arXiv subjects

Francois Boulanger

Publications and source records attributed to Francois Boulanger.

At least 19 recordsLinked to original sources

arXiv2026

PDRs4All XXII. Near-Infrared continuum in the Orion Bar

Conspicuous excess emission is present in the near-infrared (NIR) region in various objects, including reflection nebulae, planetary nebulae, and nearby galaxies. However, the spatial distribution and spectral shape of the excess emission remain poorly understood. We studied the NIR continuum emission spectroscopically and obtained its spatial distribution relative to the aromatic infrared band (AIB) at 3.3um in the Orion Bar prototypical photodissociation region (PDR). We aim to characterize its spectral shape and discuss its origin. We employed 3D spectroscopic data of the Orion Bar taken with the integrated field unit of NIRSpec on JWST from the Early Release Science program "PDRs4All." Contribution from the foreground ionized gas was estimated using the Cloudy code and subtracted. The observed regions were divided into nine physically distinct regions and an average spectrum was derived for each region. The nine regions, including the ionized gas, atomic PDR, and molecular PDR, clearly show remaining continuum in the region 1--4.5um. The continuum at wavelengths longer than 2.7um shows good correlations with the 3.3um AIB, while the correlation of the continuum at 1.2um is not significant. We further find that the NIR continuum in the Orion Bar can be approximated by a summation of two blackbodies. The low-temperature component correlates with the AIB well, while the high-temperature component does not. The average spectra also show absorption features at 3.0 and 4.27um, which are attributed to the presence in the spectra of water ice and CO2 ice. We discuss possible origins of the NIR continuum, among which recurrent fluorescence from carbon clusters better explains the observed low-temperature component. The presence of ice species suggests a contribution from a deeper layer of the PDR along the line of sight producing characteristic ice absorption features.

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arXiv2024

Bayesian inference methodology to characterize the dust emissivity at far-infrared and submillimeter frequencies

We present a Bayesian inference method to characterise the dust emission properties using the well-known dust-HI correlation in the diffuse interstellar medium at Planck frequencies $ν\ge 217$ GHz. We use the Galactic HI map from the Galactic All-Sky Survey (GASS) as a template to trace the Galactic dust emission. We jointly infer the pixel-dependent dust emissivity and the zero level present in the Planck intensity maps. We use the Hamiltonian Monte Carlo technique to sample the high dimensional parameter space ($D \sim 10^3$). We demonstrate that the methodology leads to unbiased recovery of dust emissivity per pixel and the zero level when applied to realistic Planck sky simulations over a 6300 deg$^2$ area around the Southern Galactic pole. As an application on data, we analyse the Planck intensity map at 353 GHz to jointly infer the pixel-dependent dust emissivity at Nside=32 resolution (1.8° pixel size) and the global offset. We find that the spatially varying dust emissivity has a mean of 0.031 MJysr$^{-1} (10^{20} \mathrm{cm^{-2}})^{-1}$ and $1σ$ standard deviation of 0.007 MJysr$^{-1} (10^{20} \mathrm{cm^{-2}})^{-1}$. The mean dust emissivity increases monotonically with increasing mean HI column density. We find that the inferred global offset is consistent with the expected level of Cosmic Infrared Background (CIB) monopole added to the Planck data at 353 GHz. This method is useful in studying the line-of-sight variations of dust spectral energy distribution in the multi-phase interstellar medium.

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arXiv2024

Wavelet Based Statistics for Enhanced 21cm EoR Parameter Constraints

We propose a new approach to improve the precision of astrophysical parameter constraints for the 21cm signal from the epoch of reionisation (EoR). Our method introduces new sets of summary statistics, hereafter `evolution compressed' statistics, which quantify the spectral evolution of the 2D spatial statistics computed at fixed redshift. We defined such compressed statistics for power spectrum (PS), wavelet scattering transforms (WST), and wavelet moments (WM), which also characterise non-Gaussian features. To compare these different statistics, along with the 3D power spectrum, we estimated their Fisher information on three cosmological parameters from an ensemble of simulations of 21cm EoR data, both in noiseless and noisy scenarios using Square Kilometre Array (SKA) noise levels equivalent to 100 and 1000 hours of observations. We also compare wavelet statistics, in particular WST, built from standard directional Morlet wavelets, as well as from a set of isotropic wavelets derived from the binning window function of the 2D power spectrum. For the noiseless case, the compressed wavelet statistics give constraints that are up to five times more precise than those obtained from the 3D isotropic power spectrum. At the same time, for 100h SKA noise, from which it is difficult to extract non-Gaussian features, compressed wavelet statistics still give over 30\% tighter constraints. We find that the wavelet statistics with wavelets derived from the power-spectrum binning window function provide the tightest constraints of all the statistics, with the WSTs seemingly performing better than the WMs, in particular when working with noisy data. The findings of this study demonstrate that evolution-compressed statistics extract more information than usual 3D isotropic power-spectra approaches and that our wavelet-based statistics can consistently outmatch power-spectrum-based statistics.

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arXiv2024

PDRs4All VIII: Mid-IR emission line inventory of the Orion Bar

Mid-infrared emission features probe the properties of ionized gas, and hot or warm molecular gas. The Orion Bar is a frequently studied photodissociation region (PDR) containing large amounts of gas under these conditions, and was observed with the MIRI IFU aboard JWST as part of the "PDRs4All" program. The resulting IR spectroscopic images of high angular resolution (0.2") reveal a rich observational inventory of mid-IR emission lines, and spatially resolve the substructure of the PDR, with a mosaic cutting perpendicularly across the ionization front and three dissociation fronts. We extracted five spectra that represent the ionized, atomic, and molecular gas layers, and measured the most prominent gas emission lines. An initial analysis summarizes the physical conditions of the gas and the potential of these data. We identified around 100 lines, report an additional 18 lines that remain unidentified, and measured the line intensities and central wavelengths. The H I recombination lines originating from the ionized gas layer bordering the PDR, have intensity ratios that are well matched by emissivity coefficients from H recombination theory, but deviate up to 10% due contamination by He I lines. We report the observed emission lines of various ionization stages of Ne, P, S, Cl, Ar, Fe, and Ni, and show how certain line ratios vary between the five regions. We observe the pure-rotational H$_2$ lines in the vibrational ground state from 0-0 S(1) to 0-0 S(8), and in the first vibrationally excited state from 1-1 S(5) to 1-1 S(9). We derive H$_2$ excitation diagrams, and approximate the excitation with one thermal (~700 K) component representative of an average gas temperature, and one non-thermal component (~2700 K) probing the effect of UV pumping. We compare these results to an existing model for the Orion Bar PDR and highlight the differences with the observations.

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arXiv2024

A far-ultraviolet-driven photoevaporation flow observed in a protoplanetary disk

Most low-mass stars form in stellar clusters that also contain massive stars, which are sources of far-ultraviolet (FUV) radiation. Theoretical models predict that this FUV radiation produces photo-dissociation regions (PDRs) on the surfaces of protoplanetary disks around low-mass stars, impacting planet formation within the disks. We report JWST and Atacama Large Millimetere Array observations of a FUV-irradiated protoplanetary disk in the Orion Nebula. Emission lines are detected from the PDR; modelling their kinematics and excitation allows us to constrain the physical conditions within the gas. We quantify the mass-loss rate induced by the FUV irradiation, finding it is sufficient to remove gas from the disk in less than a million years. This is rapid enough to affect giant planet formation in the disk.

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arXiv2023

PDRs4All III: JWST's NIR spectroscopic view of the Orion Bar

(Abridged) We investigate the impact of radiative feedback from massive stars on their natal cloud and focus on the transition from the HII region to the atomic PDR (crossing the ionisation front (IF)), and the subsequent transition to the molecular PDR (crossing the dissociation front (DF)). We use high-resolution near-IR integral field spectroscopic data from NIRSpec on JWST to observe the Orion Bar PDR as part of the PDRs4All JWST Early Release Science Program. The NIRSpec data reveal a forest of lines including, but not limited to, HeI, HI, and CI recombination lines, ionic lines, OI and NI fluorescence lines, Aromatic Infrared Bands (AIBs including aromatic CH, aliphatic CH, and their CD counterparts), CO2 ice, pure rotational and ro-vibrational lines from H2, and ro-vibrational lines HD, CO, and CH+, most of them detected for the first time towards a PDR. Their spatial distribution resolves the H and He ionisation structure in the Huygens region, gives insight into the geometry of the Bar, and confirms the large-scale stratification of PDRs. We observe numerous smaller scale structures whose typical size decreases with distance from Ori C and IR lines from CI, if solely arising from radiative recombination and cascade, reveal very high gas temperatures consistent with the hot irradiated surface of small-scale dense clumps deep inside the PDR. The H2 lines reveal multiple, prominent filaments which exhibit different characteristics. This leaves the impression of a "terraced" transition from the predominantly atomic surface region to the CO-rich molecular zone deeper in. This study showcases the discovery space created by JWST to further our understanding of the impact radiation from young stars has on their natal molecular cloud and proto-planetary disk, which touches on star- and planet formation as well as galaxy evolution.

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arXiv2023

PDRs4All IV. An embarrassment of riches: Aromatic infrared bands in the Orion Bar

(Abridged) Mid-infrared observations of photodissociation regions (PDRs) are dominated by strong emission features called aromatic infrared bands (AIBs). The most prominent AIBs are found at 3.3, 6.2, 7.7, 8.6, and 11.2 $μ$m. The most sensitive, highest-resolution infrared spectral imaging data ever taken of the prototypical PDR, the Orion Bar, have been captured by JWST. We provide an inventory of the AIBs found in the Orion Bar, along with mid-IR template spectra from five distinct regions in the Bar: the molecular PDR, the atomic PDR, and the HII region. We use JWST NIRSpec IFU and MIRI MRS observations of the Orion Bar from the JWST Early Release Science Program, PDRs4All (ID: 1288). We extract five template spectra to represent the morphology and environment of the Orion Bar PDR. The superb sensitivity and the spectral and spatial resolution of these JWST observations reveal many details of the AIB emission and enable an improved characterization of their detailed profile shapes and sub-components. While the spectra are dominated by the well-known AIBs at 3.3, 6.2, 7.7, 8.6, 11.2, and 12.7 $μ$m, a wealth of weaker features and sub-components are present. We report trends in the widths and relative strengths of AIBs across the five template spectra. These trends yield valuable insight into the photochemical evolution of PAHs, such as the evolution responsible for the shift of 11.2 $μ$m AIB emission from class B$_{11.2}$ in the molecular PDR to class A$_{11.2}$ in the PDR surface layers. This photochemical evolution is driven by the increased importance of FUV processing in the PDR surface layers, resulting in a "weeding out" of the weakest links of the PAH family in these layers. For now, these JWST observations are consistent with a model in which the underlying PAH family is composed of a few species: the so-called 'grandPAHs'.

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arXiv2023

PDRs4All II: JWST's NIR and MIR imaging view of the Orion Nebula

The JWST has captured the most detailed and sharpest infrared images ever taken of the inner region of the Orion Nebula, the nearest massive star formation region, and a prototypical highly irradiated dense photo-dissociation region (PDR). We investigate the fundamental interaction of far-ultraviolet photons with molecular clouds. The transitions across the ionization front (IF), dissociation front (DF), and the molecular cloud are studied at high-angular resolution. These transitions are relevant to understanding the effects of radiative feedback from massive stars and the dominant physical and chemical processes that lead to the IR emission that JWST will detect in many Galactic and extragalactic environments. Due to the proximity of the Orion Nebula and the unprecedented angular resolution of JWST, these data reveal that the molecular cloud borders are hyper structured at small angular scales of 0.1-1" (0.0002-0.002 pc or 40-400 au at 414 pc). A diverse set of features are observed such as ridges, waves, globules and photoevaporated protoplanetary disks. At the PDR atomic to molecular transition, several bright features are detected that are associated with the highly irradiated surroundings of the dense molecular condensations and embedded young star. Toward the Orion Bar PDR, a highly sculpted interface is detected with sharp edges and density increases near the IF and DF. This was predicted by previous modeling studies, but the fronts were unresolved in most tracers. A complex, structured, and folded DF surface was traced by the H2 lines. This dataset was used to revisit the commonly adopted 2D PDR structure of the Orion Bar. JWST provides us with a complete view of the PDR, all the way from the PDR edge to the substructured dense region, and this allowed us to determine, in detail, where the emission of the atomic and molecular lines, aromatic bands, and dust originate.

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arXiv2022

PDRs4All: A JWST Early Release Science Program on radiative feedback from massive stars

Massive stars disrupt their natal molecular cloud material through radiative and mechanical feedback processes. These processes have profound effects on the evolution of interstellar matter in our Galaxy and throughout the Universe, from the era of vigorous star formation at redshifts of 1-3 to the present day. The dominant feedback processes can be probed by observations of the Photo-Dissociation Regions (PDRs) where the far-ultraviolet photons of massive stars create warm regions of gas and dust in the neutral atomic and molecular gas. PDR emission provides a unique tool to study in detail the physical and chemical processes that are relevant for most of the mass in inter- and circumstellar media including diffuse clouds, proto-planetary disks and molecular cloud surfaces, globules, planetary nebulae, and star-forming regions. PDR emission dominates the infrared (IR) spectra of star-forming galaxies. Most of the Galactic and extragalactic observations obtained with the James Webb Space Telescope (JWST) will therefore arise in PDR emission. In this paper we present an Early Release Science program using the MIRI, NIRSpec, and NIRCam instruments dedicated to the observations of an emblematic and nearby PDR: the Orion Bar. These early JWST observations will provide template datasets designed to identify key PDR characteristics in JWST observations. These data will serve to benchmark PDR models and extend them into the JWST era. We also present the Science-Enabling products that we will provide to the community. These template datasets and Science-Enabling products will guide the preparation of future proposals on star-forming regions in our Galaxy and beyond and will facilitate data analysis and interpretation of forthcoming JWST observations.

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arXiv2020

Dust polarization modeling at large-scale over the Northern Galactic cap using EBHIS and Planck data

The primary source of systematic uncertainty in the quest for the B-mode polarization of the Cosmic Microwave Background (CMB) introduced by primordial gravitational waves is polarized thermal emission from Galactic dust. Therefore, accurate characterization and separation of the polarized thermal dust emission is an essential step in distinguishing such a faint CMB B-mode signal. We provide a modelling framework to simulate polarized thermal dust emission based on the model described in Ghosh et al. (2017), making use of both the Planck dust and Effelsberg-Bonn HI surveys over the northern Galactic cap. Our seven-parameter dust model, incorporating both HI gas in three different column density templates as a proxy for spatially variable dust intensity and a phenomenological model of Galactic magnetic field, is able to reproduce both 1- and 2-point statistics of the observed dust polarization maps seen by Planck at 353 GHz over a selected low-column density region in the northern Galactic cap. This work has important applications in assessing the accuracy of component separation methods and in quantifying the confidence level of separating polarized Galactic emission and the CMB B-mode signal, as is needed for ongoing and future CMB missions.

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arXiv2019

The link between E-B polarization modes and gas column density from interstellar dust emission

The analysis of the Planck polarization E and B mode power spectra of interstellar dust emission at 353 GHz recently raised new questions on the impact of Galactic foregrounds to the detection of the polarization of the Cosmic Microwave Background (CMB) and on the physical properties of the interstellar medium (ISM). In the diffuse ISM a clear E-B asymmetry is observed, with twice as much power in E modes than in B modes; as well as a positive correlation between the total power, T, and both E and B modes, presently interpreted in terms of the link between the structure of interstellar matter and that of the Galactic magnetic field. In this paper we aim at extending the Planck analysis of the high-latitude sky to low Galactic latitude, investigating the correlation between the TEB power spectra with the gas column density from the diffuse ISM to molecular clouds. We divide the sky between Galactic latitude |b|>5 deg and |b|<60 deg in 552 circular patches and we study the cross-correlations between the TEB power spectra and the column density of each patch using the latest release of the Planck polarization data. We find that the B-to-E power ratio (BB/EE) and the TE correlation ratio (rTE) depend on column density. While the former increases going from the diffuse ISM to molecular clouds in the Gould Belt, the latter decreases. This systematic variation must be related to actual changes in ISM properties. The data show significant scatter about this mean trend. The variations of BB/EE and rTE are observed to be anti-correlated for all column densities. In the diffuse ISM, the variance of these two ratios is consistent with a stochastic non-Gaussian model in which the values of BB/EE and rTE are fixed. We finally discuss the dependencies of TB and EB with column density, which are however hampered by instrumental noise.

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arXiv2019

Precision measurement of magnetic field from near to far, from fine to large scales in ISM

Magnetic fields have important or dominant effects in many areas of astrophysics, but have been very difficult to quantify. Spectropolarimetry from Ground State Alignment (GSA) has been suggested as a direct tracer of magnetic field in interstellar diffuse medium. Owing to the long life of the atoms on ground states the Larmor precession in an external magnetic field imprints the direction of the field onto the polarization of absorbing species. This provides a unique tool for studies of sub-gauss magnetic fields using polarimetry of UV, optical and radio lines. Many spectral lines with strong signals from GSA are in the UV band. By discerning magnetic fields in gas with different dynamical properties, high spectral resolution measurement of spectral polarization will allow the study of 3D magnetic field distribution and interstellar turbulence. GSA provides also a unique chance to map 3D direction of magnetic field on small scales, e.g., disks, where grain alignment is unreliable. The range of objects suitable for studies is extremely wide and includes magnetic fields in the interplanetary medium, in the interstellar medium, and in circumstellar regions as well as diffuse media in extragalactic objects.

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arXiv2019

Warm H$_2$ as a probe of massive accretion and feedback through shocks and turbulence across cosmic time

Galaxy formation depends on a complex interplay between gravitational collapse, gas accretion, merging, and feedback processes. Yet, after many decades of investigation, these concepts are poorly understood. This paper presents the argument that warm H$_2$ can be used as a tool to unlock some of these mysteries. Turbulence, shocks and outflows, driven by star formation, AGN activity or inflows, may prevent the rapid buildup of star formation in galaxies. Central to our understanding of how gas is converted into stars is the process by which gas can dissipate its mechanical energy through turbulence and shocks in order to cool. H$_2$ lines provide direct quantitative measurements of kinetic energy dissipation in molecular gas in galaxies throughout the Universe. Based on the detection of very powerful H$_2$ lines from z = 2 galaxies and proto-clusters at the detection limits of {\it Spitzer}, we are confident that future far-IR and UV H$_2$ observations will provide a wealth of new information and insight into galaxy evolution to high-z. Finally, at the very earliest epoch of star and galaxy formation, warm H$_2$ may also provide a unique glimpse of molecular gas collapse at 7 $<$ z $<$ 12 in massive dark matter (DM) halos on their way to forming the very first galaxies. Such measurements are beyond the reach of existing and planned observatories.

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arXiv2018

IMAGINE: A comprehensive view of the interstellar medium, Galactic magnetic fields and cosmic rays

In this white paper we introduce the IMAGINE Consortium and its scientific background, goals and structure. Our purpose is to coordinate and facilitate the efforts of a diverse group of researchers in the broad areas of the interstellar medium, Galactic magnetic fields and cosmic rays, and our goal is to develop more comprehensive insights into the structures and roles of interstellar magnetic fields and their interactions with cosmic rays. To achieve a higher level of self-consistency, depth and rigour can only be achieved by the coordinated efforts of experts in diverse areas of astrophysics involved in observational, theoretical and numerical work. We present our view of the present status of this topic, identify its key unsolved problems and suggest a strategy that will underpin our work. The backbone of the consortium is the Interstellar MAGnetic field INference Engine, a publicly available Bayesian platform that employs robust statistical methods to explore the multi-dimensional likelihood space using any number of modular inputs. It provides an interpretation and modelling framework that has the power and flexibility to include a variety of observational, theoretical and numerical lines of evidence into a self-consistent and comprehensive picture of the thermal and non-thermal interstellar media. An important innovation is that a consistent understanding of the phenomena that are directly or indirectly influenced by the Galactic magnetic field, such as the deflection of ultra-high energy cosmic rays or extragalactic backgrounds, is made an integral part of the modelling. The IMAGINE Consortium, which is informal by nature and open to new participants, hereby presents a methodological framework for the modelling and understanding of Galactic magnetic fields that is available to all communities whose research relies on a state-of-the-art solution to this problem. (Abridged.)

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arXiv2018

BFORE: A CMB Balloon Payload to Measure Reionization, Neutrino Mass, and Cosmic Inflation

BFORE is a high-altitude ultra-long-duration balloon mission to map the cosmic microwave background (CMB). During a 28-day mid-latitude flight launched from Wanaka, New Zealand, the instrument will map half the sky to improve measurements of the optical depth to reionization tau. This will break parameter degeneracies needed to detect neutrino mass. BFORE will also hunt for the gravitational wave B-mode signal, and map Galactic dust foregrounds. The mission will be the first near-space use of TES/mSQUID multichroic detectors (150/217 GHz and 280/353 GHz bands) with low-power readout electronics.

astro-ph.IM↗

arXiv2017

Measuring Reionization, Neutrino Mass, and Cosmic Inflation with BFORE

BFORE is a NASA high-altitude ultra-long-duration balloon mission proposed to measure the cosmic microwave background (CMB) across half the sky during a 28-day mid-latitude flight launched from Wanaka, New Zealand. With the unique access to large angular scales and high frequencies provided by the balloon platform, BFORE will significantly improve measurements of the optical depth to reionization tau, breaking parameter degeneracies needed for a measurement of neutrino mass with the CMB. The large angular scale data will enable BFORE to hunt for the large-scale gravitational wave B-mode signal, as well as the degree-scale signal, each at the r~0.01 level. The balloon platform allows BFORE to map Galactic dust foregrounds at frequencies where they dominate, in order to robustly separate them from CMB signals measured by BFORE, in addition to complementing data from ground-based telescopes. The combination of frequencies will also lead to velocity measurements for thousands of galaxy clusters, as well as probing how star-forming galaxies populate dark matter halos. The mission will be the first near-space use of TES multichroic detectors (150/217 GHz and 280/353 GHz bands) using highly-multiplexed mSQUID microwave readout, raising the technical readiness level of both technologies.

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arXiv2017

Modelling and simulation of large-scale polarized dust emission over the southern Galactic cap using the GASS HI data

The Planck survey has quantified polarized Galactic foregrounds and established that they are a main limiting factor in the quest for the cosmic microwave background (CMB) B-mode signal induced by primordial gravitational waves during cosmic inflation. Accurate separation of the Galactic foregrounds therefore binds this quest to our understanding of the magnetized interstellar medium (ISM). The two most relevant empirical results from analysis of Planck data are line of sight depolarization arising from fluctuations of the Galactic magnetic field orientation and alignment of filamentary dust structures with the magnetic field at high Galactic latitude. Furthermore, Planck and HI emission data in combination indicate that most of the filamentary dust structures are in the cold neutral medium. The goal of this paper is to test whether these salient observational results, taken together, can account fully for the statistical properties of the dust polarization over a selected low column density region comprising 34 % of the southern Galactic cap ($b \le -30°$). To do this, we construct a dust model that incorporates HI column density maps as tracers of the dust intensity structures and a phenomenological description of the Galactic magnetic field. By adjusting the parameters of the dust model, we were able to reproduce the Planck dust observations at 353 GHz in the selected region. Realistic simulations of the polarized dust emission enabled by such a dust model are useful for testing the accuracy of component separation methods, studying non-Gaussianity, and constraining the amount of decorrelation with frequency.

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arXiv2017

The impact of a massive star cluster on its surrounding matter in the Antennae overlap region

Super star clusters (SSCs), likely the progenitors of globular clusters, are one of the most extreme forms of star formation. Understanding how SSCs form is an observational challenge. Theoretical studies establish that, to form such clusters, the dynamical timescale of their parent clouds has to be shorter than the timescale of the disruption of their parent clouds by stellar feedback. However, due to insufficient observational support, it is still unclear how feedback from SSCs acts on their surrounding matter. Studying feedback in SSCs is essential to understand how such clusters form. Based on ALMA and VLT observations, we study this process in a SSC in the overlap region of the Antennae galaxies. We analyze a unique massive (~10^7 Msun) and young (1-3.5 Myr) SSC, still associated with compact molecular and ionized gas emission. The cluster has two CO velocity components, a low velocity one spatially associated with the cluster and a high velocity one distributed in a bubble-like shape around the cluster. Our results on the low velocity component suggest that this gas did not participate in the formation of the SSC. We propose that most of the parent cloud has already been blown away, accelerated at the early stages of the SSC evolution by radiation pressure, in a timescale ~1 Myr. The high velocity component may trace outflowing molecular gas from the parent cloud. Supporting evidence is found in shock heated H2 gas. The low velocity component may be gas that was near the SSC when it formed but not part of its parent cloud or clumps that migrated from the SGMC environment. This gas would be dispersed by stellar winds and supernova explosions. Within ~100 pc from the cluster, we estimate SFE>17%, smaller than the theoretical limit of 30% needed to form a bound cluster. Further higher spatial resolution observations are needed to test and quantify our proposed scenario.

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