The accreted nuclear clusters of the Milky Way

A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf,. Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster-cluster merger.

The relation between star formation, morphology, and local density in high-redshift clusters and groups

Pascale Jablonka, Anja Von der Linden

We investigate how the [O II] properties and the morphologies of galaxies in clusters and groups at z = 0.4-0.8 depend on projected local galaxy density, and compare with the field at similar redshifts and clusters at low z. In both nearby and distant clusters, higher density regions contain proportionally fewer star-forming galaxies, and the average [O II] equivalent width of star-forming galaxies is independent of local density. However, in distant clusters the average current star formation rate (SFR) in star-forming galaxies seems to peak at densities similar to 15-40 galaxies Mpc(-2). At odds with low-z results, at high z the relation between star-forming fraction and local density varies from high- to low-mass clusters. Overall, our results suggest that at high z the current star formation ( SF) activity in star-forming galaxies does not depend strongly on global or local environment, though the possible SFR peak seems at odds with this conclusion. We find that the cluster SFR normalized by cluster mass anticorrelates with mass and correlates with the star-forming fraction. These trends can be understood given (1) that the average star-forming galaxy forms about 1M(circle dot) yr(-1) (uncorrected for dust) in all clusters; (2) that the total number of galaxies scales with cluster mass; and (3) the dependence of star-forming fraction on cluster mass. We present the morphology-density ( MD) relation for our z 0.4 - 0.8 clusters, and uncover that the decline of the spiral fraction with density is entirely driven by galaxies of type Sc or later. For galaxies of a given Hubble type, we see no evidence that SF properties depend on local environment. In contrast with recent findings at low z, in our distant clusters the SF-density relation and the MD relation are equivalent, suggesting that neither of the two is more fundamental than the other.

2008

An extragalactic chromosome map: the intermediate-age SMC cluster Lindsay 1

Carmela Lardo

The discovery of star-to-star abundance variations (a.k.a. multiple populations - MPs) within globular clusters (GCs), which are generally not found in the field or in lower mass open clusters, has led to a search for the unique property of GCs that allow them to host this phenomenon. Recent studies have shown that MPs are not limited to the ancient GCs but are also found in massive clusters with ages down to (at least) 2 Gyr. This finding is important for understanding the physics of the MP phenomenon, as these young clusters can provide much stronger constraints (e.g. on potential age spreads within the clusters) than older ones. However, a direct comparison between ancient GCs and intermediate clusters has not yet been possible due to the different filters adopted in their studies. Here we present new Hubble Space Telescope UV photometry of the 7.5 Gyr, massive Small Magellanic Cloud cluster, Lindsay 1, in order to compare its pseudo colour-colour diagram to that of Galactic GCs. We find that they are almost identical and conclude that the MPs phenomenon is the same, regardless of cluster age and host galaxy.

OXFORD UNIV PRESS2019

Do the majority of stars form as gravitationally unbound?

Richard Irving Anderson

Context. Some of the youngest stars (age less than or similar to 10 Myr) are clustered, while many others are observed scattered throughout star forming regions or in complete isolation. It has been intensively debated whether such scattered or isolated stars originate in star clusters or whether they form in truly isolated conditions. Exploring these scenarios could help set constraints on the conditions in which massive stars are formed. Aims. We adopted the assumption that all stars form in gravitationally bound star clusters embedded in molecular cloud cores (Gamma-1 model), which expel their natal gas early after their formation. Then we compared the proportion (fraction) of stars found in clusters with observational data. Methods. The star clusters are modelled by the code NBODY6, which includes binary stars, stellar and circumbinary evolution, gas expulsion, and the external gravitational field of their host galaxy. Results. We find that small changes in the assumptions in the current theoretical model estimating the fraction, Gamma, of stars forming in embedded clusters have a large influence on the results, and we present a counterexample as an illustration. This calls into question theoretical arguments about Gamma in embedded clusters and it suggests that there is no firm theoretical ground for low Gamma in galaxies with lower star formation rates (SFRs). Instead, the assumption that all stars form in embedded clusters is in agreement with observational data for the youngest stars (age less than or similar to 10 Myr). In the Gamma-1 scenario, the observed fraction of the youngest stars in clusters increases with the SFR only weakly; the increase is caused by the presence of more massive clusters in galaxies with higher SFRs, which release fewer stars to the field in proportion to their mass. The Gamma-1 model yields a higher fraction of stars in clusters for older stars (ages between 10 Myr and 300 Myr) than what is observed. This discrepancy can be caused by initially less compact clusters or a slightly lower star-formation efficiency than originally assumed in the Gamma-1 model, or by interactions of the post-gas-expulsion revirialised open clusters with molecular clouds.