Magellanic Bridge Research Articles

New quasars behind the Magellanic Clouds

Context. Quasi-stellar objects (QSOs) are a basis for an absolute reference system for astrometric studies. A system like this at the far side of nearby galaxies is required to facilitate measuring of the proper motions of these galaxies. However, the foreground contamination from the galaxies themselves is a problem for the QSO identification. Aims. We search for new QSOs behind the two Magellanic Clouds, the Magellanic Bridge, and the Magellanic Stream. Methods. We identify QSO candidates with a combination of near–infrared colors and variability criteria from the public ESO Visual and Infrared Survey Telescope for Astronomy (VISTA) Magellanic Clouds (VMC) survey. We confirm their nature from broad emission lines with low-resolution optical spectroscopy. Results. We confirmed the QSO nature of 136 objects. They are distributed as follows: 12 behind the Large Magellanic Cloud, 37 behind the Small Magellanic Cloud, 63 behind the Bridge, and 24 behind the Stream. The QSOs span a redshift range from z~0.1 to z~2.9. A comparison of our quasar selection with the Quaia quasar catalog, based on Gaia low-resolution spectra, yields a selection and confirmation success rate of 6–19%, depending on whether the quality of the photometry, the magnitude ranges, and the colors are considered. Our candidate list is rather incomplete, but the objects in it are likely to be confirmed as quasars with a probability of ~90%. Finally, we report a list of 3609 objects in the entire VMC survey that match our color and variability selection criteria; only 1249 of them have Gaia counterparts. Conclusions. Our combined infrared color and variability criteria for the QSO selection prove to be efficient: ~90% of the observed candidates are bona fide QSOs and allow us to generate a list of new high-probability quasar candidates.

Exploring Molecular Gas toward the Magellanic Bridge

The Magellanic Bridge (MB) is a diffuse gas structure that connects the Large Magellanic Cloud with the Small Magellanic Cloud. The MB is the closest tidal interaction system between two galaxies to the Milky Way. Taking into account that young stellar objects are embedded in such gaseous structure, it is a unique laboratory to investigate gas dynamics and star formation processes in a very low metallicity environment. In this work we present a preview of the results obtained from observations made with the Atacama Pathfinder Experiment telescope toward some regions of the MB. Through the emission of the 12CO J = 2–1 line we characterize the molecular gas of these regions.

α-element enhancements in the ISM of the LMC and SMC: Evidence of recent star formation

Context. Important questions regarding the chemical composition of the neutral interstellar medium (ISM) in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) are still open. It is usually assumed that their metallicity is uniform and equal to that measured in hot stars and H II regions, but direct measurements of the neutral ISM metallicity had not been performed until now. Deriving the metallicity from the observed metal abundances is not straightforward because the abundances depend on the depletion of metals into dust and on nucleosynthesis effects such as α-element enhancement. Aims. Our aim is to measure the metallicity of the neutral ISM in the LMC and SMC, dust depletion, and any nucleosynthesis effects. Methods. We collected literature column densities of Ti II, Ni II, Cr II, Fe II, Mn II, Si II, Cu II, Mg II, S II, P II, Zn II, and O I in the neutral ISM towards 32 hot stars in the LMC and 22 in the SMC. We determined dust depletion from the relative abundances of different metals because they deplete with different strengths. This includes a ‘golden sample’ of sightlines where Ti and other α-elements are available. We fit linear relations to the observed abundance patterns so that the slopes determined the strengths of dust depletion and the normalizations determined the metallicities. We investigated α-element enhancements in the gas from the deviations from the linear fits and compared them with stars. Results. In our golden sample we find α-element enhancement in the neutral ISM in most systems, on average 0.26 dex (0.35 dex) for the LMC (SMC), and an Mn underabundance in the SMC (on average −0.35 dex). Measurements of Mn II are not available for the LMC. These are higher than for stars at similar metallicities. We find total neutral ISM metallicities that are mostly consistent with hot star metallicity values, on average [M/H]tot = −0.33 (−0.83), with standard deviations of 0.30 (0.30), in the LMC (the SMC). In six systems, however, we find significantly lower metallicities, 2 out of 32 in the LMC (with ~16% solar) and 4 out of 22 in the SMC (3 and 10% solar), two of which are in the outskirts of the SMC near the Magellanic Bridge, a region known for having a lower metallicity. Conclusions. The observed a-element enhancements and Mn underabundance are likely due to bursts of star formation, more recently than ~1 Gyr ago, that enriched the ISM from core-collapse supernovae. With the exception of lines of sight towards the Magellanic Bridge, the neutral gas in the LMC and SMC appears fairly well mixed in terms of metallicity.

XMM-Newton and INTEGRAL Observations of the Bright GRB 230307A: Vanishing of the Local Absorption and Limits on the Dust in the Magellanic Bridge

GRB 230307A is the second brightest gamma-ray burst detected in more than 50 years of observations and is located in the direction of the Magellanic Bridge. Despite its long duration, it is most likely the result of the merger of a compact binary ejected from a galaxy in the local universe (redshift z = 0.065). Our XMM-Newton observation of its afterglow at 4.5 days shows a power-law spectrum with photon index Γ = 1.73 ± 0.10, unabsorbed flux F 0.3−10 keV = (8.8 ± 0.5) × 10−14 erg cm−2 s−1, and no absorption in excess of that produced in our Galaxy and in the Magellanic Bridge. We derive a limit of cm−2 on the absorption at the GRB redshift, which is a factor ∼5 below the value measured during the prompt phase. We searched for the presence of dust scattering rings with negative results and set an upper limit of the order of A V < 0.05 on the absorption from dust in the Magellanic Bridge.

The VISCACHA survey

The chemical evolution history of the Small Magellanic Cloud (SMC) has been a matter of debate for decades. The challenges in understanding the SMC chemical evolution are related to a very slow star formation rate (SFR) combined with bursts triggered by the multiple interactions between the SMC and the Large Magellanic Cloud, a significant (∼0.5 dex) metallicity dispersion for the SMC cluster population younger than about 7.5 Gyr, and multiple chemical evolution models tracing very different paths through the observed age–metallicity relation of the SMC. There is no doubt that these processes were complex. Therefore, a step-by-step strategy is required in order to better understand the SMC chemical evolution. We adopted an existing framework to split the SMC into regions on the sky, and we focus on the west halo in this work, which contains the oldest and most metal-poor stellar populations and is moving away from the SMC, that is, in an opposite motion with respect to the Magellanic Bridge. We present a sample containing ∼60% of all west halo clusters to represent the region well, and we identify a clear age–metallicity relation with a tight dispersion that exhibits a 0.5 dex metallicity dip about 6 Gyr ago. We ran chemical evolution models and discuss possible scenarios to explain this metallicity dip, the most likely being a major merger accelerating the SFR after the event. This merger should be combined with inefficient internal gas mixing within the SMC and different SFRs in different SMC regions because the same metallicity dip is not seen in the AMR of the SMC combining clusters from all regions. We try to explain the scenario to better understand the SMC chemo-dynamical history.

The VISCACHA survey – VII. Assembly history of the Magellanic Bridge and SMC Wing from star clusters

ABSTRACT The formation scenario of the Magellanic Bridge during an encounter between the Large and Small Magellanic Clouds ∼200 Myr ago, as proposed by N-body models, would be imprinted in the chemical enrichment and kinematics of its stars and sites of ongoing star formation along its extension. We present an analysis of 33 Bridge star clusters using photometry obtained with the SOAR 4-m telescope equipped with adaptive optics for the VISCACHA survey. We performed a membership selection and derived self-consistent ages, metallicities, distances, and reddening values via statistical isochrone fitting, as well as tidal radii and integrated masses from structure analysis. Two groups are clearly detected: 13 well-studied clusters older than the Bridge, with 0.5–6.8 Gyr and $\hbox{[Fe/H]} &amp;lt; -0.6$ dex; and 15 clusters with &amp;lt;200 Myr and $\rm {[Fe/H]} &amp;gt; -0.5$ dex, probably formed in situ. The old clusters follow the overall age and metallicity gradients of the SMC, whereas the younger ones are uniformly distributed along the Bridge. The main results are as follows: (i) we derive ages and metallicities for the first time for 9 and 18 clusters, respectively; (ii) we detect two metallicity dips in the age-metallicity relation of the Bridge at ∼200 Myr and 1.5 Gyr ago for the first time, possibly chemical signatures of the formation of the Bridge and Magellanic Stream; (iii) we estimate a minimum stellar mass for the Bridge of 3–5 × 105 M⊙; (iv) we confirm that all the young Bridge clusters at ${\rm RA} &amp;lt; 3^h$ are metal-rich with $\rm {[Fe/H]} \sim -0.4$ dex.

ERASSt J040515.6 − 745202, an X-ray burster in the Magellanic Bridge

Context. During the third all-sky survey (eRASS3), eROSITA, the soft X-ray instrument aboard Spectrum-Roentgen-Gamma, detected a new hard X-ray transient, eRASSt J040515.6 − 745202, in the direction of the Magellanic Bridge. Aims. We arranged follow-up observations and searched for archival data to reveal the nature of the transient. Methods. Using X-ray observations with XMM-Newton, NICER, and Swift, we investigated the temporal and spectral behaviour of the source for over about 10 days. Results. The X-ray light curve obtained from the XMM-Newton observation with an ∼28 ks exposure revealed a type-I X-ray burst with a peak bolometric luminosity of at least 1.4 × 1037 erg s−1. The burst energetics are consistent with a location of the burster at the distance of the Magellanic Bridge. The relatively long exponential decay time of the burst of ∼70 s indicates that it ignited in a H-rich environment. The non-detection of the source during the other eROSITA surveys, twelve and six months before and six months after eRASS3, suggests that the burst was discovered during a moderate outburst which reached 2.6 × 1036 erg s−1 in persistent emission. During the NICER observations, the source showed alternating flux states with the high level at a similar brightness as during the XMM-Newton observation. This behaviour is likely caused by dips as also seen during the last hour of the XMM-Newton observation. Evidence for a recurrence of the dips with a period of ∼21.8 h suggests eRASSt J040515.6 − 745202 is a low-mass X-ray binary (LMXB) system with an accretion disk seen nearly edge on. We identify a multi-wavelength counterpart to the X-ray source in UVW1 and g, r, i, and z images obtained by the optical/UV monitor on XMM-Newton and the Dark Energy Camera at the Cerro Tololo Inter-American Observatory. The spectral energy distribution is consistent with radiation from an accretion disk which dominates the UV and from a cool late-type star detected in the optical to infrared wavelengths. Conclusions. After the discovery of X-ray bursts in M 31, the Magellanic Bridge is only the second location outside of the Milky Way where an X-ray burster was found. The burst uniquely identifies eRASSt J040515.6 − 745202 as an LMXB system with a neutron star. Its location in the Magellanic Bridge confirms the existence of an older stellar population which is expected if the bridge was formed by tidal interactions between the Magellanic Clouds, which stripped gas and stars from the clouds.

Lessons from the Magellanic System and its modeling

ABSTRACTThe prominent Magellanic Stream that dominates the H i sky provides a tantalizing number of observations that potentially constrains the Magellanic Clouds and the Milky Way (MW) outskirts. Here we show that the ‘ram-pressure plus collision’ model naturally explain these properties, and is able to predict some of the most recent observations made after the model was made. These include the complexity of the stellar populations in the Magellanic Bridge, for which kinematics, ages, and distances are well measured, and the Northern Tidal Arm, for which the model predicts its formation from the MW tidal forces. It appears that this over-constrained model provides a good path to investigate the Stream properties. This contrasts with tidal models that reproduce only half of the Stream’s main properties, in particular a tidal tail cannot reproduce the observed inter-twisted filaments, and its gas content is not sufficiently massive to provide the large amount of H i and H ii gas associated to the Stream. Despite the efforts made to reproduce the large amounts of gas brought by the Clouds, it seems that no viable solution for the tidal model could be foreseen. Since the ‘ram-pressure plus collision’ model has not succeeded for a Large Magellanic Cloud mass above 2 × 1010 M⊙, we conjecture that a low mass is required to form the Stream.

The VISCACHA survey – IV. The SMC West Halo in 8D

ABSTRACT The structure of the Small Magellanic Cloud (SMC) is very complex, in particular in the periphery that suffers more from the interactions with the Large Magellanic Cloud (LMC). A wealth of observational evidence has been accumulated revealing tidal tails and bridges made up of gas, stars, and star clusters. Nevertheless, a full picture of the SMC outskirts is only recently starting to emerge with a 6D phase-space map plus age and metallicity using star clusters as tracers. In this work, we continue our analysis of another outer region of the SMC, the so-called West Halo, and combined it with the previously analysed Northern Bridge. We use both structures to define the Bridge and Counter-bridge trailing and leading tidal tails. These two structures are moving away from each other, roughly in the SMC–LMC direction. The West Halo form a ring around the SMC inner regions that goes up to the background of the Northern Bridge shaping an extended layer of the Counter-bridge. Four old Bridge clusters were identified at distances larger than 8 kpc from the SMC centre moving towards the LMC, which is consistent with the SMC–LMC closest distance of 7.5 kpc when the Magellanic Bridge was formed about 150Myr ago; this shows that the Magellanic Bridge was not formed only by pulled gas, but it also removed older stars from the SMC during its formation. We also found age and metallicity radial gradients using projected distances on sky, which are vanished when we use the real 3D distances.

A radio polarization study of magnetic fields in the Small Magellanic Cloud

ABSTRACT Observing the magnetic fields of low-mass interacting galaxies tells us how they have evolved over cosmic time and their importance in galaxy evolution. We have measured the Faraday rotation of 80 extra-galactic radio sources behind the Small Magellanic Cloud (SMC) using the CSIRO Australia Telescope Compact Array (ATCA) with a frequency range of 1.4–3.0 GHz. Both the sensitivity of our observations and the source density are an order-of-magnitude improvement on previous Faraday rotation measurements of this galaxy. The SMC generally produces negative rotation measures after accounting for the Milky Way foreground contribution, indicating that it has a mean coherent line-of-sight magnetic field strength of $-0.3\pm 0.1\, \mu$G, consistent with previous findings. We detect signatures of magnetic fields extending from the north and south of the Bar of the SMC. The random component of the SMC magnetic field has a strength of $\sim 5\, \mu$G with a characteristic size-scale of magneto-ionic turbulence &amp;lt;250 pc, making the SMC like other low-mass interacting galaxies. The magnetic fields of the SMC and Magellanic Bridge appear similar in direction and strength, hinting at a connection between the two fields as part of the hypothesized ‘pan-Magellanic’ magnetic field.

Residual velocities of Small Magellanic Cloud star clusters

ABSTRACT We analysed the largest Small Magellanic Cloud (SMC) cluster sample (32) with proper motions and radial velocity measurements, from which we obtained their space velocity components. By adopting as a reference, the recent best-fitting rotating disc of SMC star clusters as a function of the position angle, we computed their residual velocity vectors, and compared their magnitudes ($\Delta \, V$) with that of a cluster with residual velocity components equal to the velocity dispersions along the three independent SMC rotating disc axes of motion ($\Delta \, V$ = 60 km s−1). We found that clusters that belong to SMC tidally induced structures have $\Delta \, V\, \gt $ 50 km s−1, which suggests that space velocities of clusters in the process of escaping the rotating disc kinematics are measurably different. Studied clusters pertaining to a northern branch of the Magellanic Bridge, the main Magellanic Bridge, the Counter-Bridge, and the West Halo give support these findings. NGC 121, the oldest known SMC cluster, does not belong to any SMC tidal feature, and has $\Delta \, V$ = 64 km s−1, slightly above the boundary between bound and kinematically perturbed clusters.

The kinematics of Small Magellanic Cloud star clusters

We report results of the proper motions of 25 known Small Magellanic Cloud (SMC) clusters (ages ∼ 1–10 Gyr old) derived from Gaia EDR3 data sets. When these mean proper motions were gathered with existent radial velocity measurements to compose the clusters’ velocity vectors, we found the parameter values of a rotation disk that best resemble their observed motions, namely: central coordinates and distance, inclination and position angle of the line-of-nodes, proper motion in right ascension and declination, systemic velocity, rotation velocity, and velocity dispersion. The SMC cluster rotation disk seems to be, at some level, kinematically synchronized with the rotation of the field red giants recently modeled using DR2 data sets. Such a rotation disk is seen in the sky as a tilted edge-on disk, with a velocity dispersion perpendicular to it that is twice as large as that in the plane of the disk. Because the direction perpendicular to the disk is nearly aligned with the Magellanic Bridge, we interpret the larger velocity dispersion as a consequence of the SMC velocity stretching caused by the tidal interaction with the Large Magellanic Cloud. Rotation alone does not seem sufficient to explain the observed kinematic behaviors in the SMC.

Gaia Early Data Release 3

Context. This work is part of the Gaia Data Processing and Analysis Consortium papers published with the Gaia Early Data Release 3 (EDR3). It is one of the demonstration papers aiming to highlight the improvements and quality of the newly published data by applying them to a scientific case. Aims. We use the Gaia EDR3 data to study the structure and kinematics of the Magellanic Clouds. The large distance to the Clouds is a challenge for the Gaia astrometry. The Clouds lie at the very limits of the usability of the Gaia data, which makes the Clouds an excellent case study for evaluating the quality and properties of the Gaia data. Methods. The basis of our work are two samples selected to provide a representation as clean as possible of the stars of the Large Magellanic Cloud (LMC) and the Small Magellanic Cloud (SMC). The selection used criteria based on position, parallax, and proper motions to remove foreground contamination from the Milky Way, and allowed the separation of the stars of both Clouds. From these two samples we defined a series of subsamples based on cuts in the colour-magnitude diagram; these subsamples were used to select stars in a common evolutionary phase and can also be used as approximate proxies of a selection by age. Results. We compared the Gaia Data Release 2 and Gaia EDR3 performances in the study of the Magellanic Clouds and show the clear improvements in precision and accuracy in the new release. We also show that the systematics still present in the data make the determination of the 3D geometry of the LMC a difficult endeavour; this is at the very limit of the usefulness of the Gaia EDR3 astrometry, but it may become feasible with the use of additional external data. We derive radial and tangential velocity maps and global profiles for the LMC for the several subsamples we defined. To our knowledge, this is the first time that the two planar components of the ordered and random motions are derived for multiple stellar evolutionary phases in a galactic disc outside the Milky Way, showing the differences between younger and older phases. We also analyse the spatial structure and motions in the central region, the bar, and the disc, providing new insightsinto features and kinematics. Finally, we show that the Gaia EDR3 data allows clearly resolving the Magellanic Bridge, and we trace the density and velocity flow of the stars from the SMC towards the LMC not only globally, but also separately for young and evolved populations. This allows us to confirm an evolved population in the Bridge that is slightly shift from the younger population. Additionally, we were able to study the outskirts of both Magellanic Clouds, in which we detected some well-known features and indications of new ones.

The VISCACHA survey

Context. The interactions between the Small and Large Magellanic Clouds (SMC and LMC) created the Magellanic Bridge; a stream of gas and stars pulled out of the SMC towards the LMC about 150 Myr ago. The tidal counterpart of this structure, which should include a trailing arm, has been predicted by models but no compelling observational evidence has confirmed the Counter-Bridge so far. Aims. The main goal of this work is to find the stellar counterpart of the Magellanic Bridge and Counter-Bridge. We use star clusters in the SMC outskirts as they provide a 6D phase-space vector, age, and metallicity which help characterise the outskirts of the SMC. Methods. Distances, ages, and photometric metallicities were derived from fitting isochrones to the colour-magnitude diagrams from the VISCACHA survey. Radial velocities and spectroscopic metallicities were derived from the spectroscopic follow-up using GMOS in the CaII triplet region. Results. Among the seven clusters analysed in this work, five belong to the Magellanic Bridge, one belongs to the Counter-Bridge, and the other belongs to the transition region. Conclusions. The existence of the tidal counterpart of the Magellanic Bridge is evidenced by star clusters. The stellar component of the Magellanic Bridge and Counter-Bridge are confirmed in the SMC outskirts. These results are an important constraint for models that seek to reconstruct the history of the orbit and interactions between the LMC and SMC as well as constrain their future interaction including with the Milky Way.

Astrophysical properties of newly discovered Magellanic Cloud star clusters

New star cluster candidates projected toward the Large and Small Magellanic Clouds (LMC, SMC) have been recently discovered from relatively deep imaging surveys. Here, we conduct a sound analysis of 24 star cluster candidates located in the outer regions of the LMC and SMC using point spread function photometry produced by the Survey of the Magellanic Stellar History. With only one exception, the studied objects were shown to be genuine stellar aggregates. We drew our conclusions on their physical characteristics once their observed color-magnitude diagrams (CMDs) were statistically decontaminated by the presence of field stars. The resulting cleaned CMDs, for stars with assigned membership probabilities higher than 50%, were compared with synthetic CMDs generated for thousands of combinations of ages, distances, metallicities, star cluster masses, and binary fractions. The parameters of the best-matched synthetic CMDs obtained from a likelihood approach were adopted as the star cluster astrophysical properties. The present star cluster sample spans a wide range of distances, from those star clusters located in front of the LMC through those along the onset of the Magellanic Bridge up to those behind the SMC. Their ages reveal different formation episodes that took place over the course of galaxy formation and others as a consequence of interactions among galaxies. From their estimated metallicities and ages, we speculate on the possibility that relatively metal-deficient gaseous flows have existed between these galaxies during nearly the last one Gyr (log(age yr−1) ≈ 9.0), which facilitated the formation of young star clusters in the galaxy peripheries. Despide the LMC-SMC interactions, the studied star clusters are similar or more massive than their counterparts in the Milky Way, suggesting that tidal effects are relatively more important in our Galaxy.

Magellanic Mayhem: Metallicities and Motions

Abstract We assemble a catalog of Magellanic Cloud red giants from Data Release 2 of the Gaia mission and, utilizing machine-learning methods, obtain photometric metallicity estimates for them. In doing so, we are able to chemically map the entirety of the Magellanic System at once. Our maps reveal a plethora of substructure within our red giant sample, with the Large Magellanic Cloud (LMC) bar and spiral arm being readily apparent. We uncover a curious spiral-like feature in the southern portion of the LMC disk, hosting relatively metal-rich giants and likely a by-product of historic encounters with the Small Magellanic Cloud (SMC). Modeling the LMC as an inclined thin disk, we find a shallow metallicity gradient of −0.048 ± 0.001 dex kpc−1 out to ∼12° from the center of the dwarf. We see evidence that the SMC is disrupting, with its outer isodensity contours displaying the S-shape symptomatic of tidal stripping. On studying the proper motions of the SMC giants, we observe a population of them being violently dragged toward the larger Cloud. The perturbed stars predominantly lie in front of the SMC, and we interpret that they exist as a tidal tail of the dwarf, trailing in its motion and undergoing severe disruption from the LMC. We find the metallicity structure in the Magellanic Bridge region to be complex, with evidence for a composite nature in this stellar population, consisting of both LMC and SMC debris.

Discovery of O stars in the tidal Magellanic Bridge

The Magellanic Bridge, stretching between the Small and the Large Magellanic Cloud (SMC and LMC), is the nearest tidally stripped intergalactic environment. The Bridge has a significantly low average metallicity of Z ≲ 0.1 Z⊙. Here we report the first discovery of O-type stars in the Magellanic Bridge. Three massive O stars were identified thanks to the archival spectra obtained by the ESO’s Very Large Telescope FLAMES instrument. We analyze the spectra of each star using the Potsdam Wolf-Rayet (PoWR) non-local thermodynamic equilibrium model atmosphere code, which provides the physical parameters, ionizing photon fluxes, and surface abundances. The ages of the newly discovered O stars suggest that star formation in the Bridge is ongoing. Furthermore, the discovery of O stars in the Bridge implies that tidally stripped galactic tails containing low-density but highly dynamical gas are capable of producing massive O stars. The multi-epoch spectra indicate that all three O stars are binaries. Despite their spatial proximity to one another, these O stars are chemically distinct. One of them is a fast-rotating giant with nearly LMC-like abundances. The other two are main-sequence stars that rotate extremely slowly and are strongly metal depleted. We discover the most nitrogen-poor O star known to date. Taking into account the previous analyses of B stars in the Bridge, we interpret the various metal abundances as the signature of a chemically inhomogeneous interstellar medium (ISM), suggesting that the Bridge gas might have accreted during multiple episodes of tidal interaction between the Clouds. Attributing the lowest derived metal content to the primordial gas, the time of the initial formation of the Bridge may date back several billion years. Using the Gaia and Galex color-magnitude diagrams, we roughly estimate the total number of O stars in the Bridge and their total ionizing radiation. Comparing this with the energetics of the diffuse ISM, we find that the contribution of the hot stars to the ionizing radiation field in the Bridge is less than 10% and conclude that the main sources of ionizing photons are leaks from the LMC and SMC. We estimate a lower limit for the fraction of ionizing radiation that escapes from these two dwarf galaxies.

The VMC survey – XLI. Stellar proper motions within the Small Magellanic Cloud

ABSTRACT We used data from the near-infrared Visible and Infrared Telescope for Astronomy (VISTA) survey of the Magellanic Cloud system (VMC) to measure proper motions (PMs) of stars within the Small Magellanic Cloud (SMC). The data analysed in this study comprise 26 VMC tiles, covering a total contiguous area on the sky of ∼40 deg2. Using multi-epoch observations in the Ks band over time baselines between 13 and 38 months, we calculated absolute PMs with respect to ∼130 000 background galaxies. We selected a sample of ∼2160 000 likely SMC member stars to model the centre-of-mass motion of the galaxy. The results found for three different choices of the SMC centre are in good agreement with recent space-based measurements. Using the systemic motion of the SMC, we constructed spatially resolved residual PM maps and analysed for the first time the internal kinematics of the intermediate-age/old and young stellar populations separately. We found outward motions that point either towards a stretching of the galaxy or stripping of its outer regions. Stellar motions towards the North might be related to the ‘Counter Bridge’ behind the SMC. The young populations show larger PMs in the region of the SMC Wing, towards the young Magellanic Bridge. In the older populations, we further detected a coordinated motion of stars away from the SMC in the direction of the Old Bridge as well as a stream towards the SMC.

Kinematic complexity around NGC 419: resolving the proper motion of the cluster, the Small Magellanic Cloud, and the Magellanic bridge

ABSTRACT We present Hubble Space Telescope proper motions in the direction of the star cluster NGC 419 in the Small Magellanic Cloud. Because of the high precision of our measurements, for the first time it is possible to resolve the complex kinematics of the stellar populations located in the field, even along the tangential direction. In fact, the proper motions we measured allow us to separate cluster stars, which move on average with ($\mu _{\alpha }\cos \delta ^{\rm NGC\, 419}, \mu _{\delta }^{\rm NGC\, 419}$) = (+0.878 ± 0.055, −1.246 ± 0.048) mas yr−1, from those of the Small Magellanic Cloud and those belonging to a third kinematic feature that we recognize as part of the Magellanic Bridge. Resolving such a kinematic complexity enables the construction of decontaminated colour–magnitude diagrams, as well as the measurement of the absolute proper motion of the three separate components. Our study therefore sets the first steps towards the possibility of dynamically investigating the Magellanic system by exploiting the resolved kinematics of its stellar clusters.

Gaia view of a stellar sub-structure in front of the Small Magellanic Cloud

ABSTRACT Recent observational studies identified a foreground stellar sub-structure traced by red clump (RC) stars (∼12 kpc in front of the main body) in the eastern regions of the Small Magellanic Cloud (SMC) and suggested that it formed during the formation of the Magellanic Bridge (MB), due to the tidal interaction of the Magellanic Clouds. Previous studies investigated this feature only up to 4${_{.}^{\circ}}$0 from the centre of the SMC due to the limited spatial coverage of the data and hence could not find a physical connection with the MB. To determine the spatial extent and properties of this foreground population, we analysed data from the Gaia Data Release 2 (DR2) of a ∼314 deg2 region centred on the SMC, which cover the entire SMC and a significant portion of the MB. We find that the foreground population is present only between 2${_{.}^{\circ}}$5 to ∼5°–6° from the centre of the SMC in the eastern regions, towards the MB and hence does not fully overlap with the MB in the plane of the sky. The foreground stellar population is found to be kinematically distinct from the stellar population of the main body with ∼35 km s−1 slower tangential velocity and moving to the north-west relative to the main body. Though the observed properties are not fully consistent with the simulations, a comparison indicates that the foreground stellar structure is most likely a tidally stripped counterpart of the gaseous MB and might have formed from the inner disc (dominated by stars) of the SMC. A chemical and 3D kinematic study of the RC stars along with improved simulations, including both tidal and hydro-dynamical effects, are required to understand the offset between the foreground structure and MB.