Characterization and history of the Helmi streams with Gaia DR2

Abstract

Context. The halo of the Milky Way has long been hypothesized to harbour significant amounts of merger debris. This view has been supported over more than a decade by wide-field photometric surveys which have revealed the outer halo to be lumpy. Aims. The recent release of Gaia DR2 is allowing us to establish that mergers also have been important and possibly built up the majority of the inner halo. In this work we focus on the Helmi streams, a group of streams crossing the Solar vicinity and known for almost two decades. We characterize their properties and relevance for the build-up of the Milky Way's halo. Methods. We identify new members of the Helmi streams in an unprecedented dataset with full phase-space information combining Gaia DR2, and the APOGEE DR2, RAVE DR5 and LAMOST DR4 spectroscopic surveys. Based on the orbital properties of the stars, we find new stream members up to a distance of 5 kpc from the Sun, which we characterize using photometry and metallicity information. We also perform N-body experiments to constrain the time of accretion and properties of the progenitor of the streams. Results. We find nearly 600 new members of the Helmi streams. Their HR diagram reveals a broad age range, from approximately 11 to 13 Gyr, while their metallicity distribution goes from −2.3 to −1.0, and peaks at [Fe/H] ∼ −1.5. These findings confirm that the streams originate in a dwarf galaxy. Furthermore, we find 7 globular clusters to be likely associated, and which follow a well-defined age-metallicity sequence whose properties suggest a relatively massive progenitor object. Our N-body simulations favour a system with a stellar mass of ∼ 10 8 M accreted 5 − 8 Gyr ago. Conclusions. The debris from the Helmi streams is an important donor to the Milky Way halo, contributing approximately 15% of its mass in field stars and 10% of its globular clusters.

High- and Low-α Disk Stars Separate Dynamically at All Ages

“…The retrograde stars are also younger (than the prograde stars) with (age ± σ err ) = (6.23 ± 0.59)Gyr, whereas for the prograde stars it is (7.75 ± 0.39)Gyr. This anomalous low-α halo population may be associated with the coherent population of infall discovered in the Gaia data by a number of groups (Myeong et al 2018;Koppelman et al 2018;Helmi et al 2018). In particular, Helmi et al (2018) find a retrograde population in the halo within 5kpc of the Sun which they call the "Gaia Enceladus" and show that it might have merged with the Milky Way ∼ 10 Gyr ago.…”

Chemical Abundance Signature of J0023+0307: A Second-generation Main-sequence Star with [Fe/H] < −6*

“…Note that of the other two stars with no detected iron, SMSS0313−6707 also has a pericenter < 1 kpc and is probably also old; but SDSSJ1035+0641 does not, with a pericenter at 6.5 kpc. The highly eccentric radial orbit suggests a possible association with the recently discovered Gaia sausage/blob structure (Belokurov et al 2018;Koppelman et al 2018), although such orbits could also just reflect typical virialized halo star orbits. We caution that our simple orbit integrations do not account for effects like the Galactic bar, which can significantly influence halo star orbits (e.g., Price-Whelan et al 2016).…”

Multiple retrograde substructures in the Galactic halo: A shattered view of Galactic history

“…We use here an augmented version of the Gaia RVS sample, extended with radial velocities from APOGEE DR14 (Abolfathi et al 2018), LAMOST (Cui et al 2012), and RAVE DR5 (Kunder et al 2017), see Sect. 2.1 and 2.2 of Koppelman et al (2019) for more details. Because the metallicity scales of the three different surveys are not necessarily the same, we will use the LAMOST values, unless mentioned otherwise.…”