Substructures, resonances, and debris streams

Dodd, Emma; Helmi, Amina; Koppelman, Helmer H.

doi:10.1051/0004-6361/202141354

Cited by 18 publications

(16 citation statements)

References 63 publications

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“…As described in Section 2.2, symbols in Figures 3 to 6 are hatched according to the subgroups discussed by Dodd et al (2022). Both subgroups show similar abundance peculiarities compared to other halo stars (lower abundances of α-elements and Y).…”

Section: Resultsmentioning

confidence: 99%

“…One is by the sign of v z ; Kepley et al (2007) and Koppelman et al (2019) used the asymmetry in the number of stars in each subgroup to estimate the accretion time of the progenitor of the Helmi streams. Another subdivision follows that by Dodd et al (2022) in the space of L z and L perp . They show that these two groups of stars with slightly different kinematics reflect the effect of an orbital resonance in the Milky Way potential.…”

Section: Targetsmentioning

confidence: 99%

“…In either definition, the two subgroups are considered parts of the same substructure, the Helmi streams. In all figures, symbols are hatched according to the subdivision by Dodd et al (2022).…”

Section: Targetsmentioning

confidence: 99%

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High-precision chemical abundances of Galactic building blocks

Matsuno

Dodd

Koppelman

et al. 2022

A&A

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Context. The Helmi streams are a kinematic substructure whose progenitor is likely a dwarf galaxy. Although 20 years have passed since their discovery, it is still unclear whether their members are chemically distinguishable from other halo stars in the Milky Way. Aims. We aim to precisely characterize the chemical properties of the Helmi streams. Methods. We analyzed high-resolution, high signal-to-noise ratio spectra for 11 Helmi stream stars through a line-by-line abundance analysis. We compared the derived abundances to homogenized literature abundances of the other halo stars, including those belonging to other kinematic substructures, such as Gaia-Enceladus and Sequoia. Results. Compared to typical halo stars, the Helmi stream members clearly show low values of [X/Fe] in elements produced by massive stars, such as Na and α-elements. This tendency is seen down to metallicities of at least [Fe/H] ∼ − 2.2, suggesting type Ia supernovae already started to contribute to the chemical evolution at this metallicity. We find that the [α/Fe] ratio does not evolve significantly with metallicity, making the Helmi stream stars less distinguishable from Gaia-Enceladus stars at [Fe/H] ≳ − 1.5. The almost constant but low value of [α/Fe] might be indicative of quiescent star formation with low efficiency at the beginning and bursty star formation at later times. We also find extremely low values of [Y/Fe] at low metallicity, providing further support for the claim that light neutron-capture elements are deficient in Helmi streams. While Zn is deficient at low metallicity, it shows a large spread at high metallicity. The origin of the extremely low Y abundances and Zn variations remains unclear. Conclusions. The Helmi stream stars are distinguishable from the majority of the halo stars if homogeneously derived abundances are compared.

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Section: Resultsmentioning

confidence: 99%

Section: Targetsmentioning

confidence: 99%

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High-precision chemical abundances of Galactic building blocks

Matsuno

Dodd

Koppelman

et al. 2022

A&A

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“…Using the criteria from Koppelman et al (2019b), Dodd et al (2022), andLövdal et al (2022), we identify 646 candidate members of the Helmi streams in the 6D halo sample. Ideally, one would compute the SFH of the progenitor of the Helmi streams from this subset.…”

Section: Data and Sample Selectionmentioning

confidence: 99%

Unveiling the past evolution of the progenitor of the Helmi streams

Ruiz-Lara¹,

Helmi²,

Gallart³

et al. 2022

Preprint

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Aims. We aim to determine the star formation history (SFH) of the progenitor of the Helmi streams. Methods. From the 5D Gaia EDR3 data set, we extract local samples of stars dominated by the Helmi streams, and the Galactic (thick and thin) disc and halo. We do this by identifying regions in a pseudo-Cartesian velocity space (obtained by setting line-ofsight velocities to zero), where stars belonging to these components, as identified in samples with 6D phase-space information, are predominantly found. We make use of an updated absolute colour-magnitude diagram (aCMD) fitting methodology to contrast, for the first time, the SFH of a disrupted accreted system -the Helmi streams-to that of the average Milky Way inner halo. To this end, special attention is given to the correct characterisation of Gaia completeness effects and observational errors on the aCMD. Results. We find that the progenitor of the Helmi streams experienced an early star formation but which was sustained for longer (until 7-9 Gyr ago) than for the Milky Way halo (10-11 Gyr ago). As a consequence, half of its stellar mass was in place ∼ 0.7 Gyr later. The quenching of star formation in the Helmi streams progenitor, together with some hints of a star formation burst at ∼ 8 Gyr, suggest it was accreted by the Milky Way around this time, in concert with previous estimates based on the dynamics of the streams.

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“…3) of the Helmi streams unjoined. However, a KS test on the metallicity distributions of these two groups confirms that the two are consistent and thus should be joined as one group (see also Dodd et al 2022). This consists of 3 clusters (1 with higher L ⊥ and 2 from the lower L ⊥ clumps) with a total of 282 stars.…”

Section: Main Groupsmentioning

confidence: 93%

The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun

Dodd¹,

Callingham²,

Helmi³

et al. 2022

Preprint

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Context. The debris from past merger events is expected and, to some extent, known to populate the stellar halo near the Sun. Aims. We aim to identify and characterise such merger debris using Gaia DR3 data supplemented by metallicity and chemical abundance information from LAMOST LRS and APOGEE for halo stars within 2.5 kpc from the Sun. Methods. We utilise a single linkage-based clustering algorithm to identify over-densities in Integrals of Motion space that could be due to merger debris. Combined with metallicity information and chemical abundances, we characterise these statistically significant over-densities. Results. We find that the local stellar halo contains 8 main dynamical groups, some of in-situ and some of accreted origin, most of which are already known. We report the discovery of a new substructure, which we name ED-1. In addition, we find evidence for 9 independent smaller clumps, 4 of which are new: ED-2, 3, 4 and 5 are typically rather tight dynamically, depict a small range of metallicities, and their abundances when available, as well as their location in Integrals of Motion space suggest an accreted origin. Conclusions. The local halo contains an important amount of substructure, of both in-situ and accreted origin.

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Substructures, resonances, and debris streams

Cited by 18 publications

References 63 publications

High-precision chemical abundances of Galactic building blocks

High-precision chemical abundances of Galactic building blocks

Unveiling the past evolution of the progenitor of the Helmi streams

The Gaia DR3 view of dynamical substructure in the stellar halo near the Sun

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