The puzzling assembly of the Milky Way halo – contributions from dwarf Spheroidals and globular clusters

Hansen

Kunder

2017

A&A

Star clusters, particularly those objects in the disk-bulge-halo interface are as of yet poorly charted, albeit carrying important information about the formation and the structure of the Milky Way. Here, we present a detailed chemical abundance study of the recently discovered object Gaia 1. Photometry has previously suggested it as an intermediate-age, moderately metal-rich system, although the exact values for its age and metallicity remained ambiguous in the literature. We measured detailed chemical abundances of 14 elements in four red giant members, from high-resolution (R=25000) spectra that firmly establish Gaia 1 as an object associated with the thick disk. The resulting mean Fe abundance is −0.62±0.03(stat.)±0.10(sys.) dex, which is more metal-poor than indicated by previous spectroscopy from the literature, but it is fully in line with values from isochrone fitting. We find that Gaia 1 is moderately enhanced in the α-elements, which allowed us to consolidate its membership with the thick disk via chemical tagging. The cluster's Fe-peak and neutron-capture elements are similar to those found across the metal-rich disks, where the latter indicate some level of s-process activity. No significant spread in iron nor in other heavy elements was detected, whereas we find evidence of light-element variations in Na, Mg, and Al. Nonetheless, the traditional Na-O and Mg-Al (anti-)correlations, typically seen in old globular clusters, are not seen in our data. This confirms that Gaia 1 is rather a massive and luminous open cluster than a low-mass globular cluster. Finally, orbital computations of the target stars bolster our chemical findings of Gaia 1's present-day membership with the thick disk, even though it remains unclear, which mechanisms put it in that place.

Section: Ironsupporting

confidence: 89%

Detailed chemical abundance analysis of the thick disk star cluster Gaia 1

Hansen

Kunder

2017

A&A

“…The dSph metallicities cover a full range of −2.2 to −3.2 dex -a broad spread that is fully in line with other metal-poor, faint dSphs (e.g., Norris et al 2010a; see also Fig. 1 in Koch et al 2012b) and that emphasizes that also Boo II is likely a dark matter dominated system with a potential well that is deep enough to allow for continuous metal enrichment.…”

Section: Reanalysis Of the Metallicities From The Calcium Tripletsupporting

confidence: 70%

A Chemical Confirmation of the Faint Boötes Ii Dwarf Spheroidal Galaxy

Rich

2014

ApJ

We present a chemical abundance study of the brightest confirmed member star of the ultrafaint dwarf galaxy Boötes II from Keck/HIRES high-resolution spectroscopy at moderate signal-to-noise ratios. At [Fe/H] = −2.93 ± 0.03(stat.)±0.17(sys.) this star chemically resembles metal-poor halo field stars and the signatures of other faint dwarf spheroidal galaxies at the same metallicities in that it shows enhanced [α/Fe] ratios, Solar Fe-peak element abundances, and low upper limits on the neutron-capture element Ba. Moreover, this star shows no chemical peculiarities in any of the eight elements we were able to measure. This implies that the chemical outliers found in other systems remain outliers pertaining to the unusual enrichment histories of the respective environments, while Boo II appears to have experienced an enrichment history typical of its very low mass. We also re-calibrated previous measurements of the galaxy's metallicity from the calcium triplet (CaT) and find a much lower value than reported before. The resulting broad metallicity spread, in excess of one dex, the very metal poor mean, and the chemical abundance patterns of the present star imply that Boötes II is a low-mass, old, metal poor dwarf galaxy and not an overdensity associated with the Sagittarius Stream as has been previously suggested based on its sky position and kinematics. The low, mean CaT metallicity of −2.7 dex falls right on the luminosity-metallicity relation delineated over four orders of magnitude from the more luminous to the faintest galaxies. Thus Boötes II's chemical enrichment appears representative of the galaxy's original mass, while tidal stripping and other mass loss mechanisms were probably not significant as for other low-mass satellites.

“…The mean Fe-abundance of the three bona fide GC member candidates (star IDs 55, 204, and 362) is −0.88 ± 0.03 dex with a metallicity dispersion of 0.03 ± 0.04 dex; this is higher than the value predicted by the CaT by 0.31 dex. As for the radial velocity dispersion of this system, also the very low spread in Fe-abundances is fully compatible with the low mass of ESO452 (Carretta et al 2009;Koch et al 2012).…”

Section: Iron Abundancesupporting

confidence: 59%

“…As above, we consider the three most central stars as best members, and obtain a mean CaT metallicity of −1.19 dex (scatter of 0.05 dex) for them. Comparison with the median measurement error of 0.20 dex indicates that this scatter in the metallicities is most likely only due to the uncertainty in our CaT determination and is not an intrinsic spread, in line with the very low overall mass of the system (e.g., Koch et al 2012). We did not attempt to assign any metallicities from the CaT to the Galactic contaminants, since the above calibrations are only valid at the assumed red clump magnitude, thus the distance of the GC.…”

Section: Calcium Triplet Metallicitymentioning

confidence: 62%

Spectroscopic study of the elusive globular cluster ESO452-SC11 and its surroundings

Hansen

Kunder

2017

A&A

Globular clusters (GCs) have long been recognized as being amongst the oldest objects in the Galaxy. As such, they have the potential of playing a pivotal role in deciphering the Milky Way's early history. Here we present the first spectroscopic study of the low-mass system ESO452-SC11 using the AAOmega multifibre spectrograph at medium resolution. Given the stellar sparsity of this object and the high degree of foreground contamination due to its location toward the Galactic bulge, very few details are known for this cluster -there is no consensus, for instance, about its age, metallicity, or its association with the disk or bulge. We identify five member candidates based on common radial velocity, calcium-triplet metallicity, and position within the GC. Using spectral synthesis, the measurement of accurate Fe-abundances from Fe-lines, and abundances of several α-, Fe-peak, and neutron-capture elements (Si, Ca, Ti,Cr, Co, Ni, Sr, and Eu) is carried out, albeit with large uncertainties. We find that two of the five cluster candidates are likely non-members, as they have deviating iron abundances and [α/Fe] ratios. The cluster mean heliocentric velocity is 19±2 km s −1 with a velocity dispersion of 2.8±3.4 km s −1 , a low value in line with its sparse nature and low mass. The mean Fe-abundance from spectral fitting is −0.88 ± 0.03 dex, where the spread is driven by observational errors. Furthermore, the α-elements of the GC candidates are marginally lower than expected for the bulge at similar metallicities. As spectra of hundreds of stars were collected in a 2-degree field centered on ESO452-SC11, a detailed abundance study of the surrounding field was also enabled. The majority of the non-members have slightly higher [α/Fe] ratios, in line with the typical nearby bulge population. A subset of the spectra with measured Fe-peak abundance ratios shows a large scatter around solar values, albeit with large uncertainties. Furthermore, our study provides the first systematic measurements of strontium abundances in a Galactic bulge GC. Here, the Eu and Sr abundances of the GC candidates are broadly consistent with a disk or bulge association. Recent proper motions and our orbital calculations place ESO452 on an elliptical orbit in the central 3 kpc of the Milky Way, establishing a firm connection with the bulge. Finally, while the radial velocities and preferential position of a dozen of stars outside the GC radius appear to imply the presence of extra-tidal stars, their significantly different chemical composition refutes this hypothesis.