Bridge Between Observation and Theory of Outer-Disk Substructures Around the Milky Way

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1. Bridge Between Observations and Theory of Outer-Disk Substructures Around the Milky Way Diffuse Structures Background Methodology & Results M-Giant Substructures in the Milky Way Conclusions Colin T. Slater, Eric F. Bell, Edward F. Schlafly, Eric Morganson., et al. 
 2014, ApJ, 791. References Stellar Populations Anastasios Tzanidakis1, Allyson Sheffield2, Kathryn Johnston1, Chervin Laporte1, Adrian Price-Whelan3 1: Department of Astronomy, Columbia University, New York, NY. 2: Department of Natural Sciences, LaGuardia Community College, Long Island City, NY. 3: Department of Astronomy, Princeton University, NJ. AndyTza at3156@columbia.edu Affiliations The Milky Way’s Halo is filled with a network substructures indicating its complex nature. Evidence is mounting that some of these substructures formed in-situ in the Galactic disk and were dynamically perturbed to their current location (e.g., Price- Whelan et al. 2015) Ting S. Li, Allyson A. Sheffield, Kathryn V. Johnston, Jennifer L. Marshall et al. 2017, ApJ, 384. Figure 2— 2MASS Survey of the Milky Way Galaxy. The internal view of the Milky Way galaxy reveals the primary components of our Galaxy : the halo, budge and disk, accompanied by the large and small Magellanic clouds — dwarf satellite galaxies. • Only massive dwarf satellite galaxies (i.e 
 Sagittarius, LMC and SMC) have a range of metallicities to contain both M giants and RR Lyrae stars. Figure 1— Star Count Map of Pan-STARRS1 3π Survey at the Galactic Anti-Center. The Pan-STARRS1 Survey reveals rich substructure in the Galactic halo. The schematic represents a star count map in Galactic coordinates centered on the Galactic anti-center overplotted with TriAnd (magenda), Monoceros (red) and A13 (orange). • The Galactic disk, which is more metal rich, contains a substantially higher number of M giants than RR Lyrae ( ~5-10 per kpc3 Amrose et al. 2001) Figure 3 — Radial Motion of Mon/GASS and A13.
 Distribution of radial velocities in the galactic standard of rest frame as a function of galactic longitude (Li et al. 2017 & Sheffield et al. 2017) of the M-Giants (top panel) and RR Lyrae (bottom panel). The black and gray points represent the posterior probability of the star belonging to the “cold” M-Giant substructure. Figure 5 — Log Number Density Maps in Galactic Coordinates of N-Body Simulations of the Milky Way. 
 Log number density map in Galactic coordinates of N-Body simulations of the Galactic disk interacting with the Sagittarius dwarf satellite galaxy. At 5.56 Gyr (approximate present position) we find that the satellite-host interaction is capable of “kick-out” stars from the Galactic disk into high Galactic latitudes. The yellow rectangles represent the fields that Mon/GASS and A13 are observed in. Figure 4 — Posterior Probability Samples from Fraction of M-Giants over RR Lyrae stars. 
 Binned posterior probability samples indicate that both substructures have fraction of M-Giants to RR Lyrae close to ~0. The dashed line line shows the expected fraction of the Disk populations and conversely the solid line for relatively large dwarf satellite galaxies (Sgr & LMC) (Price-Wheelan et al. 2015) . • We took medium resolution spectra of 150 RR Lyrae stars — 199 in Mon/GASS and 72 in A13 using MDM’s 2.4 meter Hiltner telescope • As seen in Figure 3, while the M-Giant velocity gradients follow a well-defined linear trend, the overplotted RR Lyrae seem randomly scattered, suggesting no kinematic association Contact AndyTza Andy_Tzanidakis • Using a Monte Carlo Markov Chain, we derive the posterior probability distribution of the fraction of M-Giants to RR lyrae that belong to both substructures: Mon/GASS and A13. • From the posterior probability distribution, we find that the fraction of M-Giants to RR Lyrae stars resembles more of a disk-like populations, hence suggesting a “kicked-out” formation scenario. Allyson A. Sheffield, Adrian M. Price-Whelan, Anastasios Tzanidakis, 
 Kathryn V. Johnston et al. 2017, ApJ. Chervin F. P. Laporte, Facundo A. Gomez , Gurtina Besla , Kathryn V. 
 Johnston et al. 2017, ApJ, 743. Adrian M. Price-Whelan , Kathryn V. Johnston , Allyson A. Sheffield , Chervin 
 F. P. Laporte et al. 2015, ApJ, 780. • N-body simulations of the Galactic disk interacting with dwarf satellite galaxy, Sagittarius, suggest that such dynamic interaction is able to perturb the Galactic disk and “kick-out” stars from within the disk • We find observationally that Mon/GASS and A13 have a very small number of RR Lyrae associated with the substructure — excluding the accreted dwarf satellite galaxy scenario • N-body simulations of the Milky Way also demonstrate that massive dwarf satellite galaxies are able to “kick-out” stars from the Disk into the Galactic halo Thank you to Dr. Sheffield, Dr. Johnston, Dr. Price-Wheelan and Dr. Laporte for the endless hours of guidance, support and superb advising. We also thank the Kitt Peak National Observatory and the CUNY Research Scholars Program for making this research possible. Adrian M. Price-Whelan, Kathryn V. Johnston, Allyson A. Sheffield, Chervin F. P. Laporte, & Brandimir Sesar, 2015, MNRAS, 452, 676 Simulations Time dependent Smooth Particle Hydrodynamic (SPH) simulation of Sagittarius Dwarf Galaxy perturbing the Galactic disk