Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userThe minimum metallicity of globular clusters and its physical origin -- implications for the galaxy mass-metallicity relation and observations of proto-globular clusters at high redshift
79
0
0.0
(
0
)
Authors
J. M. Diederik Kruijssen
Ask ChatGPT about the research
No Arabic abstract
In the local Universe, globular clusters (GCs) with metallicities $[{rm Fe}/{rm H}]<-2.5$ are extremely rare. In this Letter, the close connection between GC formation and galaxy evolution is used to show that this GC metallicity `floor results from the galaxy mass-metallicity relation of ultra low-luminosity galaxies (ULLGs) at high redshift, where the most metal-poor GCs must have formed. Galaxies with metallicities $[{rm Fe}/{rm H}]lesssim-2.5$ have too low masses to form GCs with initial masses $M_{rm i}gtrsim10^5~{rm M}_odot$, needed to survive for a Hubble time. This translates the galaxy mass-metallicity relation into a maximum initial cluster mass-metallicity relation for $[{rm Fe}/{rm H}]lesssim-1.8$, which naturally leads to the observed colour-magnitude relation of metal-poor GCs at $z=0$ (the `blue tilt). Its strength traces the slope of the gas phase mass-metallicity relation of ULLGs. Based on the observed blue tilt of GCs in the Virgo and Fornax Clusters, the galaxy mass-metallicity relation is predicted to have a slope of $alpha=0.4pm0.1$ for $10^5lesssim M_star/{rm M}_odotlesssim10^7$ at $zgtrsim2$. The GC metallicity floor implies a minimum host galaxy mass and a maximum redshift for GC formation. Any proto-GCs that may be detected at $z>9$ are most likely to end up in galaxies presently more massive than the Milky Way, whereas GCs in low-mass galaxies such as the Fornax dSph ($M_starapprox4times10^7~{rm M}_odot$) formed at $zlesssim3$.
rate research
Read More
We use recently derived ages for 61 Milky Way (MW) globular clusters (GCs) to show that their age-metallicity relation (AMR) can be divided into two distinct, parallel sequences at [Fe/H] $ga -1.8$. Approximately one-third of the clusters form an offset sequence that spans the full range in age ($sim 10.5$--13 Gyr), but is more metal rich at a given age by $sim 0.6$ dex in [Fe/H]. All but one of the clusters in the offset sequence show orbital properties that are consistent with membership in the MW disk. They are not simply the most metal-rich GCs, which have long been known to have disk-like kinematics, but they are the most metal-rich clusters at all ages. The slope of the mass-metallicity relation (MMR) for galaxies implies that the offset in metallicity of the two branches of the AMR corresponds to a mass decrement of 2 dex, suggesting host galaxy masses of $M_{*} sim 10^{7-8} msol$ for GCs that belong to the more metal-poor AMR. We suggest that the metal-rich branch of the AMR consists of clusters that formed in-situ in the disk, while the metal-poor GCs were formed in relatively low-mass (dwarf) galaxies and later accreted by the MW. The observed AMR of MW disk stars, and of the LMC, SMC and WLM dwarf galaxies are shown to be consistent with this interpretation, and the relative distribution of implied progenitor masses for the halo GC clusters is in excellent agreement with the MW subhalo mass function predicted by simulations. A notable implication of the bifurcated AMR, is that the identical mean ages and spread in ages, for the metal rich and metal poor GCs are difficult to reconcile with an in-situ formation for the latter population.
Globular cluster (GC) systems of massive galaxies often show a bimodal colour distribution. This has been interpreted as a metallicity bimodality, created by a two-stage galaxy formation where the red, metal-rich GCs were formed in the parent halo and the blue metal-poor GCs were accreted. This interpretation, however, crucially depends on the assumption that GCs are exclusively old stellar systems with a linear colour-metallicity relation (CZR). The shape of the CZR and range of GC ages are currently under debate, because their study requires high quality spectra to derive reliable stellar population properties. We determined metallicities with full spectral fitting from a sample of 187 GCs with high spectral signal-to-noise ratio in 23 galaxies of the Fornax cluster that were observed as part of the Fornax 3D project. The derived CZR from this sample is non-linear and can be described by a piecewise linear function with a break point at ($g - z$) $sim$ 1.1 mag. The less massive galaxies in our sample ($M_ast < 10^{10} M_odot$) appear to have slightly younger GCs, but the shape of the CZR is insensitive to the GC ages. Although the least massive galaxies lack red, metal-rich GCs, a non-linear CZR is found irrespective of the galaxy mass, even in the most massive galaxies ($M_ast geq 10^{11} M_odot$). Our CZR predicts narrow unimodal GC metallicity distributions for low mass and broad unimodal distributions for very massive galaxies, dominated by a metal-poor and metal-rich peak, respectively, and bimodal distributions for galaxies with intermediate masses (10$^{10}$ $leq$ $M_ast < 10^{11} M_odot$) as a consequence of the relative fraction of red and blue GCs. The diverse metallicity distributions challenge the simple differentiation of GC populations solely based on their colour.
Observed mass-to-light ratios (M/L) of metal-rich globular clusters (GCs) disagree with theoretical predictions. This discrepancy is of fundamental importance since stellar population models provide the stellar masses that underpin most of extragalactic astronomy, near and far. We have derived radial velocities for 1,622 stars located in the centres of 59 Milky Way GCs - twelve of which have no previous kinematic information - using integral-field unit data from the WAGGS project. Using N-body models, we then determine dynamical masses and M/L ratios for the studied clusters. Our sample includes NGC 6528 and NGC 6553, which extend the metallicity range of GCs with measured M/L up to [Fe/H] ~ -0.1 dex. We find that metal-rich clusters have M/L more than two times lower than what is predicted by simple stellar population models. This confirms that the discrepant M/L-[Fe/H] relation remains a serious concern. We explore how our findings relate to previous observations, and the potential causes for the divergence, which we conclude is most likely due to dynamical effects.
We use high-resolution cosmological zoom-in simulations from the Feedback in Realistic Environment (FIRE) project to study the galaxy mass-metallicity relations (MZR) from z=0-6. These simulations include explicit models of the multi-phase ISM, star formation, and stellar feedback. The simulations cover halo masses Mhalo=10^9-10^13 Msun and stellar mass Mstar=10^4-10^11 Msun at z=0 and have been shown to produce many observed galaxy properties from z=0-6. For the first time, our simulations agree reasonably well with the observed mass-metallicity relations at z=0-3 for a broad range of galaxy masses. We predict the evolution of the MZR from z=0-6 as log(Zgas/Zsun)=12+log(O/H)-9.0=0.35[log(Mstar/Msun)-10]+0.93 exp(-0.43 z)-1.05 and log(Zstar/Zsun)=[Fe/H]-0.2=0.40[log(Mstar/Msun)-10]+0.67 exp(-0.50 z)-1.04, for gas-phase and stellar metallicity, respectively. Our simulations suggest that the evolution of MZR is associated with the evolution of stellar/gas mass fractions at different redshifts, indicating the existence of a universal metallicity relation between stellar mass, gas mass, and metallicities. In our simulations, galaxies above Mstar=10^6 Msun are able to retain a large fraction of their metals inside the halo, because metal-rich winds fail to escape completely and are recycled into the galaxy. This resolves a long-standing discrepancy between sub-grid wind models (and semi-analytic models) and observations, where common sub-grid models cannot simultaneously reproduce the MZR and the stellar mass functions.
Recent spectroscopy on the globular cluster (GC) system of M31 with unprecedented precision witnessed a clear bimodality in absorption-line index distributions of old GCs. Such division of extragalactic GCs, so far asserted mainly by photometric color bimodality, has been viewed as the presence of merely two distinct metallicity subgroups within individual galaxies and forms a critical backbone of various galaxy formation theories. Given that spectroscopy is a more detailed probe into stellar population than photometry, the discovery of index bimodality may point to the very existence of dual GC populations. However, here we show that the observed spectroscopic dichotomy of M31 GCs emerges due to the nonlinear nature of metallicity-to-index conversion and thus one does not necessarily have to invoke two separate GC subsystems. We take this as a close analogy to the recent view that metallicity-color nonlinearity is primarily responsible for observed GC color bimodality. We also demonstrate that the metallicity-sensitive magnesium line displays non-negligible metallicity-index nonlinearity and Balmer lines show rather strong nonlinearity. This gives rise to bimodal index distributions, which are routinely interpreted as bimodal metallicity distributions, not considering metallicity-index nonlinearity. Our findings give a new insight into the constitution of M31s GC system, which could change much of the current thought on the formation of GC systems and their host galaxies.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
