Searcharxiv⌕ Search

arXiv subjects

M. Bradač

Publications and source records attributed to M. Bradač.

13 recordsLinked to original sources
arXiv2026

Resolving dust and Lyα emission in a lensed galaxy at the epoch of reionization with JWST/CANUCS

Lyman $α$ emission is highly sensitive to dust and neutral hydrogen and is therefore expected to be strongly suppressed in dusty or gas-rich galaxies during the epoch of reionization (EoR). Nevertheless, numerous moderately dusty Ly$α$ emitters (LAEs) are observed at this epoch, suggesting that complex interstellar medium (ISM) geometries and feedback-driven outflows may facilitate Ly$α$ escape. We investigate the dust, gas, and stellar properties of the gravitationally lensed LAE HCM 6A at $z=6.5676$ to characterize its multiphase ISM and the physical conditions regulating Ly$α$ escape. We combine JWST/NIRISS slitless spectroscopy, HST+JWST/NIRCam imaging, and JWST/NIRSpec slit spectra from the CANUCS program. Using a customized BAGPIPES SED-fitting framework with a flexible attenuation law, we derive stellar, nebular, and dust properties on integrated ($\sim$kpc), slit-level ($\sim$0.1 kpc), and pixel-level ($\sim$25 pc) scales, enabled by strong lensing ($μ\approx 8.3$-$9.1$). A Ly$α$ map from SLEUTH traces the spatial distribution of Ly$α$ emission. We measure an unlensed stellar mass of $\log M_\ast = 8.3$-$8.4$ and an intrinsic UV magnitude of $M_{\rm UV} = -19.8 \pm 0.1$. The older region (S1) is moderately dusty with consistent stellar and nebular attenuation indicators, implying a uniform ISM geometry, while the youngest region (S3) shows strong discrepancies among dust tracers, indicating a feedback-shaped multiphase ISM. Ly$α$ emission arises primarily from S3, where a dust-cleared central clump enables efficient Ly$α$ escape. We find Calzetti-like attenuation curves with a UV bump that strengthens with stellar age and decreasing $A_V$. Our observations provide a uniquely detailed, spatially resolved view of a moderately dusty LAE during the EoR, demonstrating how feedback and multiphase ISM structure govern Ly$α$ escape.

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 $0.8-5.3~μm$, reveals a pronounced $\rm Lyα$-continuum break at $λ\simeq 1.3~μm$, as well as weak optical $\rm Hβ$ and $\rm [OIII]\lambda4959$ emission lines at $z=9.625\pm0.002$, located in the reddest part of the spectrum ($λ> 5.1~μm$). No additional ultraviolet or optical emission lines are reliably detected. A weak Balmer break is measured alongside a very blue ultraviolet slope ($β\leq-2.5$, $\rm F_λ \sim λ^β$). 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↗
arXiv2025

A Lyman continuum analysis for $\sim 100$ galaxies at $z_{\text{spec}} \sim 3$ in the Abell 2744 Cluster Field

Identifying Lyman continuum (LyC) leakers at intermediate redshifts is crucial for understanding the properties of cosmic reionizers, as the opacity of the intergalactic medium (IGM) prevents direct detection of LyC emission from sources during the Epoch of Reionization (EoR). In this study, we confirm two new LyC candidate leakers at $z \sim 3$ in the Abell 2744 cluster field, with absolute escape fractions ($f_{\text{esc}}$) of $0.90^{+0.07}_{-0.86}$ and $0.60^{+0.37}_{-0.56}$, respectively. The LyC emission was detected using HST/WFC3/F275W and F336W imaging. These two candidate leakers appear faint ($ M_{\text{UV}} = -18.1 \pm 0.1 \text{ and } -17.81 \pm 0.11$), exhibit blue UV continuum slopes ($β= -2.42 \pm 0.05 \text{ and } -1.78 \pm 0.19$), have low masses ($M_\star \sim 10^{7.73} \pm 0.1 \text{ and } 10^{7.07} \pm 0.05 M_\odot$) and show \lya\ equivalent widths of $90 \pm 3$ Å and $28 \pm 12$ Å, respectively. The discovery of these two LyC candidate leakers was achieved in a catalog of 91 spectroscopically confirmed sources using JWST and/or MUSE public spectra. We also analyze properties that have been proposed as indirect indicators of LyC emission, like \lya, O32 ratio, and $M_\star$: we create subsample of galaxies selected according to such properties, stack the LyC observations of these subsample and assess the limits in escape fractions in the stacks. By analysing the individual candidates and the stacks, in the context of the currently limited sample of known LyC leakers at $z \sim 3$, we aim to enhance our understanding of LyC escape mechanisms and improve our predictions of the LyC $f_{\text{esc}}$ during the EoR.

astro-ph.GA↗
arXiv2025

Anatomy of a z=6 Lyman-α emitter down to parsec scales: extreme UV slopes, metal-poor regions and possibly leaking star clusters

We present a detailed JWST/NIRSpec and NIRCam analysis of a gravitationally-lensed galaxy ($\rm μ=17-21$) at redshift 6.14 magnified by the Hubble Frontier Field galaxy cluster MACS J0416. The target galaxy is overall a typical compact and UV-faint ($\rm M_{UV}=-17.8$) Lyman-$α$ emitter, yet the large magnification allows the detailed characterization of structures on sub-galactic scales (down to a few parsecs). Prominent optical $\rm Hα$, $\rm Hβ$ and [OIII]$λ\lambda4959,5007$ lines are spatially resolved with the high spectral resolution grating (G395H, R~2700), with large equivalent widths, EW($\rm Hβ$+[OIII])$\gtrsim1000$ Å, and elevated ionising photon production efficiencies $\rm log(ξ_{ion}/erg^{-1}Hz)=25.2-25.7$. NIRCam deep imaging reveals the presence of compact rest-UV bright regions along with individual star clusters of $\rm R_{eff}=3-8~pc$ in size and $\rm M\sim2\cdot10^5-5\cdot10^{6}~M_\odot$ in mass. These clusters are characterised by steep UV slopes, $\rmβ_{UV}\lesssim-2.5$, which in some cases are associated with a dearth of line emission, indicating possible leaking of the ionizing radiation, as also supported by a Lyman-$\rm α$ emission peaking at $\rm \sim100~km~s^{-1}$ from the systemic redshift. While the entire system is characterised by low-metallicity, $\sim0.1~Z_\odot$, the NIRSpec-IFU map also reveals the presence of a low-luminosity, metal-poor region with $\rm Z\lesssim2\%~Z_\odot$, barely detected in NIRCam imaging; this region is displaced by $\rm >200~pc$ from one of the brightest structures of the system in UV, and would have been too faint to detect if not for the large magnification of the system.

astro-ph.GA↗
arXiv2024

The GLASS-JWST Early Release Science Program. IV. Data release of 263 spectra from 245 unique sources

We release fully reduced spectra obtained with NIRSpec onboard JWST as part of the GLASS-JWST Early Release Science Program and a follow-up Director's Discretionary Time program 2756. From these 263 spectra of 245 unique sources, acquired with low ($R =30-300$) and high dispersion ($R\sim2700$) gratings, we derive redshifts for 200 unique sources in the redshift range $z=0-10$. We describe the sample selection and characterize its high completeness as a function of redshift and apparent magnitude. Comparison with independent estimates based on different methods and instruments shows that the redshifts are accurate, with 80\% differing less than 0.005. We stack the GLASS-JWST spectra to produce the first high-resolution ($R \sim 2700$) JWST spectral template extending in the rest frame wavelength from 2000~Å to 20, 000~Å. Catalogs, reduced spectra, and template are made publicly available to the community.

astro-ph.GA↗
arXiv2021

The Size and Pervasiveness of Ly$α$-UV Spatial Offsets in Star-Forming Galaxies at $z\sim6$

We study the projected spatial offset between the ultraviolet continuum and Ly$α$ emission for 65 lensed and unlensed galaxies in the Epoch of Reionization ($5\leq z\leq7$), the first such study at these redshifts, in order to understand the potential for these offsets to confuse estimates of the Ly$α$ properties of galaxies observed in slit spectroscopy. While we find that ~40% of galaxies in our sample show significant projected spatial offsets ($|Δ_{Lyα-UV}|$), we find a modest average offset of 0.61$\pm$0.08 kpc. A small fraction of our sample, ~10%, exhibits offsets of 2-4 kpc, sizes that are larger than the effective radii of typical galaxies at these redshifts. An internal comparison and a comparison to studies at lower redshift yielded no significant evidence of evolution of $|Δ_{Lyα-UV}|$ with redshift. In our own sample, UV-bright galaxies showed offsets a factor of three greater than their fainter counterparts, 0.89$\pm$0.18 vs. 0.27$\pm$0.05 kpc, respectively. We argue that offsets are likely not the result of merging processes, but are rather due to internal anisotropic processes resulting from stellar feedback facilitates Ly$α$ fluorescence and/or backscattering from nearby or outflowing gas. The reduction in the Ly$α$ flux due to offset effects for various observational setups was quantified through mock observations of simple simulations. It was found that the loss of Ly$α$ photons for galaxies with average offsets is not, if corrected for, a limiting factor for all but the narrowest slit widths (<0.4''). However, for the largest offsets, if such offsets are mostly perpendicular to the slit major axis, slit losses were found to be extremely severe in cases where slit widths of $\leq$1'' were employed, such as those planned for James Webb Space Telescope/NIRSpec observations. (abridged)

astro-ph.GA↗
arXiv2017

The Grism Lens-Amplified Survey from Space (GLASS). XII. Spatially Resolved Galaxy Star Formation Histories and True Evolutionary Paths at z > 1

Modern data empower observers to describe galaxies as the spatially and biographically complex objects they are. We illustrate this through case studies of four, $z\sim1.3$ systems based on deep, spatially resolved, 17-band + G102 + G141 Hubble Space Telescope grism spectrophotometry. Using full spectrum rest-UV/-optical continuum fitting, we characterize these galaxies' observed $\sim$kpc-scale structures and star formation rates (SFRs) and reconstruct their history over the age of the universe. The sample's diversity---passive to vigorously starforming; stellar masses $\log M_*/M_\odot=10.5$ to $11.2$---enables us to draw spatio-temporal inferences relevant to key areas of parameter space (Milky Way- to super-Andromeda-mass progenitors). Specifically, we find signs that bulge mass-fractions ($B/T$) and SF history shapes/spatial uniformity are linked, such that higher $B/T$s correlate with "inside-out growth" and central specific SFRs that peaked above the global average for all starforming galaxies at that epoch. Conversely, the system with the lowest $B/T$ had a flat, spatially uniform SFH with normal peak activity. Both findings are consistent with models positing a feedback-driven connection between bulge formation and the switch from rising to falling SFRs ("quenching"). While sample size forces this conclusion to remain tentative, this work provides a proof-of-concept for future efforts to refine or refute it: JWST, WFIRST, and the 30-m class telescopes will routinely produce data amenable to this and more sophisticated analyses. These samples---spanning representative mass, redshift, SFR, and environmental regimes---will be ripe for converting into thousands of sub-galactic-scale empirical windows on what individual systems actually looked like in the past, ushering in a new dialog between observation and theory.

astro-ph.GA↗
arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). III. A census of Lyα Emission at $z\gtrsim$7 from HST Spectroscopy

[abbreviated] We present a census of Lyα emission at $z\gtrsim7$ utilizing deep near infrared HST grism spectroscopy from the first six completed clusters of the Grism Lens-Amplified Survey from Space (GLASS). In 24/159 photometrically selected galaxies we detect emission lines consistent with Lyα in the GLASS spectra. Based on the distribution of signal-to-noise ratios and on simulations we expect the completeness and the purity of the sample to be 40-100% and 60-90%, respectively. For the objects without detected emission lines we show that the observed (not corrected for lensing magnification) 1$σ$ flux limits reaches $5\times10^{-18}$erg/s/cm$^{2}$ per position angle over the full wavelength range of GLASS (0.8-1.7$μ$m). Based on the conditional probability of Lyα emission measured from the ground at $z\sim7$ we would have expected 12-18 Lyα emitters. This is consistent with the number of detections, within the uncertainties, confirming the drop in Lyα emission with respect to $z\sim6$. These candidates include a promising source at $z=8.1$. The spatial extent of Lyα in a deep stack of the most convincing Lyα emitters with $\langle z\rangle=7.2$ is consistent with that of the rest-frame UV continuum. Extended Ly$α$ emission, if present, has a surface brightness below our detection limit, consistent with the properties of lower redshift comparison samples. From the stack we estimate upper limits on rest-frame UV emission line ratios and find $f_\textrm{CIV} / f_\textrm{Ly$α$} \lesssim 0.32$ and $f_\textrm{CIII]} / f_\textrm{Ly$α$} \lesssim 0.23$ in good agreement with other values published in the literature.

astro-ph.GA↗
arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). I. Survey overview and first data release

We give an overview of the Grism Lens Amplified Survey from Space (GLASS), a large Hubble Space Telescope program aimed at obtaining grism spectroscopy of the fields of ten massive clusters of galaxies at redshift z=0.308-0.686, including the Hubble Frontier Fields (HFF). The Wide Field Camera 3 yields near infrared spectra of the cluster cores, covering the wavelength range 0.81-1.69mum through grisms G102 and G141, while the Advanced Camera for Surveys in parallel mode provides G800L spectra of the infall regions of the clusters. The WFC3 spectra are taken at two almost orthogonal position angles in order to minimize the effects of confusion. After summarizing the scientific drivers of GLASS, we describe the sample selection as well as the observing strategy and data processing pipeline. We then utilize MACSJ0717.5+3745, a HFF cluster and the first one observed by GLASS, to illustrate the data quality and the high-level data products. Each spectrum brighter than H_AB=23 is visually inspected by at least two co-authors and a redshift is measured when sufficient information is present in the spectra. Furthermore, we conducted a thorough search for emission lines through all the GLASS WFC3 spectra with the aim of measuring redshifts for sources with continuum fainter than H_AB=23. We provide a catalog of 139 emission-line based spectroscopic redshifts for extragalactic sources, including three new redshifts of multiple image systems (one probable, two tentative). In addition to the data itself we also release software tools that are helpful to navigate the data.

astro-ph.GA↗
arXiv2015

The Grism Lens-Amplified Survey from Space (GLASS). IV. Mass reconstruction of the lensing cluster Abell 2744 from frontier field imaging and GLASS spectroscopy

We present a strong and weak lensing reconstruction of the massive cluster Abell 2744, the first cluster for which deep Hubble Frontier Field (HFF) images and spectroscopy from the Grism Lens-Amplified Survey from Space (GLASS) are available. By performing a targeted search for emission lines in multiply imaged sources using the GLASS spectra, we obtain 5 high-confidence spectroscopic redshifts and 2 tentative ones. We confirm 1 strongly lensed system by detecting the same emission lines in all 3 multiple images. We also search for additional line emitters blindly and use the full GLASS spectroscopic catalog to test reliability of photometric redshifts for faint line emitters. We see a reasonable agreement between our photometric and spectroscopic redshift measurements, when including nebular emission in photometric redshift estimations. We introduce a stringent procedure to identify only secure multiple image sets based on colors, morphology, and spectroscopy. By combining 7 multiple image systems with secure spectroscopic redshifts (at 5 distinct redshift planes) with 18 multiple image systems with secure photometric redshifts, we reconstruct the gravitational potential of the cluster pixellated on an adaptive grid, using a total of 72 images. The resulting mass map is compared with a stellar mass map obtained from the deep Spitzer Frontier Fields data to study the relative distribution of stars and dark matter in the cluster. We find that the stellar to total mass ratio varies substantially across the cluster field, suggesting that stars do not trace exactly the total mass in this interacting system. The maps of convergence, shear, and magnification are made available in the standard HFF format.

astro-ph.GA↗
arXiv2015

New observations of z~7 galaxies: evidence for a patchy reionization

We present new results from our search for z~7 galaxies from deep spectroscopic observations of candidate z-dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only 2 galaxies have robust redshift identifications, one from its Lyalpha emission line at z=6.65, the other from its Lyman-break, i.e. the continuum discontinuity at the Lyalpha wavelength consistent with a redshift 6.42, but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyalpha EW derived from the non detections in ultra-deep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z~7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Ly$α$ emission in z~7 Lyman break galaxies compared to z~6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one.

astro-ph.CO↗
arXiv2009

Focusing Cosmic Telescopes: Exploring Redshift z~5-6 Galaxies with the Bullet Cluster 1E0657-56

The gravitational potential of clusters of galaxies acts as a cosmic telescope allowing us to find and study galaxies at fainter limits than otherwise possible and thus probe closer to the epoch of formation of the first galaxies. We use the Bullet Cluster 1E0657-56 (z = 0.296) as a case study, because its high mass and merging configuration makes it one of the most efficient cosmic telescopes we know. We develop a new algorithm to reconstruct the gravitational potential of the Bullet Cluster, based on a non-uniform adaptive grid, combining strong and weak gravitational lensing data derived from deep HST/ACS F606W-F775W-F850LP and ground-based imaging. We exploit this improved mass map to study z~5-6 Lyman Break Galaxies (LBGs), which we detect as dropouts. One of the LBGs is multiply imaged, providing a geometric confirmation of its high redshift, and is used to further improve our mass model. We quantify the uncertainties in the magnification map reconstruction in the intrinsic source luminosity, and in the volume surveyed, and show that they are negligible compared to sample variance when determining the luminosity function of high-redshift galaxies. With shallower and comparable magnitude limits to HUDF and GOODS, the Bullet cluster observations, after correcting for magnification, probe deeper into the luminosity function of the high redshift galaxies than GOODS and only slightly shallower than HUDF. We conclude that accurately focused cosmic telescopes are the most efficient way to sample the bright end of the luminosity function of high redshift galaxies and - in case they are multiply imaged - confirm their redshifts.

astro-ph.CO↗