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Register a new userLuminous Satellites versus Dark Subhaloes: Clustering in the Milky Way
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The observed population of the Milky Way satellite galaxies offer a unique testing ground for galaxy formation theory on small-scales. Our novel approach was to investigate the clustering of the known Milky Way satellite galaxies and to quantify the amount of substructure within their distribution using a two-point correlation function statistic in each of three spaces: configuration space, line-of-sight velocity space, and four-dimensional phase-space. These results were compared to those for three sets of subhaloes in the Via Lactea II Cold Dark Matter simulation defined to represent the luminous dwarfs. We found no evidence at a significance level above 2-sigma of substructure within the distribution of the Milky Way satellite galaxies in any of the three spaces. The luminous subhalo sets are more strongly clustered than are the Milky Way satellites in all three spaces and over a broader range of scales in four-dimensional phase-space. Each of the luminous subhalo sets are clustered as a result of substructure within their line-of-sight velocity space distributions at greater than 3-sigma significance, whereas the Milky Way satellite galaxies are randomly distributed in line-of-sight velocity space. While our comparison is with only one Cold Dark Matter simulation, the inconsistencies between the Milky Way satellite galaxies and the Via Lactea II subhalo sets for all clustering methods suggest a potential new small-scale tension between Cold Dark Matter theory and the observed Milky Way satellites. Future work will obtain a more robust comparison between the observed Milky Way satellites and Cold Dark Matter theory by studying additional simulations.
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We combine a series of high-resolution simulations with semi-analytic galaxy formation models to follow the evolution of a system resembling the Milky Way and its satellites. The semi-analytic model is based on that developed for the Millennium Simulation, and successfully reproduces the properties of galaxies on large scales, as well as those of the Milky Way. In this model, we are able to reproduce the luminosity function of the satellites around the Milky Way by preventing cooling in haloes with Vvir < 16.7 km/s (i.e. the atomic hydrogen cooling limit) and including the impact of the reionization of the Universe. The physical properties of our model satellites (e.g. mean metallicities, ages, half-light radii and mass-to-light ratios) are in good agreement with the latest observational measurements. We do not find a strong dependence upon the particular implementation of supernova feedback, but a scheme which is more efficient in galaxies embedded in smaller haloes, i.e. shallower potential wells, gives better agreement with the properties of the ultra-faint satellites. Our model predicts that the brightest satellites are associated with the most massive subhaloes, are accreted later (z $lta$ 1), and have extended star formation histories, with only 1 per cent of their stars made by the end of the reionization. On the other hand, the faintest satellites were accreted early, are dominated by stars with age > 10 Gyr, and a few of them formed most of their stars before the reionization was complete. Objects with luminosities comparable to those of the classical MW satellites are associated with dark matter subhaloes with a peak circular velocity $gta$ 10 km/s, in agreement with the latest constraints.
Recent studies suggest that only three of the twelve brightest satellites of the Milky Way (MW) inhabit dark matter halos with maximum circular velocity, V_max, exceeding 30km/s. This is in apparent contradiction with the LCDM simulations of the Aquarius Project, which suggest that MW-sized halos should have at least 8 subhalos with V_max>30km/s. The absence of luminous satellites in such massive subhalos is thus puzzling and may present a challenge to the LCDM paradigm. We note, however, that the number of massive subhalos depends sensitively on the (poorly-known) virial mass of the Milky Way, and that their scarcity makes estimates of their abundance from a small simulation set like Aquarius uncertain. We use the Millennium Simulation series and the invariance of the scaled subhalo velocity function (i.e., the number of subhalos as a function of u, the ratio of subhalo V_max to host halo virial velocity, V_200) to secure improved estimates of the abundance of rare massive subsystems. In the range 0.1< u<0.5, N_sub(> u) is approximately Poisson-distributed about an average given by <N_sub>=10.2x( u/0.15)^(-3.11). This is slightly lower than in Aquarius halos, but consistent with recent results from the Phoenix Project. The probability that a LCDM halo has 3 or fewer subhalos with V_max above some threshold value, V_th, is then straightforward to compute. It decreases steeply both with decreasing V_th and with increasing halo mass. For V_th=30km/s, ~40% of M_halo=10^12 M_sun halos pass the test; fewer than 5% do so for M_halo>= 2x10^12 M_sun; and the probability effectively vanishes for M_halo>= 3x 10^12 M_sun. Rather than a failure of LCDM, the absence of massive subhalos might simply indicate that the Milky Way is less massive than is commonly thought.
The spatial distribution of Milky Way (MW) subhaloes provides an important set of observables for testing cosmological models. These include the radial distribution of luminous satellites, planar configurations, and the abundance of dark subhaloes whose existence or absence is key to distinguishing amongst dark matter models. We use the COCO $N$-body simulations of cold dark matter (CDM) and 3.3keV thermal relic warm dark matter (WDM) to predict the satellite spatial distribution. We demonstrate that the radial distributions of CDM and 3.3keV-WDM luminous satellites are identical if the minimum pre-infall halo mass to form a galaxy is $>10^{8.5}$$mathrm{M}_{odot}$ The distribution of dark subhaloes is significantly more concentrated in WDM due to the absence of low mass, recently accreted substructures that typically inhabit the outer parts of a MW halo in CDM. We show that subhaloes of mass $[10^{7},10^{8}]$$mathrm{M}_{odot}$ and within 30kpc of the centre are the stripped remnants of larger haloes in both models. Therefore their abundance in WDM is $3times$ higher than one would anticipate from the overall WDM subhalo population. We estimate that differences between CDM and WDM concentration--mass relations can be probed for subhalo--stream impact parameters $<2$kpc. Finally, we find that the impact of WDM on planes of satellites is likely negligible. Precise predictions will require further work with high resolution, self-consistent hydrodynamical simulations.
Here we investigate the evolution of a Milky Way (MW) -like galaxy with the aim of predicting the properties of its progenitors all the way from $z sim 20$ to $z = 0$. We apply GAMESH (Graziani et al. 2015) to a high resolution N-Body simulation following the formation of a MW-type halo and we investigate its properties at $z sim 0$ and its progenitors in $0 < z < 4$. Our model predicts the observed galaxy main sequence, the mass-metallicity and the fundamental plane of metallicity relations in $0 < z < 4$. It also reproduces the stellar mass evolution of candidate MW progenitors in $0 lesssim z lesssim 2.5$, although the star formation rate and gas fraction of the simulated galaxies follow a shallower redshift dependence. We find that while the MW star formation and chemical enrichment are dominated by the contribution of galaxies hosted in Lyman $alpha$-cooling halos, at z > 6 the contribution of star forming mini-halos is comparable to the star formation rate along the MW merger tree. These systems might then provide an important contribution in the early phases of reionization. A large number of mini-halos with old stellar populations, possibly Population~III stars, are dragged into the MW or survive in the Local Group. At low redshift dynamical effects, such as halo mergers, tidal stripping and halo disruption redistribute the baryonic properties among halo families. These results are critically discussed in light of future improvements including a more sophisticated treatment of radiative feedback and inhomogeneous metal enrichment.
Here we examine the Milky Ways GC system to estimate the fraction of accreted versus in situ formed GCs. We first assemble a high quality database of ages and metallicities for 93 Milky Way GCs from literature deep colour-magnitude data. The age-metallicity relation for the Milky Ways GCs reveals two distinct tracks -- one with near constant old age of ~12.8 Gyr and the other branches to younger ages. We find that the latter young track is dominated by globular clusters associated with the Sagittarius and Canis Major dwarf galaxies. Despite being overly simplistic, its age-metallicity relation can be well represented by a simple closed box model with continuous star formation. The inferred chemical enrichment history is similar to that of the Large Magellanic Cloud, but is more enriched, at a given age, compared to the Small Magellanic Cloud. After excluding Sagittarius and Canis Major GCs, several young track GCs remain. Their horizontal branch morphologies are often red and hence classified as Young Halo objects, however they do not tend to reveal extended horizontal branches (a possible signature of an accreted remnant nucleus). Retrograde orbit GCs (a key signature of accretion) are commonly found in the young track. We also examine GCs that lie close to the Fornax-Leo-Sculptor great circle defined by several satellite galaxies. We find that several GCs are consistent with the young track and we speculate that they may have been accreted along with their host dwarf galaxy, whose nucleus may survive as a GC. Finally, we suggest that 27-47 GCs (about 1/4 of the entire system), from 6-8 dwarf galaxies, were accreted to build the Milky Way GC system we seen today.
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