Galactic Anticenter Stellar Structure Research Articles

A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities

Abstract The Monoceros Ring (also known as the Galactic Anticenter Stellar Structure) and A13 are stellar overdensities at estimated heliocentric distances of d ∼ 11 kpc and 15 kpc observed at low Galactic latitudes toward the anticenter of our Galaxy. While these overdensities were initially thought to be remnants of a tidally disrupted satellite galaxy, an alternate scenario is that they are composed of stars from the Milky Way (MW) disk kicked out to their current location due to interactions between a satellite galaxy and the disk. To test this scenario, we study the stellar populations of the Monoceros Ring and A13 by measuring the number of RR Lyrae and M giant stars associated with these overdensities. We obtain low-resolution spectroscopy for RR Lyrae stars in the two structures and measure radial velocities to compare with previously measured velocities for M giant stars in the regions of the Monoceros Ring and A13, to assess the fraction of RR Lyrae to M giant stars (f RR:MG) in A13 and Mon/GASS. We perform velocity modeling on 153 RR Lyrae stars (116 in the Monoceros Ring and 37 in A13) and find that both structures have very low f RR:MG. The results support a scenario in which stars in A13 and Mon/GASS formed in the MW disk. We discuss a possible association between Mon/GASS, A13, and the Triangulum-Andromeda overdensity based on their similar velocity distributions and f RR:MG.

Exploring Halo Substructure with Giant Stars. XV. Discovery of a Connection between the Monoceros Ring and the Triangulum–Andromeda Overdensity?* † ‡

Abstract Thanks to modern sky surveys, over 20 stellar streams and overdensity structures have been discovered in the halo of the Milky Way. In this paper, we present an analysis of spectroscopic observations of individual stars from one such structure, “A13,” first identified as an overdensity using the M giant catalog from the Two Micron All Sky Survey. Our spectroscopic observations show that stars identified with A13 have a velocity dispersion of ≲40 , implying that it is a genuine coherent structure rather than a chance superposition of random halo stars. From its position on the sky, distance (∼15 kpc heliocentric), and kinematical properties, A13 is likely to be an extension of another substructure at low Galactic latitude—the Galactic Anticenter Stellar Structure (also known as the Monoceros Ring)—toward smaller Galactic longitude and greater distance. Furthermore, the kinematics of A13 also connect it with another structure in the southern Galactic hemisphere—the Triangulum–Andromeda overdensity. We discuss these three connected structures within the context of a previously proposed scenario in which one or all of these features originate from the disk of the Milky Way.

Abundances and kinematics for ten anticentre open clusters

Open clusters are distributed all across the disk and are convenient tracers of its properties. In particular, outer disk clusters bear a key role for the investigation of the chemical evolution of the Galactic disk. The goal of this study is to derive homogeneous elemental abundances for a sample of ten outer disk OCs, and investigate possible links with disk structures such as the Galactic Anticenter Stellar Structure. We analyse high-resolution spectra of red giants, obtained from the HIRES@Keck and UVES@VLT archives. We derive elemental abundances and stellar atmosphere parameters by means of the classical equivalent width method. We also performed orbit integrations using proper motions. The Fe abundances we derive trace a shallow negative radial metallicity gradient of slope -0.027+/-0.007 dex.kpc-1 in the outer 12 kpc of the disk. The [alpha/Fe] gradient appears flat, with a slope of 0.006+/-0.007 dex.kpc-1 . The two outermost clusters (Be 29 and Sau 1) appear to follow elliptical orbits. Be 20 also exhibits a peculiar orbit with a large excursion above the plane. The irregular orbits of the three most metal-poor clusters (of which two are located at the edge of the Galactic disk), if confirmed by more robust astrometric measurements such as those of the Gaia mission, are compatible with an inside-out formation scenario for the Milky Way, in which extragalactic material is accreted onto the outer disk. We cannot determine if Be 20, Be 29,and Sau 1 are of extragalactic origin, as they may be old genuine Galactic clusters whose orbits were perturbed by accretion events or minor mergers in the past 5 Gyr, or they may be representants of the thick disk population. The nature of these objects is intriguing and deserves further investigations in the near future.

Chemical fingerprinting of stellar populations in the Milky Way halo

AbstractThe idea of “chemically fingerprinting“ stars to their birth systems has been discussed over the last decade. Here we present an investigation of the chemical abundance patterns of halo substructures using high-resolution spectra. In particular, we study the abundances of the α-like element titanium (Ti) and the s-process elements yttrium (Y) and lanthanum (La) for M giant candidates of the Galactic Anticenter Stellar Structure (GASS, also known as the Monoceros Ring) and the Triangulum-Andromeda (TriAnd) Star Cloud. We apply “chemical fingerprinting“ to the GASS/Monoceros Ring and TriAnd Star Cloud, to explore the origins of the two systems and the hypothesized connections between them. GASS has been debated either to originate from a part (e.g., warp) of the Galactic disk or tidal debris of a disrupted Milky Way (MW) satellite galaxy. Our exploration shows that GASS is indeed made of stars from a dwarf spheroidal (dSph) galaxy, although we still can not rule out the possibility that GASS was dynamically created out of a previously formed outer MW disk. And whereas the TriAnd Star Cloud has been assumed to come from the tidal disruption of the same accreted MW satellite as the GASS/Monoceros Ring, our comparison of the abundance patterns in GASS and TriAnd M giants suggests that the TriAnd Star Cloud is likely an independent halo substructure unrelated to the GASS/Monoceros Ring. Furthermore, our findings also suggest that the MW may have accreted other satellites in addition to the on-going, well-known Sagittarius (Sgr) dwarf galaxy.

FIRST CHEMICAL ANALYSIS OF STARS IN THE TRIANGULUM--ANDROMEDA STAR CLOUD

We undertake the first high resolution spectroscopic study of the Triangulum-Andromeda (TriAnd) star cloud --- an extended, mid-latitude Milky Way halo substructure about 20 kpc away in the second Galactic quadrant --- through six M giant star candidates selected to be both spatially and dynamically associated with this system. The abundance patterns of [Ti/Fe], [Y/Fe] and [La/Fe] as a function of [Fe/H] for these stars support TriAnd as having an origin in a dwarf galaxy with a chemical enrichment history somewhat similar to that of the Sagittarius dwarf spheroidal (dSph) galaxy. We also investigate the previously proposed hypothesis that TriAnd is an outlying, dynamically older piece of the Monoceros Stream (also known as the Galactic Anticenter Stellar Structure, "GASS") under the assumption that both features come from the tidal disruption of the same accreted Milky Way satellite and find that net differences in the above abundance patterns between the TriAnd and GASS stars studied suggest that these two systems are independent and unrelated.

High resolution HDS/SUBARU chemical abundances of the young stellar cluster Palomar 1

Context. Palomar 1 is a peculiar globular cluster (GC). It is the youngest Galactic GC and it has been tentatively associated to several of the substructures recently discovered in the Milky Way (MW), including the Canis Major (CMa) overdensity and the Galactic Anticenter Stellar Structure (GASS).

THE CHEMICAL EVOLUTION OF THE MONOCEROS RING/GALACTIC ANTICENTER STELLAR STRUCTURE

The origin of the Galactic Anticenter Stellar Structure (GASS) or "Monoceros ring" --- a low latitude overdensity at the edge of the Galactic disk spanning at least the second and third Galactic quadrants --- remains controversial. Models for the origin of GASS fall generally into scenarios where it is either a part (e.g., warp) of the Galactic disk or it represents tidal debris from the disruption of a Milky Way (MW) satellite galaxy. To further constrain models for the origin of GASS, we derive chemical abundance patterns from high resolution spectra for 21 M giants spatially and kinematically identified with it. The abundances of the (mostly) $\alpha$ element titanium and s-process elements yttrium and lanthanum for these GASS stars are found to be lower at the same [Fe/H] than those for MW stars, but similar to those of stars in the Sagittarius stream, other dwarf spheroidal galaxies, and the Large Magellanic Cloud. This demonstrates that GASS stars have a chemical enrichment history typical of dwarf galaxies --- and unlike those of typical MW stars (at least MW stars near the Sun). Nevertheless, these abundance results cannot definitively rule out the possibility that GASS was dynamically created out of a previously formed, outer MW disk because $\Lambda$CDM-based structure formation models show that galactic disks grow outward by accretion of dwarf galaxies. On the other hand, the chemical patterns seen in GASS stars do provide striking verification that accretion of dwarf galaxies has indeed happened at the edge of the MW disk.

Chemical Fingerprinting and Chemical Analysis of Galactic Halo Substructure

AbstractWe present high-resolution spectroscopic measurements of the abundances of the α-like element titanium (Ti) and s-process elements yttrium (Y) and lanthanum (La) for M giant candidates of (a) the Sagittarius (Sgr) dwarf spheroidal + tidal tail system, (b) the Triangulum-Andromeda (TriAnd) Star Cloud, and (c) the Galactic Anticenter Stellar Structure (GASS, or Monoceros Stream). All three systems show abundance patterns unlike the Milky Way but typical of dwarf galaxies. The Sgr system abundance patterns resemble those of the Large Magellanic Cloud. GASS/Mon chemically resembles Sgr but is distinct from TriAnd, a result that does not support previous suggestions that TriAnd is a piece of the Monoceros Stream.

Tombaugh 2: the first open cluster with a significant abundance spread or embedded in a cold stellar stream?★

We present new high-resolution spectroscopy from which we derive abundances and radial velocities for stars in the field of the open cluster Tombaugh 2 (To2), which has been suggested to be one of a group of clusters previously identified with the Galactic Anticenter Stellar Structure (GASS) (also known as the Monoceros stream). Using Very Large Telescope/Fibre Large Array Multi-Element Spectrograph (VLT/FLAMES) with the Ultra-Violet and Visible Echelle Spectrograph (UVES) and GIRAFFE spectrographs, we find a radial velocity (RV) of = 121 ± 0.4km s -1 using 18 To2 cluster stars; this is in agreement with previous studies, but at higher precision. We also make the first measurement of To2's velocity dispersion, which is σ int = 1.8 ± 0.3 km s -1 . Our abundance analysis of RV-selected members finds that To2 is more metal rich than previous studies have found; moreover, unlike the previous work, our larger sample also reveals that stars with the velocity of the cluster show a relatively large spread in chemical properties (e.g. A[Fe/H] > 0.2). This is the first time a possible abundance spread has been observed in an open cluster, though this is one of several possible explanations for our observations. While there is an apparent trend of [α/Fe] with [Fe/H], the distribution of abundances of these 'RV cluster members' also may hint at a possible division into two primary groups with different mean chemical characteristics - namely ( , ) ∼ (-0.06, +0.02) and (-0.28, +0.36). Isochrone fitting to the colour-magnitude distribution of apparent To2 members yields an age of 2.0 Gyr, E(B - V) = 0.3, and (m - M) 0 = 14.5 or d = 7.9 kpc for both populations - parameters that are within the range of previous findings. Based on position and kinematics To2 is a likely member of the GASS/Monoceros stream, which makes To2 the second star cluster within the originally proposed GASS/Monoceros family after NGC 2808 to show some evidence for internal population dispersions. However, we explore other possible explanations for the observed spread in abundances and two possible subpopulations, with the most likely explanation being that the metal poor ([Fe/H] = -0.28), more centrally concentrated population being the true To2 clusters stars and the metal-rich ([Fe/H] = -0.06) population being an overlapping, and kinematically associated, but 'cold' (σ v < 2 km s -1 ) stellar stream at R gc ≥ 15 kpc.

Photometry and Spectroscopy of Old, Outer Disk Star Clusters: vdB-Hagen 176, Berkeley 29, and Saurer 1

It has been previously proposed that some distant open clusters in the Milky Way may have been accreted during a dwarf galaxy merger, perhaps associated with the same event that led to the formation of the Galactic anticenter stellar structure (GASS), also known as the Monoceros Ring. We have obtained VI and Washington + DDO51 photometric and medium-resolution (R ~ 8000) multifiber spectroscopic data for the three distant old open clusters Berkeley 29, Saurer 1, and vdB-Hagen 176 (BH?176). These clusters are spatially coincident with GASS, but radial velocities and spectroscopic metallicities were not available during previous studies of the GASS candidate cluster system. Similar data for the clusters Be 20 and Be 39 have been obtained for calibration purposes. We provide the first reliable radial velocity for BH?176 (Vhelio = 11.2 ? 5.3 km s-1). We also find that Vhelio = +95.4 ? 3.6 and +28.4 ? 3.6 km s-1 for Sa 1(A) and Be 29, respectively. In contrast to previous findings, we show that ?-enhanced isochrones, while spectroscopically motivated, provide a poor fit to the Be 29 color-magnitude diagram. We find that the clusters Be 29 and Sa 1 have properties that are consistent with the previously reported characteristics for GASS candidate clusters and the GASS stellar stream as derived from M giant observations. However, the radial velocity and photometric metallicity ([Fe/H] ~ 0.0 dex) for BH?176 suggest that a connection of this cluster with the putative GASS cluster system is unlikely. We reassess the age-metallicity relation for the most likely members of the GASS clusters system for which spectroscopic metallicities are now available.

Star Clusters in the Galactic Anticenter Stellar Structure and the Origin of Outer Old Open Clusters

The Galactic anticenter stellar structure (GASS) has been identified with excess surface densities of field stars in several large-area sky surveys and with an unusual, stringlike grouping of five globular clusters. At least two of these are diffuse, young clusters between open and globular types. Here we call attention to the fact that four younger open or transitional clusters extend the previously identified, stringlike cluster grouping, with at least one having a radial velocity consistent with the previously found GASS velocity-longitude trend. All nine clusters lie close to a plane tipped 17° to the Galactic plane. This planar orientation is used to forage for additional potential cluster members in the inner Galaxy, and a number are found along the same plane and stringlike sequence, including almost all 15 known outer, old open clusters. Tidal accretion of a dwarf satellite galaxy on a low-inclination orbit—perhaps the GASS system—appears to be a plausible explanation for the origin of the outer, old open and transitional clusters of the Milky Way. We use these clusters to explore the age-metallicity relation of the putative accreted GASS progenitor. Finally, we provide the first radial velocity of a star in the cluster BH 176 and discuss its implications.