Recently, Myeong et al. Based on clustering algorithms performed over a scaled action space, these authors found five GCs likely associated to this system. We find seven GCs to be possibly associated when considering a selection box in E and L Z corresponding to the stars from Sequoia according to Myeong et al. Three clusters have known ages and these follow a low-normalisation AMR similar to the H99 GCs, which is consistent with the low stellar mass estimated for Sequoia Myeong et al.
Nonetheless, there may be a slight preference for Sequoia given their ages and metallicities. On the other hand, Myeong et al. We were not able to associate 36 of the GCs with full phase-space information to known merger events. From their distribution in the IOM space Fig. Therefore, they cannot have a common origin. Most likely instead, they have been accreted from different low-mass progenitors which have not contributed debris field stars to the Solar vicinity as otherwise we would have identified corresponding overdensities in Fig.
For convenience only, we use a single label for all these objects H-E, for high energy in Table A. Upcoming datasets, especially of field stars with full phase-space information across the Galaxy, could be key to understanding their origin given their heterogeneous properties. The various panels in Fig. Although consistency with the AMR of each group is checked after the IOM selection, it is quite remarkable that the dynamical identification of associations of GC results in AMRs that are all well-defined and depict different shapes or amplitudes.
The corresponding progenitors are marked in the labels. Individual age uncertainties are also plotted. In the AMR for Main Progenitor clusters upper-left panel , diamonds represents bulge clusters while circles describe disc clusters.
The two red-circled black symbols are the two clusters that satisfy the disc membership criteria but that are excluded because they are near the boundary of the respective IOM region and their location in the AMR.
The clusters of the Main progenitor constitute the largest group and have the highest normalisation, that is the most metal-poor oldest clusters were born in the Galaxy itself. The G-E AMR is remarkably tight and has a high normalisation, though as expected this is not as high as that of the Main progenitor. Similarly, the L-E group depicts a reasonably coherent AMR, with a high normalisation, which seems even higher than that of G-E clusters, thus possibly suggesting the presence of yet-to-be-discovered merger debris located preferentially in the Galactic bulge and originating in a more massive object.
We can describe the various AMRs with a leaky-box chemical evolution model Prantzos ; Leaman et al. We obtained this expression by assuming a constant star formation rate starting at time t i after the Big Bang and ending at time t f , which we take to be the time of accretion which is constrained by estimates in the literature and which we took to range from 3. The time t i is a free parameter which we varied for each progenitor to obtain a reasonable description of the observed points.
The only constraint we apply is that more massive progenitors should start forming stars earlier, which led to t i values in the range of 0. The resulting curves for each progenitor are shown in the panels of Fig. We stress that these curves do not represent fits, but merely show that a simple leaky box chemical evolution model is reasonably adequate to describe the AMRs found for each set of clusters associated to the individual progenitors.
This might indicate that for the clusters lacking an age uncertainty, our assumed value of 0. In this Letter we exploit the complete kinematic information for Galactic GCs, in combination with metallicity and homogeneous age estimates for a subset of 69 GCs. Our goal is to elucidate which GCs formed in situ and which could have been accreted, associating the latter to a particular progenitor based on their dynamical properties and, where needed, on the shape of the AMR.
We found that 62 GCs likely formed in the Milky Way, and we separated them into disc and bulge clusters based on their orbital parameters. Among the accreted clusters, we assessed their possible associations with the progenitor of four known merger events: Gaia -Enceladus, the Sagittarius dwarf, the Helmi streams, and the Sequoia galaxy.
We identified 26 and an additional 6 tentative GCs associated to Gaia -Enceladus. This large number as well as the high normalisation of their AMRs are consistent with Gaia -Enceladus being the most massive among these four objects. We further identified eight clusters associated with the Sagittarius dwarf, possibly ten clusters to the progenitor of the Helmi streams, and a plausible seven to Sequoia.
Despite not being very populated, the AMRs of these two groups are consistent with the lower literature estimates for their mass. There is an inherent uncertainty to these assignments, because debris from different progenitors partly overlaps in IOM space, as in the case of Sequoia and G-E, and to a lesser degree for G-E and the Helmi streams.
These were derived with different methods, using chemistry in the first case Helmi et al. In the case of Sequoia, we used the mass estimated by Myeong et al.
The 36 clusters that we have not associated to known debris can be split in two groups based on orbital energy. While the class of GCs with low binding energy is very heterogeneous and likely has several sites of origin, the low-energy highly bound group with 25 tentative members is relatively highly clustered in its dynamical properties and shows a reasonably tight and high-normalisation AMR, possibly suggesting the presence of debris towards the Galactic bulge from a large hitherto unknown galaxy.
We note however, that most of the clusters reported in Kruijssen et al. Therefore, although the L-E group is Kraken-like, the associated clusters are different. It transpires that taking into account the dynamical properties is fundamental to establishing the origin of the different GCs of our Galaxy.
The next data release of the Gaia mission will provide improved astrometry and photometry for all the GCs, as well as for a much larger sample halo stars. This will be crucial to achieving a complete and accurate sample of GCs with absolute ages. Moreover it will lead to a better understanding of the debris of the known progenitors, and possibly to the discovery of new ones.
The combination of these factors will result in significant progress in the field and allow to better pin down tentative associations and possibly also to assess under which conditions the different clusters formed, such as for example whether formation was prior to or during the different merger events. Nuclear clusters of dwarf galaxies accreted long ago could also end up on bulge-like orbits due to dynamical friction.
The current uncertainty on the proper motions of bulge GCs they are typically highly extincted and only few stars are detected by Gaia , and the lack of an age estimate for most of them, prevents us from investigating this possibility. For which we used the metallicity of the most metal-poor and oldest population, see Bellini et al.
We thank the anonymous referee for comments and suggestions which improved the quality of our paper. Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.
To put together the sample of GCs, we started from the 75 GCs analysed by Gaia Collaboration b , who combined the Gaia measured proper motions with distances and line-of-sight velocities available from the compilation by Harris , edition.
An analogy in this case may be to think of them like the tallest skyscrapers in a city. From a distance, you can see these tall buildings very easily, allowing you to determine roughly your distance from the city. You can also estimate how big the city is by how many skyscrapers it contains and how spread out they are. He measured their positions and distances and plotted their locations on a two-dimensional chart.
I have reproduced his work in the two-dimensional plot below, but using modern data for the distances and locations of all known globular clusters.
The hatched region represents the plane of the Milky Way that is, roughly the part of the sky where the Milky Way is visible to the eye , and the X located at 0,0 marks the location of the Sun in the plane of the Milky Way.
The Galaxy fills a 3D region in space, so this 2D plot only shows a slice from the top to the bottom through the plane of the Milky Way. Shapley's data was not as extensive as in the plot shown above, nor was it as accurate.
Similar to Herschel and Kapteyn, dust extinction and reddening affected his distance measurements to the clusters, and thus his conclusions as well. Because dust makes stars appear fainter than they truly are, if you do not account for the amount of extinction, you will overestimate the distances to these objects.
This is just what Shapley did. However, the data he did have allowed him to make two very important discoveries:. Globular clusters are incredibly dense structures often featuring hundreds of thousands of stars packed into a relatively small space.
One of the densest globular clusters in the Milky Way, M80 is located roughly 28, light-years from Earth and holds hundreds of thousands of stars.
Globular clusters formed from giant molecular clouds, or huge masses of gas that form stars as they collapse. Because there is less free gas available now than at the beginning of the universe, globular clusters generally cannot form today. Globular clusters GCs are spheroidal collections of , to a million stars found orbiting in the halos of all large galaxies.
Some GCs have been shown to be almost as old as the age of the Universe, making them among the oldest stellar populations known. Star cluster, either of two general types of stellar assemblages held together by the mutual gravitational attraction of its members, which are physically related through common origin. The two types are open formerly called galactic clusters and globular clusters. Stars in an open cluster have a common origin — they formed from the same initial giant molecular cloud.
Clusters typically contain a few hundred stars though this can vary from as low as a few dozen up to a few thousand. The three basic types of clusters astronomers have discovered are globular clusters, open clusters, and stellar associations. Advantages in studying stars clusters. Also, if a cluster is close enough, we can measure proper motions. The proper motions will converge toward a point in the sky, which indicates the direction of travel for the cluster.
By determining the mass of the main-sequence turnoff stars, we get the age of the cluster. The cluster age equals the main-sequence lifetime of the turnoff stars. This is one of the ways we have studied the age of the Universe and the formation history of the Galaxy. Star Clusters. When stars are born they develop from large clouds of molecular gas.
This means that they form in groups or clusters, since molecular clouds are composed of hundreds of solar masses of material. After the remnant gas is heated and blow away, the stars collect together by gravity. Eventual fate These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.
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