Stellar Overdensity Research Articles

The Power of High-precision Broadband Photometry: Tracing the Milky Way Density Profile with Blue Horizontal Branch Stars in the Dark Energy Survey

Blue horizontal branch stars (BHBs), excellent distant tracers for probing the Milky Way’s halo density profile, are distinguished in the g−r0 versus (i − z)0 color space from another class of stars, blue straggler stars. We develop a Bayesian mixture model to classify BHBs using high-precision photometry data from the Dark Energy Survey Data Release 2 (DES DR2). We select ∼2100 highly probable BHBs based on their griz photometry and the associated uncertainties, and we use these stars to map the stellar halo over the Galactocentric radial range 20 kpc ≲ R ≲ 70 kpc. After excluding known stellar overdensities, we find that the number density n ⋆ of BHBs can be represented by a power-law density profile n ⋆ ∝ R −α with an index of α=4.34−0.12+0.13±0.52 , consistent with existing literature values. In addition, we examine the impact of systematic errors and the spatial inhomogeneity on the fitted density profile. Our work demonstrates the effectiveness of high-precision griz photometry in selecting BHBs. The upcoming photometric survey from the Rubin Observatory, expected to reach depths 2–3 mag greater than DES during its 10 yr mission, will enable us to investigate the density profile of the Milky Way’s halo out to the virial radius, unraveling the complex processes of formation and evolution in our Galaxy.

EROSITA (eRASS1) study of the Canis Major overdensity: Developing a multi-wavelength algorithm for classifying faint X-ray sources

Context. Using data from eROSITA (extended Roentgen Survey with an Imaging Telescope Array) on board Spektrum-Roentgen- Gamma (Spektr-RG, SRG) taken during the first eROSITAall-sky survey (eRASS1), we performed the first X-ray classification and population study in the field of the Canis Major overdensity (CMa OD), which is an elliptical-shaped stellar overdensity located at l = -240°, b = −80°. Aims. This study aims to identify the X-ray sources in CMa OD. We developed a classification algorithm using multi-wavelength criteria as a preliminary method for the classification of faint X-ray sources, specifically in regions with a high source number density. Methods. We used the brightness of the multi-wavelength counterparts (mainly from infrared and optical catalogues), along with the X-ray flux and X-ray hardness ratios (HRs) to classify the sources. Results. Out of a total number of 8311 X-ray sources, we classified 1029 sources as Galactic stars and binaries in the foreground, 946 sources as active galactic nuclei (AGNs) and galaxies in the background, and 435 sources with stellar counterparts that may belong to either the MW or CMa OD. Among the sources with a stellar counterpart, we identified 34 symbiotic star candidates, plus 335 sources, of which the infrared (IR) counterparts have properties of M-giants in CMa OD. Moreover, there is a known high-mass X-ray binary (HMXB, 4U 0728-25) in the field of our study; according to the Gaia parallax of its companion, it appears to be a member of CMa OD. There is also a recently detected transient low-mass X-ray binary (LMXB, SRGt J071522.1-191609) is also present; it may be a member of CMa OD based on its companion, which is most likely highly absorbed and is thus located behind the Galactic disk. In addition, we present the X-ray luminosity function (XLF) of members and candidate members of CMa OD. It is dominated by sources with luminosities of <2 × 1032–1033 erg s−1 in the energy range of 0.2–2.3 keV. These sources are expected to be either accreting white dwarfs or quiescent LMXBs.

Capture of field stars by dark substructures

ABSTRACT We use analytical and N-body methods to study the capture of field stars by gravitating substructures moving across a galactic environment. The majority of stars captured by a substructure move on temporarily bound orbits that are lost to galactic tides after a few orbital revolutions. In numerical experiments where a substructure model is immersed into a sea of field particles on a circular orbit, we find a population of particles that remain bound to the substructure potential for indefinitely long times. This population is absent from substructure models, initially placed outside the galaxy on an eccentric orbit. We show that gravitational capture is most efficient in dwarf spheroidal galaxies (dSphs) on account of their low velocity dispersions and high stellar phase-space densities. In these galaxies, ‘dark’ sub-subhaloes, which do not experience in situ star formation, may capture field stars and become visible as stellar overdensities with unusual properties: (i) they would have a large size for their luminosity, (ii) contain stellar populations indistinguishable from the host galaxy, and (iii) exhibit dark matter (DM)-dominated mass-to-light ratios. We discuss the nature of several ‘anomalous’ stellar systems reported as star clusters in the Fornax and Eridanus II dSphs that exhibit some of these characteristics. DM sub-subhaloes with a mass function ${\rm d}N/{\rm d}M_\bullet \sim M_\bullet ^{-\alpha }$ are expected to generate stellar systems with a luminosity function, ${\rm d}N/{\rm d}M_\star \sim M_\star ^{-\beta }$, where $\beta =(2\alpha +1)/3=1.6$ for $\alpha =1.9$. Detecting and characterizing these objects in dSphs would provide unprecedented constraints on the particle mass and cross-section of a large range of DM particle candidates.

UVIT Study of the MAgellanic Clouds (U-SMAC) – I. Recent star formation history and kinematics of the Shell region in the north-eastern Small Magellanic Cloud

ABSTRACT The interactions between the Magellanic Clouds significantly affect the shape and distribution of the young stellar population, particularly in the periphery of the Small Magellanic Cloud (SMC). We present the first far-UV (FUV) map of the north-east SMC-Shell region using the Ultra Violet Imaging Telescope (UVIT) onboard AstroSat. The detected FUV stars are combined with Gaia Early Data Release 3 data to create a FUV–optical catalogue of ∼14 400 stars. FUV–optical colour-magnitude diagrams are used along with isochrones to estimate the stellar ages. The detected stars are formed in multiple episodes. We identified two episodes of star formation (∼60 and ∼260 Myr ago), where the episode at ∼260 Myr is linked to the recent interaction with the Large Magellanic Cloud (LMC) and the episode at ∼60 Myr is linked to the pericentric passage of the SMC around our Galaxy. The median proper motion (PM) and velocity dispersion are found to be similar to the SMC main body, indicating that this region has not experienced significant tidal effects. The FUV stellar surface density and the dispersion in PM suggest that the extent of the inner SMC in the north-east direction to be around 2.2°. We detect arm-like and arc-like structures in the FUV stellar density map, and their kinematics appear to be similar to the SMC main body. These extended outer features are the spatial stellar overdensities formed over multiple episodes of star formation, but without apparent kinematic distinction.

All-sky Kinematics and Chemistry of Monoceros Stellar Overdensity

We explore the kinematic and chemical properties of the Monoceros stellar overdensity by combining data from the Two Micron All Sky Survey, Wide-field Infrared Survey Explorer, APOGEE, and Gaia. Monoceros is a structure located toward the Galactic anticenter and close to the disk. We have identified that its stars have azimuthal velocity in the range of 200 < v ϕ (km s−1) < 250. Combining their kinematics and spatial distribution, we designed a new method to select stars from this overdensity. This method allows us to easily identify the structure in both hemispheres and estimate their distances. Our analysis was supported by comparison with simulated data from the entire sky generated by the Galaxia code. Furthermore, we characterized, for the first time, the Monoceros overdensity in several chemical abundance spaces. Our results confirm its similarity to stars found in the thin disk of the Galaxy and suggest an in situ formation. Furthermore, we demonstrate that the southern and northern regions of Monoceros exhibit indistinguishable chemical compositions.

The spatially resolved star formation history of the dwarf spiral galaxy NGC 5474

ABSTRACT We study the resolved stellar populations and derive the star formation history of NGC 5474, a peculiar star-forming dwarf galaxy at a distance of ∼7 Mpc, using Hubble Space Telescope Advanced Camera for Surveys data from the Legacy Extragalactic UV Survey (LEGUS) programme. We apply an improved colour–magnitude diagram fitting technique based on the code sfera and use the latest PARSEC–COLIBRI stellar models. Our results are the following. The off-centre bulge-like structure, suggested to constitute the bulge of the galaxy, is dominated by star formation (SF) activity initiated 14 Gyr ago and lasted at least up to 1 Gyr ago. Nevertheless, this component shows clear evidence of prolonged SF activity (lasting until ∼10 Myr ago). We estimate the total stellar mass of the bulge-like structure to be (5.0 ± 0.3) × 108 M⊙. Such a mass is consistent with published suggestions that this structure is in fact an independent system orbiting around and not within NGC 5474’s disc. The stellar overdensity located to the South–West of the bulge-like structure shows a significant SF event older than 1 Gyr, while it is characterized by two recent peaks of SF, around ∼10 and ∼100 Myr ago. In the last Gyr, the behaviour of the stellar disc is consistent with what is known in the literature as ‘gasping’. The synchronized burst at 10–35 Myr in all components might hint to the recent gravitational interaction between the stellar bulge-like structure and the disc of NGC 5474.

Galaxy mergers in Subaru HSC-SSP: A deep representation learning approach for identification, and the role of environment on merger incidence

Context.Galaxy mergers and interactions are an important process within the context of galaxy evolution, however, there is still no definitive method which identifies pure and complete merger samples is still not definitive. A method for creating such a merger sample is required so that studies can be conducted to deepen our understanding of the merger process and its impact on galaxy evolution.Aims.In this work, we take a deep-learning-based approach for galaxy merger identification in Subaru HSC-SSP, using deep representation learning and fine-tuning, with the aim of creating a pure and complete merger sample within the HSC-SSP survey. We can use this merger sample to conduct studies on how mergers affect galaxy evolution.Methods.We used Zoobot, a deep learning representation learning model pretrained on citizen science votes on Galaxy Zoo DeCALS images. We fine-tuned Zoobot for the purpose of merger classification of images of SDSS and GAMA galaxies in HSC-SSP public data release 3. Fine-tuning was done using ∼1200 synthetic HSC-SSP images of galaxies from the TNG simulation. We then found merger probabilities on observed HSC images using the fine-tuned model. Using our merger probabilities, we examined the relationship between merger activity and environment.Results.We find that our fine-tuned model returns an accuracy on the synthetic validation data of ∼76%. This number is comparable to those of previous studies in which convolutional neural networks were trained with simulation images, but with our work requiring a far smaller number of training samples. For our synthetic data, our model is able to achieve completeness and precision values of ∼80%. In addition, our model is able to correctly classify both mergers and non-mergers of diverse morphologies and structures, including those at various stages and mass ratios, while distinguishing between projections and merger pairs. For the relation between galaxy mergers and environment, we find two distinct trends. Using stellar mass overdensity estimates for TNG simulations and observations using SDSS and GAMA, we find that galaxies with higher merger scores favor lower density environments on scales of 0.5 to 8h−1Mpc. However, below these scales in the simulations, we find that galaxies with higher merger scores favor higher density environments.Conclusions.We fine-tuned a citizen-science trained deep representation learning model for purpose of merger galaxy classification in HSC-SSP, and make our merger probability catalog available to the public. Using our morphology-based catalog, we find that mergers are more prevalent in lower density environments on scales of 0.5–8h−1Mpc.

Loss cone shielding

ABSTRACT A star wandering close enough to a massive black hole can be ripped apart by the tidal forces of the black hole. The advent of wide-field surveys at many wavelengths has quickly increased the number of tidal disruption events (TDEs) observed, and has revealed that (i) observed TDE rates are lower than theoretical predictions and (ii) E+A galaxies are significantly overrepresented. This overrepresentation further worsens the tension between observed and theoretically predicted TDEs for non-E+A galaxies. Classical loss cone theory focuses on the cumulative effect of many weak scatterings. However, a strong scattering can remove a star from the distribution before it can get tidally disrupted. Most stars undergoing TDEs come from within the radius of influence, the densest environments of the Universe. In such environments, close encounters rare elsewhere become non-negligible. We revise the standard loss cone theory to take into account classical two-body interactions as well as strong scattering, collisions, tidal captures, and study under which conditions close encounters can shield the loss cone. We (i) analytically derive the impact of strong scattering and other close encounters, (ii) compute time-dependent loss cone dynamics including both weak and strong encounters, and (iii) derive analytical solutions to the Fokker–Planck equation with strong scattering. We find that (i) TDE rates can be reduced to up to an order of magnitude and (ii) strong shielding preferentially reduces deeply plunging stars. We also show that stellar overdensities, one possible explanation for the E + A preference, can fail to increase TDE rates when taking into account strong scattering.

Exploring the structure and kinematics of the Milky Way through A stars

Context. Despite their relatively high intrinsic brightness and the fact that they are more numerous than younger OB stars and kinematically colder than older red giants, A-type stars have rarely been used as Galactic tracers. They may, in fact, be used to fill the age gap between these two tracers, thereby allowing us to evaluate the evolutionary and dynamic processes underlying the transition between them. Aims. We analyse Galactic disc structure and kinematic perturbations up to 6 kpc from the Sun based on observations of A-type stars. Methods. This work presents a catalogue of A-type stars selected using the IGAPS photometric survey. It covers the Galactic disc within 30° ≤l ≤ 215° and |b|≤5° up to a magnitude of r ≤ 19 mag with about 3.5 million sources. We used Gaia Data Release 3 parallaxes and proper motions, as well as the line-of-sight velocities, to analyse the large-scale features of the Galactic disc. We carried out a study of the completeness of the detected density distributions, along with a comparison between the b &lt; 0° and b &gt; 0° regions. Possible biases caused by interstellar extinction or by the usage of some kinematic approximations were examined as well. Results. We find stellar overdensities associated with the Local and the Perseus spiral arms, as well as with the Cygnus region. We find that A-type stars also provide kinematic indications of the Galactic warp towards the anticentre, which displays a median vertical motion of ∼6 − 7 km s−1 at a Galactocentric radius of R = 14 kpc. It starts at R ≈ 12 kpc, which supports the scenario where the warp begins at larger radii for younger tracers when compared with other samples in the literature. We also detect a region with downward mean motion extending beyond 2 kpc from the Sun towards 60° ≲l ≲ 75° that may be associated with a compression breathing mode. Furthermore, A-type stars reveal very clumpy inhomogeneities and asymmetries in the VZ − Vϕ velocity space plane.

A deep dive into the Type II Globular Cluster NGC 1851

Abstract About one-fifth of the Galactic globular clusters (GCs), dubbed Type II GCs, host distinct stellar populations with different heavy elements abundances. NGC 1851 is one of the most studied Type II GCs, surrounded by several controversies regarding the spatial distribution of its populations and the presence of star-to-star [Fe/H], C+N+O, and age differences. This paper provides a detailed characterization of its stellar populations through Hubble Space Telescope (HST), ground-based, and Gaia photometry. We identified two distinct populations with different abundances of s-process elements along the red-giant branch (RGB) and the sub-giant branch (SGB) and detected two sub-populations among both s-poor (canonical) and s-rich (anomalous) stars. To constrain the chemical composition of these stellar populations, we compared observed and simulated colors of stars with different abundances of He, C, N, and O. It results that the anomalous population has a higher CNO overall abundance compared to the canonical population and that both host stars with different light-element abundances. No significant differences in radial segregation between canonical and anomalous stars are detected, while we find that among their sub-populations, the two most chemical extremes are more centrally concentrated. Anomalous and canonical stars show different 2D spatial distributions outside ∼3 arcmin, with the latter developing an elliptical shape and a stellar overdensity in the northeast direction. We confirm the presence of a stellar halo up to ∼80 arcmin with Gaia photometry, tagging 14 and five of its stars as canonical and anomalous, respectively, finding a lack of the latter in the south/southeast field.

A possible dwarf galaxy satellite-of-satellite problem in ΛCDM

Dark matter clusters on all scales, and it is therefore expected that even substructure should host its own substructure. Using the Extragalactic Distance Database, we searched for dwarf-galaxy satellites of dwarf galaxies, that is, satellite-of-satellite galaxies, corresponding to these substructures of substructure. From investigation of Hubble Space Telescope data for 117 dwarf galaxies, we report the discovery of a previously unknown dwarf galaxy around the ultra-diffuse M96 companion M96-DF6 at 10.2 Mpc in the Leo-I group. We confirm its dwarf-galaxy nature as a stellar overdensity. Modeling its structural parameters with a growth-curve analysis, we find that it is an ultrafaint dwarf galaxy with a luminosity of 1.5 × 105 L⊙, which is 135 times fainter than its host. Based on its close projection to M96-DF6, it is unlikely that their association occurs simply by chance. We compare the luminosity ratio of this and three other known satellite-of-satellite systems with results from two different cosmological sets of ΛCDM simulations. For the observed stellar mass range of the central dwarf galaxies, the simulated dwarfs have a higher luminosity ratio between the central dwarf and its first satellite (≈10 000) than observed (≈100), excluding the Large and Small Magellanic Cloud (LMC/SMC) system. No simulated dwarf analog at these observed stellar masses has the observed luminosity ratio. This cannot be due to missing resolution, because it is the brightest subhalos that are missing. This may indicate that there is a satellite-of-satellite (SoS) problem for ΛCDM in the stellar-mass range between 106 and 108 M⊙, the regime of the classical dwarf galaxies. However, simulated dwarf models at both a lower (&lt; 106 M⊙) and higher (&gt; 108 M⊙) stellar mass have comparable luminosity ratios. For the higher-stellar-mass systems, the LMC/SMC system is reproduced by simulations; for the lower stellar masses, no observed satellite-of-satellite system has been observed to date. More observations and simulations of satellite-of-satellite systems are needed to assess whether the luminosity ratio is at odds with ΛCDM.

Assessing the physical reality of Milky Way open cluster candidates

ABSTRACT We report results on the analysis of 11 new Milky Way open cluster candidates, recently discovered from the detection of stellar overdensities in the Vector Point diagram, by employing Xtreme deconvolution Gaussian mixture models. We treated these objects as real open clusters and derived their fundamental properties with their associated intrinsic dispersions by exploring the parameter space through the minimization of likelihood functions on the generated synthetic colour–magnitude diagrams. The intrinsic dispersions of the resulting ages turned out to be much larger than those usually obtained for open clusters. Indeed, they resemble the ages and metallicities of composite star field populations. We also traced their stellar number density profiles and mass functions and derived their total masses and Jacobi and tidal radii, which helped us as criteria while assessing their physical nature as real open clusters. Because the 11 candidates show a clear gathering of stars in the proper-motion plane and some hint for similar distances, we concluded that they are possibly sparse groups of stars.

The Unmixed Debris of Gaia-Sausage/Enceladus in the Form of a Pair of Halo Stellar Overdensities

In the first billion years after its formation, the Galaxy underwent several mergers with dwarf satellites of various masses. The debris of Gaia-Sausage/Enceladus (GSE), the galaxy responsible for the last significant merger of the Milky Way, dominates the inner halo and has been suggested to be the progenitor of both the Hercules-Aquila Cloud (HAC) and Virgo Overdensity (VOD). We combine SEGUE, APOGEE, Gaia, and StarHorse distances to characterize the chemodynamical properties and verify the link between HAC, VOD, and GSE. We find that the orbital eccentricity distributions of the stellar overdensities and GSE are comparable. We also find that they have similar, strongly peaked, metallicity distribution functions, reinforcing the hypothesis of common origin. Furthermore, we show that HAC and VOD are indistinguishable from the prototypical GSE population within all chemical-abundance spaces analyzed. All these evidences combined provide a clear demonstration that the GSE merger is the main progenitor of the stellar populations found within these halo overdensities.

A Tilt in the Dark Matter Halo of the Galaxy

Recent observations of the stellar halo have uncovered the debris of an ancient merger, Gaia–Sausage–Enceladus (GSE), estimated to have occurred ≳8 Gyr ago. Follow-up studies have associated GSE with a large-scale tilt in the stellar halo that links two well-known stellar overdensities in diagonally opposing octants of the Galaxy (the Hercules–Aquila Cloud and Virgo Overdensity; HAC and VOD). In this paper, we study the plausibility of such unmixed merger debris persisting over several gigayears in the Galactic halo. We employ the simulated stellar halo from Naidu et al., which reproduces several key properties of the merger remnant, including the large-scale tilt. By integrating the orbits of these simulated stellar halo particles, we show that adoption of a spherical halo potential results in rapid phase mixing of the asymmetry. However, adopting a tilted halo potential preserves the initial asymmetry in the stellar halo for many gigayears. The asymmetry is preserved even when a realistic growing disk is added to the potential. These results suggest that HAC and VOD are long-lived structures that are associated with GSE and that the dark matter halo of the Galaxy is tilted with respect to the disk and aligned in the direction of HAC–VOD. Such halo–disk misalignment is common in modern cosmological simulations. Lastly, we study the relationship between the local and global stellar halo in light of a tilted global halo comprised of highly radial orbits. We find that the local halo offers a dynamically biased view of the global halo due to its displacement from the Galactic center.

Hunting for open clusters inGaiaEDR3: 628 new open clusters found with OCfinder

Context.The improvements in the precision of the published data inGaiaEDR3 with respect toGaiaDR2, particularly for parallaxes and proper motions, offer the opportunity to increase the number of known open clusters in the Milky Way by detecting farther and fainter objects that have thus far gone unnoticed.Aims.Our aim is to continue to complete the open cluster census in the Milky Way with the detection of new stellar groups in the Galactic disc. We useGaiaEDR3 up to magnitudeG= 18 mag, increasing the magnitude limit and therefore the search volume explored in one unit with respect to our previous studies.Methods.We used theOCfindermethod to search for new open clusters inGaiaEDR3 using a big data environment. As a first step,OCfinderidentified stellar statistical overdensities in five-dimensional astrometric space (position, parallax, and proper motions) using theDBSCANclustering algorithm. Then, these overdensities were classified into random statistical overdensities or real physical open clusters using a deep artificial neural network trained on well-characterisedG,GBP–GRPcolour-magnitude diagrams.Results.We report the discovery of 628 new open clusters within the Galactic disc, with most of them being located beyond 1 kpc from the Sun. From the estimation of ages, distances, and line-of-sight extinctions of these open clusters, we see that young clusters align following the Galactic spiral arms while older ones are dispersed in the Galactic disc. Furthermore, we find that most open clusters are located at low Galactic altitudes with the exception of a few groups older than 1 Gyr.Conclusions.We show the success of theOCfindermethod leading to the discovery of a total of 1274 open clusters (joining the discoveries here with the previous ones based onGaiaDR2), which represents almost 50% of the known population. Our ability to perform big data searches on a large volume of the Galactic disc, together with the higher precision inGaiaEDR3, enable us to keep completing the census with the discovery of new open clusters.

The role of the cosmic web in the scatter of the galaxy stellar mass–gas metallicity relation

The large-scale structure of the Universe can be understood in terms of features such as filaments, nodes and walls, which we collectively term the cosmic web. Galaxies evolve within the cosmic web, naturally raising the question of its impact on that process. There are two main mechanisms by which the cosmic web can influence galaxies: one is by modulating the growth of haloes, and the other is by regulating the gas ecosystem around galaxies. Disentangling the two is difficult, but key to deriving a holistic picture of galaxy formation and observational constraints on the growth of haloes. Here we report a detection of the effect of the cosmic web on the galaxy stellar mass–gas-phase metallicity relation of low-redshift star-forming galaxies using data from the Sloan Digital Sky Survey. The proximity of a galaxy to a node, independently of stellar mass and overdensity, influences its gas-phase metallicity, with galaxies closer to nodes displaying higher chemical enrichment than those farther away. We find a similar, but notably weaker, effect with respect to filaments. We find qualitative agreement in the cosmological hydrodynamical simulation IllustrisTNG (TNG300). Using IllustrisTNG, our results can be explained by both halo assembly bias and gas supply combining in nodes in a way that markedly modulates the metallicity of the gas, contributing to the scatter of this fundamental relation in galaxy evolution.

Pearls on a String: Numerous Stellar Clusters Strung Along the Same Orbit

Abstract Stars originate from the dense interstellar medium, which exhibits filamentary structure to scales of ∼1 kpc in galaxies like our Milky Way. We explore quantitatively how much resulting large-scale correlation there is among different stellar clusters and associations in orbit-phase space, characterized here by actions and angles. As a starting point, we identified 55 prominent stellar overdensities in the 6D space of orbit (actions) and orbit-phase (angles) among the ∼2.8 million stars with radial velocities from Gaia EDR3 and with d ≤ 800 pc. We then explored the orbit-phase distribution of all sample stars in the same orbit patch as any one of these 55 overdensities. We find that very commonly numerous other distinct orbit-phase overdensities exist along these same orbits, like pearls on a string. These “pearls” range from known stellar clusters to loose, unrecognized associations. Among orbit patches defined by one initial orbit-phase overdensity, 50% contain at least 8 additional orbit-phase pearls of 10 cataloged members; 20% of them contain 20 additional pearls. This is in contrast to matching orbit patches sampled from a smooth mock catalog, or offset nearby orbit patches, where there are only 2 (or 5, respectively) comparable pearls. Our findings quantify for the first time how common it is for star clusters and associations to form at distinct orbit-phases of nearly the same orbit. This may eventually offer a new way to probe the 6D orbit structure of the filamentary interstellar medium.

The impact of stellar clustering on the observed multiplicity of super-earth systems: outside–in cascade of orbital misalignments initiated by stellar flybys

ABSTRACT A recent study suggests that the observed multiplicity of super-Earth (SE) systems is correlated with stellar overdensities: field stars in high phase-space density environments have an excess of single-planet systems compared to stars in low-density fields. This correlation is puzzling as stellar clustering is expected to influence mostly the outer part of planetary systems. Here, we examine the possibility that stellar flybys indirectly excite the mutual inclinations of initially coplanar SEs, breaking their co-transiting geometry. We propose that flybys excite the inclinations of exterior substellar companions, which then propagate the perturbation to the inner SEs. Using analytical calculations of the secular coupling between SEs and companions, together with numerical simulations of stellar encounters, we estimate the expected number of ‘effective’ flybys per planetary system that lead to the destruction of the SE co-transiting geometry. Our analytical results can be rescaled easily for various SE and companion properties (masses and semimajor axes) and stellar cluster parameters (density, velocity dispersion, and lifetime). We show that for a given SE system, there exists an optimal companion architecture that leads to the maximum number of effective flybys; this results from the trade-off between the flyby cross-section and the companion’s impact on the inner system. Subject to uncertainties in the cluster parameters, we conclude that this mechanism is inefficient if the SE system has a single exterior companion, but may play an important role in ‘SE + two companions’ systems that were born in dense stellar clusters. Whether this effect causes the observed correlation between planet multiplicity and stellar overdensities remains to be confirmed.

Linking nearby stellar streams to more distant halo overdensities

Context. It has recently been shown that the halo near the Sun contains several kinematic substructures associated with past accretion events. For the more distant halo, there is evidence of large-scale density variations – in the form of stellar clouds or overdensities. Aims. We study the link between the local halo kinematic groups and three of these stellar clouds: the Hercules-Aquila cloud, the Virgo Overdensity, and the Eridanus-Phoenix overdensity. Methods. We perform orbital integrations in a standard Milky Way potential of a local halo sample extracted from Gaia EDR3 with the goal of predicting the location of the merger debris elsewhere in the Galaxy. We specifically focus on the regions occupied by the three stellar clouds and compare their kinematic and distance distributions with those predicted from the orbits of the nearby debris. Results. We find that the local halo substructures have families of orbits that tend to pile up in the regions where the stellar clouds have been found. The distances and velocities of the cloud’s member stars are in good agreement with those predicted from the orbit integrations, particularly for Gaia-Enceladus stars. This is the dominant contributor of all three overdensities, with a minor part stemming from the Helmi streams and to an even smaller extent from Sequoia. The orbital integrations predict no asymmetries in the sky distribution of halo stars, and they pinpoint where additional debris associated with the local halo substructures may be located.

A panoramic view of the Local Group dwarf galaxy NGC 6822

ABSTRACT We present a panoramic survey of the isolated Local Group dwarf irregular galaxy NGC 6822. Our photometry reaches ∼2–3 mag deeper than most previous studies and spans the widest area around the dwarf compared to any prior work. We observe no stellar overdensities in the outskirts of NGC 6822 to V ∼ 30 mag arcsec−2 and a projected radius of 16.5 kpc. This indicates that NGC 6822 has not experienced any recent interaction with a companion galaxy, despite previous suggestions to the contrary. Similarly, we find no evidence for any dwarf satellites of NGC 6822 to a limiting luminosity MV ≈ −5. NGC 6822 contains a disc of H i gas and young stars, oriented at ∼60○ to an extended spheroid composed of old stellar populations. We observe no correlation between the distribution of young stars and spheroid members. Our imaging allows us to trace the spheroid to nearly 11 kpc along its major axis, commensurate with the extent of the NGC 6822 globular cluster system. We find that the spheroid becomes increasingly flattened at larger radii, and its position angle twists by up to 40○. We use Gaia EDR3 astrometry to measure a proper motion for NGC 6822, and then sample its orbital parameter space. While this galaxy has spent the majority of its life in isolation, we find that it likely passed within the virial radius of the Milky Way ∼3–4 Gyr ago. This may explain the apparent flattening and twisting observed in the outskirts of its spheroid.