Stellar Stream Research Articles

Scalable inference with autoregressive neural ratio estimation

ABSTRACT In recent years, there has been a remarkable development of simulation-based inference (SBI) algorithms, and they have now been applied across a wide range of astrophysical and cosmological analyses. There are a number of key advantages to these methods, centred around the ability to perform scalable statistical inference without an explicit likelihood. In this work, we propose two technical building blocks to a specific sequential SBI algorithm, truncated marginal neural ratio estimation (TMNRE). In particular, first, we develop autoregressive ratio estimation with the aim to robustly estimate correlated high-dimensional posteriors. Secondly, we propose a slice-based nested sampling algorithm to efficiently draw both posterior samples and constrained prior samples from ratio estimators, the latter being instrumental for sequential inference. To validate our implementation, we carry out inference tasks on three concrete examples: a toy model of a multidimensional Gaussian, the analysis of a stellar stream mock observation, and finally, a proof-of-concept application to substructure searches in strong gravitational lensing. In addition, we publicly release the code for both the autoregressive ratio estimator and the slice sampler.

Extending the chemical reach of the H3 survey: detailed abundances of the dwarf-galaxy stellar stream Wukong/LMS-1

ABSTRACT We present the first detailed chemical-abundance analysis of stars from the dwarf-galaxy stellar stream Wukong/LMS-1 covering a wide metallicity range ($-3.5 \lt \rm [Fe/H] \lesssim -1.3$). We find abundance patterns that are effectively indistinguishable from the bulk of Indus and Jhelum, a pair of smaller stellar streams proposed to be dynamically associated with Wukong/LMS-1. We confirmed a carbon-enhanced metal-poor star ($\rm [C/Fe] \gt +0.7$ and $\rm [Fe/H] \sim -2.9$) in Wukong/LMS-1 with strong enhancements in Sr, Y, and Zr, which is peculiar given its solar-level [Ba/Fe]. Wukong/LMS-1 stars have high abundances of α elements up to $\rm [Fe/H] \gtrsim -2$, which is expected for relatively massive dwarfs. Towards the high-metallicity end, Wukong/LMS-1 becomes α-poor, revealing that it probably experienced fairly standard chemical evolution. We identified a pair of N- and Na-rich stars in Wukong/LMS-1, reminiscent of multiple stellar populations in globular clusters. This indicates that this dwarf galaxy contained at least one globular cluster that was completely disrupted in addition to two intact ones previously known to be associated with Wukong/LMS-1, which is possibly connected to similar evidence found in Indus. From these ≥3 globular clusters, we estimate the total mass of Wukong/LMS-1 to be ${\approx }10^{10} \, \mathrm{M}_\odot$, representing ∼1 per cent of the present-day Milky Way. Finally, the [Eu/Mg] ratio in Wukong/LMS-1 continuously increases with metallicity, making this the first example of a dwarf galaxy where the production of r-process elements is clearly dominated by delayed sources, presumably neutron-star mergers.

Where do they come from?

Globular clusters (GCs), as old as our Galaxy, constantly lose their members to the field as they cross through the Milky Way (MW). These escaped GC stars (or escapees) are thought to contribute significantly to the MW halo. If a star left the host GC a long time ago, chemical finger prints (e.g., N enrichment) may reveal its origin. In this work we aim to establish dynamical connections between N-rich field stars recently identified by LAMOST and the existing MW GCs. By constructing the full action distribution in combination with metallicity, we found 29 potential GC progenitors for 15 N-rich field stars. In particular, some of them may be related to MW accretion events. On the other hand, if a star has recently left its host GC via tidal evaporation, it still maintains the kinematic properties of the cluster. Here we identify extra-tidal candidates based on their spatial locations, proper motions (PMs), and their positions on color-magnitude diagrams (CMDs). We successfully identified more than 1600 extra-tidal candidates in the vicinity of six Gaia-Enceladus (GE)-related GCs: NGC 1851, NGC 1904, NGC 6205, NGC 6341, NGC 6779, NGC 7089. The density map of the extra-tidal candidates is confirmed to be an efficient way to find extra-tidal structures. The two possible density peaks at opposite sides of the inner boundary is a good indicator for a long stellar stream. Among 95 extra-tidal candidates with spectroscopic radial velocities and metallicity, 54 of them are confirmed to be GC escaped stars as they share similar properties to host GCs. These extra-tidal candidates are ideal targets for follow-up spectroscopic observation as it greatly improves the scientific outcome. Once a statistically significant number of spectroscopic radial velocities and metallicities are available, the GC dynamical evolution (e.g., mass loss, rotation) can be carefully investigated.

A New Tidal Stream Discovered in Gaia DR3

Thanks to the precise astrometric measurements of proper motions by the Gaia mission, a new tidal stellar stream has been discovered in the northern hemisphere. The distribution of star count shows that the stream is approximately 80° long and 1.°70 wide. Observations of 21 member stars, including 14 RR Lyrae stars, indicate that the stream has an eccentric and retrograde orbit with e = 0.58. The low metallicity, high total energy, and large angular momentum suggest that it is associated with the merging event Sequoia. This discovery suggests the possibility of finding more substructures with high eccentricity orbits, even in the inner halo.

Chemical Cartography of the Sagittarius Stream with Gaia

The stellar stream connected to the Sagittarius (Sgr) dwarf galaxy is the most massive tidal stream that has been mapped in the Galaxy, and is the dominant contributor to the outer stellar halo of the Milky Way (MW). We present metallicity maps of the Sgr stream, using 34,240 red giant branch stars with inferred metallicities from Gaia BP/RP spectra. This sample is larger than previous samples of Sgr stream members with chemical abundances by an order of magnitude. We measure metallicity gradients with respect to Sgr stream coordinates (Λ, B), and highlight the gradient in metallicity with respect to stream latitude coordinate B, which has not been observed before. Including the core, we find ∇[M/H] = −2.48 ± 0.08 × 10−2 dex deg−1 above the stream track (B > B 0, where B 0 = 1.5° is the latitude of the Sgr remnant) and ∇[M/H] = −2.02 ± 0.08 × 10−2 dex deg−1 below the stream track (B < B 0). By painting metallicity gradients onto a tailored N-body simulation of the Sgr stream, we find that the observed metallicities in the stream are consistent with an initial radial metallicity gradient in the Sgr dwarf galaxy of ∼−0.1 to −0.2 dex kpc−1, well within the range of observed metallicity gradients in Local Group dwarf galaxies. Our results provide novel observational constraints for the internal structure of the dwarf galaxy progenitor of the Sgr stream. Leveraging new large data sets in conjunction with tailored simulations, we can connect the present-day properties of disrupted dwarfs in the MW to their initial conditions.

Swarming in stellar streams: Unveiling the structure of the Jhelum stream with ant colony-inspired computation

The halo of the Milky Way galaxy hosts multiple dynamically coherent substructures known as stellar streams that are remnants of tidally disrupted orbiting systems such as globular clusters (GCs) and dwarf galaxies (DGs). A particular case is that of the Jhelum stream, which is known for its unusual and complex morphology. Using the available data from the Gaia DR3 catalog, we extracted a region on the sky that contains Jhelum, and fine-tuned this selection by enforcing limits on the magnitude and proper motion of the selected stars. We then applied the novel Locally Aligned Ant Technique (LAAT) on the position and proper motion space of stars belonging to the selected region to highlight the stars that are closely aligned with a local manifold in the data and the stars belonging to regions of high local density. We find that the overdensity representing the stream in proper motion space is composed of two components, and show the correspondence of these two signals to the previously reported narrow and broad spatial components of Jhelum. We then made use of the radial velocity measurements provided by the S5 survey and confirm, for the first time, a separation between the stars belonging to the two components in radial velocity. We show that the narrow and broad components have velocity dispersions of 4.84+1.23-0.79 km s-1 and 19.49+2.19-1.84 km s−1, and metallicity dispersions of 0.15+0.18-0.10 and 0.34+0.13-0.09, respectively. These measurements, as well as the given difference in component widths, could be explained with a probable scenario where Jhelum is the remnant of a GC embedded within a DG and where both were accreted onto the Milky Way during their infall. Although the properties of Jhelum could be explained with this merger scenario, other progenitors of the narrow component remain possible such as a nuclear star cluster or a DG. To rule these possibilities out, we would need more observational data of member stars of the stream. Our analysis shows that the internal structure of streams holds great information on their past formation history, and therefore provides further insight into the merger history of the Milky Way.

Introducing the Condor array telescope II – deep imaging observations of the edge-on spiral galaxy NGC 5907 and the NGC 5866 Group: yet another view of the iconic stellar stream

ABSTRACT We used the Condor array telescope to obtain deep imaging observations through the luminance filter of the entirety of the NGC 5866 Group, including a very extended region surrounding the galaxy NGC 5907 and its stellar stream. We find that the stellar stream consists of a single curved structure that stretches 220 kpc from a brighter eastern stream to a fainter western stream that bends to the north and then curls back toward the galaxy. This result runs contrary to a previous claim of a second loop of the stellar stream but is consistent with another previous description of the overall morphology of the stream. We further find that: (1) an extension of the western stream appears to bifurcate near its apex, (2) there is an apparent gap of ≈6 kpc in the western stream due east of the galaxy, (3) contrary to a previous claim, there is no evidence of the remnant of a progenitor galaxy within the eastern stream, although (4) there are many other possible progenitor galaxies, (5) there is another structure that, if it is at the distance of the galaxy, stretches 240 kpc and contains two very large, very low-surface-brightness ‘patches’ of emission, one of which was noted previously and another of which was not. We note the number and variety of stellar streams in the vicinity of NGC 5907 and the apparent gap in the western stream, which may be indicative of a dark sub-halo or satellite in the vicinity of the galaxy.

Via Machinae 2.0: Full-sky, model-agnostic search for stellar streams in Gaia DR2

ABSTRACT We present an update to Via Machinae, an automated stellar stream-finding algorithm based on the deep learning anomaly detector ANODE. Via Machinae identifies stellar streams within Gaia, using only angular positions, proper motions, and photometry, without reference to a model of the Milky Way potential for orbit integration or stellar distances. This new version, Via Machinae 2.0, includes many improvements and refinements to nearly every step of the algorithm, that altogether result in more robust and visually distinct stream candidates than our original formulation. In this work, we also provide a quantitative estimate of the false positive rate of Via Machinae 2.0 by applying it to a simulated Gaia-mock catalogue based on galaxia, a smooth model of the Milky Way that does not contain substructure or stellar streams. Finally, we perform the first full-sky search for stellar streams with Via Machinae 2.0, identifying 102 streams at high significance within the Gaia Data Release 2, of which only 10 have been previously identified. While follow-up observations for further confirmation are required, taking into account the false positive rate presented in this work, we expect approximately 90 of these stream candidates to correspond to real stellar structures.

Stellar Population Properties in the Stellar Streams around SPRC047

We have investigated the properties (e.g., age, metallicity) of the stellar populations of a ringlike tidal stellar stream (or streams) around the edge-on galaxy SPRC047 (z = 0.031) using spectral energy distribution (SED) fits to integrated broadband aperture flux densities. We used visual images in six different bands and Spitzer/IRAC 3.6 μm data. We have attempted to derive best-fit stellar population parameters (metallicity, age) in three noncontiguous segments of the stream. Due to the very low surface brightness of the stream, we have performed a deconvolution with a Richardson–Lucy–type algorithm of the low spatial resolution 3.6 μm IRAC image, thereby reducing the effect of the point-spread function aliased emission from the bright edge-on central galaxy at the locations of our three stream segments. Our SED fits that used several different star formation (SF) history priors, from an exponentially decaying SF burst to continuous SF, indicate that the age–metallicity–dust degeneracy is not resolved, most likely because of inadequate wavelength coverage and low signal-to-noise ratios of the low surface brightness features. We also discuss how future deep visual–near-infrared observations, combined with absolute flux calibration uncertainties at or below the 1% level, complemented by equally well absolute flux-calibrated observations in ultraviolet and mid-infrared bands, would improve the accuracy of broadband SED fitting results for low surface brightness targets, such as stellar streams around nearby galaxies that are not resolved into stars.

Prospects for Detecting Gaps in Globular Cluster Stellar Streams in External Galaxies with the Nancy Grace Roman Space Telescope

Stellar streams form through the tidal disruption of satellite galaxies or globular clusters orbiting a host galaxy. Globular cluster streams are exciting since they are thin (dynamically cold) and therefore sensitive to perturbations from low-mass subhalos. Since the subhalo mass function differs depending on the dark matter composition, these gaps can provide unique constraints on dark matter models. However, current samples are limited to the Milky Way. With its large field of view, deep imaging sensitivity, and high angular resolution, the upcoming Nancy Grace Roman Space Telescope (Roman) presents a unique opportunity to increase the number of observed streams and gaps significantly. This paper presents a first exploration of the prospects for detecting gaps in streams in M31 and other nearby galaxies with resolved stars. We simulate the formation of gaps in a Palomar 5–like stream and generate mock observations of these gaps with background stars in M31 and foreground Milky Way stellar fields. We assess Roman's ability to detect gaps out to 10 Mpc through visual inspection and with the gap-finding tool FindTheGap. We conclude that gaps of ≈1.5 kpc in streams that are created from subhalos of masses ≥5 × 106 M ⊙ are detectable within a 2–3 Mpc volume in exposure times of 1000 s to 1 hr. This volume contains ≈150 galaxies, including ≈eight galaxies with luminosities >109 L ⊙. Large samples of stream gaps in external galaxies will open up a new era of statistical analyses of gap characteristics in stellar streams and help constrain dark matter models.

A catalogue of Galactic GEMS: Globular cluster Extra-tidal Mock Stars

ABSTRACT This work presents the Globular cluster Extra-tidal Mock Star (GEMS) catalogue of extra-tidal stars and binaries created via three-body dynamical encounters in globular cluster cores. Using the particle-spray code Corespray, we sample $N=50\, 000$ extra-tidal stars and escaped recoil binaries for 159 Galactic globular clusters. Sky positions, kinematics, stellar properties, and escape information are provided for all simulated stars. Stellar orbits are integrated in seven different static and time-varying Milky Way gravitational potential models where the structure of the disc, perturbations from the Large Magellanic Cloud and the mass and sphericity of the Milky Way’s dark matter halo are all investigated. We find that the action coordinates of the mock extra-tidal stars are largely Galactic model independent, where minor offsets and broadening of the distributions between models are likely due to interactions with substructure. Importantly, we also report the first evidence for stellar stream contamination by globular cluster core stars and binaries for clusters with pericentre radii larger than five kiloparsecs. Finally, we provide a quantitative tool that uses action coordinates to match field stars to host clusters with probabilities. Ultimately, combining data from the GEMS catalogue with information of observed stars will allow for association of extra-tidal field stars with any Galactic globular cluster; a requisite tool for understanding population-level dynamics and evolution of clusters in the Milky Way.

Multiple populations and a CH star found in the 300S globular cluster stellar stream

ABSTRACT Milky Way globular clusters (GCs) display chemical enrichment in a phenomenon called multiple stellar populations (MSPs). While the enrichment mechanism is not fully understood, there is a correlation between a cluster’s mass and the fraction of enriched stars found therein. However, present-day GC masses are often smaller than their masses at the time of formation due to dynamical mass-loss. In this work, we explore the relationship between mass and MSPs using the stellar stream 300S. We present the chemical abundances of eight red giant branch member stars in 300S with high-resolution spectroscopy from Magellan/MIKE. We identify one enriched star characteristic of MSPs and no detectable metallicity dispersion, confirming that the progenitor of 300S was a GC. The fraction of enriched stars (12.5 per cent) observed in our 300S stars is less than the 50 per cent of stars found enriched in Milky Way GCs of comparable present-day mass (∼104.5 $\mathrm{\, {\rm M}_{\odot }}$). We calculate the mass of 300S’s progenitor and compare it to the initial masses of intact GCs, finding that 300S aligns well with the trend between the system mass at formation and enrichment. 300S’s progenitor may straddle the critical mass threshold for the formation of MSPs and can therefore serve as a benchmark for the stellar enrichment process. Additionally, we identify a CH star, with high abundances of s-process elements, probably accreted from a binary companion. The rarity of such binaries in intact GCs may imply stellar streams permit the survival of binaries that would otherwise be disrupted.

The effect of relativistic precession on light curves of tidal disruption events

ABSTRACT The disruption of a star by the tidal forces of a spinning black hole causes the stellar stream to precess, affecting the conditions for triggering the tidal disruption event (TDE). In this work, we study the effect that precession imprints on TDE light curves due to the interaction of the TDE wind and luminosity with the stream wrapped around the black hole. We perform two-dimensional radiation-hydrodynamic simulations using the moving-mesh hydrodynamic code jet with its radiation treatment module. We study the impact of black hole mass, accretion efficiency, and inclination between the orbital and spin planes. From our results, we identified two behaviours: (i) models with low-mass black holes (Mh ∼ 106 M⊙), low inclination (i ∼ 0), and low accretion efficiency (η ∼ 0.01) show light curves with a short early peak caused by the interaction of the wind with the inner edge of the stream. The line of sight has little effect on the light curve, since the stream covers a small fraction of the solid angle due to the precession occurring in the orbital plane; and (ii) models with high-mass black holes (Mh ≳ 107 M⊙), high inclination (i ∼ 90°), and high accretion efficiency (η ∼ 0.1) produce light curves with luminosity peaks that can be delayed by up to 50–100 d depending on the line of sight due to presence of the precessed stream blocking the radiation in the early phase of the event. Our results show that black hole spin and misalignment do not imprint recognizable features on the light curves but rather can add complications to their analysis.

Tidal Disruption Events through the Lens of the Cooling Envelope Model

The cooling envelope model for tidal disruption events (TDEs) postulates that while the stellar debris streams rapidly dissipate their bulk kinetic energy (“circularize”), this does not necessarily imply rapid feeding of the supermassive black hole (SMBH). The bound material instead forms a large pressure-supported envelope that powers optical/UV emission as it undergoes gradual Kelvin–Helmholtz contraction. We present results interpreting a sample of 15 optical TDEs within the cooling envelope model in order to constrain the SMBH mass M BH, stellar mass M ⋆, and orbital penetration factor β. The distributions of inferred properties from our sample broadly follow the theoretical expectations of loss-cone analysis assuming a standard stellar initial mass function. However, we find a deficit of events with M BH ≲ 5 × 105 and M ⋆ ≲ 0.5 M ⊙, which could result in part from the reduced detectability of TDEs with these properties. Our model fits also illustrate the predicted long delay between the optical light-curve peak and when the SMBH accretion rate reaches its maximum. The latter occurs only once the envelope contracts to the circularization radius on a timescale of months to years, consistent with delayed-rising X-ray and nonthermal radio flares seen in a growing number of TDEs.

The Age Distribution of Stellar Orbit Space Clumps

The orbit distribution of young stars in the Galactic disk is highly structured, from well-defined clusters to streams of stars that may be widely dispersed across the sky, but are compact in orbital action-angle space. The age distribution of such groups can constrain the timescales over which conatal groups of stars disperse into the “field.” Gaia data have proven powerful in identifying such groups in action-angle space, but the resulting member samples are often too small and have too narrow a color–magnitude diagram (CMD) coverage to allow robust age determinations. Here, we develop and illustrate a new approach that can estimate robust stellar population ages for such groups of stars. This first entails projecting the predetermined action-angle distribution into the 5D space of positions, parallaxes, and proper motions, where much larger samples of likely members can be identified over a much wider range of the CMD. It then entails isochrone fitting that accounts for: (a) widely varying distances and reddenings; (b) outliers and binaries; (c) sparsely populated main-sequence turnoffs, by incorporating the age information of the low-mass main sequence; and (d) the possible presence of an intrinsic age spread in the stellar population. When we apply this approach to 92 nearby stellar groups identified in 6D orbit space, we find that they are predominantly young (≲1 Gyr), mono-age populations. Many groups are established (known) localized clusters with possible tidal tails, while others tend to be widely dispersed and manifestly unbound. This new age-dating tool offers a stringent approach to understanding on which orbits stars form in the solar neighborhood and how quickly they disperse into the field.

Chemical Analysis of the Brightest Star of the Cetus II Ultrafaint Dwarf Galaxy Candidate* * This paper includes data gathered with the 6.5 m Magellan Telescopes located at Las Campanas Observatory, Chile.

We present a detailed chemical abundance analysis of the brightest star in the ultrafaint dwarf (UFD) galaxy candidate Cetus II from high-resolution Magellan/MIKE spectra. For this star, DES J011740.53-173053, abundances or upper limits of 18 elements from carbon to europium are derived. Its chemical abundances generally follow those of other UFD galaxy stars, with a slight enhancement of the α-elements (Mg, Si, and Ca) and low neutron-capture element (Sr, Ba, and Eu) abundances supporting the classification of Cetus II as a likely UFD. The star exhibits lower Sc, Ti, and V abundances than Milky Way (MW) halo stars with similar metallicity. This signature is consistent with yields from a supernova originating from a star with a mass of ∼11.2 M ⊙. In addition, the star has a potassium abundance of [K/Fe] = 0.81, which is somewhat higher than the K abundances of MW halo stars with similar metallicity, a signature that is also present in a number of UFD galaxies. A comparison including globular clusters and stellar stream stars suggests that high K is a specific characteristic of some UFD galaxy stars and can thus be used to help classify objects as UFD galaxies.

Weakly supervised anomaly detection in the Milky Way

ABSTRACT Large-scale astrophysics data sets present an opportunity for new machine learning techniques to identify regions of interest that might otherwise be overlooked by traditional searches. To this end, we demonstrate how Classification Without Labels (CWoLa), a weakly supervised anomaly detection method, can help identify cold stellar streams within the more than one billion Milky Way stars observed by the Gaia satellite. CWoLa operates without the use of labelled streams or knowledge of astrophysical principles. Instead, it uses a classifier to distinguish between mixed samples for which the proportions of signal and background samples are unknown. As a proof of concept, we demonstrate that this computationally lightweight strategy is able to detect both simulated streams and the known stream GD-1 in data. Originally designed for high-energy collider physics, this technique may have broad applicability within astrophysics as well as other domains interested in identifying localized anomalies.

The origin of the metallicity distributions of the NE and W stellar shelves in the Andromeda Galaxy

ABSTRACT Tidal streams and stellar shells are naturally formed in galaxy interactions and mergers. The Giant Stellar Stream (GSS), the North-East (NE), and Western (W) stellar shelves observed in Andromeda Galaxy (M31) are examples of these structures and were formed through the merger of M31 and a satellite galaxy. Recent observational papers have provided strong evidence that the shells and GSS originate from a single progenitor. In this paper, we investigate the formation of these two stellar shelves and the detailed nature of their relationship to the GSS. We present numerical simulations of tidal disruption of a satellite galaxy assuming that it is a progenitor of the GSS and the shell system. We represent the progenitor as a dwarf spheroidal galaxy with the stellar mass of 109 M⊙ and evolve its merger with M31 for 3 Gyr to reproduce the chemodynamical properties of the NE and W shelves. We find that an initial metallicity of the progenitor with a negative radial gradient of ΔFeH = −0.3 ± 0.2 successfully reproduces observed metallicities of the NE, W shelves, and the GSS, showing that all these structures can originate from the same merger event.

A giant thin stellar stream in the Coma Galaxy Cluster

The study of dynamically cold stellar streams reveals information about the gravitational potential where they reside and provides important constraints on the properties of dark matter. However, the intrinsic faintness of these streams makes their detection beyond Local environments highly challenging. Here, we report the detection of an extremely faint stellar stream (μg, max = 29.5 mag arcsec−2) with an extraordinarily coherent and thin morphology in the Coma Galaxy Cluster. This Giant Coma Stream spans ∼510 kpc in length and appears as a free-floating structure located at a projected distance of 0.8 Mpc from the center of Coma. We do not identify any potential galaxy remnant or core, and the stream structure appears featureless in our data. We interpret the Giant Coma Stream as being a recently accreted, tidally disrupting passive dwarf. Using the Illustris-TNG50 simulation, we identify a case with similar characteristics, showing that, although rare, these types of streams are predicted to exist in Λ-CDM. Our work unveils the presence of free-floating, extremely faint and thin stellar streams in galaxy clusters, widening the environmental context in which these objects are found ahead of their promising future application in the study of the properties of dark matter.

The Metallicity Variations Along the Chromosome Maps: The Globular Cluster 47 Tucanae* *Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO program 105.20NB, and on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.

The “chromosome maps” (ChMs) of globular clusters (GCs) have revealed that these ancient structures are inhomogeneous in metallicity in various ways and in different natures. Type II GCs generally display larger variations, sometimes coupled with slow neutron-capture (s) element enrichments on the ChMs redder sequences, which have been interpreted as due to multiple generations of stars. On the other hand, most GCs have inhomogeneous first populations (1P) in the form of large ranges in the ΔF275W,F814W values, pointing toward a not fully mixed pristine molecular cloud. We analyze the chemical composition of GC 47 Tucanae, which shows both inhomogeneous 1P stars and, although not formally a Type II GC, hosts a small number of stars distributed on the red side of the main stream of ChM stars. Our results suggest that 1P stars are inhomogeneous in the overall metallicity, with variations on the order of ∼0.10 dex in all the chemical species. The anomalous stars distributed on a redder sequence of the ChM are further enriched in metals but have no evidence for a significant enrichment in the s elements. Our three second population stars located on the normal component of the map have metallicities similar to those of the metal-richer 1P oup, suggesting that this population formed from these stars. Although three stars is a too-small sample to draw strong conclusions, the low spread in metals of these objects might point toward formation in a fully mixed medium, possibly after a cooling flow phase.