arXiv subjects

D. Paris

Publications and source records attributed to D. Paris.

At least 19 recordsLinked to original sources

arXiv2026

Binary Stars as Dynamical Tracers in Globular Clusters .I. First Observations of Bimodal Spatial Distributions

We present the first homogeneous study of the radial distribution of the binary fraction across the full extent of six Galactic globular clusters (GCs) spanning a wide range of dynamical ages, from dynamically young systems to core-collapsed clusters. We measured the radial variation of the binary fraction using a combination of deep optical HST observations and wide-field ground-based data. For the first time, we provide evidence that the binary fraction in GCs does not decrease monotonically with radius, as commonly assumed, but instead exhibits a bimodal distribution characterized by an excess in the outer regions. Specifically, the binary fraction displays a central peak, followed by a minimum at intermediate radii and a rising branch beyond approximately 1-2 half-light radii. The position of this minimum correlates with the cluster relaxation timescale, indicating that it is shaped by long-term dynamical effects of two-body relaxation driving the binary evolution, segregation, and disruption. The minimum radius also correlates with the A+ parameter derived from the radial distribution of blue straggler stars, an empirical indicator of the cluster dynamical age, further supporting the interpretation that this feature is due to internal dynamical processes. Numerical simulations presented in a companion paper show that such bimodal distributions naturally arise from the combined effects of binary disruption and mass segregation of the surviving binaries in clusters.

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arXiv2025

The Early Maturity of High-Redshift Galaxies: Insights from sSFR, M/L and SFHs at z~7-14

The James Webb Space Telescope (JWST) has revealed an unexpected excess of UV-bright galaxies at z>10, unaccounted for by extrapolations from pre-JWST observations and theoretical models. Understanding the physical properties and star formation histories (SFHs) of high-redshift systems is key to distinguishing between the proposed scenarios. We identified and analysed a sample of 2420 robust candidates at z~7-14 drawn from the ASTRODEEP-JWST dataset over ~0.2 deg^2, and modelled their properties with non-parametric SFHs to derive the specific star formation rate (sSFR) and stellar population properties. We find that the median sSFR and M/L remain roughly constant across the probed redshift range. We show that this result is robust against potential systematics unless a hidden population of dust-enshrouded starbursts, undetectable in current data, exists at these redshifts. In any case, the absence of observed high-sSFR systems at the highest redshifts suggests that any dust-free starburst phase must be short-lived. The observed sSFR evolution is in tension with most theoretical models, making it a key quantity for discriminating among competing scenarios. The sample shows a wide range of physical conditions and galaxy classes, including systems with low sSFRs and high mass-to-light ratios (M/L) up to z~10, indicative of already-evolved galaxies only a few hundred Myr after the Big Bang, and different degrees of dust attenuation. We finally reconstructed the assembly histories of two sub-samples, namely the highest-M/L galaxies at z~7-8, which appear to have formed the bulk of their stars at least 500 Myr before observation, implying progenitors observable as UV-bright sources at z>20, and the full sample of z>11 galaxies, which formed through stochastic SFHs, remaining UV-faint for most of their early evolution, before undergoing recent (~50 Myr old) episodes of major growth.

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arXiv2025

Investigating ionising sources and the complex interstellar medium of GHZ2 at $z=12.3$

An accurate characterisation of the physical properties of galaxies at cosmic dawn is key to understanding the origin of the high abundance of UV-bright galaxies at z$\gtrsim$10. We exploit deep NIRSpec PRISM observations of GHZ2 to constrain the sources of ionising radiation and the properties of the ISM in this bright, compact, and highly ionising galaxy at z=12.3. We measure with high significance the prominent N IV, C IV, He II, O III, C III, O II, and Ne III emission features previously detected in shallower observations, and confirm the detection of the N III] $\lambda 1750$ multiplet, yielding tight constraints on the N/O ratio, which is found to be $\simeq$2 times the solar value. We also detect the Mg II $\lambda 2800$, [Fe IV] $\lambda 2833$ and Si II $\lambda 1812$ doublets, the H8+HeI $\lambda\lambda 3889$ blend, and the Si IV+O IV] $\lambda\lambda 1400$ absorption complex. The O III $\lambda 3133$ fluorescence line is only detected in the first observing epoch, implying variability on a rest-frame time span of 19 days, strongly suggesting the presence of an active nucleus. Combining the NIRSpec dataset with available optical and far-infrared constraints from MIRI and ALMA, we show that the emission spectrum of GHZ2 cannot be reproduced by single-density spectro-photometric models. Multi-zone photoionisation modelling performed with the HOMERUN code demonstrates that star formation must be occurring in a strongly stratified ISM, where both low-/intermediate-density gas and high-density regions (log($n_e$/cm$^{-3}) \gtrsim 4$) coexist. The GHZ2 emission landscape is consistent with either a composite star-formation plus AGN scenario, or with star formation occurring in a combination of radiation- and matter-bounded regions. Purely radiation-bounded stellar models fail to reproduce the observed He II emission, making an additional hard ionising component unavoidable.

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arXiv2025

RGB Tip distance to the faint gas-rich dwarf KK 153

KK 153 is a star-forming dwarf galaxy that has been recently proposed as a new member of the sparsely populated class of gas-rich ultra faint dwarfs, lying in the outskirts of the Local Group. We used the Large Binocular Telescope under sub-arcsec seeing conditions to resolve for the first time the outer regions of KK 153 into individual stars, reaching the red giant branch. The magnitude of the red giant branch tip was used to measure a distance of D=3.06 (+0.17/-0.14) Mpc, much more accurate and precise than the estimate previously available in the literature, based on the baryonic Tully-Fisher relation (D=2.0 (+1.7/-0.8) Mpc). The new distance places KK 153 clearly beyond the boundaries of the Local Group, and, together with a new measure of the integrated magnitude, implies a stellar mass of M_*=2.4 \pm 0.2 X 10^6 M_{\sun}. The dwarf populates the extreme low-mass tail of the M_* distribution of gas-rich galaxies but it is significantly more massive than the faintest local gas-rich dwarfs, Leo T and Leo P. In analogy with similar systems, the star formation history of KK 153 may have been impacted by the re-ionisation of the Universe while keeping a sufficient gas reservoir to form new stars several Gyr later.

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arXiv2025

From precursor to afterglow: The complex evolution of GRB 210312B

Long gamma-ray bursts (GRBs) are characterized by a brief gamma-ray flash followed by a longer-lasting multiwavelength afterglow. The basic mechanism is largely understood, and the early afterglow evolution often shows complex features that provide crucial insights into the transition between prompt and afterglow phases. We present a detailed analysis of GRB 210312B, detected by INTEGRAL, which exhibits both a precursor and a complex optical afterglow evolution. Through careful modeling using Markov chain Monte Carlo methods, we disentangled the contributions of an early optical flare and forward shock emission. Our analysis reveals a gamma-ray precursor 17 s before the main pulse with a significantly softer spectrum (hardness ratio 0.37 +/- 0.12 versus 1.9 +/- 0.4). The optical afterglow shows an early peak at 76.0^{+4.4}{-5.1} s characterized by a steep rise ({\alpha}{flare,1} = -4.1^{+1.6}{-2.3}) and decay ({\alpha}{flare,2} = 4.0^{+2.1}{-1.5}), followed by forward shock emission with a broad hydrodynamic peak at around 150 s. In the subsequent plateau phase, the afterglow initially has a complex structure before settling into a final power law decay consistent with an electron distribution index p = 2.36^{+0.18}{-0.15}. The negligible host extinction (A_{V,host} = -0.073^{+0.100}_{-0.078}) suggests we are observing the intrinsic afterglow spectrum. The host system consists of two luminous (M_B ~ -21.7) components separated by 11.5 kpc at z = 1.069, which are possibly an interacting galaxy pair. GRB 210312B provides a rare opportunity to study the prompt-to-afterglow transition in detail. The consistency of the forward shock component with standard afterglow theory supports our physical interpretation despite the lack of X-ray coverage.

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arXiv2025

EP 250108a/SN 2025kg: Observations of the most nearby Broad-Line Type Ic Supernova following an Einstein Probe Fast X-ray Transient

With a small sample of fast X-ray transients (FXTs) with multi-wavelength counterparts discovered to date, the progenitors of FXTs and their connections to gamma-ray bursts (GRBs) and supernovae (SNe) remain ambiguous. Here, we present photometric and spectroscopic observations of SN 2025kg, the supernova counterpart to the FXT EP 250108a. At $z=0.17641$, this is the closest known SN discovered following an Einstein Probe (EP) FXT. We show that SN 2025kg's optical spectra reveal the hallmark features of a broad-lined Type Ic SN. Its light curve evolution and expansion velocities are also comparable to those of GRB-SNe, including SN 1998bw, and several past FXT SNe. We present JWST/NIRSpec spectroscopy taken around SN 2025kg's maximum light, and find weak absorption due to He I $\lambda 1.0830, \lambda 2.0581$ $\mu$m and a broad, unidentified feature at $\sim$ 4-4.5 $\mu$m. Further, we observe clear evidence for broadened H$\alpha$ in optical data at 42.5 days that is not detected at other epochs, indicating interaction with hydrogen-rich material. From its light curve, we derive a $^{56}$Ni mass of 0.2 - 0.6 $M_{\odot}$. Together with our companion paper (Eyles-Ferris et al. 2025), our broadband data of EP 250108a/SN 2025kg are consistent with a trapped or low energy ($\lesssim 10^{51}$ ergs) jet-driven explosion from a collapsar with a zero-age main sequence mass of 15-30 $M_{\odot}$. Finally, we show that the sample of EP FXT SNe support past rate estimates that low-luminosity jets seen through FXTs are more common than successful (GRB) jets, and that similar FXT-like signatures are likely present in at least a few percent of the brightest Ic-BL SNe.

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arXiv2025

Pushing JWST to the extremes: search and scrutiny of bright galaxy candidates at z$\simeq$15-30

We designed customized Lyman-break color selection techniques to identify galaxy candidates in the redshift ranges $15 \leq z \leq 20$ and $20 \leq z \leq 28$. The selection was performed on the ASTRODEEP-JWST multi-band catalogs of the CEERS, Abell-2744, JADES, NGDEEP, and PRIMER survey fields, covering a total area of $\sim0.2$ sq. deg. We identify five candidates at $15 \leq z \leq 20$, while no objects are found based on the $z\gtrsim20$ color selection criteria. Despite exhibiting a $>$1.5 mag break, all the objects display multimodal redshift probability distributions across different SED-fitting codes and methodologies. The alternative solutions correspond to poorly understood populations of low-mass quiescent or dusty galaxies at z$\sim$3-7. This conclusion is supported by the analysis of five F200W-dropout objects that we find to be interlopers on the basis of NIRSpec PRISM spectra: four dusty star-forming galaxies at z$\sim$2.2-6.6, and a passive galaxy at z=4.91 with log$(M_{\rm star}/{\rm M}_{\odot}) \lesssim$ 9. We measured the UV luminosity function under different assumptions on the contamination level within our sample. We find that if even a fraction of the candidates is indeed at $z\gtrsim15$, the resulting UV LF points to a very mild evolution compared to estimates at $z<15$, implying a significant tension with existing theoretical models. In particular, confirming our bright ($M_{\text{UV}}<-21$) candidates would require substantial revisions to the theoretical framework. In turn, if all these candidates will be confirmed to be interlopers, we conclude that future surveys may need ten times wider areas to select $M_{\text{UV}}\lesssim-20$ galaxies at $z>15$. Observations in the F150W and F200W filters at depths comparable to those in the NIRCam LW bands are also required to mitigate contamination from rare red objects at z$\lesssim$8.

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arXiv2024

The puzzling long GRB 191019A: Evidence for Kilonova Light

GRB 191019A was a long Gamma-ray burst (GRB) lasting about 65 s and, as such, originally thought to be linked to a core-collapse supernova. However, even though follow-up observations identified the optical counterpart close to the bright nucleus of a nearby ancient galaxy (z=0.248), no associated supernova was found. This led to the suggestion that the burst was caused by the merger of two compact stellar objects, likely in a dense circumnuclear environment. By using a recently developed diagnostic tool based on prompt emission temporal properties, we noticed that GRB 191019A falls among those long GRBs which are associated with compact mergers and with evidence of kilonova light. We thus re-analyzed unpublished GROND multi-color (g'r'i'z'JHK_s) data obtained between 0.4 and 15 days post trigger. Image subtraction confirmed the optical counterpart in all four optical bands, with GROND tracking its fading until 1.5 days post-burst. Incorporating publicly available Swift-XRT data, a joint fit of an afterglow plus a kilonova model revealed a better match than an afterglow-only scenario. The resulting kilonova properties resemble those of AT2017gfo associated with the binary neutron star merger GW170817, with a total ejected mass of about 0.06 solar mass. Contrary to previous findings inferring a high-density circumburst environment (n0=10^7-10^8 cm^-3), our analysis finds standard conditions (n0 = 1 cm^-3), suggesting the long duration of GRB 191019A was intrinsic rather than due to jet interaction with a dense external medium.

astro-ph.HE↗

arXiv2024

Seven wonders of Cosmic Dawn: JWST confirms a high abundance of galaxies and AGNs at z $\simeq$ 9-11 in the GLASS field

We present JWST/NIRSpec PRISM follow-up of candidate galaxies at z=9-11 selected from deep JWST/NIRCam photometry in GLASS-JWST Early Release Science data. We spectroscopically confirm six sources with secure redshifts at z = 9.52-10.43, each showing multiple emission lines. An additional object is likely at z = 10.66, based on its Lya-break and a single emission feature, while one source is a lower redshift interloper. The sample includes the first JWST-detected candidate at z=10, GHZ1/GLASS-z10, which we confirm at z = 9.875, and the X-ray detected AGN GHZ9 confirmed at z = 10.145. Three objects in our sample, including GHZ9, have EW(CIII])>20A and occupy a region compatible with AGN emission in the EW(CIII]) vs CIV/CIII] diagram. The spectroscopic sample confirms a high abundance of galaxies at z > 9. We measure a number density of z=10 galaxies in the GLASS-JWST ERS field that is a factor of >3 higher than other JWST-based estimates at demagnified rest-frame magnitudes of -21 < Muv < -19. We find that the positions of these galaxies in redshift and angular space are not consistent with all of them being part of a unique progenitor of present-day galaxy clusters. The high density of objects in the GLASS region can be explained either by clustering on large scales or by a superposition of different forming structures of which we observe only the brightest members. By considering all the spectroscopic z=10 sources in the Abell-2744 field, we identify two potential galaxy proto-clusters centered around GHZ9 and JD1, with relative separations between their members of 1-2 pMpc. The potential AGN nature of three of the sources in our sample lends support to a scenario in which the high abundance of bright sources determined by JWST surveys at cosmic dawn may be affected by AGN contribution to their UV luminosity.

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arXiv2024

ASTRODEEP-JWST: NIRCam-HST multiband photometry and redshifts for half a million sources in six extragalactic deep fields

We present a set of photometric catalogs primarily aimed at providing the community with a comprehensive database for the study of galaxy populations in the high redshift Universe. The set gathers data from eight JWST NIRCam observational programs, targeting the Abell 2744 (GLASS-JWST, UNCOVER, DDT2756 and GO3990), EGS (CEERS), COSMOS and UDS (PRIMER), and GOODS North and South (JADES and NGDEEP) deep fields, for a total area of $\sim$0.2 sq. degrees. Photometric estimates are obtained by means of well-established techniques, including tailored improvements designed to enhance the performance on the specific dataset. We also include new measurements from HST archival data, thus collecting 16 bands spanning from 0.44 to 4.44 $\mu$m. A grand total of $\sim$530 thousand sources is detected on stacks of NIRCam 3.56 and 4.44 $\mu$m mosaics. We assess the photometric accuracy by comparing fluxes and colors against archival catalogs. We also provide photometric redshift estimates, statistically validated against a large set of robust spectroscopic data. The catalogs are publicly available on the Astrodeep website.

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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~\AA\ to 20, 000~\AA. Catalogs, reduced spectra, and template are made publicly available to the community.

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arXiv2024

Gaia23bab : a new EXor

On March 6 2023, the Gaia telescope has alerted a 2-magnitude burst from Gaia23bab, a Young Stellar Object in the Galactic plane. We observed Gaia23bab with the Large Binocular Telescope obtaining optical and near-infrared spectra close in time to the peak of the burst, and collected all public multi-band photometry to reconstruct the historical light curve. This latter shows three bursts in ten years (2013, 2017 and 2023), whose duration and amplitude are typical of EXor variables. We estimate that, due to the bursts, the mass accumulated on the star is about twice greater than if the source had remained quiescent for the same period of time. Photometric analysis indicates that Gaia23bab is a Class,II source with age < 1 Myr, spectral type G3-K0, stellar luminosity 4.0 L_sun, and mass 1.6 M_sun. The optical/near infrared spectrum is rich in emission lines. From the analysis of these lines we measured the accretion luminosity and the mass accretion rate L_acc(burst)=3.7 L_sun, M_acc(burst) 2.0 10 $^(-7) M_sun/yr, consistent with those of EXors. More generally, we derive the relationships between accretion and stellar parameters in a sample of EXors. We find that, when in burst, the accretion parameters become almost independent of the stellar parameters and that EXors, even in quiescence, are more efficient than classical T Tauri stars in assembling mass.

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arXiv2023

Closing in on the sources of cosmic reionization: first results from the GLASS-JWST program

The escape fraction of Lyman-continuum (LyC) photons ($f_{esc}$) is a key parameter for determining the sources of cosmic reionization at $z\geq 6$. At these redshifts, owing to the opacity of the intergalactic medium, the LyC emission cannot be measured directly. However, LyC leakers during the epoch of reionization could be identified using indirect indicators that have been extensively tested at low and intermediate redshifts. These include a high [OIII]/[OII] flux ratio, high star-formation surface density, and compact sizes. In this work, we present observations of 29 $4.5 \leq z \leq 8$ gravitationally lensed galaxies in the Abell 2744 cluster field. From a combined analysis of JWST-NIRSpec and NIRCam data, we accurately derived their physical and spectroscopic properties: our galaxies have low masses $(\log(M_\star)\sim 8.5)$, blue UV spectral slopes ($\beta \sim -2.1$), compact sizes ($r_e \sim 0.3-0.5$ kpc), and high [OIII]/[OII] flux ratios. We confirm that these properties are similar to those characterizing low-redshift LyC leakers. Indirectly inferring the fraction of escaping ionizing photons, we find that more than 80% of our galaxies have predicted $f_{esc}$ values larger than 0.05, indicating that they would be considered leakers. The average predicted $f_{esc}$ value of our sample is 0.12, suggesting that similar galaxies at $z\geq 6$ have provided a substantial contribution to cosmic reionization.

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arXiv2022

The Stellar Mass Function in CANDELS and Frontier Fields: the build-up of low mass passive galaxies since z~3

Despite significant efforts in the recent years, the physical processes responsible for the formation of passive galaxies through cosmic time remain unclear. The shape and evolution of the Stellar Mass Function (SMF) give an insight into these mechanisms. Taking advantage from the CANDELS and the deep Hubble Frontier Fields (HFF) programs, we estimated the SMF of total, star-forming and passive galaxies from z=0.25 to z=2.75 to unprecedented depth, and focus on the latter population. The density of passive galaxies underwent a significant evolution over the last 11 Gyr. They account for 60% of the total mass in the nearby Universe against ~20% observed at z~2.5. The inclusion of the HFF program allows us to detect, for the first time at z>1.5, the characteristic upturn in the SMF of passive galaxies at low masses, usually associated with environmental quenching. We observe two separate populations of passive galaxies evolving on different timescales: roughly half of the high mass systems were already quenched at high redshift, while low mass passive galaxies are gradually building-up over the redshift range probed. In the framework of environmental-quenching at low masses, we interpret this finding as evidence of an increasing role of the environment in the build-up of passive galaxies as a function of time. Finally, we compared our findings with a set of theoretical predictions. Despite good agreement in some redshift and mass intervals, none of the models are able to fully reproduce the observations. This calls for further investigation into the involved physical mechanisms, both theoretically and observationally, especially with the brand new JWST data.

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arXiv2022

Early Results From GLASS-JWST. XVII: Building the First Galaxies -- Chapter 1. Star Formation Histories at 5<z < 7

JWST observations of high redshift galaxies are used to measure their star formation histories - the buildup of stellar mass in the earliest galaxies. Here we use a novel analysis program, SEDz*, to compare near-IR spectral energy distributions for galaxies with redshifts 5 < z < 7 to combinations of stellar population templates evolved from z = 12. We exploit NIRCam imaging in 7 wide bands covering 1-5 mu m, taken in the context of the GLASS-JWST-ERS program, and use SEDz* to solve for well-constrained star formation histories for 24 exemplary galaxies. In this first look we find a variety of histories, from long, continuous star formation over 5 < z < 12 to short but intense starbursts - sometimes repeating, and, most commonly, contiguous mass buildup lasting ~ 0.5 Myr,possibly the seeds of today's typical, M* galaxies.

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arXiv2022

Early results from GLASS-JWST XV: properties of the faintest red sources in the NIRCAM deep fields

We present a first look at the reddest 2-5$\mu\rm m$ sources found in deep images from the GLASS Early Release Science program. We undertake a general search, i.e. not looking for any particular spectral signatures, for sources detected only in bands redder than reachable with the Hubble Space Telescope, and which would likely not have been identified in pre-JWST surveys. We search for sources down to AB $\sim 27$ (corresponding to $>10\sigma$ detection threshold) in any of the F200W to F444W filters,with a $>1$ magnitude excess relative to F090W to F150W bands. Fainter than F444W$>25$ we find 56 such sources of which 37 have reasonably constrained spectral energy distributions to which we can fit photometric redshifts. We find the majority of this population ($\sim$ 65%) as $2 $2$\mu \rm m$ boosted by a combination of the Balmer break and emission lines. The typical implied rest equivalent widths are $\sim200\unicode{0x212B}$ with some extreme objects up to $\sim 1000\unicode{0x212B}$. This is in contrast with brighter magnitudes where the red sources tend to be $z<3$ quiescent galaxies and dusty star forming objects. Our general selection criteria for red sources allow us to independently identify other phenomena as diverse as extremely low mass ($\sim 10^8$ M$_\odot$) quiescent galaxies at $z<1$, recover recently identified $z>11$ galaxies and a very cool brown dwarf.

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arXiv2022

Early results from GLASS-JWST. XIII. A faint, distant, and cold brown dwarf

We present the serendipitous discovery of a late T-type brown dwarf candidate in JWST NIRCam observations of the Early Release Science Abell 2744 parallel field. The discovery was enabled by the sensitivity of JWST at 4~$\mu$m wavelengths and the panchromatic 0.9--4.5~$\mu$m coverage of the spectral energy distribution. The unresolved point source has magnitudes F115W = 27.95$\pm$0.15 and F444W = 25.84$\pm$0.01 (AB), and its F115W$-$F444W and F356W$-$F444W colors match those expected for other, known T dwarfs. We can exclude it as a reddened background star, high redshift quasar, or very high redshift galaxy. Comparison with stellar atmospheric models indicates a temperature of $T_{eff}$ $\approx$ 600~K and surface gravity $\log{g}$ $\approx$ 5, implying a mass of 0.03~M$_{\odot}$ and age of 5~Gyr. We estimate the distance of this candidate to be 570--720~pc in a direction perpendicular to the Galactic plane, making it a likely thick disk or halo brown dwarf. These observations underscore the power of JWST to probe the very low-mass end of the substellar mass function in the Galactic thick disk and halo.

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arXiv2022

Early Results From GLASS-JWST. XII: The Morphology of Galaxies at the Epoch of Reionization

Star-forming galaxies can exhibit strong morphological differences between the rest-frame far-UV and optical, reflecting inhomogeneities in star-formation and dust attenuation. We exploit deep, high resolution NIRCAM 7-band observations to take a first look at the morphology of galaxies in the epoch of reionization ($z>7$), and its variation in the rest-frame wavelength range between Lyman $\alpha$ and 6000-4000\AA, at $z=7-12$. We find no dramatic variations in morphology with wavelength -- of the kind that would have overturned anything we have learned from the Hubble Space Telescope. No significant trends between morphology and wavelengths are detected using standard quantitative morphology statistics. We detect signatures of mergers/interactions in 4/19 galaxies. Our results are consistent with a scenario in which Lyman Break galaxies -- observed when the universe is only 400-800 Myrs old - are growing via a combination of rapid galaxy-scale star formation supplemented by accretion of star forming clumps and interactions.

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