arXiv subjects

B. Welch

Publications and source records attributed to B. Welch.

8 recordsLinked to original sources

arXiv2025

Lyman Continuum escaping from in-situ formed stars in a tidal bridge at z = 3

We present an analysis of archival JWST NIRSpec IFS and HST imaging observations of the z = 3 Lyman-Continuum Emitter (LCE) candidate LACES104037. We show that a nearby galaxy, denoted LACES104037-S, has a redshift offset from the main galaxy by only $\sim 450$ km/s. Together with the identification of tidal bridge features between the galaxies, this indicates that the galaxies are interacting and most likely in the early stages of a merger. We show that the restframe LCE cluster sits in a tidal bridge towards the companion, about 2.7 kpc from the galaxy's core. It is faint in non-ionizing stellar continuum, and shows faint but non-negligible H{\alpha} and [O III] emission, suggesting that much of the gas surrounding the LCE cluster has been dispersed by feedback in the shallower gravitational potential of the tidal bridge. From comparing the direct LyC escape and the local H{\alpha} emission, we find a total ionizing LyC escape fraction of $f_{\mathrm{esc}} = 57 \pm 8\%$ from the LCE cluster. We estimate the age of the LCE cluster to be $\lesssim 6.5$ Myr, indicating that the cluster must have formed in situ in the tidal bridge well after the time of closest interaction. LyC escape from tidal stripping or in-situ formed stars in tidal features would depend less on intrinsic galaxy properties than typically observed in low-z LCE surveys, and could help explain the higher cosmic escape fraction as well as the enhanced diversity of LCE galaxy properties observed at Cosmic Noon.

astro-ph.GA↗

arXiv2025

The z = 9.625 Cosmic Gems Galaxy was a "Compact Blue Monster" Propelled by Massive Star Clusters

The recent discovery of five massive stellar clusters at z=9.625 in the Cosmic Gems has raised the question about the formation mechanism of star clusters in the first half Gyr after the Big-Bang. We infer the total stellar mass in clusters by normalizing and integrating the stellar cluster mass function (SCMF, dn(M)/dM ~ (n$_0$) $M^\beta$), assuming three different slopes $\beta$ = -1.5, -2.0 and -2.5 and different lower-mass limits between $10^2$ and $10^5$ Msun. The total integrated cluster stellar mass is compared to the stellar mass inferred from the counter-image of the Cosmic Gems, which provides the best, modestly magnified ($\mu$ = 1.84$\pm$0.05) representation of the entire galaxy. The delensed stellar mass of the Cosmic Gems galaxy is estimated as 3.5$_{-1.8}^{+3.3}$ x$10^7$ Msun, with an effective radius of Reff = 103$_{-15}^{+13}$ parsec and a stellar surface mass density of $\Sigma$mass = 520$_{-225}^{+340}$ Msun pc$^{-2}$. Accounting for normalization uncertainties - including different lensing magnification scenarios for the arc - a modified SCMF, combined with a significantly high star cluster formation efficiency (approaching 100%), appears to be a necessary condition to explain the relatively short formation timescale of both the star clusters and the counter-image, without exceeding the galaxy's stellar mass. By extrapolating the physical properties at the peak of the burst we find that in its recent past (<~ 30 Myr) the Cosmic Gems galaxy has likely experienced a specific star formation rate (sSFR) exceeding 25 Gyr$^{-1}$ and luminosity approaching the ``blue monster'' regime (M$_{UV}$ < -20). Our study provides insights into the extreme clustered nature of star formation in early galaxies and shed light into the formation of bound star clusters that might survive to z = 0 as globular clusters, older than 13 Gyr.

astro-ph.GA↗

arXiv2025

JWST spectroscopic confirmation of the Cosmic Gems arc at z=9.625 -- Insights into the small scale structure of a post-burst system

We present JWST/NIRSpec integral field spectroscopy of the Cosmic Gems arc, strongly magnified by the galaxy cluster SPT-CL J0615$-$5746. Six-hour integration using NIRSpec prism spectroscopy (resolution $\rm R\simeq 30-300$), covering the spectral range $0.8-5.3~\mu m$, reveals a pronounced $\rm Ly\alpha$-continuum break at $\lambda \simeq 1.3~\mu m$, as well as weak optical $\rm H\beta$ and $\rm [OIII]\lambda4959$ emission lines at $z=9.625\pm0.002$, located in the reddest part of the spectrum ($\lambda > 5.1~\mu m$). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope ($\beta \leq-2.5$, $\rm F_{\lambda} \sim \lambda^{\beta}$). Spectral fitting with $\tt Bagpipes$ suggests that the Cosmic Gems galaxy is in a post-starburst phase, making it the highest-redshift system currently observed in a mini-quenched state. Spatially resolved spectroscopy at tens of parsecs shows relatively uniform features across subcomponents of the arc. These findings align well with the physical properties previously derived from JWST/NIRCam photometry of the stellar clusters, now corroborated by spectroscopic evidence. In particular, five observed star clusters exhibit ages of $\rm 7-30~Myr$. An updated lens model constrains the intrinsic sizes and masses of these clusters, confirming they are extremely compact and denser than typical star clusters in local star-forming galaxies. Additionally, four compact stellar systems consistent with star clusters ($\lesssim10$ pc) are identified along the extended tail of the arc. A sub-parsec line-emitting HII region straddling the critical line, lacking a NIRCam counterpart, is also serendipitously detected.

astro-ph.GA↗

arXiv2024

The Sunburst Arc with JWST: II. Observations of an Eta Carinae Analog at $z=2.37$

"Godzilla" is a peculiar object within the gravitationally lensed Sunburst Arc at $z=2.37$. Despite being very bright, it appears in only one of the twelve lensed images of the source galaxy, and shows exotic spectroscopic properties not found in any other clumps. We use JWST's unique combination of spatial resolution and spectroscopic sensitivity to provide a unified, coherent explanation of the physical nature of Godzilla. We measure fluxes and kinematic properties of rest-optical emission lines in Godzilla and surrounding regions. Using standard line ratio-based diagnostic methods in combination with NIRCam imaging and ground based rest-UV spectra, we characterize Godzilla and its surroundings. Among around 60 detected lines, we find a cascade of strong O I lines pumped by intense Ly$\beta$ emission, as well as Ly$\alpha$-pumped rest-optical Fe II lines, reminiscent of the Weigelt blobs in the local LBV star Eta Carinae. Spectra and images of Godzilla and two faint adjacent images, and the detection of a low-surface brightness foreground galaxy in the NIRCam data, support the interpretation that Godzilla is an extremely magnified object due to the alignment with lensing caustics. We find that Godzilla is part of a previously identified clump, comprising $\sim10 - 25$ % of it, with magnifications in the range of $\approx600-25,000$ depending on the models and images in comparison. The unique O I source in Godzilla is well explained by a non-erupting LBV accompanied by a hotter companion and/or gas condensations exposed to more intense radiation compared to the Weigelt blobs. If Godzilla is confirmed to contain an LBV star, it expands the distance to the furthest known LBV from a dozen Mpc to several Gpc.

astro-ph.GA↗

arXiv2024

The Sunburst Arc with JWST: I. Detection of Wolf-Rayet stars injecting nitrogen into a low-metallicity, $z=2.37$ proto-globular cluster leaking ionizing photons

We report the detection of a population of Wolf-Rayet (WR) stars in the Sunburst Arc, a strongly gravitationally lensed galaxy at redshift $z=2.37$. As the brightest known lensed galaxy, the Sunburst Arc has become an important cosmic laboratory for studying star and cluster formation, Lyman $\alpha$ radiative transfer, and Lyman Continuum (LyC) escape. Here, we present the first results of JWST/NIRSpec IFU observations of the Sunburst Arc, focusing on a stacked spectrum of the 12-fold imaged LyC-emitting (Sunburst LCE) cluster. In agreement with previous studies, we find that the cluster is massive and compact, with $M_{\text{dyn}} = (9\pm1) \times 10^{6} M_{\odot}$, Our age estimate of 4.2--4.5 Myr is much larger than the crossing time of $t_{\text{cross}} = 183 \pm 9 $ kyr, indicating that the cluster is dynamically evolved and consistent with being gravitationally bound. We find a significant nitrogen enhancement of the low ionization state ISM, with $\log(N/O) = -0.74 \pm 0.09$, which is $\approx 0.8$ dex above typical values for H II regions of similar metallicity in the local Universe. We find broad stellar emission complexes around He II$\lambda 4686$ and C IV$\lambda 5808$ with associated nitrogen emission -- this is the first time WR signatures have been directly observed at redshifts above $\sim 0.5$. The strength of the WR signatures cannot be reproduced by stellar population models that only include single-star evolution. While models with binary evolution better match the WR features, they still struggle to reproduce the nitrogen-enhanced WR features. JWST reveals the Sunburst LCE to be a highly ionized, proto-globular cluster with low oxygen abundance and extreme nitrogen enhancement that hosts a population of Wolf-Rayet stars, and possibly Very Massive stars (VMSs), which are rapidly enriching the surrounding medium.

astro-ph.GA↗

arXiv2024

Copacabana: A Probabilistic Membership Assignment Method for Galaxy Clusters

Cosmological analyses using galaxy clusters in optical/NIR photometric surveys require robust characterization of their galaxy content. Precisely determining which galaxies belong to a cluster is crucial. In this paper, we present the COlor Probabilistic Assignment of Clusters And BAyesiaN Analysis (Copacabana) algorithm. Copacabana computes membership probabilities for {\it all} galaxies within an aperture centred on the cluster using photometric redshifts, colours, and projected radial probability density functions. We use simulations to validate Copacabana and we show that it achieves up to 89\% membership accuracy with a mild dependency on photometric redshift uncertainties and choice of aperture size. We find that the precision of the photometric redshifts has the largest impact on the determination of the membership probabilities followed by the choice of the cluster aperture size. We also quantify how much these uncertainties in the membership probabilities affect the stellar mass--cluster mass scaling relation, a relation that directly impacts cosmology. Using the sum of the stellar masses weighted by membership probabilities ($\mu_{\star}$) as the observable, we find that Copacabana can reach an accuracy of 0.06 dex in the measurement of the scaling relation. These results indicate the potential of Copacabana and $\mu_{\star}$ to be used in cosmological analyses of optically selected clusters in the future.

astro-ph.CO↗

arXiv2022

JWST/NIRCam Probes Young Star Clusters in the Reionization Era Sunrise Arc

Star cluster formation in the early universe and their contribution to reionization remains to date largely unconstrained. Here we present JWST/NIRCam imaging of the most highly magnified galaxy known at z ~ 6, the Sunrise arc. We identify six young massive star clusters (YMCs) with measured radii spanning ~ 20 pc down to ~ 1 pc (corrected for lensing magnification), estimated stellar masses of ~ $10^{(6-7)}$ Msun, and with ages 1-30 Myr based on SED fitting to photometry measured in 8 filters extending to rest-frame 7000A. The resulting stellar mass surface densities are higher than 1000 Msun pc$^{-2}$ (up to a few $10^5$ Msun pc$^{-2}$) and their inferred dynamical ages qualify the majority of these systems as gravitationally-bound stellar clusters. The star cluster ages map the progression of star formation along the arc, with to evolved systems (>~ 10 Myr old) followed by very young clusters. The youngest stellar clusters (< 5 Myr) show evidence of prominent Hbeta + [OIII]4959,5007 emission, based on photometry, with equivalent widths larger than 1000 A rest-frame, and are hosted in a 200 pc sized star-forming complex. Such a region dominates the ionizing photon production, with a high efficiency log($\xi_{ion}$ [Hz erg$^{-1}$]) ~ 25.7. A significant fraction of the recently formed stellar mass of the galaxy (> 10-30 %) occurred in these YMCs. We speculate that such sources of ionizing radiation boost the ionizing photon production efficiency which eventually carve ionized channels that might favor the escape of Lyman continuum radiation. The survival of some of the clusters would make them the progenitors of massive and relatively metal-poor globular clusters in the local Universe.

astro-ph.GA↗

arXiv2019

Stellar mass as a galaxy cluster mass proxy: application to the Dark Energy Survey redMaPPer clusters

We introduce a galaxy cluster mass observable, $μ_\star$, based on the stellar masses of cluster members, and we present results for the Dark Energy Survey (DES) Year 1 observations. Stellar masses are computed using a Bayesian Model Averaging method, and are validated for DES data using simulations and COSMOS data. We show that $μ_\star$ works as a promising mass proxy by comparing our predictions to X-ray measurements. We measure the X-ray temperature-$μ_\star$ relation for a total of 150 clusters matched between the wide-field DES Year 1 redMaPPer catalogue, and Chandra and XMM archival observations, spanning the redshift range $0.1<z<0.7$. For a scaling relation which is linear in logarithmic space, we find a slope of $α= 0.488\pm0.043$ and a scatter in the X-ray temperature at fixed $μ_\star$ of $σ_{{\rm ln} T_X|μ_\star}=0.266^{+0.019}_{-0.020}$ for the joint sample. By using the halo mass scaling relations of the X-ray temperature from the Weighing the Giants program, we further derive the $μ_\star$-conditioned scatter in mass, finding $σ_{{\rm ln} M|μ_\star}=0.26^{+ 0.15}_{- 0.10}$. These results are competitive with well-established cluster mass proxies used for cosmological analyses, showing that $μ_\star$ can be used as a reliable and physically motivated mass proxy to derive cosmological constraints.

astro-ph.CO↗