arXiv subjects

Sébastien Comerón

Publications and source records attributed to Sébastien Comerón.

At least 19 recordsLinked to original sources

arXiv2026

The Infrared Glow of Galactic Outskirts: A New Window with NASIM

The outskirts of galaxies retain fossil records of their assembly history and are dominated by old stellar populations shaped by tidal interactions, accretion events, and environmental processes. Observing these low-surface-brightness (LSB) regions in the near-infrared (NIR), particularly in the $K_s$ band, is crucial for tracing evolved stars and minimising the effects of dust. Yet, ground-based NIR studies of faint extended structures remain challenging because of the bright and variable sky background and complex detector systematics. To overcome these limitations, we developed NASIM, the Near-infrared Automated low Surface brightness reduction In Maneage pipeline, a fully automated and reproducible framework designed to recover faint structures in VISTA/VIRCAM data. Using GNU Astronomy Utilities (Gnuastro), NASIM corrects large-scale flat-field patterns and instrumental signatures across the VIRCAM detectors while preserving LSB emission that can be strongly suppressed in conventional NIR reductions. We apply NASIM to $K_s$-band imaging of the Euclid Deep Field South through the KEDFS programme, a key region also targeted by Rubin/LSST and Euclid. The final reduction reaches a surface-brightness limit of $μ_{K_s} \simeq 27.7$ mag arcsec$^{-2}$, measured at $3σ$ over 100 arcsec$^2$, approximately 67 times deeper than 2MASS and 11 times deeper than VHS in surface-brightness sensitivity. Direct comparisons with publicly available VISTA products show that NASIM preserves diffuse emission without substantially compromising compact-source depth. These results open a deep NIR view of faint galaxy outskirts, LSB galaxies, and intracluster light, providing a reproducible ground-based foundation for future studies of the faint Universe.

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arXiv2026

HI-bearing dark galaxies predictions from constrained Local Group simulations: how many and where to find them

Dark galaxies are small, DM-dominated halos whose gas remains in hydrostatic and thermal equilibrium and has never formed stars. They are of particular interest because they represent a strong prediction of the LCDM model. As of today, only a handful of candidates have been detected, the most intriguing of which being Cloud-9. Using several state-of-the-art hydrodynamical simulations, we aim to predict the abundance of dark galaxies expected within our Local Group (LG), characterise their properties and provide guidance for their potential detection. We analyse LG simulations with constrained initial conditions, run with different codes, implementing different baryonic physics, feedback prescriptions, and employing two distinct values of SF density threshold, n_th=0.13 and 10 cm^-3, to select samples of dark and bright galaxies harboured in haloes of similar mass. We demonstrate that dark galaxies exist in such simulations, though their number is larger in simulations that use a higher, more realistic n_th. These galaxies, whose gas remains diffuse and never forms stars, predominantly inhabit less-concentrated, higher-spin DM halos than their luminous counterparts. Dark galaxies are typically found in low-density regions at the outskirts of the LG, and their evolution across z indicate that both the DM and gas densities in their surroundings were consistently lower than those found around bright galaxies, making them less susceptible to interactions, mergers, or gas inflows. We estimate that up to 8 dark galaxies should be detectable in HI emission within 2.5 Mpc of the MW, with the FAST telescope, accounting for its sky coverage and minimum M_HI and N_HI. Current hydrodynamical simulations of galaxies, combined with upcoming HI surveys, will offer a direct and powerful test of LCDM through their ability to predict and measure properties of dark galaxies within and beyond the LG.

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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

NASIM: Revealing the low surface brightness Universe from legacy VISTA data

Near-infrared imaging is a powerful technique in observational astronomy, but the bright background, primarily from the Earthś atmosphere, makes the detection of faint features particularly challenging. To recover low surface brightness (LSB) structures in such data, we present NASIM (Near-infrared Automated low Surface brightness reduction In Maneage), a fully automated and reproducible data reduction pipeline optimised for VISTA/VIRCAM observations. NASIM builds on GNU Astronomy Utilities (Gnuastro) to effectively remove large-scale instrumental artefacts while preserving faint, diffuse emission. As a key science application, we focus on deep Ks-band observations of the Euclid Deep Field South (KEDFS), one of the deepest VISTA/VIRCAM datasets and a high-priority field for synergy with current and future facilities, including Euclid, JWST, LSST, Roman, Spitzer, and ALMA. With VIRCAM no longer operational, KEDFS now stands as a unique legacy dataset. We release selected tiles from the KEDFS survey and highlight science cases, including galaxy outskirts, LSB galaxies, and intracluster light, that demonstrate NASIMś ability to recover diffuse structures. A direct comparison with conventional VISTA data reduction pipelines demonstrates the advantages of NASIM in preserving diffuse emission without compromising compact source detection. All quantitative results presented in this paper are fully reproducible with Maneage (commit 4d32667).

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arXiv2025

The Intrinsic Flattening of Galaxy Disks

Highly inclined (edge-on) disk galaxies offer the unique perspective to constrain their intrinsic flattening, $c/a$, where $c$ and $a$ are respectively the vertical and long radial axes of the disk measured at suitable stellar densities. The ratio $c/a$ is a necessary quantity in the assessment of galaxy inclinations, three-dimensional structural reconstructions, total masses, as well as a constraint to galaxy formation models. 3.6 micron maps of 133 edge-on spiral galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) and its early-type galaxy extension are used to revisit the assessment of $c/a$ free from dust extinction and away from the influence of a stellar bulge. We present a simple definition of $c/a$ and explore trends with other galactic physical parameters: total stellar mass, concentration index, total HI mass, mass of the central mass concentration, circular velocity, model-dependent scales, as well as Hubble type. Other than a dependence on early/late Hubble types, and a related trend with light concentration, no other parameters were found to correlate with the intrinsic flattening of spiral galaxies. The latter is mostly constant with $\langle c/a \rangle$ = 0.124 $\pm$ 0.001 (stat) $\pm$ 0.033 (intrinsic/systematic) and greater for earlier types.

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arXiv2024

Assessing the accuracy of the star formation rate measurements by direct star count in molecular clouds

Star formation estimates based on the counting of YSOs is commonly applied to nearby star-forming regions in the Galaxy. With this method, the SFRs are measured using the counts of YSOs in a particular protostellar Class, a typical protostellar mass, and the lifetime associated with this Class. However, the assumptions underlying the validity of the method such as that of a constant star formation history (SFH) and whether the method is valid for all protostellar Classes has never been fully tested. In this work, we use Monte Carlo models to test the validity of the method. We build synthetic clusters in which stars form at times that are randomly drawn from a specified SFH. The latter is either constant or time-dependent with a burst like behavior. The masses of the protostars are randomly drawn from an IMF which can be either similar to that of the Milky Way field or be variable . For each star in every cluster, the lifetimes associated with the different protostellar classes are also randomly drawn from Gaussian distribution functions centered around their most likely value as suggested by the observations. We find that only the SFR derived using the Class 0 population can reproduce the true SFR at all epochs, and this is true irrespective of the shape of the SFH. For a constant SFH, the SFR derived using the more evolved populations of protostars (Classes I, F, II, and III) reproduce the real SFR only at later epochs which correspond to epochs at which their numbers have reached a steady state. For a time-dependent burst-like SFH, all SFR estimates based on the number counts of the evolved populations fail to reproduce the true SFR. We also show how the offsets between Class I and Class II based SFRs and the true SFR plotted as a function of the number ratios of Class I and Class II versus Class III YSOs can be used in order to constrain the SFH of observed molecular clouds.

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arXiv2024

Gas-Phase metallicity for the Seyfert galaxy NGC 7130

Metallicity measurements in galaxies can give valuable clues about galaxy evolution. One of the mechanisms postulated for metallicity redistribution in galaxies is gas flows induced by AGN, but the details of this process remain elusive. We report the discovery of a positive radial gradient in the gas-phase metallicity of the narrow line region of the Seyfert 2 galaxy NGC 7130, which is not found when considering the star-forming components in the galaxy disk. To determine gas-phase metallicities for each kinematic component, we use both active galactic nuclei (AGN) and star-forming (SF) strong-line abundance relations, as well as BPT diagnostic diagrams. These relations involve sensitive strong emission lines, namely [OIII]5007, [NII]6584, H$α$, H$β$, [SII]6716, and [SII]6731, observed with the adaptive-optics-assisted mode of the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT). The presence of a positive radial metallicity gradient only in the AGN ionized component suggests that metals may be transported from central areas to its purlieus by AGN activity.

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arXiv2023

The Tully-Fisher relation and the Bosma effect

We show that the rotation curves of 16 nearby disc galaxies in the THINGS sample and the Milky Way can be described by the NFW halo model and by the Bosma effect at approximately the same level of accuracy. The latter effect suggests that the behavior of the rotation curve at large radii is determined by the rescaled gas component and thus that dark matter and gas distributions are tightly correlated. By focusing on galaxies with exponential decay in their gas surface density, we can normalize their rotation curves to match the exponential thin disc model at large enough radii. This normalization assumes that the galaxy mass is estimated consistently within this model, assuming a thin disc structure. We show that this rescaling allows us to derive a new version of the Tully-Fisher (TF) relation, the Bosma TF relation that nicely fit the data. In the framework of this model, the connection between the Bosma Tully-Fisher (TF) relation and the baryonic TF relation can be established by considering an additional empirical relation between the baryonic mass and the total mass of the disc, as measured in the data.

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arXiv2023

Mapping non-axisymmetric velocity fields of external galaxies

Disk galaxies are typically in a stable configuration where matter moves in almost closed circular orbits. However, non-circular motions caused by distortions, warps, lopsidedness, or satellite interactions are common and leave distinct signatures on galaxy velocity maps. We develop an algorithm that uses an ordinary least square method for fitting a non-axisymmetric model to the observed two-dimensional line-of-sight velocity map of an external galaxy, which allows for anisotropic non-circular motions. The method approximates a galaxy as a flat disk, which is an appropriate assumption for spiral galaxies within the optical radius where warps are rare. In the outer parts of HI distributions, which may extend well into the warp region, we use this method in combination with a standard rotating tilted ring model to constrain the range of radii where the flat disk assumption can be conservatively considered valid. Within this range, the transversal and radial velocity profiles, averaged in rings, can be directly reconstructed from the velocity map. The novelty of the algorithm consists in using arc segments in addition to rings: in this way spatial velocity anisotropies can be measured in both components, allowing for the reconstruction of angularly resolved coarse-grained two-dimensional velocity maps. We applied this algorithm to 25 disk galaxies from the THINGS sample for which we can provide 2D maps of both velocity components.

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arXiv2023

The massive relic galaxy NGC 1277 is dark matter deficient. From dynamical models of integral-field stellar kinematics out to five effective radii

According to the $Λ$CDM cosmology, present-day galaxies with stellar masses $M_\star>10^{11} {\rm M}_\odot$ should contain a sizable fraction of dark matter within their stellar body. Models indicate that in massive early-type galaxies (ETGs) dark matter should account for $\sim60\%$ of the dynamical mass within five effective radii ($5 R_{\rm e}$). Most massive ETGs have been shaped through a two-phase process: the rapid growth of a compact core was followed by the accretion of an extended envelope through mergers. The exceedingly rare galaxies that have avoided the second phase, the so-called relic galaxies, are thought to be the frozen remains of the massive ETG population at $z\gtrsim2$. The best relic galaxy candidate discovered to date is NGC 1277, in the Perseus cluster. We used deep integral field GCMS data to revisit NGC 1277 out to an unprecedented radius of 6 kpc (corresponding to $5 R_{\rm e}$). By using Jeans anisotropic modelling we find a negligible dark matter fraction within $5 R_{\rm e}$ ($f_{\rm DM}(5 R_{\rm e})<0.05$; two-sigma confidence level), which is in tension with the expectation. Since the lack of an extended envelope would reduce dynamical friction and prevent the accretion of an envelope, we propose that NGC 1277 lost its dark matter very early or that it was dark matter deficient ab initio. We discuss our discovery in the framework of recent proposals suggesting that some relic galaxies may result from dark matter stripping as they fell in and interacted within galaxy clusters. Alternatively, NGC 1277 might have been born in a high-velocity collision of gas-rich proto-galactic fragments, where dark matter left behind a disc of dissipative baryons. We speculate that the relative velocities of $\approx2000 {\rm km/s}$ required for the latter process to happen were possible in the progenitors of the present-day rich galaxy clusters.

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arXiv2022

Launching the VASCO citizen science project

The Vanishing & Appearing Sources during a Century of Observations (VASCO) project investigates astronomical surveys spanning a time interval of 70 years, searching for unusual and exotic transients. We present herein the VASCO Citizen Science Project, which can identify unusual candidates driven by three different approaches: hypothesis, exploratory, and machine learning, which is particularly useful for SETI searches. To address the big data challenge, VASCO combines three methods: the Virtual Observatory, user-aided machine learning, and visual inspection through citizen science. Here we demonstrate the citizen science project and its improved candidate selection process, and we give a progress report. We also present the VASCO citizen science network led by amateur astronomy associations mainly located in Algeria, Cameroon, and Nigeria. At the moment of writing, the citizen science project has carefully examined 15,593 candidate image pairs in the data (ca. 10% of the candidates), and has so far identified 798 objects classified as "vanished". The most interesting candidates will be followed up with optical and infrared imaging, together with the observations by the most potent radio telescopes.

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arXiv2022

Linking star formation thresholds and truncations in the thin and thick disks of the low-mass galaxy UGC 7321

Thin and thick disks are found in most spiral galaxies, yet their formation scenarios remain uncertain. Whether thick disks form through slow or fast, internal or environmental, processes is unclear. The physical origin of outer truncations in thin and thick disks, observed as a drop in optical and near-infrared (NIR) surface brightness profiles, is also a much debated topic. These truncations have been linked to star formation (SF) thresholds in Milky-Way type galaxies, but no such connection has been made for their low-mass counterparts or in thick disks. Our photometric analysis of the edge-on galaxy UGC 7321 offers a possible breakthrough. This well-studied diffuse, isolated, bulgeless, ultra-thin galaxy is thought to be under-evolved both dynamically and in SF. It is an ideal target to disentangle internal effects in the formation of thick disks and truncations. Our axial light profiles from deep far- and near-ultraviolet (UV; GALEX) images, tracing recent SF, and optical (DESI grz) and NIR (Spitzer 3.6 microns) images, tracing old stellar populations, enable a detailed identification of an outer truncation in all probed wavelengths in both the thin and thick disks. After deprojecting to a face-on view, a sharp truncation signature is found at a stellar density of roughly 1.5 solar masses per square parsec, in agreement with theoretical expectations of gas density SF thresholds. The redder colours beyond the truncation radius are indicative of stellar migration towards the outer regions. We thus show that thick disks and truncations can form via internal mechanisms alone, given the pristine nature of UGC 7321. We report the discovery of a truncation at and above the mid-plane of a diffuse galaxy that is linked to a SF threshold; this poses a constraint on physically-motivated disk size measurements among low-mass galaxies.

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arXiv2020

The distribution of star formation in galactic bars as seen with H$α$ and stacked GALEX UV imaging

We investigate the spatial distribution of star formation (SF) within bars of nearby disk galaxies (inclination $< 65^{\circ}$) from the S$^4$G survey. We use archival GALEX far- and near-UV imaging for 772 barred galaxies. We also assemble a compilation of continuum-subtracted H$α$ images for 433 barred galaxies, of which 70 are produced by ourselves from ancillary photometry and MUSE/CALIFA IFU data cubes. We employ two complementary approaches: i) the analysis of bar/disk stacks built from co-added UV images of hundreds of galaxies; and ii) the classification of the morphology of ionised regions in galaxies into three main SF classes: A) only circumnuclear SF, B) SF at the bar ends, but not along the bar, and C) SF along the bar. Lenticular galaxies typically belong to SF class A: this is probably related to bar-induced SF quenching. The distribution of SF class B peaks for early- and intermediate-type spirals: this most likely results from the interplay of gas flow, shocks, and enhanced shear in centrally concentrated galaxies with large bar amplitudes. Late-type galaxies are mainly assigned to SF class C: we argue that this is a consequence of low shear. In bar stacks of spirals, the UV emission traces the stellar bars and dominates on their leading side, as witnessed in simulations. For early-types, the central UV emission is $\sim$0.5 mag brighter in strongly barred galaxies, relative to their weakly barred counterparts: this is related to the efficiency of strong bars sweeping the disk gas and triggering central starbursts. We also show that the distributions of SF in inner ringed galaxies are broadly the same in barred and non-barred galaxies, including a UV/H$α$ deficit in the middle part of the bar: this hints at the effect of resonance rings trapping gas. Distinct distributions of SF within bars are reported in galaxies of different morphological types (Abridged).

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arXiv2020

The reports of thick discs' deaths are greatly exaggerated: thick discs are NOT artefacts caused by diffuse scattered light

Recent studies have made the community aware of scattered light when examining low-surface-brightness galaxy features such as thick discs. In our past studies of the thick discs of edge-on galaxies in the Spitzer Survey of Stellar Structure in Galaxies (S$^4$G) we modelled the point spread function as a Gaussian. We re-examine our results using a revised point spread function model that accounts for extended wings out to more than 2.5arcmin. We study the $3.6μ{\rm m}$ images of 141 edge-on galaxies from the S$^4$G. We decompose the surface brightness profiles of the galaxies perpendicular to their mid-planes assuming that discs are made of two stellar discs in hydrostatic equilibrium. We decompose the axial surface brightness profiles of galaxies to model the central mass concentration - described by a Sérsic function - and the disc - described by a broken exponential disc. Our improved treatment confirms the ubiquity of thick discs. The main difference between our current fits and those presented before is that now the scattered light from the thin disc dominates the surface brightness at levels below $μ\sim26\,{\rm mag\,arcsec^{-2}}$. This does not affect drastically any of our previously presented results: 1) Thick discs are nearly ubiquitous. They are not an artefact caused by scattered light as has been suggested elsewhere. 2) Thick discs have masses comparable to those of thin discs in low-mass galaxies - circular velocities $v_{\rm c}<120\,{\rm km\,s^{-1}}$ - whereas they are typically less massive than the thin discs in high-mass galaxies. 3) Thick discs and central mass concentrations seem to have formed at the same epoch from a common material reservoir. 4) Roughly 60% of the up-bending breaks in face-on galaxies are caused by the superposition of a thin and a thick disc where the scale-length of the latter is the largest. (Abridged)

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arXiv2020

Examining supernova events in Type 1 active galactic nuclei

A statistical study of intermediate Palomar Transient Factory supernovae (SNe) in Type 1 AGN has shown a major deficit of supernovae around Type 1 AGN host galaxies, with respect to Type 2 AGN hosts. The aim of this work is to test whether there is any preference for Type 1 AGN to host SN of a specific kind. Through the analysis of SN occurrence and their type (thermonuclear vs core-collapse), we can directly link the type of stars producing the SN events, thus this is an indirect way to study host galaxies in Type 1 AGN. We examine the detection fractions of SNe, the host galaxies and compare the sample properties to typical host galaxies in the Open Supernova Catalog (OSC; Guillochon et al. 2017). The majority of the host galaxies in the AGN sample are late-type, similar to typical galaxies hosting SN within the OSC. The findings are supportive of a deficiency of SNe near Type 1 AGN, although we cannot with certainty assess the overall detection fractions of SNe in Type 1 AGN relative to other SN host galaxies. We can state that Type 1 AGN has equal detection fractions of thermonuclear vs core-collapse SNe. However, we note the possibility of a higher detection rate of core-collapse supernovae in Type-1 AGN with insecure AGN classifications.

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arXiv2020

On the origins of up-bending breaks in disk galaxies

Using SPITZER 3.6$μ$m imaging, we investigate the physical and data-driven origins of up-bending (Type III) disk breaks. We apply a robust new break-finding algorithm to 175 low-inclination disk galaxies previously identified as containing Type III breaks, classify each galaxy by its outermost re-classified (via our new algorithm) break type, and compare the local environments of each resulting subgroup. Using three different measures of the local density of galaxies, we find that galaxies with extended outer spheroids (Type IIIs) occupy the highest density environments in our sample, while those with extended down-bending (Type II) disks and symmetric outskirts occupy the lowest density environments. Among outermost breaks, the most common origin of Type III breaks in our sample is methodological; the use of elliptical apertures to measure the radial profiles of asymmetric galaxies usually results in features akin to Type III breaks.

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arXiv2019

The Vanishing & Appearing Sources during a Century of Observations project: I. USNO objects missing in modern sky surveys and follow-up observations of a "missing star"

In this paper we report the current status of a new research program. The primary goal of the "Vanishing & Appearing Sources during a Century of Observations" (VASCO) project is to search for vanishing and appearing sources using existing survey data to find examples of exceptional astrophysical transients. The implications of finding such objects extend from traditional astrophysics fields to the more exotic searches for evidence of technologically advanced civilizations. In this first paper we present new, deeper observations of the tentative candidate discovered by Villarroel et al. (2016). We then perform the first searches for vanishing objects throughout the sky by comparing 600 million objects from the US Naval Observatory Catalogue (USNO) B1.0 down to a limiting magnitude of $\sim 20 - 21$ with the recent Pan-STARRS Data Release-1 (DR1) with a limiting magnitude of $\sim$ 23.4. We find about 150,000 preliminary candidates that do not have any Pan-STARRS counterpart within a 30 arcsec radius. We show that these objects are redder and have larger proper motions than typical USNO objects. We visually examine the images for a subset of about 24,000 candidates, superseding the 2016 study with a sample ten times larger. We find about $\sim$ 100 point sources visible in only one epoch in the red band of the USNO which may be of interest in searches for strong M dwarf flares, high-redshift supernovae or other catagories of unidentified red transients.

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arXiv2019

On the varied origins of up-bending breaks in galaxy disks

Aims: Using a sample of 175 low-inclination galaxies from the S$^{4}$G, we investigate the origins of up-bending (Type III) breaks in the 3.6 $μ$m surface brightness profiles of disk galaxies. Methods: We re-analyze a sample of previously identified Type III disk break-hosting galaxies using a new, unbiased break-finding algorithm, which uncovered many new, sometimes subtle disk breaks across the whole sample. We classify each break by its likely origin through close examination of the galaxy images across wavelengths, and compare samples of galaxies separated by their outermost identified break types in terms of their stellar populations and local environments. Results: We find that more than half of the confirmed Type III breaks in our sample can be attributed to morphological asymmetry in the host galaxies. As these breaks are mostly an artifact of the azimuthal averaging process, their status as physical "breaks" is questionable. Such galaxies occupy some of the highest density environments in our sample, implying that much of this asymmetry is the result of tidal disturbance. Additionally, we find that Type III breaks related to extended spiral arms or star formation often host down-bending (Type II) breaks at larger radius which were previously unidentified. Such galaxies reside in the lowest density environments in our sample, in line with previous studies that found a lack of Type II breaks in clusters. Galaxies occupying the highest density environments most often show Type III breaks associated with outer spheroidal components. Conclusions: We find that Type III breaks in the outer disks of galaxies arise most often through environmental influence: either tidal disturbance (resulting in disk asymmetry) or heating through, e.g., galaxy harrassment (leading to spheroidal components). Galaxies hosting the latter break types also show... (abstract continues)

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