arXiv subjects

Varun Bajaj

Publications and source records attributed to Varun Bajaj.

At least 19 recordsLinked to original sources

arXiv2026

Updates to WFC3/UVIS Encircled Energy Values in Select Filters

We present updated encircled energy (EE) curves for a subset of UVIS filters. These are derived from a reanalysis of the drizzled Point Spread Function (PSF) data underlying the current EE calibration together with new measurements from deep observations of the PSF wings. Improved centroiding and analysis techniques applied to the drizzled PSFs produce more accurate EE values at small radii ($r \le 10$ pixels), bringing the results into closer agreement with a large archival PSF study (Huynh et al. 2025). At large radii, we leverage deep observations of the PSF wings in six filters and compare the fraction of light between 2" and 6" with predictions from an optical model of the PSF (Hartig 2009). For filters with pivot wavelengths $ \gtrsim{4000} \mathrm{\mathring{A}}$, the model agrees with the empirical data, but over-estimates the EE for UV filters by $\sim0.5$% at 2". The revised EE solutions affect the UVIS zeropoints, which are derived from aperture photometry in a 0.4" (10 pixel) radius and corrected to 6" using EE tables (Calamida et al. 2022). Overall, the impact is small; the EE at 0.4" is larger by $ \gtrsim{0.5}$ % (0.005 mag) for several filters (F218W, F225W, F275W, F775W, F814W, F845M), especially for UVIS2. In contrast, the EE value is generally smaller at 0.4" for long-pass filters (F200LP, F350LP, F850LP), with F850LP differing by $\sim2 $ % (0.02 mag). Updated EE tables will be delivered together with a revised set of UVIS inverse sensitivity tables and zeropoints later in 2026. In the interim, we provide EE tables for commonly used filters in Appendix A.

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arXiv2026

The James Webb Space Telescope Absolute Flux Calibration. VI. Near-Infrared Camera Imaging and Coronagraphy

We present an updated flux calibration for all imaging modes of the Near-Infrared Camera on JWST that converts instrumental units to physical surface brightness units of MJy sr^-1. This calibration includes observations of 19 flux standard stars spanning 3.5 years, with a mix of A dwarfs, solar analogs, and hot stars. All 5 coronagraphic setups, all 29 filters, and both weak lenses are calibrated. This is the first on-sky calibration for the weak lenses used in Time Series observations and for secondary coronagraphic configurations (long wavelength masks paired with short wavelength filters, and vice versa). We also assess count-rate differences between subarrays and the full frame, finding differences of up to ~1%. These differences are incorporated into the calibration factors. We find that the scatter in the calibration factor is typically <2%, with about half of the filter+detector[+mask] combinations reaching <1% scatter. There are no trends with detector effects such as the count rate and well depth. Images in a handful of filters of the Large Magellanic Cloud and globular cluster 47 Tuc show that residual detector-to-detector offsets are typically small (<1%), but can be as high as 4-5%. The NIRCam detectors are found to be quite stable, with possible count-rate decreases of <0.4% per year, which is within the calibration uncertainties. These new calibration factors were incorporated into the JWST pipeline in 2026 March.

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arXiv2026

The HII Regions' Molecular Law of Star Formation

We combine imaging data from the HST, JWST, and ground-based millimeter facilities to investigate the correlation between star formation rate (SFR) and molecular gas at the ~100 pc scale of HII regions in three nearby galaxies: NGC628, NGC5194 and NGC5236. The JWST 21 micron maps of the three galaxies offer a unique insight into the dust-absorbed SFR at high resolution. We find that the relation between the surface densities of SFR and molecular gas has a slope of ~1.85, in log-log scale, significantly steeper than previous results for nearby galaxies but closer to the trends found for molecular clouds in the Milky Way. The steep relation also holds on larger, ~500 pc, scales, and results from the high-resolution imaging that cleanly isolates the star-forming region emission from the underlying galaxy's diffuse contribution. The diffuse emission at 21 micron is, in fact, found to correlate with the galaxy's stellar mass. Comparisons with physical models of star formation are inconclusive; they overlap with the locus of the 100 pc data, but have difficulties in reproducing the data scatter. Possible exceptions are models that add a power law tail to the gas density probability distribution, due to the large range of free parameters allowed. We find that local HII regions, high redshift star-forming clumps, and low and high redshift starburst galaxies form a single sequence of star formation over three orders of magnitude in gas surface density.

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arXiv2026

Updates to the WFC3/IR Photometric Stability Stellar Cluster Study

The WFC3/IR channel has been shown to lose sensitivity over time. The sensitivity loss rate has been quantified in past works and we extend those measurements into 2026 using photometry of uncrowded regions of star clusters 47 Tucanae and Messier 4 in the F110W and F160W filters. This study adds 4-7 additional visits since Bajaj et al. (2022) for each target/filter combination. Using F160W data of 47Tuc, we find that the IR channel continues to lose sensitivity at a rate of -0.07% $\pm$ 0.01% per year using the full dataset, and -0.06% $\pm$ 0.02% per year using only the first exposure of every visit. Using M4 data observed in F110W, we find a sensitivity loss rate of -0.13% $\pm$ 0.02% per year using the full dataset, and -0.07% $\pm$ 0.03% per year using just the first exposure of every visit. We find that the rate of sensitivity loss has decreased since the previous study. This likely stems from the inclusion of more recent visits that were designed to mitigate persistence effects. These results suggest that earlier observations, which were not intended for sensitivity measurements, may have overestimated the IR channel's sensitivity due to the effects of persistence from bright sources. The IMPHTTAB reference file that contains the correction to the time-dependent sensitivity remains unchanged from the sensitivity measurements of this report.

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arXiv2026

The emerging timescale of young star clusters regulated by cluster stellar mass

Quantifying the timescales of star cluster emergence from their natal clouds remains one of the main challenges in understanding the star formation process. These timescales are fundamental measurements of the star formation cycle within galaxies, yet are difficult to constrain due to the complex interplay between stellar feedback and star formation across multiple physical scales. Here we present Hubble Space Telescope and James Webb Space Telescope observations of thousands of young star clusters in four nearby galaxies (M51, M83, NGC 628 and NGC 4449). A substantial fraction of these clusters are still embedded within their natal gas and remain invisible at optical wavelengths. We constrain their emergence process by measuring the timescales required to disperse the surrounding material. We find a strong correlation between dispersal timescale and cluster stellar mass, with massive clusters emerging faster than their lower-mass counterparts. This is a critical constraint on star formation and stellar feedback simulations, which struggle to fully reproduce star clusters formation and emergence. Our results emphasize the central role of massive clusters in driving the escape of ionizing radiation into the galactic medium. Finally, they impose time limitations for planet formation in massive cluster environments where disks get exposed to ultraviolet irradiation and further gas infall is halted.

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arXiv2026

New Time-Dependent WFC3/IR Inverse Sensitivities

We present new time-dependent inverse sensitivities for the WFC3/IR channel. These were calculated using the sensitivity change slopes measured by \citet{2024wfc..rept....6M} and photometry of five CALSPEC standards (the white dwarfs GRW+70~5824, GD~153, GD~71, G191B2B, and the G-type star P330E) collected from 2009 to 2023. The new inverse sensitivities account for losses of 1-2\% over 15 years, depending on wavelength, and provide an internal photometric precision better than 0.5\% for all wide--, medium--, and narrow-band filters. An updated version of \texttt{calwf3} (v3.7.2) has been developed for use with a new time-dependent image photometry table (IMPHTTAB) and will be used to update the image header photometric keywords following MAST reprocessing, expected in late-2024. Alternatively, the new inverse sensitivities may be computed by the user for a specific observation date by running \texttt{stsynphot}.

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arXiv2026

WFC3/UVIS Geometric Distortion -- Time Evolution of Linear Terms w.r.t Gaia

We align more than 7,400 WFC3/UVIS exposures to the Gaia DR3 catalog to examine the time evolution of the linear terms (shift, rotation, scale and skew) of the geometric distortion solution between 2009 and 2022. We find small linear temporal changes in the scale and skew terms (less than 0.2 pixels in 13 years) which are generally dominated by intrinsic scatter (up to $\pm$ 0.3 pixels). Concurrently, a larger filter-dependent offset in the scale term is observed, with a maximum difference of 0.3 pixels between F275W and F814W images at all epochs. A small rotation offset to Gaia of 0.003 $\pm$ 0.004 degrees is measured from 2009 to mid-2017, after which the offsets are as large as 0.01 degrees, with a large scatter. MAST pipeline processing includes an additional alignment step which corrects UVIS images for any residual linear terms with respect to Gaia DR3 when there are at least 10 matched sources. In addition to any pointing offsets, this step accounts for any evolution in the distortion linear terms described here. For observers requiring high-precision astrometry, we recommend using the tweakreg routine to realign images using a 4-parameter fit (x-shift, y-shift, rotation, and scale) or a 6-parameter fit (x-shift, y-shift, x-rotation, y-rotation, x-scale, and y-scale) depending on the number of matched sources. We provide links to DrizzlePac tutorials for improving both absolute and relative astrometry in WFC3 images.

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arXiv2025

Pixel-Based Non-Linearity Correction for the WFC3 IR Detector

The current non-linearity correction for the Wide Field Camera 3 Infrared (WFC3/IR) channel is based on ground-based data acquired during WFC3's Thermal Vacuum 3 (TV3) testing campaign. In the current reference file, the correction coefficients derived for each pixel are averaged over each of the four detector quadrants. In this work, we compute a new pixel-based non-linearity correction using in-flight calibration observations with the internal tungsten lamp flats acquired between 2011 and 2013. We derive the new correction coefficients by fitting a third-order polynomial to the accumulated signal ``up-the-ramp" for each pixel. Approximately 2\% of IR detector pixels are flagged as bad, and a solution cannot be computed. For these, we use the quadrant averages of the new correction coefficients. An accompanying report (\cite{huynh2025}) provides detailed testing results using both internal flats and external science targets acquired in a variety of observing modes. The report highlights improvements in photometry derived from ``\_flt.fits" data products calibrated using the new reference file, with the largest improvement for pixels with fluence levels approaching the full well limit of $\sim$80,000 $e^- $. A new NLINFILE reference file was delivered in October 2025 and will be used to reprocess all WFC3/IR imaging and grism observations in the MAST archive.

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arXiv2025

Ashes of Creation: JWST Uncovers Silicate Dust in Massive Star Clusters

Dust production is a fundamental aspect of the baryonic cycle of star formation. It is known that dust is injected into the interstellar medium during early star formation by supernovae and later on by evolved stars. From individual objects, these mechanisms are well understood, but the overall dust production in star clusters at different evolutionary stages is still challenging to quantify. We present 22 massive (> 105M$_{\odot}$) extra galactic star clusters with ages between 3 and 100 Myr exhibiting a compact dust morphology seen with JWST-MIRI. We only find PAH features associated with one star cluster and nineteen have already cleared themselves from their natal dust. Their main characteristic is a significant enhancement at 10${\mu}$m, which is likely due to silicate emission and cannot be explained by ionized gas. We discuss several possible explanations including dust production from evolved stars such as red super giants, more exotic star types like yellow hypergiants and luminous blue variable stars. Stochastic dust injection from supernovae or a single supernova in dense gas can also create significant silicate emission. However, for this scenario secondary tracers such as a X-ray signal are expected which we only observe in three star clusters. We find the most luminous 10${\mu}$m emitter to be the three most massive star clusters (> 106M$_{\odot}$) which is at least a magnitude stronger than any known stellar sources indicating a rare mechanism that only appears at extreme masses and a short lifetime.

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arXiv2025

WFC3/IR Geometric Distortion - Time Evolution of Linear Terms w.r.t. Gaia DR3

We examine the relative offsets of the linear terms in the geometric distortion between WFC3/IR and the Gaia DR3 catalog using the Mikulski Archive for Space Telescopes (MAST) pipeline WFC3/IR to Gaia DR3 alignment solutions to assess temporal stability over the lifetime of the WFC3 instrument (2009-2024). We find a period of increased uncertainty and offsets in the rotation term between 2018 and 2021, as seen in a previous analysis of WFC3/UVIS linear geometric distortion (O'Connor et al., 2024), corresponding with a period of increased jitter. We find a similar pattern of increased uncertainty between 2018 and 2021 in the skew offsets to Gaia DR3, as well. We find no significant linear temporal evolution in the rotation, skew, or scale offsets between the WFC3/IR IDCTAB distortion solution and Gaia DR3 over the 16-year lifetime of the WFC3 instrument; however, we do see temporal evolution in the shift offsets (the difference -in pixels- between the IDCTAB and Gaia WCS positions), which are dominated by telescope pointing inaccuracy external to the WFC3/IR geometric distortion solution. For observers requiring high-precision astrometry, we continue to recommend that observers verify or improve image alignment using the tweakreg routine.

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arXiv2025

The Spatial Evolution of Star Clusters in NGC 628 with JWST

We examine the spatial distribution of star clusters in NGC 628 using the statistical tool INDICATE to quantify clustering tendencies. Our sample, based on HST and JWST observations, is the most complete to date, spanning ages from 1 Myr to >100 Myr. We find cluster spatial behaviour varies with galactic position, age, and mass. Most emerging young clusters are tightly spatially associated with each other, while fully emerged clusters are in \sim1.5 times looser spatial associations, irrespective of age. Young Massive Clusters (YMCs \ge 10^4 M_{\odot}) tend to associate with lower-mass clusters but not strongly with other YMCs, implying that intense star formation regions produce a few YMCs alongside many lower-mass clusters rather than multiple YMCs together. Young concentrated clusters show a wide radial distribution in the galactic disk, which narrows with age; with concentrated clusters >100 Myr mostly residing between 2-6 kpc. This pattern may reflect either faster dispersal of isolated tight cluster spatial "structure" in a lower gas density outer disk or gradual inside-out growth, with the formation of this structure shifting outward over time. We also detect distinct spatial behaviours for clusters within 2 kpc, linked to the inner Lindblad resonance (\le1 kpc), nuclear ring (\sim0.5-1 kpc), and the start of spiral arms (\sim1.25-2 kpc), suggesting these regions exhibit strong radial motions that could hinder clusters from forming and remaining in tight concentrations. Our results highlight how spatially-resolved studies of clusters can reveal the influence of galactic dynamics on star formation and cluster evolution.

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arXiv2025

Isolated massive star candidates in NGC 4242 with GULP

$\textit{Context.}$ There is considerable debate on how massive stars form, including whether a high-mass star must always form with a population of low-mass stars or whether it can also form in isolation. Massive stars found in the field are often considered to be runaways from star clusters or OB associations. However, there is evidence in the Milky Way and the Small Magellanic Cloud of high-mass stars that appear isolated in the field and cannot be related to any known star cluster or OB association. Studies of more distant galaxies have been lacking so far. $\textit{Aims.}$ In this work, we identified massive star candidates that appear isolated in the field of the nearby spiral galaxy NGC 4242 (distance: 5.3 Mpc), to explore how many candidates for isolated star formation we find in a galaxy outside the Local Group. $\textit{Methods.}$ We identified 234 massive ($M_{ini}\geq15M_{\odot}$) and young ($\leq 10$ Myr) field stars in NGC 4242 using the Hubble Space Telescope's Solar Blind Channel of the Advanced Camera for Surveys, the UVIS channel of the Wide Field Camera 3 from the Galaxy UV Legacy Project (GULP) and optical data from the Legacy ExtraGalactic UV Survey (LEGUS). We investigated the surroundings of our targets within the range of projected distances expected for runaway stars, $74$ pc and $204$ pc. $\textit{Results.}$ We find that between $9.8\%$ and $34.6\%$ of our targets have no young stellar groups or massive stars within the threshold radii, making them appear isolated. This fraction reduces to $3.2\%-11.5\%$ when we consider the total number of massive stars expected from the observed UV star formation rate. $\textit{Conclusions.}$ Our results show that there is a small population of young and massive, potentially isolated field stars in NGC 4242.

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arXiv2025

The Calibration of Short Wavelength Polycyclic Aromatic Hydrocarbon Emission as Star Formation Rate Indicators with JWST

We use JWST/NIRCam and MIRI imaging acquired by the Feedback in Emerging extrAgalactic Star clusTers (FEAST) program along with archival HST imaging to map ionized gas (Pa$\alpha$, Br$\alpha$, and H$\alpha$) and Polycyclic Aromatic Hydrocarbon (PAH) emission (3.3 and 7.7 $\mu$m) across a sample of four nearby galaxies (NGC 5194, 5236, 628, and 4449). These maps are utilized to calibrate the PAH features as star formation rate (SFR) indicators in 40 pc size regions around massive emerging young star clusters (eYSCs). We find a tight, sub-linear (power-law exponent, $\alpha{\,}{\sim}{\,}0.8$) relation between the PAH luminosities (3.3 and 7.7 $\mu$m) and SFR (extinction corrected Pa$\alpha$) in near solar metallicity environments. PAH destruction in more intense ionizing environments and/or variations in the age of our sources may drive the deviation from a linear relation. In the metal-poor environment of NGC 4449 (${\sim}$1/3 Z$_{\odot}$), we see substantial deficits in the PAH feature strengths at fixed SFR and significantly higher scatter in the PAH-SFR relations. We determine that the 3.3/7.7 $\mu$m PAH luminosity ratio increases towards lower metallicity environments. This is interpreted as a result of a shift in the size distribution towards smaller PAHs at lower metallicities, possibly due to inhibited grain growth. Focusing on the regions in NGC 4449, we observe a decreasing 3.3/7.7 $\mu$m ratio towards higher SFR, which could indicate that small PAHs are preferentially destroyed relative to larger PAHs in significantly sub-solar metallicity conditions. We estimate that ${\sim}$2/3 of the PAH emission in typical local star-forming galaxies is excited by older stars and unrelated to recent ($<$10 Myr) star formation.

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arXiv2025

Updates to the WFC3/UVIS Saturation Map

The calwf3 software for WFC3/UVIS utilizes a reference file to flag pixels that are saturated beyond their full-well depth. Previously, this was accomplished using a constant threshold of 65,500 e$^{-}$ across the entire detector. In this study, we retrieved $\sim$1 million stars from the Mikulski Archive for Space Telescopes (MAST) to determine the flux level at which the Point Spread Function begins to flatten, which occurs as the central pixel saturates. We quantified the saturation limit as a function of position on the detector in 1,024 discrete regions, and interpolated to a pixel-by-pixel saturation map to construct a spatially-variable saturation map reference file that is now implemented in the calwf3 calibration pipeline. We find the saturation varies by 13% across the UVIS detectors, from 63,465 e$^{-}$ to 72,356 e$^{-}$. These values agree well with earlier studies using sparser datasets, with the current analysis leading to improved characterization on small scales. Critically, the revised saturation values are larger than the previous constant threshold over 87% of the UVIS detector, leading to the recovery of usable science pixels near bright sources. This update greatly improves the robustness of saturation flags in the Data Quality arrays of observations obtained with WFC3/UVIS, and users are encouraged to redownload their data from MAST to benefit from the improved flags.

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arXiv2025

The near infrared SED of young star clusters in the FEAST galaxies: Missing ingredients at 1-5 $\mu$m

We present a combined HST and JWST 0.2 - to - 5 $\mu$m analysis of the spectral energy distributions (SEDs) of emerging young star clusters (eYSCs) in four nearby galaxies from the Feedback in Emerging extrAgalactic Star clusTers (FEAST) survey: M51, M83, NGC 628, and NGC 4449. These clusters, selected for their bright Pa$\alpha$ and 3.3 $\mu$m polycyclic aromatic hydrocarbon (PAH) emission, are still associated to their natal gas cloud and have been largely missed in previous HST optical campaigns. We modeled their SEDs using the CIGALE fitting code and identified: i) a systematic flux excess at 1.5 - 2.5 $\mu$m that is not accounted for by current stellar population models; ii) the preference for a set of dust model parameters that is not aligned with expectations from self-consistent analyses of star-forming regions, suggesting model shortcomings also in the 3 - 5 $\mu$m. The near-infrared (NIR) excess is most prominent in low-mass ($\leq 3000$ M$_\odot$) and young ($\leq 6$ Myr) clusters. Additionally, we see that the SED fitting analysis wrongly assigns ages $\geq 6$ Myr to a fraction of strong Pa$\alpha$ emitters with equivalent widths suggestive of significantly younger ages. A parallel analysis with the slug code suggests that stochastic initial mass function (IMF) sampling of pre-main-sequence stars combined with extinction might partially reduce the gap. We conclude that the inclusion of young stellar object SEDs, along with more realistic sampling of the cluster IMF, might be needed to fully account for the stellar population and dust properties of eYSCs.

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arXiv2025

Quantification of The Age Dependence of Mid-Infrared Star Formation Rate Indicators

We combine James Webb Space Telescope images of the nearby galaxy NGC 5194 in the hydrogen recombination line Pa-alpha (lambda=1.8756 micron) from the Cycle 1 program JWST-FEAST with 21 micron dust continuum images from the Cycle 2 Treasury program JWGT to quantify the difference in the calibration of mid-infrared star formation rates (SFR) between HII regions and galaxies. We use the archival HST H-alpha image to correct the Pa-alpha emission for the effects of dust attenuation. Our data confirm previous results that the dust-corrected Pa-alpha flux is tightly correlated with the 21 micron emission at the scales of HII regions. When combined with published JWST data for the HII regions of the galaxy NGC 628 and Spitzer 24 micron data for whole galaxies and for kpc-size galaxy regions, we show that the L(24)-L(Pa-alpha) correlation has exponent >1 across six decades in luminosity. In addition, the hybrid 24 micron+H-alpha SFR indicator has a scaling constant about 4.4 times higher for HII regions than for whole galaxies, also in agreement with previous results. Models of stellar populations with a range of star formation histories reveal that the observed trends can be entirely ascribed to and quantified with the contribution to the IR emission by stellar populations older than ~5-6 Myr. Based on the models' results, we provide: (1) a calibration for the infrared SFR across six orders of magnitude in L(24), from HII regions to luminous galaxies, and (2) a prescription for the scaling constant of the hybrid infrared SFR indicators as a function of the star formation timescale.

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arXiv2025

FEAST: JWST uncovers the emerging timescales of young star clusters in M83

We present JWST NIRCam observations of the emerging young star clusters (eYSCs) detected in the nearby spiral galaxy M83. The NIRcam mosaic encompasses the nuclear starburst, the bar, and the inner spiral arms. The eYSCs, detected in Pa$\alpha$ and Br$\alpha$ maps, have been largely missed in previous optical campaigns of young star clusters (YSCs). We distinguish between eYSCI, if they also have compact 3.3~$\mu$m PAH emission associated to them, and eYSCII, if they only appear as compact Pa$\alpha$ emitters. We find that the variations in the 3.3~$\mu$m PAH feature are consistent with an evolutionary sequence where eYSCI evolve into eYSCII and then optical YSCs. This sequence is clear in the F300M-F335M (tracing the excess in the \PAHlambda\ feature) and the F115W-F187N (tracing the excess in Pa$\alpha$) colors which become increasingly bluer as clusters emerge. The central starburst stands out as the region where the most massive eYSCs are currently forming in the galaxy. We estimate that only about 20~\% of the eYSCs will remain detectable as compact YSCs. Combining eYSCs and YSCs ($\leq$10 Myr) we recover an average clearing timescale of 6~Myr in which clusters transition from embedded to fully exposed. We see evidence of shorter emergence timescales ($\sim$5~Myr) for more massive ($>5\times10^3$ \msun) clusters, while star clusters of $\sim 10^3$ \msun\ about 7~Myr. We estimate that eYSCs remain associated to the \PAHlambda\ emission 3--4~Myr. Larger samples of eYSC and YSC populations will provide stronger statistics to further test environmental and cluster mass dependencies on the emergence timescale.

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arXiv2025

GULP II: Hierarchical Distribution and Evolution of Young Stellar Structures in NGC 4449

We investigate the hierarchical distribution and evolution of young stellar structures in the dwarf starburst galaxy NGC 4449 using data from the GULP survey. By analyzing the spatial distribution of field stars younger than 100 Myr, we identify large-scale stellar complexes and substructures using HDBSCAN -- a density-based clustering algorithm -- and trace their evolution over time. While comparing these stellar structures in different regions of the galaxy, we find that the central bar-like region shows a clear expansion of the structures within the first $\sim$ 60 Myrs, while the arm-like structure in the NE shows no discernible trend, possibly due to external perturbations from tidal interactions with a neighboring galaxy. An age-dependent two point correlation function (TPCF) analysis shows that young stars exhibit a strong hierarchical distribution, with clustering strength decreasing over time. The power-law slope of the TPCF, which starts at $\alpha \sim 0.65$ for stars younger than 5 Myr, shows a slight decline to $\alpha \sim 0.4$ for stars older than 50 Myr, though it does not reach a completely flat (random) distribution. This trend indicates a subtle weakening of structural hierarchy among young ($<$100 Myr) stars, which is primarily driven by internal stellar motions. Future work will extend this analysis to the remaining 26 galaxies in the GULP survey to better constrain the role of the galactic environment in shaping the hierarchical evolution of young stellar populations.

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