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Register a new userGlobular Cluster and Galaxy Formation: M31, the Milky Way and Implications for Globular Cluster Systems of Spiral Galaxies
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D. Burstein
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The globular cluster (GC) systems of the Milky Way and of our neighboring spiral galaxy, M31, comprise 2 distinct entities, differing in 3 respects. 1. M31 has young GCs, ages from ~100 Myr to 5 Gyr old, as well as old globular clusters. No such young GCs are known in the Milky Way. 2. We confirm that the oldest M31 GCs have much higher nitrogen abundances than do Galactic GCs at equivalent metallicities. 3. Morrison et al. found M31 has a subcomponent of GCs that follow closely the disk rotation curve of M31. Such a GC system in our own Galaxy has yet to be found. These data are interpreted in terms of the hierarchical-clustering-merging (HCM) paradigm for galaxy formation. We infer that M31 has absorbed more of its dwarf systems than has the Milky Way. This inference has 3 implications: 1. All spiral galaxies likely differ in their GC properties, depending on how many companions each galaxy has, and when the parent galaxy absorbs them. The the Milky Way ties down one end of this spectrum, as almost all of its GCs were absorbed 10-12 Gyr ago. 2. It suggests that young GCs are preferentially formed in the dwarf companions of parent galaxies, and then absorbed by the parent galaxy during mergers. 3. Young GCs seen in tidally-interacting galaxies might come from dwarf companions of these galaxies, rather than be made a-new in the tidal interaction. There is no ready explanation for the marked difference in nitrogen abundance for old M31 GCs relative to the oldest Galactic GCs. The predictions made by Li & Burstein regarding the origin of nitrogen abundance in globular clusters are consistent with what is found for the old M31 GCs compared to that for the two 5 Gyr-old M31 GCs.
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Globular clusters are compact, gravitationally bound systems of up to a million stars. The GCs in the Milky Way contain some of the oldest stars known, and provide important clues to the early formation and continuing evolution of our Galaxy. More generally, GCs are associated with galaxies of all types and masses, from low-mass dwarf galaxies to the most massive early-type galaxies which lie in the centres of massive galaxy clusters. GC systems show several properties which connect tightly with properties of their host galaxies. For example, the total mass of GCs in a system scales linearly with the dark matter halo mass of its host galaxy. Numerical simulations are at the point of being able to resolve globular cluster formation within a cosmological framework. Therefore, GCs link a range of scales, from the physics of star formation in turbulent gas clouds, to the large-scale properties of galaxies and their dark matter. In this Chapter we review some of the basic observational approaches for GC systems, some of their key observational properties, and describe how GCs provide important clues to the formation of their parent galaxies.
We review spectroscopic results concerning multiple stellar populations in globularclusters. The cluster initial mass is the most important parameter determining the fraction of second generation stars. The threshold for the onset of the multiple population phenomenon is 1-3x10^5 MSun. Nucleosynthesis is influenced by metallicity: Na/O and Mg/Al anti-correlations are more extended in metal-poor than in metal-rich clusters. Massive clusters are more complex systems than the smaller ones, with several populations characterized by different chemical compositions. The high Li abundance observed in the intermediate second generation stars strongly favours intermediate mass AGB stars as polluters for this class of stars; however, it is well possible that the polluters of extreme second generation stars, that often do not have measurable Li, may be fast rotating massive stars or super-massive stars. The mass budget factor should be a function of the cluster mass, and needs to be large only in massive clusters.
We present new metallicity estimates for globular cluster (GC) candidates in the Sd spiral NGC 300, one of the nearest spiral galaxies outside the Local Group. We have obtained optical spectroscopy for 44 Sculptor Group GC candidates with the Boller and Chivens (B&C) spectrograph on the Baade Telescope at Las Campanas Observatory. There are 2 GCs in NGC 253 and 12 objects in NGC 300 with globular-cluster-like spectral features, 9 of which have radial velocities above 0 km/s. The remaining three, due to their radial velocities being below the expected 95% confidence limit for velocities of NGC 300 halo objects, are flagged as possible foreground stars. The non-clusterlike candidates included 13 stars, 15 galaxies, and an HII region. One GC, four galaxies, two stars, and the HII region from our sample were identified in archival Hubble Space Telescope images. For the GCs, we measure spectral indices and estimate metallicities using an empirical calibration based on Milky Way GCs. The GCs of NGC 300 appear similar to those of the Milky Way. Excluding possible stars and including clusters from the literature, the GC system (GCS) has a velocity dispersion of 68 km/s, and has no clear evidence of rotation. The mean metallicity for our full cluster sample plus one literature object is [Fe/H] = -0.94, lying above the relationship between mean GC metallicity and overall galaxy luminosity. Excluding the three low-velocity candidates, we obtain a mean [Fe/H] = -0.98, still higher than expected, raising the possibility of significant foreground star contamination even in this sample. Visual confirmation of genuine GCs using high-resolution space-based imagery could greatly reduce the potential problem of interlopers in small samples of GCSs in low-radial-velocity galaxies.
We use the age-metallicity distribution of 96 Galactic globular clusters (GCs) to infer the formation and assembly history of the Milky Way (MW), culminating in the reconstruction of its merger tree. Based on a quantitative comparison of the Galactic GC population to the 25 cosmological zoom-in simulations of MW-mass galaxies in the E-MOSAICS project, which self-consistently model the formation and evolution of GC populations in a cosmological context, we find that the MW assembled quickly for its mass, reaching ${25,50}%$ of its present-day halo mass already at $z={3,1.5}$ and half of its present-day stellar mass at $z=1.2$. We reconstruct the MWs merger tree from its GC age-metallicity distribution, inferring the number of mergers as a function of mass ratio and redshift. These statistics place the MWs assembly $textit{rate}$ among the 72th-94th percentile of the E-MOSAICS galaxies, whereas its $textit{integrated}$ properties (e.g. number of mergers, halo concentration) match the median of the simulations. We conclude that the MW has experienced no major mergers (mass ratios $>$1:4) since $zsim4$, sharpening previous limits of $zsim2$. We identify three massive satellite progenitors and constrain their mass growth and enrichment histories. Two are proposed to correspond to Sagittarius (few $10^8~{rm M}_odot$) and the GCs formerly associated with Canis Major ($sim10^9~{rm M}_odot$). The third satellite has no known associated relic and was likely accreted between $z=0.6$-$1.3$. We name this enigmatic galaxy $textit{Kraken}$ and propose that it is the most massive satellite ($M_*sim2times10^9~{rm M}_odot$) ever accreted by the MW. We predict that $sim40%$ of the Galactic GCs formed ex-situ (in galaxies with masses $M_*=2times10^7$-$2times10^9~{rm M}_odot$), with $6pm1$ being former nuclear clusters.
We analyse the globular cluster (GC) systems of a sample of 15 massive, compact early-type galaxies (ETGs), 13 of which have already been identified as good relic galaxy candidates on the basis of their compact morphologies, old stellar populations and stellar kinematics. These relic galaxy candidates are likely the nearby counterparts of high redshift red nugget galaxies. Using F814W (~I) and F160W (~H) data from the WFC3 camara onboard the Hubble Space Telescope we determine the total number, luminosity function, specific frequency, colour and spatial distribution of the GC systems. We find lower specific frequencies (SN<2.5 with a median of SN=1) than ETGs of comparable mass. This is consistent with a scenario of rapid, early dissipative formation, with relatively low levels of accretion of low-mass, high-SN satellites. The GC half-number radii are compact, but follow the relations found in normal ETGs. We identify an anticorrelation between the specific angular momentum (lambda_R) of the host galaxy and the (I-H) colour distribution width of their GC systems. Assuming that lambda_R provides a measure of the degree of dissipation in massive ETGs, we suggest that the (I-H) colour distribution width can be used as a proxy for the degree of complexity of the accretion histories in these systems.
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