arXiv subjects

Katja Fahrion

Publications and source records attributed to Katja Fahrion.

At least 19 recordsLinked to original sources

arXiv2026

Semiresolved Stellar Populations as Distance Indicators

Galaxy distances are central to our understanding of the Universe. Despite the success of existing approaches, independent and complementary methods remain valuable for testing systematic effects and extending the applicability range of different distance metrics. Here we demonstrate that the spectrum of an individual semi-resolved stellar population encodes direct information about its distance and that valuable distance constraints can, in principle, be obtained by combining multiple independent measurements. When applied to optical spectra of bulge of the Andromeda galaxy (M\,31), we derive a stellar population-based distance of 756$\pm$57 kpc ($μ_0 = 24.39 \pm 0.15$), in agreement with state-of-the-art measurements. Crucially, semi-resolved stellar population distances does not require secondary calibrations, although its absolute scale remains conditional on the adopted stellar population models. Our findings provide a first assessment of the feasibility of deriving stellar population-based distance estimates from simple stellar population models, motivating further tests of the broader applicability and precision of the method.

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arXiv2026

The Fornax Cluster VLT Spectroscopic Survey - V. Mass modelling of the BCG NGC 1399 out to 150 kpc

NGC 1399, the bright central galaxy of the Fornax cluster, hosts an extensive population of globular clusters (GCs) extending beyond 200 kpc into its outer halo, about 6.5 effective radii (Re). We conducted dynamical mass modelling of NGC 1399 out to 5 Re (~150 kpc), using a comprehensive radial velocity catalogue of GCs, combining data from the Fornax cluster VLT spectroscopic survey (FVSS) with prior measurements from the literature. Applying spherical Jeans equations, we performed dispersion-kurtosis modelling of NGC 1399's GC kinematics to derive its mass profile and GC orbital anisotropy. We also investigated the impact of including intra-cluster GCs in the mass modelling, selected based on spatial segregation and a velocity $σ$-clipping method. Our findings indicate that both cusp-like (NFW) and core-like (Burkert) halos can reproduce the observed kinematics of GCs. Including the intra-cluster GCs in the mass modelling yields broadly consistent mass profiles for both halo types, with the full GC sample giving a virial mass of log $\log M_{\rm vir} = 13.81 \pm 0.09~M_{\odot}$ for the NFW halo, slightly higher than the value inferred for the Burkert halo. Regardless of the dark matter halo profile adopted, we observe that the intra-cluster GCs exhibit radial anisotropy in the outskirts, especially among the blue, metal-poor GCs. These results suggest that GCs in NGC 1399's outer halo are influenced by an additional halo component associated with the galaxy cluster potential. The radial anisotropy observed in these outer-halo GCs provides strong dynamical evidence supporting their accretion from external sources, consistent with prior photometric and spectroscopic studies of GCs. This study highlights the importance of including intra-cluster GCs in dynamical mass modelling to fully account for the contribution of dark matter to the cluster gravitational potential.

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arXiv2026

FASTAR -- II. Semi-resolved evolutionary stellar population models

Standard evolutionary synthesis models rely on the assumption of a fully sampled stellar initial mass function (IMF). Under this assumption, the age, chemical composition, and IMF uniquely define the predicted absorption spectra. However, with current instrumentation pushing observations towards higher spatial resolutions and lower surface brightnesses, the assumption of a fully sampled IMF does not always hold true. Here we present the semi-resolved version of the FASTAR models, a comprehensive set of evolutionary synthesis predictions able to reproduce the stochastic behavior of discretely-sampled IMFs. Semi-resolved FASTAR predictions share the same evolutionary principles, ingredients, and features of the integral (fully sampled IMF) version of the FASTAR models, expanding a range of ages from 20 Myr to 14 Gyr, metallicities between -2.5 < [M/H] < +0.3, and several IMF functional forms. Detailed spectroscopic measurements can be carried out within the 3,540-7,400 A wavelength range, and low-resolution spectral energy distributions can also be synthesized over a wider 2,000-to-12,000 A coverage. Semi-resolved FASTAR models also depend on the number of stars contributing to the observed spectra, which determines the effective sampling of the different stellar evolutionary phases along the isochrones. This incomplete sampling implies that semi-resolved FASTAR models are inevitably stochastic. On top of the inherent stochasticity of the models, derived quantities such as equivalent widths, colors, or mass-to-light ratios might present strong deviations compared to standard fully sampled simple stellar population models. This stochasticity dilutes the boundary between model predictions and data, promoting new sampling-based inference approaches. FASTAR semi-resolved models allow for the effective exploration of the parameter space thanks to their optimized, JAX-based computation.

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arXiv2026

FASTAR -- I. Continuous and differentiable evolutionary stellar population models

The development of evolutionary stellar population models is central to interpreting observations of galaxies in terms of astrophysical quantities. Stellar population models must therefore be both accurate and compatible with inversion algorithms in order to extract meaningful information from the observed data. Here we present FASTAR, a fully differentiable stellar population synthesis code. Contrary to traditional, grid-based single stellar population models, FASTAR can be continuously evaluated at any age (between 20 Myr and 14 Gyr), metallicity (-2.5 < [M/H] < +0.3), and initial mass function (IMF). Changes in the IMF parameterization are straightforward, allowing for consistent conversions of colors, magnitudes, and mass-to-light ratios, as well as the synthesis of models under the assumption of arbitrary IMF functional forms. FASTAR provides detailed spectroscopic predictions over the MILES wavelength range (3,540-7,400 A) as well as more coarsely sampled spectral energy distributions across a wider 2,000-to-12,000 A, which can be directly convolved with any arbitrary set of photometric filters. FASTAR performs at the same level of state-of-the-art simple stellar population models benchmarked against observations of globular clusters and high signal-to-noise spectra of early-type galaxies, but it is faster, lighter, and more flexible. Moreover, its differentiable nature allows for a quantitative understanding of model behavior and uncertainties, as well as a natural framework for gradient descent inference algorithms.

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arXiv2026

The complex history of NGC 1427A revealed by its star clusters and star formation history

Star-forming low-mass galaxies in the dense environments of galaxy clusters provide opportunities to study how environmental effects such as ram-pressure stripping, tidal interactions, or galaxy mergers shape a galaxy's star formation history. We combined integral-field spectroscopic observations with the Multi Unit Spectroscopic Explorer (MUSE) and available multi-band imaging of the star-forming galaxy NGC 1427A, located near the centre of the Fornax galaxy cluster, at a distance of 20 Mpc. Our aim was to trace the evolutionary history of NGC 1427A using the star formation history reconstructed from the integrated spectra and employing star clusters as surviving tracers of past star formation episodes. We fitted the spectral energy distribution of 222 star cluster candidates using archival $u,g,r$, and $i$ photometry to derive the ages and masses. For 58 clusters, we additionally incorporated their MUSE spectra in the fits and found an encouraging agreement between the photometric and spectroscopic results. The comparison of the age distribution of star clusters with star formation histories from a full spectrum fitting of the MUSE data found a reasonable agreement, with evidence for multiple episodes of star formation throughout the history of NGC 1427A. In particular, we found a population of young clusters ($\sim$ 10 Myr) that is located along the star formation edge and within the northern object, and a population of intermediate-age clusters ($\sim$ 100 - 300 Myr) with corresponding peaks in the star formation history of NGC 1427A. We interpret these populations in the context of the orbital evolution of NGC 1427A in the Fornax cluster and conclude that this galaxy has experienced not only ram-pressure stripping, but also tidal interactions or even a minor galaxy merger. The northern object is likely a regular component of the galaxy.

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arXiv2026

Orbital decomposition of the nuclear regions in the early-type galaxy FCC 47: Unveiling the nuclear cluster origin

Nuclear star clusters (NSCs) are among the densest stellar systems in the Universe and often coexist with supermassive black holes (SMBHs) at galaxy centres. While SMBH formation histories are essentially lost, NSCs preserve evolutionary imprints through their stellar populations and stellar kinematics, reflecting the cumulative effects of mergers, accretion, and internal dynamical evolution. We aim to investigate the orbital structure of the unusually large NSC in FCC 47 (NGC 1336) by decomposing its stellar orbits into dynamically distinct components. We extract stellar kinematics, and in particular the line-of-sight velocity distributions (LOSVDs), from VLT/MUSE integral-field spectroscopy using the non-parametric Bayes-LOSVD approach, and apply triaxial Schwarzschild orbit-superposition modelling with the DYNAMITE software. We decompose the orbit library into hot, warm, cold, and counter-rotating components. We detect triple-peaked LOSVDs in the nucleus, indicating a complex orbital structure. The NSC forms a counter-rotating, kinematically decoupled component. A hot pressure-supported component, a warm counter-rotating structure and a counter-rotating cold disk in the centre suggest hierarchical assembly via early star cluster accretion and later in situ star formation. Our orbital decomposition of FCC 47 supports a hybrid formation scenario for this NSC. Dynamically distinct substructures reflect the interplay of accretion and in situ star formation during galaxy evolution.

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arXiv2026

Distance measurements from the internal dynamics of globular clusters: Application to the Sombrero galaxy (M104)

Globular clusters (GCs) are dense star clusters found in all massive galaxies. Recent work has established that they follow a tight relation between their internal stellar velocity dispersion $σ$ and luminosity, enabling accurate distance measurements. In this work, we aim to apply this GC velocity dispersion (GCVD) distance method to measure the distance to M104 (NGC 4594, the Sombrero galaxy). We have measured internal stellar velocity dispersions for 85 globular clusters (GCs) and one ultra-compact dwarf galaxy around M104 using high-resolution multi-object integrated-light spectroscopy with FLAMES/GIRAFFE on the Very Large Telescope. The measured velocity dispersions range from $σ= 4 - 30$ km s$^{-1}$, with a mean uncertainty of $Δσ= 2.5$ km s$^{-1}$. For a subset of 77 GCs with $V$-band magnitudes and reliable velocity dispersion measurements above $σ> 4$ km s$^{-1}$, we constructed the $M_V$-$σ$ relation to measure the distance to M104, finding $D=9.00\pm0.29$ (stat.)~$\pm0.26$ (sys.) Mpc. The GCs follow the Milky Way and M31 $M_V-σ$ relation closely, with the exception of the luminous ultra-compact dwarf SUCD1, which is nearly one magnitude brighter than the mean relation. 29 GCs in the sample have sizes determined from Hubble Space Telescope imaging which allowed us to determine their masses and $V$-band dynamical mass-to-light ratios (M/L$_V$). We find a mean $ = 2.6 M_{\odot}/L_{\odot}$ for the luminous ($M_V < -8$ mag) M 104 GCs, which is higher than the Milky Way GCs, but is reminiscent of the brightest GCs in Centaurus A. With the exception of SUCD1, the GCs of M104 follow the GCVD relation irrespective of their mass-to-light ratio.

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arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: A Low Surface Brightness Science Case

The Extragalactic Low Surface Brightness (LSB, $μ_V\gtrsim 27$ mag/arcsec$^2$) Universe represents a crucial, yet largely unseen, frontier in modern astrophysics. This faint realm holds the keys to completing our understanding of galaxy evolution, hierarchical assembly, and even the fundamental nature of dark matter. Our current theoretical models are inherently incomplete, largely mirroring the properties of the brightest, most easily observed objects. To overcome this critical bias and unlock the secrets of this realm, a transformative leap in observational capability is required. A 30 to 40m class telescope, leveraging unprecedented sensitivity and spatial resolution, especially with adaptive optics, is the essential tool to fundamentally probe these faint, low-density stellar regimes. This white paper details the transformative LSB science that such a facility, strategically positioned in the Northern Hemisphere (NH) to access crucial nearby structures and rich environments, can achieve.

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arXiv2025

Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: Mapping formation pathways of nuclear star clusters across galaxies

Nuclear star clusters (NSCs) are dense, compact stellar systems only a few parsecs across, located at galaxy centers. Their small sizes make them difficult to resolve spatially. NSCs often coexist with massive black holes, and both trace the dynamical state and evolution of their host galaxies. Dense stellar environments such as NSCs are also ideal sites for forming intermediate-mass black holes (IMBHs). To date, spatially resolved NSC properties, crucial for reconstructing dynamical and star-formation histories, have only been obtained for galaxies within 5 Mpc, using the highest-resolution instruments on the current class of very large telescopes. This severely limits spectroscopic studies, and a systematic, unbiased survey has never been accomplished. Because the vast majority of known NSCs are located in the Northern Hemisphere, only a 30-m-class telescope in the North can provide the statistical power needed to study their physical properties and measure the mass of coexisting central black holes. We propose leveraging the capabilities of a 30-m-class Northern telescope to obtain the first comprehensive, spatially resolved survey of NSCs, finally allowing us to unveil their formation pathways and their yet unknown connection with central massive black holes.

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arXiv2025

Looking into the faintEst WIth MUSE (LEWIS): Exploring the nature of ultra-diffuse galaxies in the Hydra-I cluster IV. A study of the Globular Cluster population in four UDGs

As old stellar systems, globular clusters (GCs) are key fossil tracers of galaxy formation and interaction histories. This paper is part of the LEWIS project, an integral-field spectroscopic survey of ultra-diffuse galaxies (UDGs) in the Hydra I cluster. We use MUSE spectroscopy and new VIRCAM $H$-band imaging data to study the GC populations and dark matter content in four dwarf galaxies. We retrieved line-of-sight velocities for all sources in the observed MUSE fields. Since the spectroscopic measurements are limited to relatively bright sources, we developed a multi-band photometric procedure to identify additional GC candidates too faint for spectroscopic confirmation. GC candidates were selected using a combination of photometric properties and morphometric criteria. Additionally, the $H$-band observations were used to constrain the stellar masses of the studied galaxies. Based on the spectroscopic classification, we confirm one GC in UDG3, two in UDG7, and four in UDG11, while UDG9 has no spectroscopically confirmed bright GCs. We identify four intra-cluster GCs in the vicinity of UDG3 and UDG11, and one ultra-compact dwarf with a radial velocity only $Δv = -85 \pm 10\mathrm{km\ s^{-1}}$ relative to UDG7, suggesting it may be bound to it. Considering completeness corrections and accounting for possible contamination, from photometry we estimate that the number of GCs ranges between 0 and $\sim40$ for the investigated UDGs. Their specific frequencies suggest that three out of four UDGs are either GC-rich, similar to those in the Coma cluster, or belong to an intermediate population as seen in the Perseus cluster. Dark matter content estimates, inferred from GC counts and stellar mass, indicate that these galaxies are dark-matter dominated, with dynamical-to-stellar mass ratios of $M_{\mathrm{dyn}} / M_\star \sim 10-1000$.

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arXiv2025

First detection of ices in intermediate-mass young stellar objects beyond the Milky Way

Using NIRSpec on JWST, we studied a sample of 15 intermediate-mass (1.8-4.1 Msun) young stellar objects (YSOs) previously identified with MIRI photometry in the low-metallicity NGC 346 star-forming cluster in the Small Magellanic Cloud (SMC). All objects, observed in the 1.7-5.3 micron range, show strong hydrogen recombination lines in the Paschen, Brackett, Pfund, and Humphreys series, confirming their very young ages. The spectra of 11 YSOs show prominent absorption bands from the three most important ice species (H2O, CO2, CO), marking the first detection of these ices in intermediate-mass YSOs beyond our Galaxy. In three YSOs, water ice appears to be in crystalline form. In some objects, we also detect 13CO2 and OCS ices -- never before observed beyond the Milky Way (MW) -- and methanol ice in at least one star. We compared the column densities of H2O, CO2, and CO ices with those measured in more and less massive protostars in the MW and Large Magellanic Cloud, finding that in NGC 346 ice column densities reach values nearly an order of magnitude lower than in more massive objects (~1x10^{17} cm-2 for water and ~1x10^{16} cm-2 for CO2 and CO). However, the relative proportions of the ice species abundances do not differ from those in massive MW YSOs. This suggests that metallicity may not significantly affect ice chemistry in protoplanetary discs and that, shielded by the protostellar envelope or deep in the midplane, circumstellar material is likely impervious to the radiation environment.

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arXiv2025

Globular clusters in M104: Tracing kinematics and metallicities from the centre to the halo

As ancient star clusters, globular clusters (GCs) are regarded as powerful tracers of galaxy evolution and assembly. Due to their brightness and compact sizes, GCs are employed to probe the kinematics and stellar population properties of galaxies, from the central regions out into the halo where the underlying stellar light becomes too faint for spectroscopic studies. In this work, we present a comprehensive study of the GC system of M 104 (NGC 4594, also known as the Sombrero galaxy) based on literature spectroscopic catalogues and newly collected data from Very Large Telescope (VLT) MUSE integral-field spectroscopy combined with multi-object spectroscopy from VLT FLAMES and OSIRIS at the Gran Telescopio de Canarias (GTC). We present a new catalogue of 499 GCs with radial velocity measurements that span from the inner disc region out to $\sim$ 70 kpc (24$^{\prime}$). In addition to velocities, we measure metallicities from the MUSE, OSIRIS, and FLAMES spectra of 190 GCs. Together with literature values, we collected a sample of 278 metallicities. Comparing GCs observed with multiple instruments, we find a good agreement of velocity and metallicity measurements. Studying GC kinematics with a simple model confirms a decreasing velocity dispersion profile and low rotation velocities. The blue GCs appear to be more dispersion-dominated, while the red GCs follow the kinematics of the stars more closely. We find a large scatter of GC metallicities with distance from the centre and metal-rich GCs are found over all radii. We discuss that the GC metallicity distribution with a broad metal-poor component likely reflects the complex assembly history of M 104.

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arXiv2025

GAMA 526784: the progenitor of a globular cluster-rich ultra-diffuse galaxy? I. Star clusters, stellar body and ionised gas properties

Context. Ultra-diffuse galaxies (UDGs) are an intriguing population of galaxies. Despite their dwarf-like stellar masses and low surface brightness, they have large half-light radii and exhibit a diverse range of globular cluster (GC) populations. Some UDGs host many GCs while others have none, raising questions about the conditions under which star clusters (SC) form in dwarfs. GAMA526784, an isolated UDG with both an old stellar body and an extended star-forming (SF) front, including many young SCs, provides an exceptional case to explore the link between UDG evolution and star cluster formation. Aims. This study investigates the stellar populations, SCs, ionised gas, and kinematics of GAMA526784, focusing on its potential to form massive GCs and its connection to broader UDG formation scenarios. Methods. Imaging from HST and Subaru/HSC, alongside MUSE spectroscopy, were used to analyse the galaxy's morphology, chemical composition, and kinematics. A combination of SED fitting and full spectral fitting was applied. Results. GAMA526784's central stellar body exhibits a low-metallicity ([M/H] ~ -1.0 dex) and an old age (~9.9 Gyr). The outskirts are much younger (~0.9 Gyr), but slightly more metal-poor ([M/H] ~ -1.2 dex). The stellar kinematics show low velocity dispersions (~10 km/s) and a coherent rotational field, while the ionised gas exhibits higher dispersions (~50 km/s), a misaligned rotation axis (~20 deg) and localised SF, what could be suggestive of a recent interaction. The young SCs span ages of 8-11 Myr and masses of log(M*/Mo)~5.0, while the old GCs have ~9 Gyr and stellar masses of log(M*/Mo)~5.5. Conclusions. GAMA526784's properties point to interactions that triggered localised SF, leading to the formation of young SCs. Future observations of its molecular and neutral gas will help assess its environment, and the trigger of this SF episode.

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arXiv2025

The JWST/NIRSpec view of the nuclear region in the prototypical merging galaxy NGC 6240

Merger events are thought to be an important phase in the assembly of massive galaxies. At the same time, Active Galactic Nuclei (AGN) play a fundamental role in the evolution of their star formation histories. Both phenomena can be observed at work in NGC 6240, a local prototypical merger, classified as an UltraLuminous InfraRed Galaxy (ULIRG) thanks to its elevated infrared luminosity. Interestingly, NGC 6240 hosts two AGN separated by 1.5''(~ 735 pc), detected in both X-ray and radio band. Taking advantage of the unprecedented sensitivity and wavelength coverage provided by the Integral Field Unit (IFU) of the NIRSpec instrument onboard JWST, we observed the nuclear region of NGC 6240 in a FoV of 3.7'' x 3.7''(1.9 x 1.9 kpc^2), to investigate gas kinematics and InterStellar Medium (ISM) properties with a high spatial resolution of ~ 0.1'' (or ~ 50 pc). We separated the different gas kinematic components through multi-Gaussian fitting and studied the excitation properties of the ISM from the NIR diagnostic diagram based on the H_2 1-0 S(1)/BrGamma and [Fe II]1.257micron/PaBeta lines ratios. We isolated the ionization cones of the two nuclei, and detected coronal lines emission from both of them. Using H_2 line ratios, we found that the molecular hydrogen gas is excited mostly by thermal processes. We computed a hot molecular gas mass of 1.3 x 10^5 M_sun and an ionized gas mass in the range of 10^5 - 10^7 M_sun. We studied with unprecedented spatial resolution and sensitivity the kinematics of the molecular and ionized gas phases. We revealed the complex structure of the molecular gas and found a blueshifted outflow near the Southern nucleus, together with filaments connecting a highly redshifted H_2 cloud with the two nuclei. We speculate on the possible nature of this H_2 cloud and propose two possible scenarios: either outflowing gas, or a tidal cloud falling onto the nuclei.

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arXiv2025

MUSE observations of dwarf galaxies and a stellar stream in the M83 group

Spectroscopy for faint dwarf galaxies outside of our own Local Group is challenging. Here, we present MUSE spectroscopy to study the properties of four known dwarf satellites and one stellar stream (KK208) surrounding the nearby grand spiral M83, which resides together with the lenticular galaxy Cen A in the Centaurus group. This data complete the phase-space information for all known dwarf galaxies around M83 down to a completeness of $-$10 mag in the $V$ band. All studied objects have an intermediate to old and metal-poor stellar population and follow the stellar luminosity-metallicity relation as defined by the Local Group dwarfs. For the stellar stream we serendipitously identify a previously unknown globular cluster, which is old and metal-poor. Two dwarf galaxies (NGC5264 and dw1341-29) may be a bound satellite of a satellite system due to their proximity and shared velocities. Having access to the positions and velocities of 13 dwarfs around M83, we estimate the mass of the group with different estimators. Ranging between 1.3 and $3.0 \times 10^{12}$ M$_\odot$ for the halo mass we find it to be larger than previously assumed. This may impact the previously reported tension for cold dark matter cosmology with the count of dwarf galaxies. In contrast to Cen A, we do not find a co-rotating plane-of-satellites around M83.

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arXiv2025

A new way to measure the distance to NGC1052-DF2

We employ a new way to measure the distance to NGC1052-DF2 via internal stellar velocity dispersions ($σ$) of its globular clusters (GCs). We obtained deep (15.1h), R=18,200, Ca Triplet integrated-light spectra for 10 GCs in NGC1052-DF2 using FLAMES GIRAFFE on VLT. For five GCs we measure $σ$, along with precision velocities for the whole sample. We also present a new photometric analysis based on 40 orbits of archival Hubble Space Telescope imaging for 16 spectroscopically confirmed GCs. Assuming that the NGC1052-DF2 GCs obey the $M_V$ -- log($σ$) relation followed by the Milky Way and M31 GCs, the NGC1052-DF2 GCs give a distance, $d=16.2\pm1.3$ (stat.) $\pm1.7$ (sys.) Mpc. By contrast, using a literature distance of $d=21.7$ Mpc from forward modelling of the TRGB, the GCs lie above the Milky Way + M31 relation by $\sim0.6$ magnitudes. For a shorter literature distance of 13 Mpc, the GCs fall below the relation by $\sim0.4$ mag. At $d = 16.2$ Mpc, we obtain mean dynamical $M/L_V = 1.61\pm0.44 M_\odot/L_\odot$, and median half-light radii, $r_h =3.0\pm0.5$ pc. This is entirely consistent with Milky Way GCs, with mean $M/L_V = 1.77\pm0.10 M_\odot/L_\odot$, median $r_h =3.2\pm0.6$ pc. For the further distance of 21.7 Mpc, we obtain low $M/L_V$ ratios ($M/L_V = 1.19\pm0.33 M_\odot/L_\odot$) which could suggest ages of $\sim6$ Gyr.. Such young ages are inconsistent with our MUSE stellar population (companion paper, Fahrion et al.) analysis of the NGC1052-DF2 GCs which indicates they are $\sim10$ Gyr old. For $d = 16.2$ Mpc, coupled with our new photometry, we find that the properties of the GCs in NGC1052-DF2 appear entirely consistent with those in the Milky Way and other Local Group galaxies. In order to reconcile the further distance with our results, a mass function more dwarf-depleted than the Milky Way GCs must be invoked for the GCs of NGC1052-DF2.

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arXiv2024

Protoplanetary discs around sun-like stars appear to live longer when the metallicity is low

Previous Hubble Space Telescope (HST) observations of the star-forming cluster NGC 346 in the Small Magellanic Cloud (SMC) had revealed a large population of pre-main sequence (PMS) candidates, characterised by Halpha excess emission in their photometry. However, without access to spectroscopy, the nature of these objects remained unclear. Using the NIRSpec instrument on board JWST, we studied a sample of these stars, with masses in the range ~0.9-1.8 Msun, effective temperatures in the range 4,500-8,000 K, and PMS ages between ~0.1 and 30 Myr. Here we present the first spectra of solar-mass PMS stars in the metal-poor SMC (Z=1/8 Zsun) and discuss the physical properties of ten representative sources with good signal-to-noise ratio. The observations indicate that even the oldest of these PMS candidates are still accreting gas with typical rates of ~10^{-8} Msun/yr for stars older than ~10 Myr, confirming their PMS nature. The spectra also reveal near-infrared excess and molecular hydrogen excitation lines consistent with the presence of discs around these stars. These findings suggest that in a low-metallicity environment circumstellar discs can live longer than previously thought.

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arXiv2024

Young Stellar Objects in NGC 346: A JWST NIRCam/MIRI Imaging Survey

We present a JWST imaging survey with NIRCam and MIRI of NGC 346, the brightest star-forming region in the Small Magellanic Cloud (SMC). By combining aperture and point spread function (PSF) photometry of eleven wavelength bands across these two instruments, we have detected more than 200,000 unique sources. Using near-infrared (IR) color analysis, we observe various evolved and young populations, including 196 young stellar objects (YSOs) and pre-main sequence stars suitable for forthcoming spectroscopic studies. We expand upon this work, creating mid-IR color-magnitude diagrams and determining color cuts to identify 833 reddened sources which are YSO candidates. We observe that these candidate sources are spatially associated with regions of dusty, filamentary nebulosity. Furthermore, we fit model YSO spectral energy distributions (SEDs) to a selection of sources with detections across all of our MIRI bands. We classify with a high degree of confidence 23 YSOs in this sample and estimate their radii, bolometric temperatures, luminosities, and masses. We detect YSOs approaching 1 solar mass, the lowest-mass extragalactic YSOs confirmed to date.

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