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Register a new userThe Sausage Globular Clusters
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G.C. Myeong
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The Gaia Sausage is an elongated structure in velocity space discovered by Belokurov et al. (2018) using the kinematics of metal-rich halo stars. It was created by a massive dwarf galaxy ($sim 5 times 10^{10} M_odot$) on a strongly radial orbit that merged with the Milky Way at a redshift $zlesssim 3$. We search forthe associated Sausage Globular Clusters by analysing the structure of 91 Milky Way globular clusters (GCs) in action space using the Gaia Data Release 2 catalogue, complemented with Hubble Space Telescope proper motions. There is a characteristic energy $E_{rm crit}$ which separates the in situ objects, such as the bulge/disc clusters, from the accreted objects, such as the young halo clusters. There are 15 old halo GCs that have $E > E_{rm crit}$. Eight of the high energy, old halo GCs are strongly clumped in azimuthal and vertical action, yet strung out like beads on a chain at extreme radial action. They are very radially anisotropic ($beta sim 0.95$) and move on orbits that are all highly eccentric ($e gtrsim 0.80$). They also form a track in the age-metallicity plane distinct from the bulk of the Milky Way GCs and compatible with a dwarf spheroidal origin. These properties are consistent with GCs associated with the merger event that gave rise to the Gaia Sausage.
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We study the escape fraction of ionizing photons (f_esc) in two cosmological zoom-in simulations of galaxies in the reionization era with halo mass M_halo~10^10 and 10^11 M_sun (stellar mass M*~10^7 and 10^9 M_sun) at z=5 from the Feedback in Realistic Environments project. These simulations explicitly resolve the formation of proto-globular clusters (GCs) self-consistently, where 17-39% of stars form in bound clusters during starbursts. Using post-processing Monte Carlo radiative transfer calculations of ionizing radiation, we compute f_esc from cluster stars and non-cluster stars formed during a starburst over ~100 Myr in each galaxy. We find that the averaged f_esc over the lifetime of a star particle follows a similar distribution for cluster stars and non-cluster stars. Clusters tend to have low f_esc in the first few Myrs, presumably because they form preferentially in more extreme environments with high optical depths; the f_esc increases later as feedback starts to disrupt the natal cloud. On the other hand, non-cluster stars formed between cluster complexes or in the compressed shell at the front of a superbubble can also have high f_esc. We find that cluster stars on average have comparable f_esc to non-cluster stars. This result is robust across several star formation models in our simulations. Our results suggest that the fraction of ionizing photons from proto-GCs to cosmic reionization is comparable to the cluster formation efficiency in high-redshift galaxies and hence proto-GCs likely contribute an appreciable fraction of photons but are not the dominant sources for reionization.
We analyse a set of cosmological magneto-hydrodynamic simulations of the formation of Milky Way-mass galaxies identified to have a prominent radially anisotropic stellar halo component similar to the so-called Gaia Sausage found in the Gaia data. We examine the effects of the progenitor of the Sausage (the Gaia-Enceladus-Sausage, GES) on the formation of major galactic components analogous to the Galactic thick disc and inner stellar halo. We find that the GES merger is likely to have been gas-rich and contribute 10-50$%$ of gas to a merger-induced centrally concentrated starburst that results in the rapid formation of a compact, rotationally supported thick disc that occupies the typical chemical thick disc region of chemical abundance space. We find evidence that gas-rich mergers heated the proto-disc of the Galaxy, scattering stars onto less-circular orbits such that their rotation velocity and metallicity positively correlate, thus contributing an additional component that connects the Galactic thick disc to the inner stellar halo. We demonstrate that the level of kinematic heating of the proto-galaxy correlates with the kinematic state of the population before the merger, the progenitor mass and orbital eccentricity of the merger. Furthermore, we show that the mass and time of the merger can be accurately inferred from local stars on counter-rotating orbits.
It has long been argued that the radial distribution of globular clusters (GCs) in the Fornax dwarf galaxy requires its dark matter halo to have a core of size $sim 1$ kpc. We revisit this argument by investigating analogues of Fornax formed in E-MOSAICS, a cosmological hydrodynamical simulation that self-consistently follows the formation and evolution of GCs in the EAGLE galaxy formation model. In EAGLE, Fornax-mass haloes are cuspy and well described by the Navarro-Frenk-White profile. We post-process the E-MOSAICS to account for GC orbital decay by dynamical friction, which is not included in the original model. Dynamical friction causes 33 per cent of GCs with masses $M_{rm GC}geq4times10^4 {~rm M_odot}$ to sink to the centre of their host where they are tidally disrupted. Fornax has a total of five GCs, an exceptionally large number compared to other galaxies of similar stellar mass. In the simulations, we find that only 3 per cent of the Fornax analogues have five or more GCs, while 30 per cent have only one and 35 per cent have none. We find that GC systems in satellites are more centrally concentrated than in field dwarfs, and that those formed in situ (45 per cent) are more concentrated than those that were accreted. The survival probability of a GC increases rapidly with the radial distance at which it formed ($r_{rm init}$): it is 37 per cent for GCs with $r_{rm init} leq 1$ kpc and 92 per cent for GCs with $r_{rm init} geq 1$ kpc. The present-day radial distribution of GCs in E-MOSAICS turns out to be indistinguishable from that in Fornax, demonstrating that, contrary to claims in the literature, the presence of five GCs in the central kiloparsec of Fornax does not exclude a cuspy DM halo.
Thanks to the outstanding capabilites of the HST, our current knowledge about the M31 globular clusters (GCs) is similar to our knowledge of the Milky Way GCs in the 1960s-1970s, which set the basis for studying the halo and galaxy formation using these objects as tracers, and established their importance in defining the cosmic distance scale. We intend to derive a new calibration of the M_V(HB)-[Fe/H] relation by exploiting the large photometric database of old GCs in M31 in the HST archive. We collected the BVI data for 48 old GCs in M31 and analysed them by applying the same methods and procedures to all objects. We obtained a set of homogeneous colour-magnitude diagrams (CMDs) that were best-fitted with the fiducial CMD ridge lines of selected Milky Way template GCs. Reddening, metallicity, Horizontal Branch (HB) luminosity and distance were determined self-consistently for each cluster. There are three main results of this study: i) the relation M_V(HB)=(0.25+/-0.02)[Fe/H]+(0.89+/-0.03), which is obtained from the above parameters and is calibrated on the distances of the template Galactic GCs; ii) the distance modulus to M31 of (m-M)_0=24.42+/-0.06 mag, obtained by normalising this relation at the reference value of [Fe/H]=-1.5 to a similar relation using V_0(HB). This is the first determination of the distance to M31 based on the characteristics of its GC system which is calibrated on Galactic GCs; iii) the distance to the Large Magellanic Cloud (LMC), which is estimated to be 18.54+/-0.07 mag as a consequence of the previous results. These values agree excellently with the most recent estimate based on HST parallaxes of Galactic Cepheid and RR Lyrae stars, as well as with recent methods.
The dominant systematic uncertainty in the age determination of galactic globular clusters is the depth of the convection envelope of the stars. This parameter is partially degenerate with metallicity which is in turn degenerate with age. However, if the metal content, distance and extinction are known, the position and morphology of the red giant branch in a color-magnitude diagram are mostly sensitive to the value of the depth of the convective envelope. Therefore, using external, precise metallicity determinations this degeneracy and thus the systematic error in age, can be reduced. Alternatively, the morphology of the red giant branch of globular clusters color magnitude diagram can also be used to achieve the same. We demonstrate that globular cluster red giant branches are well fitted by values of the depth of the convection envelope consistent with those obtained for the Sun and this finding is robust to the adopted treatment of the stellar physics. With these findings, the uncertainty in the depth of the convection envelope is no longer the dominant contribution to the systematic error in the age determination of the oldest globular clusters, reducing it from $0.5$ to $0.23$ or $0.33$ Gyr, depending on the methodology adopted: i.e., whether resorting to external data (spectroscopic metallicity determinations) or relying solely on the morphology of the clusterss color-magnitude diagrams. This results in an age of the Universe $t_{rm U}=13.5^{+0.16}_{-0.14} {rm (stat.)} pm 0.23(0.33) ({rm sys.})$ at 68% confidence level, accounting for the formation time of globular clusters and its uncertainty. An uncertainty of 0.27(0.36) Gyr if added in quadrature. This agrees well with $13.8 pm 0.02$ Gyr, the cosmological model-dependent value inferred by the Planck mission assuming the $Lambda$CDM model.
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