the central regions of galaxies have experienced, at least one, rapid star-formation episodes, which leads to the formation of the bulge, while their outer regions (e.g., disks) grow via more smoothly varying SFHs. These results demonstrate the ability to study resolved star formation in distant galaxies with JWST NIRISS.

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arXiv2024

JWST NIRCam Photometry: A Study of Globular Clusters Surrounding Bright Elliptical Galaxy VV 191a at z=0.0513

James Webb Space Telescope NIRCam images have revealed 154 reliable globular cluster (GC) candidates around the $z = 0.0513$ elliptical galaxy VV~191a after subtracting 34 likely interlopers from background galaxies inside our search area. NIRCam broadband observations are made at 0.9-4.5 $μ$m using the F090W, F150W, F356W, and F444W filters. Using PSF-matched photometry, the data are analyzed to present color-magnitude diagrams (CMDs) and color distributions that suggest a relatively uniform population of GCs, except for small fractions of reddest (5-8%) and bluest (2-4%) outliers. GC models in the F090W vs. (F090-F150W) diagram fit the NIRCam data well and show that the majority of GCs detected have a mass of approximately $\sim$$10^{6.5}$$M_{\odot}$, with metallicities [Fe/H] spanning the typical range expected for GCs (-2.5$\le$ [Fe/H]$\le$ 0.5). However, the models predict $\sim$0.3-0.4 mag bluer (F356W-F444W) colors than the NIRCam data for a reasonable range of GC ages, metallicities, and reddening. Although our data does not quite reach the luminosity function turnover, the measured luminosity function is consistent with previous measurements, suggesting an estimated peak at $m_{\rm AB}$$\sim$-9.4 mag, $\pm$0.2 mag in the F090W filter.

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arXiv2024

Magellanic System Stars Identified in SMACS J0723.3-7327 James Webb Space Telescope Early Release Observations Images

We identify 71 distant stars in JWST/NIRCam ERO images of the field of galaxy cluster SMACS J0723.3-7327 (SMACS 0723). Given the relatively small ($\sim$$10^{\circ}$) angular separation between SMACS 0723 and the Large Magellanic Cloud, it is likely that these stars are associated with the LMC outskirts or Leading Arm. This is further bolstered by a spectral energy distribution analysis, which suggests an excess of stars at a physical distance of $40-100$ kpc, consistent with being associated with or located behind the Magellanic system. In particular, we find that the overall surface density of stars brighter than 27.0 mag in the field of SMACS 0723 is $\sim$2.3 times that of stars in a blank field with similar galactic latitude (the North Ecliptic Pole Time Domain Field), and that the density of stars in the SMACS 0723 field with SED-derived distances consistent with the Magellanic system is $\sim$6.1 times larger than that of the blank field. The candidate stars at these distances are consistent with a stellar population at the same distance modulus with [Fe/H] $= -1.0$ and an age of $\sim$$5.0$ Gyr. On the assumption that all of the 71 stars are associated with the LMC, then the stellar density of the LMC at the location of the SMACS 0723 field is $\sim$$740$ stars kpc$^{-3}$, which helps trace the density of stars in the LMC outskirts.

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arXiv2024

A search for high-redshift direct-collapse black hole candidates in the PEARLS north ecliptic pole field

Direct-collapse black holes (DCBHs) of mass $\sim 10^4$-$10^5 {M}_\odot$ that form in HI-cooling halos in the early Universe are promising progenitors of the $\gtrsim 10^9 {M}_\odot$ supermassive black holes that fuel observed $z \gtrsim 7$ quasars. Efficient accretion of the surrounding gas onto such DCBH seeds may render them sufficiently bright for detection with the JWST up to $z\approx 20$. Additionally, the very steep and red spectral slope predicted across the $\approx 1$-5 $μ$m wavelength range of the JWST/NIRSpec instrument during their initial growth phase should make them photometrically identifiable up to very high redshifts. In this work, we present a search for such DCBH candidates across the 34 arcmin$^{2}$ in the first two spokes of the JWST cycle-1 PEARLS survey of the north ecliptic pole time-domain field covering eight NIRCam filters down to a maximum depth of $\sim$ 29 AB mag. We identify two objects with spectral energy distributions consistent with the Pacucci et al. (2016) DCBH models. However, we also note that even with data in eight NIRCam filters, objects of this type remain degenerate with dusty galaxies and obscured active galactic nuclei over a wide range of redshifts. Follow-up spectroscopy would be required to pin down the nature of these objects. Based on our sample of DCBH candidates and assumptions on the typical duration of the DCBH steep-slope state, we set a conservative upper limit of $\lesssim 5\times 10^{-4}$ comoving Mpc$^{-3}$ (cMpc$^{-3}$) on the comoving density of host halos capable of hosting DCBHs with spectral energy distributions similar to the Pacucci et al. (2016) models at $z\approx 6$-14.

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arXiv2024

JWST's PEARLS: Improved Flux Calibration for NIRCam

The Prime Extragalactic Areas for Reionization and Lensing Science (PEARLS), a JWST GTO program, obtained a set of unique NIRCam observations that have enabled us to significantly improve the default photometric calibration across both NIRCam modules. The observations consisted of three epochs of 4-band (F150W, F200W, F356W, and F444W) NIRCam imaging in the Spitzer IRAC Dark Field (IDF). The three epochs were six months apart and spanned the full duration of Cycle 1. As the IDF is in the JWST continuous viewing zone, we were able to design the observations such that the two modules of NIRCam, modules A and B, were flipped by 180 degrees and completely overlapped each other's footprints in alternate epochs. We were therefore able to directly compare the photometry of the same objects observed with different modules and detectors, and we found significant photometric residuals up to ~ 0.05 mag in some detectors and filters, for the default version of the calibration files that we used (jwst_1039.pmap). Moreover, there are multiplicative gradients present in the data obtained in the two long-wavelength bands. The problem is less severe in the data reduced using the latest pmap (jwst_1130.pmap as of September 2023), but it is still present, and is non-negligible. We provide a recipe to correct for this systematic effect to bring the two modules onto a more consistent calibration, to a photometric precision better than ~ 0.02 mag.

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arXiv2024

Galaxies Going Bananas: Inferring the 3D Geometry of High-Redshift Galaxies with JWST-CEERS

The 3D geometry of high-redshift galaxies remains poorly understood. We build a differentiable Bayesian model and use Hamiltonian Monte Carlo to efficiently and robustly infer the 3D shapes of star-forming galaxies in JWST-CEERS observations with $\log M_*/M_{\odot}=9.0-10.5$ at $z=0.5-8.0$. We reproduce previous results from HST-CANDELS in a fraction of the computing time and constrain the mean ellipticity, triaxiality, size and covariances with samples as small as $\sim50$ galaxies. We find high 3D ellipticities for all mass-redshift bins suggesting oblate (disky) or prolate (elongated) geometries. We break that degeneracy by constraining the mean triaxiality to be $\sim1$ for $\log M_*/M_{\odot}=9.0-9.5$ dwarfs at $z>1$ (favoring the prolate scenario), with significantly lower triaxialities for higher masses and lower redshifts indicating the emergence of disks. The prolate population traces out a ``banana'' in the projected $b/a-\log a$ diagram with an excess of low $b/a$, large $\log a$ galaxies. The dwarf prolate fraction rises from $\sim25\%$ at $z=0.5-1.0$ to $\sim50-80\%$ at $z=3-8$. If these are disks, they cannot be axisymmetric but instead must be unusually oval (triaxial) unlike local circular disks. We simultaneously constrain the 3D size-mass relation and its dependence on 3D geometry. High-probability prolate and oblate candidates show remarkably similar Sérsic indices ($n\sim1$), non-parametric morphological properties and specific star formation rates. Both tend to be visually classified as disks or irregular but edge-on oblate candidates show more dust attenuation. We discuss selection effects, follow-up prospects and theoretical implications.

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arXiv2024

PEARLS: A Potentially Isolated Quiescent Dwarf Galaxy with a TRGB Distance of 30 Mpc

A wealth of observations have long suggested that the vast majority of isolated classical dwarf galaxies ($M_*=10^7$-$10^9$ M$_\odot$) are currently star-forming. However, recent observations of the large abundance of "Ultra-Diffuse Galaxies" beyond the reach of previous large spectroscopic surveys suggest that our understanding of the dwarf galaxy population may be incomplete. Here we report the serendipitous discovery of an isolated quiescent dwarf galaxy in the nearby Universe, which was imaged as part of the PEARLS GTO program. Remarkably, individual red-giant branch stars are visible in this near-IR imaging, suggesting a distance of $30\pm4$ Mpc, and a wealth of archival photometry point to an sSFR of $2\times10^{-11}$ yr$^{-1}$ and SFR of $4\times10^{-4}$ M$_\odot$ yr$^{-1}$. Spectra obtained with the Lowell Discovery Telescope find a recessional velocity consistent with the Hubble Flow and ${>}1500$ km/s separated from the nearest massive galaxy in SDSS, suggesting that this galaxy was either quenched from internal mechanisms or had a very high-velocity ($>1000$ km/s) interaction with a nearby massive galaxy in the past. This analysis highlights the possibility that many nearby quiescent dwarf galaxies are waiting to be discovered and that JWST has the potential to resolve them.

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arXiv2023

A Census from JWST of Extreme Emission Line Galaxies Spanning the Epoch of Reionization in CEERS

We present a sample of 1165 extreme emission-line galaxies (EELGs) at 4 5000 AA. JWST/NIRSpec spectroscopic observations of a subset (34) of the photometrically selected EELGs validate our selection method: all spectroscopically observed EELGs confirm our photometric identification of extreme emission, including some cases where the SED-derived photometric redshifts are incorrect. We find that the medium-band F410M filter in CEERS is particularly efficient at identifying EELGs, both in terms of including emission lines in the filter and in correctly identifying the continuum between Hb + [OIII] and Ha in the neighboring broad-band filters. We present examples of EELGs that could be incorrectly classified at ultra-high redshift (z>12) as a result of extreme Hb + [OIII] emission blended across the reddest photometric filters. We compare the EELGs to the broader (sub-extreme) galaxy population in the same redshift range and find that they are consistent with being the bluer, high equivalent width tail of a broader population of emission-line galaxies. The highest-EW EELGs tend to have more compact emission-line sizes than continuum sizes, suggesting that active galactic nuclei are responsible for at least some of the most extreme EELGs. Photometrically inferred emission-line ratios are consistent with ISM conditions with high ionization and moderately low metallicity, consistent with previous spectroscopic studies.

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arXiv2023

The JWST Discovery of the Triply-imaged Type Ia "Supernova H0pe" and Observations of the Galaxy Cluster PLCK G165.7+67.0

A Type Ia supernova (SN) at $z=1.78$ was discovered in James Webb Space Telescope Near Infrared Camera imaging of the galaxy cluster PLCK G165.7+67.0 (G165; $z = 0.35$). The SN is situated 1.5-2 kpc from the host-galaxy nucleus and appears in three different locations as a result of gravitational lensing by G165. These data can yield a value for Hubble's constant using time delays from this multiply-imaged SN Ia that we call "SN H0pe." Over the cluster, we identified 21 image multiplicities, confirmed five of them using the Near-Infrared Spectrograph, and constructed a new lens model that gives a total mass within 600 kpc of ($2.6 \pm 0.3) \times 10^{14}$ $M_{\odot}$. The photometry uncovered a galaxy overdensity coincident with the SN host galaxy. NIRSpec confirmed six member galaxies, four of which surround the SN host galaxy with relative velocity $\lesssim$900 km s$^{-1}$ and projected physical extent $\lesssim$33 kpc. This compact galaxy group is dominated by the SN host galaxy, which has a stellar mass of $(5.0 \pm 0.1) \times 10^{11}$ $M_{\odot}$. The group members have specific star-formation rates of 2-260 Gyr$^{-1}$ derived from the H$α$-line fluxes corrected for stellar absorption, dust extinction, and slit losses. Another group centered on a strongly-lensed dusty star forming galaxy is at $z=2.24$. The total (unobscured and obscured) SFR of this second galaxy group is estimated to be ($\gtrsim$100 $M_{\odot}$ yr$^{-1}$), which translates to a supernova rate of $\sim$1 SNe yr$^{-1}$, suggesting that regular monitoring of this cluster may yield additional SNe.

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arXiv2023

Evidence for a Shallow Evolution in the Volume Densities of Massive Galaxies at $z=4$ to $8$ from CEERS

We analyze the evolution of massive (log$_{10}$ [$M_\star/M_\odot$] $>10$) galaxies at $z \sim$ 4--8 selected from the JWST Cosmic Evolution Early Release Science (CEERS) survey. We infer the physical properties of all galaxies in the CEERS NIRCam imaging through spectral energy distribution (SED) fitting with dense basis to select a sample of high redshift massive galaxies. Where available we include constraints from additional CEERS observing modes, including 18 sources with MIRI photometric coverage, and 28 sources with spectroscopic confirmations from NIRSpec or NIRCam wide-field slitless spectroscopy. We sample the recovered posteriors in stellar mass from SED fitting to infer the volume densities of massive galaxies across cosmic time, taking into consideration the potential for sample contamination by active galactic nuclei (AGN). We find that the evolving abundance of massive galaxies tracks expectations based on a constant baryon conversion efficiency in dark matter halos for $z \sim$ 1--4. At higher redshifts, we observe an excess abundance of massive galaxies relative to this simple model. These higher abundances can be explained by modest changes to star formation physics and/or the efficiencies with which star formation occurs in massive dark matter halos, and are not in tension with modern cosmology.

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arXiv2023

Rest-Frame UV Colors for Faint Galaxies at $z \sim 9-16$ with the \textit{JWST} NGDEEP Survey

We present measurements of the rest-frame UV spectral slope, $β$, for a sample of 36 faint star-forming galaxies at z ~ 9-16 discovered in one of the deepest JWST NIRCam surveys to date, the Next Generation Deep Extragalactic Exploratory Public (NGDEEP) Survey. We use robust photometric measurements for UV-faint galaxies (down to $M_{UV}$ ~ -16), originally published in Leung+23, and measure values of the UV spectral slope via photometric power-law fitting to both the observed photometry and to stellar population models obtained through spectral energy distribution (SED) fitting with Bagpipes. We obtain a median and 68% confidence interval for $β$ from photometric power-law fitting of $β_{PL} = -2.7^{+0.5}_{-0.5}$ and from SED-fitting, $β_{SED} = -2.3^{+0.2}_{-0.1}$ for the full sample. We show that when only 2-3 photometric detections are available, SED-fitting has a lower scatter and reduced biases than photometric power-law fitting. We quantify this bias and find that after correction, the median $β_{SED,corr} = -2.5^{+0.2}_{-0.2}$. We measure physical properties for our galaxies with Bagpipes and find that our faint ($M_{UV} = -18.1^{+0.7}_{-0.9}$) sample is low mass (${log}[M_{\ast}/M_\odot] = 7.7^{+0.5}_{-0.5}$), fairly dust-poor ($A_{v} = 0.1^{+0.2}_{-0.1}$ mag), and modestly young (${log[age]} = 7.8^{+0.2}_{-0.8}$ yr) with a median star formation rate of $\mathrm{log(SFR)} = -0.3^{+0.4}_{-0.4} M_\odot{/yr}$. We find no strong evidence for ultra-blue UV spectral slopes ($β$ ~ -3) within our sample, as would be expected for exotically metal-poor ($Z/Z_{\odot}$ < 10$^{-3}$) stellar populations with very high LyC escape fractions. Our observations are consistent with model predictions that galaxies of these stellar masses at z~9-16 should have only modestly low metallicities ($Z/Z_{\odot}$ ~ 0.1--0.2).

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arXiv2023

A Milky Way-like barred spiral galaxy at a redshift of 3

The majority of massive disk galaxies in the local Universe show a stellar barred structure in their central regions, including our Milky Way. Bars are supposed to develop in dynamically cold stellar disks at low redshift, as the strong gas turbulence typical of disk galaxies at high redshift suppresses or delays bar formation. Moreover, simulations predict bars to be almost absent beyond $z = 1.5$ in the progenitors of Milky Way-like galaxies. Here we report observations of ceers-2112, a barred spiral galaxy at redshift $z_{\rm phot} \sim 3$, which was already mature when the Universe was only 2 Gyr old. The stellar mass ($M_{\star} = 3.9 \times 10^9 M_{\odot}$) and barred morphology mean that ceers-2112 can be considered a progenitor of the Milky Way, in terms of both structure and mass-assembly history in the first 2 Gyr of the Universe, and was the closest in mass in the first 4 Gyr. We infer that baryons in galaxies could have already dominated over dark matter at $z \sim 3$, that high-redshift bars could form in approximately 400 Myr and that dynamically cold stellar disks could have been in place by redshift $z = 4-5$ (more than 12 Gyrs ago).

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arXiv2023

CEERS Key Paper I: An Early Look into the First 500 Myr of Galaxy Formation with JWST

We present an investigation into the first 500 Myr of galaxy evolution from the Cosmic Evolution Early Release Science (CEERS) survey. CEERS, one of 13 JWST ERS programs, targets galaxy formation from z~0.5 to z>10 using several imaging and spectroscopic modes. We make use of the first epoch of CEERS NIRCam imaging, spanning 35.5 sq. arcmin, to search for candidate galaxies at z>9. Following a detailed data reduction process implementing several custom steps to produce high-quality reduced images, we perform multi-band photometry across seven NIRCam broad and medium-band (and six Hubble broadband) filters focusing on robust colors and accurate total fluxes. We measure photometric redshifts and devise a robust set of selection criteria to identify a sample of 26 galaxy candidates at z~9-16. These objects are compact with a median half-light radius of ~0.5 kpc. We present an early estimate of the z~11 rest-frame ultraviolet (UV) luminosity function, finding that the number density of galaxies at M_UV ~ -20 appears to evolve very little from z~9 to z~11. We also find that the abundance (surface density [arcmin^-2]) of our candidates exceeds nearly all theoretical predictions. We explore potential implications, including that at z>10 star formation may be dominated by top-heavy initial mass functions, which would result in an increased ratio of UV light per unit halo mass, though a complete lack of dust attenuation and/or changing star-formation physics may also play a role. While spectroscopic confirmation of these sources is urgently required, our results suggest that the deeper views to come with JWST should yield prolific samples of ultra-high-redshift galaxies with which to further explore these conclusions.

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arXiv2023

Evolution of the Size-Mass Relation of Star-forming Galaxies Since $z=5.5$ Revealed by CEERS

We combine deep imaging data from the CEERS early release JWST survey and HST imaging from CANDELS to examine the size-mass relation of star-forming galaxies and the morphology-quenching relation at stellar masses $\textrm{M}_{\star} \geq 10^{9.5} \ \textrm{M}_{\odot}$ over the redshift range $0.5 < z < 5.5$. In this study with a sample of 2,450 galaxies, we separate star-forming and quiescent galaxies based on their star-formation activity and confirm that star-forming and quiescent galaxies have different morphologies out to $z=5.5$, extending the results of earlier studies out to higher redshifts. We find that star-forming and quiescent galaxies have typical Sérsic indices of $n\sim1.3$ and $n\sim4.3$, respectively. Focusing on star-forming galaxies, we find that the slope of the size-mass relation is nearly constant with redshift, as was found previously, but shows a modest increase at $z \sim 4.2$. The intercept in the size-mass relation declines out to $z=5.5$ at rates that are similar to what earlier studies found. The intrinsic scatter in the size-mass relation is relatively constant out to $z=5.5$.

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arXiv2023

CEERS: MIRI deciphers the spatial distribution of dust-obscured star formation in galaxies at $0.1<z<2.5$

[Abridged] We combined HST images from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey with JWST images from the Cosmic Evolution Early Release Science (CEERS) survey to measure the stellar and dust-obscured star formation distributions of a mass-complete ($>10^{10}M_\odot$) sample of 69 star-forming galaxies (SFGs) at $0.1 75$; 35 galaxies). At lower $S/N$, simulations show that simultaneous measurements of the size and Sérsic index become unreliable. We extended our study to fainter sources ($S/N>10$; 69 galaxies) by fixing their Sérsic index to unity. The Sérsic index of bright galaxies ($S/N>75$) has a median value of 0.7, which, together with their axis ratio distribution, suggests a disk-like morphology in the rest-MIR. Galaxies above the main sequence (MS; i.e., starbursts) have rest-MIR sizes that are a factor 2 smaller than their rest-optical sizes. The median rest-optical to rest-MIR size ratio of MS galaxies increases with stellar mass, from 1.1 at $10^{9.8}M_\odot$ to 1.6 at $10^{11}M_\odot$. This mass-dependent trend resembles the one found in the literature between the rest-optical and rest-near-infrared sizes of SFGs, suggesting that it is due to radial color gradients affecting rest-optical sizes and that the sizes of the stellar and star-forming components of SFGs are, on average, consistent at all masses. There is, however, a small population of SFGs (15%) with a compact star-forming component embedded in a larger stellar structure. This could be the missing link between galaxies with an extended stellar component and those with a compact stellar component, the so-called blue nuggets.

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arXiv2023

JWST's PEARLS: Transients in the MACS J0416.1-2403 Field

With its unprecedented sensitivity and spatial resolution, the James Webb Space Telescope (JWST) has opened a new window for time-domain discoveries in the infrared. Here we report observations in the only field that has received four epochs (spanning 126 days) of JWST NIRCam observations in Cycle 1. This field is towards MACS J0416.1-2403, which is a rich galaxy cluster at redshift z=0.4 and is one of the Hubble Frontier Fields. We have discovered 14 transients from these data. Twelve of these transients happened in three galaxies (with z=0.94, 1.01, and 2.091) crossing a lensing caustic of the cluster,and these transients are highly magnified by gravitational lensing. These 12 transients are likely of similar nature to those previously reported based on the Hubble Space Telescope (HST) data in this field, i.e., individual stars in the highly magnified arcs. However, these twelve could not have been found by HST because they are too red and too faint. The other two transients are associated with background galaxies (z=2.205 and 0.7093) that are only moderately magnified, and they are likely supernovae. They indicate a de-magnified supernova surface density, when monitored at a time cadence of a few months to a ~3--4 micron survey limit of AB ~ 28.5 mag, of ~0.5 per sq. arcmin integrated to z ~ 2. This survey depth is beyond the capability of HST but can be easily reached by JWST.

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