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Roelof S. de Jong

Publications and source records attributed to Roelof S. de Jong.

At least 19 recordsLinked to original sources
arXiv2026

WST instrument Exposure Time Calculator: full simulation of multi-mode spectrograph performance from source to detector

We present a comprehensive Exposure Time Calculator (ETC) developed for the Wide-field Spectroscopic Telescope (WST) concept. The WST, currently in its conceptual phase, is designed as a next-generation large spectroscopic survey facility featuring three complementary observing modes: an Integral Field Spectrograph (IFS) covering 370-930 nm at R of about 4800; a high-resolution Multi-Object Spectrograph (MOS-HR) with four bands at R of about 40000; and a low-resolution Multi-Object Spectrograph (MOS-LR) with four channels at R of about 3800-4900. The ETC simulates the complete photon-propagation path from astronomical source to detector, incorporating wavelength-dependent system throughput (telescope transmission, instrumental optics, detector quantum efficiency), accurate sky background via ESO SkyCalc integration, and a comprehensive noise treatment (photon noise, sky background, read-out noise, dark current). The computational core is implemented as the "pyetc_wst" Python library built on the MPDAF framework, supporting multiple target spectral energy distributions (stellar templates, blackbody, power-law, emission lines, and user-uploaded spectra with arbitrary redshift) and spatial morphologies (point sources and Sersic extended profiles). Four operational modes enable flexible exposure-time optimization. Full spectral outputs include wavelength-dependent signal-to-noise ratio (SNR), source and sky photon counts, noise decomposition by component, and simulated extracted spectra. An interactive web interface, together with a REST API and a command-line tool, complete the user experience and enable batch survey-design workflows.

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arXiv2026

WST -- Wide-field Spectroscopic Telescope: The Next Leap in Wide-field Spectroscopy

The Wide-field Spectroscopic Telescope (WST) is a concept for a dedicated 12-m spectroscopic survey facility designed to address some of the most important questions in astrophysics in the 2040s. The WST will provide unprecedented spectroscopic survey capabilities by operating simultaneously over a 2-degree diameter field of view with 54 low-resolution spectrographs fed by 30,000 fibres, 8-16 high-resolution spectrographs fed by 2,000 fibres, and a large panoramic low-resolution integral-field spectrograph. Supported by Horizon Europe, the concept study has refined the science cases, facility architecture, operations model, sustainability strategy, and technology roadmap. The resulting reference design demonstrates that the WST is both scientifically transformative and technically feasible, while identifying the developments required to mitigate the remaining risks. The WST is designed as an ESO flagship facility for the post-ELT construction era and a key spectroscopic complement to the major imaging, time-domain, and multi-messenger facilities of the coming decades.

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arXiv2026

Exploration of small footprint stick-slip piezoelectric actuators for use in the FLEX fibre positioner system for the Wide-field Spectroscopic Telescope (WST)

This work presents a novel stick-slip piezoelectric actuator for the FLEX fibre positioner, addressing the challenge of reducing footprint while maintaining precision in next-generation multi-object spectroscopic instruments such as the Wide-field Spectroscopic Telescope (WST). The actuator combines compact geometry (3.2 mm diameter, sub 70 mm length) with high-resolution incremental motion, low power consumption, and high reliability. Three actuators can be integrated into a sub 7 mm diameter circle, enabling a simplified, single-plane assembly. This design offers a promising solution for miniaturized, accurate fibre positioning in wide-field spectroscopic applications.

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arXiv2026

The Wide-field Spectroscopic Telescope: optical designs for the low-resolution multi-object spectrographs

In MOS Low-Resolution mode, WST has a 'spectral etendue' per fiber (etendue x spectral resolution elements) twice as large as any existing MOS instrument, and a total spectral etendue ~20 times larger. Combined, these present enormous challenges for the spectrograph design. Initial designs were based on (a) a novel F/0.775 Folded Solid Schmidt camera with 6cm detectors, (b) an F/1.15 dioptric camera with 6cm detectors, and (c) an F/1.31 dioptric camera with 9cm detectors, all with YAG field-flatteners. However, the imperative of fast camera speed has greatly diminished, as the projected cost and read-noise of large format CMOS detectors is projected to decrease greatly. The adopted baseline solution has 4 dioptric F/1.64 cameras, each with two doublets, a YAG field lens and 9cm x 9cm detector. Theoretical image quality is 8 microns rms radius. The design seems readily athermalised, potentially allowing passive temperature control. Each spectrograph accepts 500+ fibers. Expected RoM costs (scaled from earlier designs) suggest a total cost <4M Euro per spectrograph.

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arXiv2026

The Star Formation History and Evolution of the Ultra-Diffuse M81 Satellite F8D1

We present deep HST imaging of one of the nearest ultra-diffuse galaxies (UDGs) outside of the Local Group: F8D1, a satellite of M81 known to be tidally disrupting. UDGs are an enigmatic and diverse population, with evolutionary pathways ranging from tidal processing to bursty feedback and high initial angular momentum. To determine F8D1's evolutionary drivers, we resolve stars in F8D1's central $\sim$1 kpc and in a parallel field $\sim$6 kpc along its major axis to deep photometric limits, reaching below the Red Clump. We also image eight shallower fields along F8D1's major and minor axes. We calculate the star formation history (SFH) in the two deep fields, finding that while currently quiescent, both regions experienced a substantial burst $\sim$2 Gyr ago and a smaller burst $\sim$500 Myr ago, which likely formed F8D1's nuclear star cluster. In the shallow fields, using the ratio of evolved Asymptotic Giant Branch and Red Giant Branch stars out to $\sim$13 kpc along F8D1's known stellar stream, we confirm that F8D1 was globally star-forming until at least $\sim$2 Gyr ago. We estimate a total progenitor stellar mass, including the stream, of $\sim$1.3$\times$10$^8 M_{\odot}$, with an average [M/H] $\sim$ $-$0.8. We compare F8D1's properties to those of Local Group galaxies with similar initial stellar mass. We find that F8D1 is consistent with a progenitor star-forming galaxy similar to NGC 6822 that is in the midst of a transition to a Sagittarius-like system. Notably, this evolutionary sequence can be accomplished through tidal processing alone in galaxies that have experienced sufficiently bursty feedback to have created cored profiles.

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arXiv2026

High multiplex and precision: the design and development of FLEX, a grid-based fiber positioner with large patrol radius and minimized telecentric error

In next-generation spectroscopic facilities, high-multiplex fiber positioning systems must operate within highly constrained focal surfaces, such as the Wide-Field Spectroscopic Telescope (WST) requiring 30,000+ fibers across a 1.4-meter surface. The Fiber Location EXtender (FLEX) positioner meets these constraints by improving fiber patrol radii while minimizing telecentric error and positioner spacing for dense clustering and high Multi-Object Spectrograph (MOS) multiplexing. The patented FLEX concept utilizes a superelastic nickel-titanium alloy (Nitinol) inside three concentric, geometrically altered tubes. This construction ensures the tip remains parallel with its base during tilting, while internal routing allows the fiber to run freely along the axis to minimize Focal Ratio Degradation (FRD). Designed for a patrol radius of 2.5x the pitch within the WST architecture, the design delivers a maximum patrol radius up to ~22.5 mm with a telecentric error of less than 0.39 degrees. FLEX utilizes three piezoelectric actuators to provide large radial displacements and precise focus adjustment. To scale this architecture, a modular focal surface layout of 90 identical curvilinear modules has been devised. This layout houses 30,240 positioners across a 2-degree hexagonal field-of-view (FoV), accommodating a central hole for an Integral Field pickoff mirror. Only three support struts are required, obscuring just 0.8% of the FoV while allowing full positioner coverage. One in 16 positioners is allocated for high-resolution spectroscopy, with the remainder split among three low-resolution spectrograph sets; all four sets achieve virtually full coverage of the FoV.

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arXiv2026

WST Multi-Object Spectrograph Fiber Positioners:Development of a 32,000-Unit Precision Robotic System

The Wide-field Spectroscopic Telescope Multi-Object Spectrograph requires an unprecedented fiber positioning system comprising 30'000 low-resolution and 2'000 high-resolution positioners across a 3.1 deg squared field of view. Each robotic positioner must achieve 5um RMS positioning accuracy in a densely packed focal plane, representing a more than sixfold scale increase over current instruments like 4MOST and DESI. To mitigate risks associated with industrial-scale production of 32'000 precision mechanisms, WST is pursuing a multi-concept development strategy. Four distinct positioner architectures are being prototyped and tested by an international collaboration (EPFL, AIP, UKATC, AAO) using 6.2 mm pitch triangular modules of 63 units each or a new inline modular concept. Performance metrics including positioning accuracy, repeatability, reconfiguration speed, collision avoidance, and manufacturability are being systematically evaluated. Down-selection to one or two concepts is planned for 2026-2027 during the HORIZON Europe-funded conceptual study phase. Current prototype testing demonstrates feasibility of meeting specifications, supporting WST's path toward first light in the early 2040s as ESO's next major spectroscopic facility.

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arXiv2026

Stars Born in the Wind II: Widespread Extra-planar Star Formation in M82's Halo

Galaxies evolve in tandem with their environments -- mergers and gas inflows drive galaxy growth while galactic outflows launched by supernovae may seed the galactic environment with gas, metals, and energy, fueling star-formation far from the main bodies of galaxies. The formation histories of young stars in the stellar halos of nearby galaxies can help understand this interplay. We thus present the most detailed map to date of young stars in the stellar halo of M82, a starburst galaxy in the M81 Group that hosts a prototypical outflow, using Hubble Space Telescope (HST) and Subaru Hyper-Suprime Cam observations. We find widespread extraplanar populations of stars with ages $\lesssim630$ Myr, with clear detections of stars up to $\sim5$ kpc to the south in unique arc-like stellar features (Southern Arcs) and in a new stellar trail up to $\sim20$ kpc to the east (M82's Tail), originating from the Southern Arcs. We estimate a total halo star formation of $\sim4\times10^6\,M_\odot$ in the last $630$ Myr. Overall, the star formation history (SFH) of the M82 Tail is correlated with periods of heightened star cluster formation in the M82 disk, which suggests the influence of the starburst outflow. Further, the fraction of young stars decreases as we move away from M82 to the east. We forward a picture where the M82 Tail formed from ram pressure stripped gas arising from M82's westward motion, triggered by shocks from the outflow.

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arXiv2025

The low-mass and structured stellar halo of M83 argues against a merger origin for its starburst and extended neutral hydrogen disk

A merger origin has been suggested for M83's massive, metal-rich extended HI disk and nuclear starburst. We observe M83's stellar halo to test this idea. We train nearest-neighbor star-galaxy separation on wide-area Subaru imaging with Hubble Space Telescope data to map M83's halo in resolved stars. We find that M83 has an extended, very low density smooth stellar halo of old and metal-poor [M/H]$\sim -1.15$ RGB stars with a mass between 15 and 40 kpc of $\log_{10}M_{*,15-40,maj}/M_{\odot}=8.02\pm0.10$. In addition to M83's well-known Northern Stream, our ground-based Subaru imaging reveals a new stream to M83's south, which modeling suggests could be its trailing arm. The combined stream masses are $\log_{10}M_{stream}/M_{\odot}=7.93\pm0.10$, with metallicity [M/H]$= -1.0\pm0.2$. The stream progenitor was only recently accreted, as its stellar populations suggest that it formed stars until $2.1\pm1.3$ Gyr ago. M83 lies on the stellar halo mass-metallicity correlation seen for other Milky Way mass galaxies, albeit with low stellar halo mass. We infer a total accreted mass of $\log_{10}M_{*,accreted}/M_{\odot}=8.78^{+0.22}_{-0.28}$, with the most massive past merger having $\log_{10}M_{*,dom}/M_{\odot}=8.5\pm0.3$. We identify plausible M83 analogs in TNG-50 with similar stellar halos, finding that while a recent accretion can create a prominent stellar stream, such accretions do not trigger starburst activity, nor do they deliver enough gas to form M83's extended Hi disk. We conclude that other non-merger mechanisms, such as secular evolution or accretion of gas from the IGM, are likely to be responsible for M83's remarkable properties.

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arXiv2025

Stars Born in the Wind: M82's Outflow and Halo Star Formation

Starburst galaxies, like M82, launch kiloparsec-scale galactic outflows that interact with the circumgalactic medium (CGM) in complex ways. Apart from enriching the CGM with metals and energy, these outflows may trigger star formation in the halo -- either by driving shocks into the CGM or transporting cold, star-forming gas. To investigate such processes, we analyze the star formation history (SFH) of the Southern Arcs -- arc-like stellar features located ~5 kpc from M82's star-forming disk along the minor axis -- using Hubble Space Telescope Wide Field Camera 3 photometry. From resolved stellar populations, we derive SFHs over the last ~500 Myr, finding that ~85% of the stellar mass formed between ~150 and ~70 Myr ago, followed by a brief pause, with the remaining ~15% forming since ~30 Myr ago. The two stellar populations are co-spatial on scales of at least ~200 pc. The timing of the ~100 Myr burst aligns with star formation in the M82 disk and the age distribution of its star clusters, suggesting a causal link between the disk starburst and halo star formation. We explore two mechanisms that could explain these observations. In the first, shocks driven by the interaction between hot outflowing gas and cooler CGM material compress dense clouds, triggering collapse and star formation. In the second, stars form directly within massive, cool clouds associated with the outflow. As these clouds move ballistically through the halo, subsequent interactions with tidal debris may trigger additional star formation, producing the observed episodic structure.

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arXiv2025

Andromeda XXXV: The Faintest Dwarf Satellite of the Andromeda Galaxy

We present the discovery of Andromeda XXXV, the faintest Andromeda satellite galaxy discovered to date, identified as an overdensity of stars in the Pan-Andromeda Archaeological Survey and confirmed via Hubble Space Telescope imaging. Located at a heliocentric distance of $927^{+76}_{-63}$ kpc and $158^{+57}_{-45}$ kpc from Andromeda, Andromeda XXXV is an extended ($r_h = 53\,^{+13}_{-11}$ pc), elliptical ($ε= 0.4\, \pm 0.2$), metal-poor ($[\text{Fe}/\text{H}]\sim-1.9$) system, and the least luminous ($M_V=-5.2 \pm 0.3$) of Andromeda's dwarf satellites discovered so far. Andromeda XXXV's properties are consistent with the known population of dwarf galaxies around the Local Group, bearing close structural resemblance to the Canes Venatici II and Hydra II Milky Way (MW) dwarf satellite galaxies. Its stellar population, characterized by a red horizontal branch or a red clump feature, mirrors that of other Andromeda satellite galaxies in showing evidence for a spread in age and metallicity, with no signs of younger stellar generations. This age-metallicity spread is not observed in MW satellites of comparable stellar mass, and highlights the persistent differences between the satellite systems of Andromeda and the MW, extending even into the ultrafaint regime.

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arXiv2024

Exploring the Diversity of Faint Satellites in the M81 Group

In the last decade, we have been able to probe further down the galaxy luminosity function than ever before and expand into the regime of ultra-faint dwarfs (UFDs), which are some of the best probes we have of small-scale cosmology and galaxy formation. Digital sky surveys have enabled the discovery and study of these incredibly low-mass, highly dark-matter dominated systems around the Local Group, but it is critical that we expand the satellite census further out to understand if Milky Way and M31 satellites are representative of dwarf populations in the local Universe. Using data from HST/ACS, we present updated characterization of four satellite systems in the M81 group. These systems - D1005+68, D1006+69, DWJ0954+6821, and D1009+68 - were previously discovered using ground-based Subaru HSC data as overdensities in M81's halo and are now confirmed with HST/ACS by this work. These are all faint (M_V >= -7.9) and consistent with old (~13 Gyr), metal-poor ([M/H] < -1.5) populations. Each system possesses relatively unusual features - including one of the most concentrated satellite galaxies with a Sersic index of n ~ 5, one of the most elliptical galaxies outside the Local Group with an e ~ 0.6, and one of the most compact galaxies for its magnitude. Two of the satellites have very low surface brightness, lower than most known galaxies in this absolute magnitude range. This work previews the scientific promise of the upcoming Rubin Observatory and Roman Telescope for illuminating the diversity of UFDs in the Local Volume and beyond.

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arXiv2024

Rediscovering the Milky Way with orbit superposition approach and APOGEE data III. Panoramic view of the bulge

The innermost parts of the Milky Way (MW) are very difficult to observe due to the high extinction along the line of sight, especially close to the disc mid-plane. However, this region contains the most massive complex stellar component of the MW, the bulge, primarily composed of disc stars whose structure is (re-)shaped by the evolution of the bar. In this work, we extend the application of the orbit superposition method to explore the present-day 3D structure, orbital composition, chemical abundance trends and kinematics of the MW bulge. Thanks to our approach, we are able to transfer astrometry from Gaia and stellar parameters from APOGEE DR 17 to map the inner MW without obscuration by the survey footprint and selection function. We demonstrate that the MW bulge is made of two main populations originating from a metal-poor, high-α thick disc and a metal-rich, low-α thin disc, with a mass ratio of 4:3, seen as two major components in the MDF. Finer MDF structures hint at multiple sub-populations associated with different orbital families of the bulge, which, however, have broad MDFs themselves. Decomposition using 2D GMMs in [Fe/H] -[Mg/Fe] identifies five components including a population with ex-situ origin. Two dominant ones correspond to the thin and thick discs and two in between trace the transition between them. We show that no universal metallicity gradient value can characterise the MW bulge. The radial gradients closely trace the X-shaped bulge density structure, while the vertical gradient variations follow the boxy component. While having, on average, subsolar metallicity, the MW bulge populations are more metal-rich compared to the surrounding disc, in agreement with extragalactic observations and state-of-the-art simulations reinforcing its secular origin.

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arXiv2024

Rediscovering the Milky Way with orbit superposition approach and APOGEE data II. Chrono-chemo-kinematics of the disc

The stellar disc is the dominant luminous component of the Milky Way (MW). Although our understanding of its structure is rapidly expanding due to advances in large-scale stellar surveys, our picture of the MW disc remains substantially obscured by selection functions and incomplete spatial coverage of observational data. In this work, we present the comprehensive chrono-chemo-kinematic structure of the MW disc, recovered using a novel orbit superposition approach combined with data from APOGEE DR 17. We detect periodic azimuthal metallicity variations within 6-8 kpc with an amplitude of 0.05-0.1 dex peaking along the bar major axis. The radial metallicity profile of the MW also varies with azimuth, displaying a pattern typical among other disc galaxies: a decline outside the solar radius and an almost flat profile in the inner region, attributed to the presence of old, metal-poor high-α populations, which comprise about 40% of the total stellar mass. The geometrically defined thick disc and the high-α populations have comparable masses, with differences in their stellar population content, which we quantify using the reconstructed 3D MW structure. The well-known [α/Fe]-bimodality in the MW disc, once weighted by stellar mass, is less pronounced at a given metallicity for the whole galaxy but distinctly visible in a narrow range of galactic radii (5-9 kpc), explaining its relative lack of prominence in external galaxies and galaxy formation simulations. Analysing a more evident double age-abundance sequence, we construct a scenario for the MW disc formation, advocating for an inner/outer disc dichotomy genetically linked to the MW's evolutionary stages. In this picture, the extended solar vicinity is a transition zone that shares chemical properties of both the inner (old age-metallicity sequence) and outer discs (young age-metallicity sequence).

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arXiv2024

First resolved stellar halo kinematics of a MW-mass galaxy outside the Local Group: A flat counter-rotating halo in NGC 4945

Stellar halos of galaxies, primarily formed through the accretion of smaller objects, are important to understand the hierarchical mass assembly of galaxies. However, the inner regions of stellar halos in disk galaxies are predicted to have an in-situ component that is expected to be prominent along the major axis. Kinematic information is crucial to disentangle the contribution of the in-situ component from the accreted stellar halos. The low surface brightness of stellar halos makes it inaccessible with traditional integrated light spectroscopy. In this work, using a novel technique, we study the kinematics of the stellar halo of the edge-on galaxy NGC 4945. We couple new deep Multi Unit Spectroscopic Explorer spectroscopic observations with existing Hubble Space Telescope imaging data to spectroscopically measure the line-of-sight (LOS) heliocentric velocity and velocity dispersion in two fields at a galactocentric distance of 12.2 kpc (outer disk field) and 34.6 kpc (stellar halo field) along NGC 4945 major axis, by stacking individual spectra of red giant branch and asymptotic giant branch stars. We obtain a LOS velocity and dispersion of 673+/-11 km/s and 73+/-14 km/s, respectively, for the outer disk field. This is consistent with the mean HI velocity of the disk at that distance. For the halo field we obtain a LOS velocity and dispersion of 519+/-12 km/s and 42+/-22 km/s. The halo fields' velocity measurement is within ~40 km/s from the systemic LOS velocity of NGC 4945, which is 563 km/s, suggesting that its stellar halo at 34.6 kpc along the major axis is counter-rotating and is of likely accretion origin. This provides the first ever kinematic measurement of the stellar halo of a Milky Way-mass galaxy outside the Local Group from its resolved stellar population, and establishes a powerful technique for measuring the velocity field of the stellar halos of nearby galaxies.

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arXiv2024

The Smallest Scale of Hierarchy Survey (SSH) III. Dwarf-dwarf satellite merging phenomena in the low-mass regime

We present new deep, wide-field Large Binocular Telescope (LBT) $g$ and $r$ imaging data from the Smallest Scale of Hierarchy Survey (SSH) revealing previously undetected tidal features and stellar streams in the outskirts of six dwarf irregular galaxies (NGC 5238, UGC 6456, UGC 6541, UGC 7605, UGC 8638, and UGC 8760) with stellar masses in the range $1.2 \times 10^7$ M$_{\odot}$ to $1.4 \times 10^8$ M$_{\odot}$. The six dwarfs are located 1-2 Mpc away from large galaxies, implying that the observed distortions are unlikely to be due to tidal effects from a nearby, massive companion. At the dwarfs' distances of $\sim$3-4 Mpc, the identified tidal features are all resolved into individual stars in the LBT images and appear to be made of a population older than 1-2 Gyr, excluding the possibility that they result from irregular and asymmetric star formation episodes that are common in gas-rich dwarf galaxies. The most plausible explanation is that we are witnessing the hierarchical merging assembling of these dwarfs with their satellite populations, a scenario also supported by the peculiar morphology and disturbed velocity field of their HI component. From the SSH sample we estimate a fraction of late type dwarfs showing signs of merging with satellites of $\sim$13\%, in agreement with other recent independent studies and theoretical predictions within the $Λ$CDM cosmological framework.

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arXiv2024

WST -- Widefield Spectroscopic Telescope: addressing the instrumentation challenges of a new 12m class telescope dedicated to widefield Multi-object and Integral Field Spectroscopy

WST - Widefield Spectroscopic Telescope: We summarise the design challenges of instrumentation for a proposed 12m class Telescope that aims to provide a large (>2.5 square degree) field of view and enable simultaneous Multi-object (> 20,000 objects) and Integral Field spectroscopy (inner 3x3 arcminutes field of view), initially at visible wavelengths. For the MOS mode, instrumentation includes the fiber positioning units, fiber runs and the high (R~40,000) and low (R~3,000 - 4,000) resolution spectrographs. For the MUSE like Integral Field Spectrograph, this includes the relay from the Telescope Focal Plane, the multi-stage splitting and slicing and almost 150 identical spectrographs. We highlight the challenge of mass production at a credible cost and the issues of maintenance and sustainable operation.

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