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Alex Ross

Publications and source records attributed to Alex Ross.

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LEGGOS III: Mapping Star Formation and Dust in Gravitationally Lensed Galaxies with $\textit{SUMAC}$, a UMAP and Clustering Framework

Strong gravitational lensing combined with JWST's spatio-spectral resolution enables resolved studies of star-forming regions in $z\sim$ 2-4 galaxies, but identifying and characterizing such regions in lensed integral-field and multi-band data remains a manual, observer-dependent process. We present $\texttt{SUMAC}$ (Software for the Uniform Manifold Approximation of Clumps), an unsupervised learning pipeline that segments JWST imaging and spectroscopy at the "spaxel" level by combining $\texttt{UMAP}$-based manifold embedding with $\texttt{HDBSCAN}$ density clustering applied to spectral energy distributions/spectra. We demonstrate the pipeline on JWST/NIRSpec PRISM IFS observations of the lensed galaxy SGAS111020.0+645950.8 at $z = 2.481$, recovering six physically distinct stellar/nebular populations. The cluster median SEDs separate cleanly on the presence and strength of H$\beta$+[OIII], H$\alpha$+[NII], $\beta_{NUV}$ slope, Balmer break strength, and the Balmer decrement, with bluer clusters tracing unobscured star-forming regions and progressively redder clusters tracing dusty star-forming regions.

astro-ph.GA

LEGGOS II: A Strong Lens Model and Source-Plane Projection of the Clumpy Star-Forming Galaxy SGASJ111020.0+645950.8 at z=2.48

Strong gravitational lensing by galaxy clusters combined with the resolution of JWST enables studies of star formation on ~10-100 pc scales in galaxies at z~2-4. As part of the LEnsing and Galaxy Growth: Observing Substructures survey (LEGGOS), we present an updated strong lensing model of the galaxy cluster SDSSJ1110+6459 (z=0.659), which lenses the clumpy star-forming galaxy SGASJ111020.0+645950.8 at z=2.481 into a highly magnified giant arc. Using JWST NIRCam imaging, NIRSpec spectroscopy, and archival HST data, we confirm and refine the identification of four multiply imaged background sources, including one newly identified system, and map over 20 luminous regions between each image of the primary arc. Spectroscopy confirms that several previously ambiguous edge "clumps" belong to the main arc at z=2.481. Despite the limited number of strongly lensed sources in the field, the resulting lens model has high precision, owing to the high density of JWST-resolved clump constraints that tightly probe the lensing potential near the giant arc. The model yields a projected lens mass of $M(<250~\mathrm{kpc}) = 1.21^{+0.09}_{-0.04} \times 10^{14}~M_\odot$, an Einstein radius of $\theta_\mathrm{E} = 10.8^{+0.3}_{-0.4}~\mathrm{arcsec}$, and a total effective magnification of $\mu_\mathrm{tot}=24.2^{+3.4}_{-1.2}$ for the giant arc. Across the arc, individual clump magnifications span $\mu_\mathrm{clump}\sim4-19$, with fractional magnification uncertainties of $\sigma_\mu/|\mu_{\rm best}|\sim0.03-0.09$. We report a $\sim2-8\times$ improvement in magnification precision over previous models. Ongoing and future analyses of this arc will enable robust measurements of star-forming structure, building on the lensing foundation established here for LEGGOS studies of galaxy growth and feedback during cosmic noon.

astro-ph.GA

LEGGOS I: The JWST LEGGOS Survey -- LEnsing and Galaxy Growth: Observing Substructures -- Unpacks the Nature of Clumpy Star Formation and Quenching in Gravitationally Lensed Galaxies beyond Cosmic Noon

We present first results from the JWST LEGGOS Survey (LEnsing and Galaxy Growth: Observing Substructures), aimed at studying the physics of clumpy star formation and quenching in eight lensed galaxies at $z\sim2$--4. LEGGOS combines multiple Cycle 2 JWST GO programs (GO 4125, GO 3843) and Cycle 1 archival data, and utilizes strong gravitational lensing with NIRCam imaging and NIRSpec integral-field spectroscopy. LEGGOS targets UV-bright, highly magnified systems to resolve $\sim$10--200 pc regions in both rest-frame optical continuum and nebular emission. This overview paper describes the survey design, data reduction and calibration strategy, and science-quality data products, and highlights early examples demonstrating how spectroscopy breaks key degeneracies inherent to photometry-only clump studies, including identifying recent quenching in previously-thought UV star forming galaxies. We introduce a uniform analysis framework that jointly models lensing reconstruction, multi-band photometry, and integral field spectroscopy to disentangle multiple stellar populations within individual clumps and their surrounding diffuse regions. Using maps of Balmer recombination lines and key emission line diagnostic ratios, we connect star formation histories, dust attenuation, and nebular conditions on sub-kpc scales -- LEGGOS galaxies range from uniform metallicities across the whole galaxy, to having higher clump metallicities and harder ionization conditions relative to diffuse regions. The full survey dataset, with simultaneous flux and morphology constraints on clumpy source-plane regions, and a flexible spectrophotometric SPS modeling approach, provides a direct bridge between parsec-scale star formation physics and galaxy assembly at and beyond cosmic noon, offering a robust and efficient means of resolving star formation in the first galaxies.

astro-ph.GA

A Semi-Supervised Variational Autoencoder for Generating Neutron Star Equations of State

We develop a semi-supervised variational autoencoder (SSVAE) framework to reconstruct and generate neutron star (NS) equations of state (EOS). The SSVAE consists of an encoder network that maps high-dimensional EOS data into a lower-dimensional latent space and a decoder network that reconstructs the full EOS from the latent representation. The latent space includes supervised NS observables derived from the training EOS data, as well as variational latent variables that capture additional EOS features learned automatically. Using a SSVAE trained on a Skyrme EOS dataset, we find that a latent space consisting of two supervised observables, the maximum mass $M_{\max}$ and the canonical radius $R_{1.4}$, together with a single variational latent variable associated mainly with the EOS near the crust-core transition, is sufficient to reconstruct Skyrme EOSs with high fidelity. The decoder reconstructed EOSs reproduce $M_{\max}$ and $R_{1.4}$ with mean absolute percentage errors within $0.14\%$. Sampling the latent space generates new EOSs that are causal, thermodynamically stable, and consistent with imposed constraints on the supervised observables. The framework therefore provides a compact and physically interpretable parameterization of the NS EOS that is well suited for Bayesian inference with multimessenger observations, including pulsar mass-radius measurements and gravitational wave data.

astro-ph.IM

Data-Driven Generation of Neutron Star Equations of State Using Variational Autoencoders

We develop a machine learning model based on a structured variational autoencoder (VAE) framework to reconstruct and generate neutron star (NS) equations of state (EOS). The VAE consists of an encoder network that maps high-dimensional EOS data into a lower-dimensional latent space and a decoder network that reconstructs the full EOS from the latent representation. The latent space includes supervised NS observables derived from the training EOS data, as well as latent random variables corresponding to additional unspecified EOS features learned automatically. Sampling the latent space enables the generation of new, causal, and stable EOS models that satisfy astronomical constraints on the supervised NS observables, while allowing Bayesian inference of the EOS incorporating additional multimessenger data, including gravitational waves from LIGO/Virgo and mass and radius measurements of pulsars. Based on a VAE trained on a Skyrme EOS dataset, we find that a latent space with two supervised NS observables, the maximum mass $(M_{\max})$ and the canonical radius $(R_{1.4})$, together with one latent random variable controlling the EOS near the crust--core transition, can already reconstruct Skyrme EOSs with high fidelity, achieving mean absolute percentage errors of approximately $(0.15\%)$ for $(M_{\max})$ and $(R_{1.4})$ derived from the decoder-reconstructed EOS.

astro-ph.HE

JWST & the Waz Arc I: Spatially Resolving the Physical Conditions within a Post-Starburst Galaxy at Redshift 5 with NIRSpec IFS

We present NIRSpec/IFS observations of a rest-frame UV-bright, massive ($M_* \sim 10^{10}$ M$_\odot$, $z_{AB}=20.5$) galaxy highly magnified by gravitational-lensing observed just after the end of the epoch of reionization ($z=5.04$, $\bar{\mu}\sim90$). With JWST accessing the restframe UV and optical spectrum of this galaxy with high fidelity, we classify this UV-bright galaxy as post-starburst in nature -- due to weak/absent emission lines and strong absorption features -- making this an example of a new class of UV-bright but significantly quenched galaxies being discovered in this epoch. With a median $E(B-V)=0.44\pm0.14$, we identify the presence of stellar absorption across the arc both in Balmer lines and the MgII doublet, indicative of older stellar populations dominated by A stars (and potentially B stars). Using spatially-resolved maps of rest-optical strong emission lines, we find a heterogeneous distribution of nebular metallicities across the arc, potentially hinting at different enrichment processes. With a low median lensing-corrected H$\alpha$ star formation rate of SFR$_{H\alpha} = 0.024 \pm 0.001$ M$_\odot$ yr$^{-1}$, we find in the most "star-forming" clumps indications of lower ionization (log$_{10}$U $\sim -3.2$), lower nebular metallicities (12+log$_{10}$O/H $\lesssim$ 8.3), and hints of higher densities that suggest a possible recent infall of more pristine (low metallicity) gas onto the galaxy. Investigating the regions with no detectable H$\beta$ emission, we find (for the first time at $z>5$) signatures of diffuse ionized gas (DIG). Separating DIG from HII regions within a galaxy has predominantly been demonstrated at lower redshifts, where such spatial resolution allows clear separation of such regions -- highlighting the immense power of gravitational lensing to enable studies at the smallest spatial scales at cosmic dawn.

astro-ph.GA