Exploring Halo Substructure With Giant Stars. Xiv. The Nature of the Triangulum-Andromeda Stellar Features

Abstract

As large-scale stellar surveys have become available over the past decade, the ability to detect and characterize substructures in the Galaxy has increased dramatically. These surveys have revealed the Triangulum-Andromeda (TriAnd) region to be rich with substructures in the distance range 20-30 kpc, and the relation of these features to each other, if any, remains unclear. An exploration using Two Micron All Sky Survey (2MASS) photometry reveals not only the faint sequence in M giants detected by Rocha-Pinto et al. spanning the range 100 • < l < 160 • and −50 • < b < −15 • , but, in addition, a second, brighter and more densely populated sequence. These sequences are likely associated with the distinct main sequences (MSs) discovered (and labeled TriAnd1 and TriAnd2) by Martin et al. in an optical survey in the direction of M31, where TriAnd2 is the optical counterpart of the fainter red giant branch (RGB)/asymptotic giant branch sequence of Rocha-Pinto et al. Here, the age, distance, and metallicity ranges for TriAnd1 and TriAnd2 are estimated by simultaneously fitting isochrones to the 2MASS RGB tracks and the optical MS/MS turn-off features. The two populations are clearly distinct in age and distance: the brighter sequence (TriAnd1) is younger (6-10 Gyr) and closer (distance of ∼15-21 kpc), whereas the fainter sequence (TriAnd2) is older (10-12 Gyr) and at an estimated distance of ∼24-32 kpc. A comparison with simulations demonstrates that the differences and similarities between TriAnd1 and TriAnd2 can simultaneously be explained if they represent debris originating from the disruption of the same dwarf galaxy, but torn off during two distinct pericentric passages.

Disk-like Chemistry of the Triangulum-Andromeda Overdensity as Seen by APOGEE

“…This is consistent with past distances found for TriAnd, e.g., the 18.2 kpc distance (Sheffield et al 2014) used to select the "TriAnd1" members that dominate our sample here. This puts the TriAnd sample at a median Galactocentric radius of ∼24 kpc (1-σ R spread of 4 kpc) and below the disk by ∼7 kpc (1-σ Z spread of 1 kpc).…”

“…While a few of the TriAnd stars do not have reliable spectrophotometric distances from StarHorse, they were selected by Sheffield et al (2014) in color-magnitude to fall along the red giant branches of 8 Gyr/10 Gyr −0.8/−1.0 metallicity isochrones at heliocentric distances around 18.2 kpc/27.5 kpc for TriAnd1/TriAnd2. Figure 2 shows that TriAnd stars have effective temperatures and surface gravities of cool red giants, supporting the isochrone-based distances used by Sheffield et al (2014).…”

“…Several studies have suggested that TriAnd may separate into two features, TriAnd1 and TriAnd2, that coincide on the sky but lie at photometrically determined heliocentric distances ∼20 kpc and ∼28 kpc, respectively (Martin et al 2007). However, because these features were shown to overlap considerably in spectrophotometric distance, radial velocity, and metallicity (Sheffield et al 2014), and we only have two stars classified as TriAnd2 members, we treat them here as a single overdensity. For a comparison, we use Sgr dSph because this dwarf galaxy is sufficiently enriched to have a considerable M giant population, like TriAnd.…”

Isotropic at the Break? 3d Kinematics of Milky Way Halo Stars in the Foreground of M31

“…In contrast, as an example of substructure that is dynamically cold in LOS velocity, an approximate CDF for the Triangulum-Andromeda Stream (TriAnd; located along the LOS toward M31) is shown in red (v 0 ∼50 km s −1 , σ∼15 km s −1 ; e.g., Deason et al 2014;Sheffield et al 2014). D13 suggested that the presence of a cold stream or TriAnd could be the reason for the relative increase in tangential pressure seen in this sample.…”

Target Selection for the SDSS-IV APOGEE-2 Survey

“…TriAnd's nature as either a satellite remnant or disk substructure, and/or the superposition of multiple structures, remains unresolved (e.g., Martin et al 2007;Chou et al 2011;Price-Whelan et al 2015;Xu et al 2015). APOGEE-2 is targeting five locations where the standard halo selection criteria, without W+D photometry (Section 4.1), select the maximum number of TriAnd member candidates from Sheffield et al (2014) and Chou et al (2011). These fields are called TRIAND-1, TRIAND-2, TRIAND-3, TRIAND-4, and TRIAND-5.…”