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Register a new userMass segregation in young compact star clusters in the Large Magellanic Cloud: II. Mass Functions
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We review the complications involved in the conversion of stellar luminosities into masses and apply a range of mass-to-luminosity relations to our Hubble Space Telescope observations of the young LMC star clusters NGC 1805 and NGC 1818. Both the radial dependence of the mass function (MF) and the dependence of the cluster core radii on mass indicate clear mass segregation in both clusters at radii r <= 20-30, for masses in excess of ~1.6-2.5 Msun. This result does not depend on the mass range used to fit the slopes or the metallicity assumed. It is clear that the cluster MFs, at any radius, are not simple power laws. The global and the annular MFs near the core radii appear to be characterised by similar slopes in the mass range (-0.15 <= log m/Msun <= 0.85), the MFs beyond r >= 30 have significantly steeper slopes. We estimate that while the NGC 1818 cluster core is between ~5 and ~30 crossing times old, the core of NGC 1805 is likely $lesssim 3-4$ crossing times old. However, since strong mass segregation is observed out to ~6 Rcore and ~3 Rcore in NGC 1805 and NGC 1818, respectively, it is most likely that significant primordial mass segregation was present in both clusters, particularly in NGC 1805.
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We have undertaken a detailed analysis of HST/WFPC2 and STIS imaging observations, and of supplementary wide-field ground-based observations obtained with the NTT of two young ~10-25 Myr) compact star clusters in the LMC, NGC 1805 and NGC 1818. The ultimate goal of our work is to improve our understanding of the degree of primordial mass segregation in star clusters. This is crucial for the interpretation of observational luminosity functions (LFs) in terms of the initial mass function (IMF), and for constraining the universality of the IMF. We present evidence for strong luminosity segregation in both clusters. The LF slopes steepen with cluster radius; in both NGC 1805 and NGC 1818 the LF slopes reach a stable level well beyond the clusters core or half-light radii. In addition, the brightest cluster stars are strongly concentrated within the inner ~4 R_hl. The global cluster LF, although strongly nonlinear, is fairly well approximated by the core or half-light LF; the (annular) LFs at these radii are dominated by the segregated high-luminosity stars, however. We present tentative evidence for the presence of an excess number of bright stars surrounding NGC 1818, for which we argue that they are most likely massive stars that have been collisionally ejected from the cluster core. We therefore suggest that the cores of massive young stars clusters undergo significant dynamical evolution, even on time-scales as short as ~25 Myr.
Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. In this Letter we demonstrate that mergers of small clumps that are either initially mass segregated, or in which mass segregation can be produced by two-body relaxation before they merge, generically lead to larger systems which inherit the progenitor clumps segregation. We conclude that clusters formed in this way are naturally mass segregated, accounting for the anomalous observations and suggesting that this process of prompt mass segregation due to initial clumping should be taken fully into account in constructing cluster dynamical models.
(Abridged) Photometry of archival Spitzer observations of the Large Magellanic Cloud (LMC) are used to search for young stellar objects (YSOs). Simple mid-infrared selection criteria were used to exclude most normal and evolved stars and background galaxies. We identify a sample of 2,910 sources in the LMC that could potentially be YSOs. We then simultaneously considered images and photometry from the optical through mid-IR wavelengths to assess the source morphology, spectral energy distribution (SED), and the surrounding interstellar environment to determine the most likely nature of each source. From this examination of the initial sample, we suggest 1,172 sources are most likely YSOs and 1,075 probable background galaxies, consistent with expectations based on SWIRE survey data. Spitzer IRS observations of 269 of the brightest YSOs from our sample have confirmed that ~>95% are indeed YSOs. A comprehensive search for YSOs in the LMC has also been carried out by the SAGE team. There are three major differences between these two searches. (1) In the common region of color-magnitude space, ~850 of our 1,172 probable YSOs are missed in the SAGE YSO catalog because their conservative point source identification criteria have excluded YSOs superposed on complex diffuse emission. (2) About 20-30% of the YSOs identified by the SAGE team are sources we classify as background galaxies. (3) the SAGE YSO catalog identifies YSO in parts of color-magnitude space that we excluded and thus contains more evolved or fainter YSOs missed by our analysis. Finally, the mid-IR luminosity functions of our most likely YSO candidates in the LMC can be well described by N(L) propto L^-1, which is consistent with the Salpeter initial mass function if a mass-luminosity relation of L propto M^2.4 is adopted.
Several dynamical scenarios have been proposed that can lead to prompt mass segregation on the crossing time scale of a young cluster. They generally rely on cool and/or clumpy initial conditions, and are most relevant to small systems. As a counterpoint, we present a novel dynamical mechanism that can operate in relatively large, homogeneous, cool or cold systems. This mechanism may be important in understanding the assembly of large mass-segregated clusters from smaller clumps.
We derive the mass-radius relation and mass function of molecular clumps in the Large Magellanic Cloud (LMC) and interpret them in terms of the simple feedback model proposed by Fall, Krumholz, and Matzner (FKM). Our work utilizes the dendrogram-based catalog of clumps compiled by Wong et al. from $^{12}$CO and $^{13}$CO maps of six giant molecular clouds in the LMC observed with the Atacama Large Millimeter Array (ALMA). The Magellanic Clouds are the only external galaxies for which this type of analysis is possible at the necessary spatial resolution ($sim1$ pc). We find that the mass-radius relation and mass function of LMC clumps have power-law forms, $R propto M^{alpha}$ and $dN/dM propto M^{beta}$, with indices $alpha = 0.36 pm 0.03$ and $beta= -1.8 pm 0.1 $ over the mass ranges $10^2 M_odot lesssim M lesssim 10^5 M_odot$ and $10^2 M_odot lesssim M lesssim 10^4 M_odot$, respectively. With these values of $alpha$ and $beta$ for the clumps (i.e., protoclusters), the predicted index for the mass function of young LMC clusters from the FKM model is $beta approx 1.7$, in good agreement with the observed index. The situation portrayed here for clumps and clusters in the LMC replicates that in the Milky Way.
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