arXiv subjects

Ivelina Momcheva

Publications and source records attributed to Ivelina Momcheva.

At least 19 recordsLinked to original sources

arXiv2026

Surveying the Universe in 4D: Beating Cosmic Variance with Wide-Field Slitless Spectroscopy from HST, JWST, Euclid, Roman, and Beyond

We summarize strategies, lessons learned, and future directions from the Space Telescope Science Institute workshop Surveying the Universe in 4D: Beating Cosmic Variance with Wide-Field Slitless Spectroscopy from HST, JWST, Euclid, Roman, and Beyond, held August 24--28, 2026. The workshop examined scientific results, observational and data analysis challenges, extraction tools, and future opportunities. Discussions highlighted (1) the transformative potential of WFSS for the study of transient phenomena, galaxy evolution --both spatially-resolved and within the broader context of the cosmic web--, and rare populations and (2) the synergies among Euclid and Roman surveys, Rubin-LSST monitoring, JWST WFSS, and high-resolution integral-field observations. Participants identified advances in forward modeling and physics-informed machine learning as essential for addressing spectral overlap, crowded fields, and upcoming, very large data volumes. Realizing WFSS's full potential will require community-wide infrastructure, science-ready data products, accessible cloud-based analysis tools, and robust benchmarking of reduction pipelines. Crucially, participants called for systemic changes to properly recognize early-career researchers who invest significant efforts in pipeline, code, and calibration developments that enable WFSS science, and stressed that progress requires collaborative, multidisciplinary practices that optimize the participation and benefits of the next generation.

astro-ph.GA↗

arXiv2026

OutThere Survey: Addressing $\mathrm{ξ_{ion}}$ and $\mathrm{f_{esc}}$ with a population of average galaxies at z$\sim$2

Constraining the major contributors to the ionisation of the early universe is an ongoing endeavour of high-redshift galaxy research. We measure the ionising photon production efficiency and Lyman Continuum escape fraction for a sample of 230 intermediate redshift ($1.3 $5. This control sample allows us to verify the correlations between ionising and spectral/physical properties suggested by previous studies. We find no significant correlations between the ionising photon production efficiency ($\mathrm{ξ_{ion}}$) with the UV slope, $\mathrm{M_{UV}}$, M$_*$ or sSFR. We do find that $\mathrm{ξ_{ion}}$ correlates with [OIII]5007Å\, equivalent width (EW) (Spearman coefficient $ρ$ =0.24; p$< 4\times10^{-4}$) and H$α$ EW ($ρ$ =0.63; p$<< 1\times10^{-6}$) hold even at low EW albeit with more scatter. We also find that our novel approach to determining the ionising photon escape fraction $\mathrm{f_{esc}}$ results in values within theoretical ranges (0-10\%) though vary substantially in comparison to the empirical results (median $\mathrm{f_{esc}} = 0.9\%^{+1.1}_{-0.5}$ including non-detections, median $\mathrm{f_{esc}} = 1.9\%^{+8.9}_{-1.8}$ above a $0.01\%$ threshold). We find that this escape fraction method has consistently significant correlations with the redshift, SFR and M$_{UV}$ and sample-dependent correlations with [OIII]5007Å\,EW,H$α$ EW and stellar mass.

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arXiv2025

The Evolution of Half-Mass Radii and Color Gradients for Young and Old Quiescent Galaxies at $0.5 < z < 3$ with JWST/PRIMER

We present a study of the size growth of the red sequence between $0.5 10$, whereas half-mass radii are derived from the F444W profiles together with the F277W-F444W color-$M_*$/L relation. We investigate the dependence of the ratio $r_{e, \mathrm{mass}}/r_{e, \mathrm{light}}$ on redshift, stellar mass, and the wavelength used to measure $r_{e, \mathrm{light}}$, also separating the sample into younger and older quiescent galaxies. Our data demonstrate that rest-frame infrared sizes accurately trace mass-weighted sizes while sizes measured at rest-frame optical wavelengths (0.5-0.7$μ$m) are 0.1-0.2 dex larger, with only minor variations in redshift. We find that the average size of young quiescent galaxies agrees with that of old quiescent galaxies at intermediate masses, $10<$log($M_*/M_{\odot}$)$<11$, within their respective uncertainties in all observed-frame half-light, rest-frame half-light and half-mass radius measurements. At face value, our results point to a combination of progenitor bias and minor mergers driving the size growth of intermediate-mass quiescent galaxies at $0.5<z<3$. Our results further indicate that the varying contributions to the general quiescent population by young and old quiescent galaxies can mimic evolution in redshift.

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arXiv2024

3D-DASH: The Evolution of Size, Shape, and Intrinsic Scatter in Populations of Young and Old Quiescent Galaxies at 0.5 < z < 3

We present a study of the growth of the quiescent galaxy population between 0.5 < z < 3 by tracing the number density and structural evolution of a sample of 4518 old and 583 young quiescent galaxies with log($M_*$/$M_{\odot}$)>10.4, selected from the COSMOS2020 catalog with complementary HST/F160W imaging from the 3D-DASH survey. Among the quiescent population at z$\sim$2, roughly 50% are recently quenched galaxies; these young quiescent galaxies become increasingly rare towards lower redshift, supporting the idea that the peak epoch of massive galaxy quenching occurred at z>2. Our data show that while the effective half-light radii of quiescent galaxies generally increases with time, young quiescent galaxies are significantly smaller than their older counterparts at the same redshift. In this work we investigate the connection between this size difference and other structural properties, including axis ratios, color gradients, stellar mass, and the intrinsic scatter in effective radii. We demonstrate that the size difference is driven by the most massive sub-population (log($M_*$/$M_{\odot}$)>11) and does not persist when restricting the sample to intermediate mass galaxies (10.4<log($M_*$/$M_{\odot}$)<11). Interestingly, the intrinsic scatter in physical size shows a strong co-evolution over the investigated time period and peaks around z$\sim$2 for both populations, only diverging at z < 1. Taken together, and assuming we are not missing a significant population of lower surface brightness galaxies, while the formation and quenching mechanisms that dominate at higher redshifts yield compact remnants, multiple evolutionary pathways may explain the diverse morphologies of galaxies that quench at z<1.

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arXiv2023

CLEAR: The Morphological Evolution of Galaxies in the Green Valley

Quiescent galaxies having more compact morphologies than star-forming galaxies has been a consistent result in the field of galaxy evolution. What is not clear is at what point this divergence happens, i.e. when do quiescent galaxies become compact, and how big of a role does the progenitor effect play in this result? Here we aim to model the morphological and star-formation histories of high redshift (0.8 $<$ z $<$ 1.65) massive galaxies (log(M/M$\odot$) $>$ 10.2) with stellar population fits using HST/WFC3 G102 and G141 grism spectra plus photometry from the CLEAR (CANDELS Lyman-alpha Emission at Reionization) survey, constraining the star-formation histories for a sample of $\sim$ 400 massive galaxies using flexible star-formation histories. We develop a novel approach to classifying galaxies by their formation activity in a way that highlights the green valley population, by modeling the specific star-formation rate distributions as a function of redshift and deriving the probability that a galaxy is quiescent (PQ). Using PQ and our flexible star-formation histories we outline the evolutionary paths of our galaxies in relation to stellar mass, Sersic index, $R_{eff}$, and stellar mass surface density. We find that galaxies show no appreciable stellar mass growth after entering the green valley (a net decrease of 4$\%$) while their stellar mass surface densities increase by $\sim$ 0.25 dex. Therefore galaxies are becoming compact during the green valley and this is due to increases in Sersic index and decreases in $R_{eff}$.

astro-ph.GA↗

arXiv2023

CLEAR: High-Ionization [Ne V] $λ$3426 Emission-line Galaxies at $1.4 <z< 2.3$

We analyze a sample of 25 [Ne V] $λ$3426 emission-line galaxies at $1.4<z<2.3$ using Hubble Space Telescope/Wide Field Camera 3 G102 and G141 grism observations from the CANDELS Lyman-$α$ Emission at Reionization (CLEAR) survey. [Ne V] emission probes extremely energetic photoionization (97.11-126.21 eV), and is often attributed to energetic radiation from active galactic nuclei (AGN), shocks from supernova, or an otherwise very hard ionizing spectrum from the stellar continuum. In this work, we use [Ne V] in conjunction with other rest-frame UV/optical emission lines ([O II] $λλ$3726,3729, [Ne III] $λ$3869, H$β$, [O III] $λλ$4959,5007, H$α$+[N II] $λλ$6548,6583, [S II] $λλ$6716,6731), deep (2--7 Ms) X-ray observations (from Chandra), and mid-infrared imaging (from Spitzer) to study the origin of this emission and to place constraints on the nature of the ionizing engine. The majority of the [Ne V]-detected galaxies have properties consistent with ionization from AGN. However, for our [Ne V]-selected sample, the X-ray luminosities are consistent with local ($z\lesssim 0.1$) X-ray-selected Seyferts, but the [Ne V] luminosities are more consistent with those from $z\sim 1$ X-ray-selected QSOs. The excess [Ne V] emission requires either reduced hard X-rays, or a $\sim$0.1 keV excess. We discuss possible origins of the apparent [Ne V] excess, which could be related to the ``soft (X-ray) excess'' observed in some QSOs and Seyferts, and/or be a consequence of a complex/anisotropic geometry for the narrow line region, combined with absorption from a warm, relativistic wind ejected from the accretion disk. We also consider implications for future studies of extreme high-ionization systems in the epoch of reionization ($z \gtrsim 6$) with JWST.

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arXiv2022

CLEAR: Spatially Resolved Emission Lines and Active Galactic Nuclei at $0.6<z<1.3$

We investigate spatially-resolved emission-line ratios in a sample of 219 galaxies ($0.6<z<1.3$) detected using the G102 grism on the \emph{Hubble Space Telescope} Wide Field Camera 3, taken as part of the CANDELS Ly$α$ Emission at Reionization (CLEAR) survey, to measure ionization profiles and search for low-luminosity active galactic nuclei (AGN). We analyze \OIII\ and \Hb\ emission-line maps, enabling us to spatially resolve the \OIIIHb\ emission-line ratio across the galaxies in the sample. We compare the \OIIIHb\ ratio in galaxy centers and outer annular regions to measure ionization gradients and investigate the potential of sources with nuclear ionization to host AGN. We investigate some of the individual galaxies that are candidates to host strong nuclear ionization and find that they often have low stellar mass and are undetected in X-rays, as expected for low-luminosity AGN in low-mass galaxies. We do not find evidence for a significant population of off-nuclear AGN or other clumps of off-nuclear ionization. We model the observed distribution of \OIIIHb\ gradients and find that most galaxies are consistent with small or zero gradients, but 6-16\% of galaxies in the sample are likely to host nuclear \OIIIHb\ that is $\sim$0.5~dex higher than in their outer regions. This study is limited by large uncertainties in most of the measured \OIIIHb\ spatial profiles, therefore deeper data, e.g, from deeper \textit{HST}/WFC3 programs or from \textit{JWST}/NIRISS, are needed to more reliably measure the spatially resolved emission-line conditions of individual high-redshift galaxies.

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arXiv2022

CLEAR: The Ionization and Chemical-Enrichment Properties of Galaxies at 1.1 < z < 2.3

We use deep spectroscopy from the Hubble Space Telescope (HST) Wide-Field-Camera 3 (WFC3) IR grisms combined with broad-band photometry to study the stellar populations, gas ionization and chemical abundances in star-forming galaxies at $z\sim 1.1-2.3$. The data stem from the CANDELS Lyman-$α$ Emission At Reionization (CLEAR) survey. At these redshifts the grism spectroscopy measure the [OII] 3727, 3729, [OIII] 4959, 5008, H-$β$ strong emission features, which constrain the ionization parameter and oxygen abundance of the nebular gas. We compare the line flux measurements to predictions from updated photoionization models (MAPPINGS (Kewley et al. 2019), which include an updated treatment of nebular gas pressure, log P/k = $n_e T_e$. Compared to low-redshift samples ($z\sim 0.2$) at fixed stellar mass, llog M / M$_\odot$ = 9.4-9.8, the CLEAR galaxies at z=1.35 (z=1.90) have lower gas-phase metallicity, $Δ$(log Z) = 0.25 (0.35) dex, and higher ionization parameters, $Δ$(log q) = 0.25 (0.35) dex, where U = q/c. We provide updated analytic calibrations between the [OIII], [OII], and H-$β$ emission line ratios, metallicity, and ionization parameter. The CLEAR galaxies show that at fixed stellar mass, the gas ionization parameter is correlated with the galaxy specific star-formation rates (sSFRs), where $Δ$ log q = 0.4 $Δ$(log sSFR), derived from changes in the strength of galaxy H-$β$ equivalent width. We interpret this as a consequence of higher gas densities, lower gas covering fractions, combined with higher escape fraction of H-ionizing photons. We discuss both tests to confirm these assertions and implications this has for future observations of galaxies at higher redshifts.

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arXiv2022

CLEAR: The Evolution of Spatially Resolved Star Formation in Galaxies between $0.5\lesssim z \lesssim1.7$ using H$α$ Emission Line Maps

Using spatially resolved H-alpha emission line maps of star-forming galaxies, we study the evolution of gradients in galaxy assembly over a wide range in redshift ($0.5<z<1.7$). Our $z\sim0.5$ measurements come from deep Hubble Space Telescope WFC3 G102 grism spectroscopy obtained as part of the CANDELS Lyman-alpha Emission at Reionization (CLEAR) Experiment. For star-forming galaxies with Log$(M_{*}/\mathrm{M}_{\odot})\geqslant8.96$, the mean H-alpha effective radius is $1.2\pm0.1$ times larger than that of the stellar continuum, implying inside-out growth via star formation. This measurement agrees within $1σ$ with those measured at $z\sim1$ and $z\sim1.7$ from the 3D-HST and KMOS-3D surveys respectively, implying no redshift evolution. However, we observe redshift evolution in the stellar mass surface density within 1 kiloparsec ($Σ_\mathrm{1kpc}$). Star-forming galaxies at $z\sim0.5$ with a stellar mass of Log$(M_{*}/\mathrm{M}_{\odot})=9.5$ have a ratio of $Σ_\mathrm{1kpc}$ in H-alpha relative to their stellar continuum that is lower by $(19\pm2)\%$ compared to $z\sim1$ galaxies. $Σ_{1\mathrm{kpc, H}α}$/$Σ_{1\mathrm{kpc,Cont}}$ decreases towards higher stellar masses. The majority of the redshift evolution in $Σ_{1\mathrm{kpc,H}α}$/$Σ_{1\mathrm{kpc,Cont}}$ versus stellar mass stems from the fact that Log($Σ_{1\mathrm{kpc, H}α}$) declines twice as much as Log($Σ_{1\mathrm{kpc, Cont}}$) from $z\sim 1$ to 0.5 (at a fixed stellar mass of Log$(M_{*}/\mathrm{M}_{\odot})=9.5$). By comparing our results to the TNG50 cosmological magneto-hydrodynamical simulation, we rule out dust as the driver of this evolution. Our results are consistent with inside-out quenching following in the wake of inside-out growth, the former of which drives the significant drop in $Σ_{1\mathrm{kpc, H}α}$ from $z\sim1$ to $z\sim0.5$.

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arXiv2022

CLEAR: Paschen-$β$ Star Formation Rates and Dust Attenuation of Low Redshift Galaxies

We use \Pab\ (1282~nm) observations from the Hubble Space Telescope ($\HST$) G141 grism to study the star-formation and dust attenuation properties of a sample of 29 low-redshift ($z < 0.287$) galaxies in the CANDELS Ly$α$ Emission at Reionization (CLEAR) survey. We first compare the nebular attenuation from $\Pab/\Ha$ with the stellar attenuation inferred from the spectral energy distribution, finding that the galaxies in our sample are consistent with an average ratio of the continuum attenuation to the nebular gas of 0.44, but with a large amount of excess scatter beyond the observational uncertainties. Much of this scatter is linked to a large variation between the nebular dust attenuation as measured by (space-based) $\Pab$ to (ground-based) $\Ha$ to that from (ground-based) $\Ha/\Hb$. This implies there are important differences between attenuation measured from grism-based / wide-aperture $\Pab$ fluxes and the ground-based / slit-measured Balmer decrement. We next compare star-formation rates (SFRs) from $\Pab$ to those from dust-corrected UV. We perform a survival analysis to infer a census of \Pab\ emission implied by both detections and non-detections. We find evidence that galaxies with lower stellar mass have more scatter in their ratio of \Pab\ to attenuation-corrected UV SFRs. When considering our \Pab\ detection limits, this observation supports the idea that lower mass galaxies experience "burstier" star-formation histories. Together, these results show that \Pab\ is a valuable tracer of a galaxy's SFR, probing different timescales of star-formation and potentially revealing star-formation that is otherwise missed by UV and optical tracers.

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arXiv2021

Diagnosing DASH: A Catalog of Structural Properties for the COSMOS-DASH Survey

We present the $H_{160}$ morphological catalogs for the COSMOS-DASH survey, the largest area near-IR survey using HST-WFC3 to date. Utilizing the "Drift And SHift" observing technique for HST-WFC3 imaging, the COSMOS-DASH survey imaged approximately 0.5 deg$^2$ of the UltraVISTA deep stripes (0.7 deg$^2$ when combined with archival data). Global structural parameters are measured for 51,586 galaxies within COSMOS-DASH using GALFIT (excluding the CANDELS area) with detection using a deep multi-band HST image. We recover consistent results with those from the deeper 3D-HST morphological catalogs, finding that, in general, sizes and Sérsic indices of typical galaxies are accurate to limiting magnitudes of $H_{160}<23$ and $H_{160}<22$ ABmag, respectively. In size-mass parameter space, galaxies in COSMOS-DASH demonstrate robust morphological measurements out to $z\sim2$ and down to $\log(M_\star/M_\odot)\sim9$. With the advantage of the larger area of COSMOS-DASH, we measure a flattening of the quiescent size-mass relation below $\log(M_\star/M_\odot)\sim10.5$ that persists out to $z\sim2$. We show that environment is not the primary driver of this flattening, at least out to $z=1.2$, whereas internal physical processes may instead govern the structural evolution.

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arXiv2021

CLEAR: Boosted Ly$α$ Transmission of the Intergalactic Medium in UV bright Galaxies

Reionization is an inhomogeneous process, thought to begin in small ionized bubbles of the intergalactic medium (IGM) around overdense regions of galaxies. Recent Lyman-alpha (Ly$α$) studies during the epoch of reionization show growing evidence that ionized bubbles formed earlier around brighter galaxies, suggesting higher IGM transmission of Ly$α$ from these galaxies. We investigate this problem using IR slitless spectroscopy from the Hubble Space Telescope (HST) Wide-Field Camera 3 (WFC3) G102 grism observations of 148 galaxies selected via photometric redshifts at $6.0 6$. Furthermore, we compare the redshift evolution of $W_0$ between galaxies at different UV luminosities. The UV-bright ($M_{\text{UV}}<-21$, or $L_{\text{UV}}>L^{*}$) galaxies show weaker evolution with a decrease of 0.4 ($\pm$0.2) dex in $W_0$ at $z>6$ while UV-faint ($M_{\text{UV}}>-21$, or $L_{\text{UV}} 6$. Our results add to the accumulating evidence that UV-bright galaxies exhibit boosted Ly$α$ transmission in the IGM, suggesting that reionization completes sooner in regions proximate to galaxies of higher UV luminosity.

astro-ph.GA↗

arXiv2021

CLEAR: Emission Line Ratios at Cosmic High Noon

We use Hubble Space Telescope WFC3 G102 and G141 grism spectroscopy to measure rest-optical emission-line ratios of 533 galaxies at $z\sim1.5$ in the CANDELS Ly$α$ Emission at Reionization (CLEAR) survey. We compare $\frac{[OIII]}{Hβ}$ vs. $\frac{[SII]}{(Hα+[NII])}$ as an "unVO87" diagram for 461 galaxies and $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ as an "OHNO" diagram for 91 galaxies. The unVO87 diagram does not effectively separate active galactic nuclei (AGN) and $[NeV]$ sources from star-forming galaxies, indicating that the unVO87 properties of star-forming galaxies evolve with redshift and overlap with AGN emission-line signatures at $z>1$. The OHNO diagram effectively separates X-ray AGN and $[NeV]$-emitting galaxies from the rest of the population. We find that the $\frac{[OIII]}{Hβ}$ line ratios are significantly anti-correlated with stellar mass and significantly correlated with $\log(L_{Hβ})$, while $\frac{[SII]}{(Hα+[NII])}$ is significantly anti-correlated with $\log(L_{Hβ})$. Comparison with MAPPINGS~V photoionization models indicates that these trends are consistent with lower metallicity and higher ionization in low-mass and high-SFR galaxies. We do not find evidence for redshift evolution of the emission-line ratios outside of the correlations with mass and SFR.Our results suggest that the OHNO diagram of $\frac{[OIII]}{Hb}$ vs. $\frac{[NeIII]}{[OII]}$ will be a useful indicator of AGN content and gas conditions in very high-redshift galaxies to be observed by the James Webb Space Telescope.

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arXiv2020

Cost Management on Commercial Cloud Platforms

Commercial cloud platforms are a powerful technology for astronomical research. Despite the benefits of cloud computing -- such as on-demand scalability and reduction of systems management overhead -- confusion over how to manage costs remains, for many, one of the biggest barriers to entry. This confusion is exacerbated by the rapid growth in services offered by commercial providers, by the growth in the number of these providers, and by storage, compute, and I/O metered at separate rates -- all of which can change without notice. As a rule, processing is very cheap, storage is more expensive, and downloading is very expensive. Thus, an application that produces large image data sets for download will be far more expensive than an application that performs extensive processing on a small data set. This Birds of a Feather (BoF) session aimed to quantify the above statement by presenting case studies of the costing of astronomy applications on commercial clouds that covered a range of processing scenarios; these presentations were the basis for discussion by the attendees.

astro-ph.IM↗

arXiv2020

CLEAR: The Gas-Phase Metallicity Gradients of Star-Forming Galaxies at 0.6 < z < 2.6

We report on the gas-phase metallicity gradients of a sample of 264 star-forming galaxies at 0.6 < z < 2.6, measured through deep near-infrared Hubble Space Telescope slitless spectroscopy. The observations include 12-orbit depth Hubble/WFC3 G102 grism spectra taken as a part of the CANDELS Lya Emission at Reionization (CLEAR) survey, and archival WFC3 G102+G141 grism spectra overlapping the CLEAR footprint. The majority of galaxies (84%) in this sample are consistent with a zero or slightly positive metallicity gradient across the full mass range probed (8.5 < log M_*/M_sun < 10.5). We measure the intrinsic population scatter of the metallicity gradients, and show that it increases with decreasing stellar mass---consistent with previous reports in the literature, but confirmed here with a much larger sample. To understand the physical mechanisms governing this scatter, we search for correlations between the observed gradient and various stellar population properties at fixed mass. However, we find no evidence for a correlation with the galaxy properties we consider---including star-formation rates, sizes, star-formation rate surface densities, and star-formation rates per gravitational potential energy. We use the observed weakness of these correlations to provide material constraints for predicted intrinsic correlations from theoretical models.

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arXiv2020

A Comparison of Rest-frame Ultraviolet and Optical Emission-Line Diagnostics in the Lensed Galaxy SDSS J1723+3411 at Redshift z=1.3293

For the extremely bright lensed galaxy SDSS J1723+3411 at z=1.3293 , we analyze spatially integrated MMT, Keck, and Hubble Space Telescope spectra that fully cover the rest-frame wavelength range of 1400 to 7200 Angstroms. We also analyze near-IR spectra from Gemini that cover H alpha for a portion of the lensed arc. We report fluxes for 42 detected emission lines, and upper limits for an additional 22. This galaxy has extreme emission line ratios and high equivalent widths that are characteristic of extreme emission-line galaxies. We compute strong emission line diagnostics from both the rest-frame optical and rest-frame ultraviolet (UV), to constrain physical conditions and test the spectral diagnostics themselves. We tightly determine the nebular physical conditions using the most reliable diagnostics, and then compare to results from other diagnostics. We find disappointing performance from the UV--only diagnostics: they either are unable to measure the metallicity or dramatically under-estimate it; they over-estimate the pressure; and the UV diagnostic of ionization parameter has a strong metallicity dependence in this regime. Based on these results, we suggest that upcoming James Webb Space Telescope spectroscopic surveys of galaxies in the reionization epoch should invest the additional integration time to capture the optical [O II] and [O III] emission lines, and not rely solely on the rest-frame UV emission lines. We make available the spectra; they represent one of the highest-quality emission line spectral atlases of star-forming galaxy available beyond the local universe, and will aid planning observations with JWST.

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arXiv2020

Spatial Variation in Strong Line Ratios and Physical Conditions in Two Strongly-Lensed Galaxies at z~1.4

For studies of galaxy formation and evolution, one of the major benefits of the James Webb Space Telescope is that space-based IFUs like those on its NIRSpec and MIRI instruments will enable spatially resolved spectroscopy of distant galaxies, including spectroscopy at the scale of individual star-forming regions in galaxies that have been gravitationally lensed. In the meantime, there is only a very small subset of lensed sources where work like this is possible even with the Hubble Space Telescope's Wide Field Camera 3 infrared channel grisms. We examine two of these sources, SDSS J1723+3411 and SDSS J2340+2947, using HST WFC3/IR grism data and supporting spatially-unresolved spectroscopy from several ground-based instruments to explore the size of spatial variations in observed strong emission line ratios like O32, R23, which are sensitive to ionization parameter and metallicity, and the Balmer decrement as an indicator of reddening. We find significant spatial variation in the reddening and the reddening-corrected O32 and R23 values which correspond to spreads of a few tenths of a dex in ionization parameter and metallicity. We also find clear evidence of a negative radial gradient in star formation in SDSS J2340+2947 and tentative evidence of one in SDSS J1723+3411, though its star formation is quite asymmetric. Finally, we find that reddening can vary enough spatially to make spatially-resolved reddening corrections necessary in order to characterize gradients in line ratios and the physical conditions inferred from them, necessitating the use of space-based IFUs for future work on larger, more statistically robust samples.

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arXiv2020

CLEAR II: Evidence for Early Formation of the Most Compact Quiescent Galaxies at High Redshift

The origin of the correlations between mass, morphology, quenched fraction, and formation history in galaxies is difficult to define, primarily due to the uncertainties in galaxy star-formation histories. Star-formation histories are better constrained for higher redshift galaxies, observed closer to their formation and quenching epochs. Here we use "non-parametric" star-formation histories and a nested sampling method to derive constraints on the formation and quenching timescales of quiescent galaxies at $0.7 $10.25, where $Σ_1$ is the stellar mass within 1~pkpc (proper kpc). Quiescent galaxies with the highest stellar-mass surface density, $\logΣ_1 / (M_\odot\ \mathrm{kpc}^{-2}) > 10.25$, } show a \textit{minimum} formation redshift: all such objects in our sample have $z_{50} > 2.9$. Quiescent galaxies with lower surface density, $\log Σ_1 / (M_\odot\ \mathrm{kpc}^{-2}) = 9.5 - 10.25$, show a range of formation epochs ($z_{50} \simeq 1.5 - 8$), implying these galaxies experienced a range of formation and assembly histories. We argue that the surface density threshold $\logΣ_1/(M_\odot\ \mathrm{kpc}^{-2})>10.25$ uniquely identifies galaxies that formed in the first few Gyr after the Big Bang, and we discuss the implications this has for galaxy formation models.

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