Probing the Dynamic Evolution of the Milky Way Using Stellar Substructures in the Galactic Anticenter

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1. Probing the Dynamic Evolution of the Milky Way Using Stellar Substructures in the Galactic Anticenter Anastasios Tzanidakis1, Kathryn Johnston1, Adrian Price-Whelan2, Allyson Sheffield3, Chervin Laporte4 Why Map the Galactic Anticenter? References Galactic Disk “Kicking-out” Stars Mapping the Galactic Anticenter with 2MASS and Gaia DR2 Why is the Disk Oscillating? The Galactic Anticenter serves as a laboratory to understand the dynamical interactions in and around the Milky Way. Recent all-sky surveys have revealed a complex network of substructures populating the Galactic Anticenter. Examples include arcs of stars at large heliocentric radii distributed below and above the Galactic mid-plane such as the Triangulum-Andromeda Clouds (TriAnd)[1], Galactic Anticenter Stellar Structure (GASS)[2], and A13[3][4] stellar substructures. Figure 2 — Posterior Probability Samples from Fraction of M-Giants over RR Lyrae stars. 
 The 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. 1Columbia University, 2Princeton University, 3LaGuardia Community College, 4University of Victoria Connecting Local to Global Structures from Simulations to Observations With the release of Gaia DR2, we are now able to map the 4D distributions of these known substructures. We select M giants from 2MASS, estimate their approximate distances photometrically, and crossmatch with Gaia DR2 to obtain their proper motion distributions. The characteristics we have observed still remain unclear. However, recent studies have revealed a density and velocity asymmetry within one kpc of the Sun[6] which could be connected to what we see at large radii. In essence, these nearby oscillations may connect to the more distant structures we observe and suggest a more global view of the Galactic disk at the Anticenter. Figure 1— Density Map of Pan-STARRS1 3π Survey at the Galactic Anti-Center. 
 The Pan-STARRS1 Survey reveals rich substructure in the Galactic halo. The following schematic represents a star count map in Galactic coordinates. The over plotted scatter shows the associated M giants that are members of the TriAnd (purple), GASS (red), A13 (orange) substructures. Source: Laporte et al. 2018 TriAnd, GASS and A13 stellar features were initially identified as over-densities in spatial distributions in M giant stars. Subsequent studies showed the identified M Giants members to display small velocity dispersions suggesting a kinematic association between the members of each substructure. Finally, Price-Whelan et al. 2015 and Sheffield et al. 2018, showed that there existed a lack of RR Lyrae stars at the same spatial footprints, suggesting that the stellar populations resembled more like the Galactic disk rather than the halo. A follow-up of the abundance pattern also revealed disk-like metallicity distribution[5]. We concluded these are structures have been “kicked-out" from the Galactic Disk. Figure 5 — Schematic Illustration of Galactic Disk Oscillations Schematic representation of GASS, A13 and Triand substructures in the R-Z plane. Each line represents an oscillatory distribution for three different line of sight. Finally, ϕ is a Galactocentric azimuth angle and increases in the opposite sense as the heliocentric Galactic longitude, and R is the Galactocentric distance.
 The preliminary results confirm the existence of the three substructures mentioned in this work with systematic offsets in proper motion in bins of Galactic radii. The M giants selected in 2MASS show systematic difference in vertical motion along various lines of sight, suggesting signals of Galactic oscillations at3156@columbia.edu Andy_Tza @Andy_Tzanidakis Figure 7 — Log Number Density and Proper Motions of M Giant Stars in 2MASS and Gaia DR2
 The top three panels represent the log number density of M Giant stars in the same spatial footprints of GASS, A13 and TriAnd. Each substructure separately emerges as an overdensity in bins of distance. The bottom row shows for the same distance bins, the median proper motion in Galactic latitude. The shaded regions highlight the distance bins that each substructure has been identified. Acknowledgements This work has made use of data from the European Space Agency (ESA) mission Gaia, and data products from the Two Micron All Sky Survey. This research was part supported by the NSF grant AST - 1614743 but…very far from disk Johnston, Price-Whelan+201 With Gaia DR2, a global picture of disk oscillations is just emerging[7]. The remaining important question is: what is exciting the oscillations? 
 One possible scenario is the interaction between the Galactic disk and the Sagittarius dwarf satellite galaxy. Laporte et al. 2018(a) shows through an N-body simulation of a MW-like Galaxy interacting with a Sgr-like merger, is enough to excite the Galactic disk at large latitudes. Moreover, the N-body simulation was also able to produce disk substructures reminiscent of those observed throughout the disk with Gaia[8]. While we do not expect the N-body simulations to exactly match what is observed around the Milky Way, they can nonetheless give us insight in connecting oscillations around the Sun to disk disturbances on larger scales. Figure 4 — Physical Projection of N-Body Simulation of MW-like Galaxy
 Using the Laporte et al. 2018 N-Body Simulation, we plot the particles in the XY-plane color coded by their height above and below the disk (Z). It becomes apparent that the Galactic disk exhibits systematic oscillatory like distribution as a function of radius. Figure 5 — Cartesian Projections of GASS, A13 and TriAnd 
 Projected z-R positions of the stellar substructures GASS, A13, and TriAnd structures in the Z-R plane. 
 The doted circle at z=0 is the position of the sun, and the gray lines are the associated errors in distance. These stellar substructures show displacement above and below the Galactic mid-plane. In the Laporte et. al 2018 simulations we explore how the local disk substructures connect to a more global view the Milky Way Figure 5. from A Disk Origin for the Monoceros Ring and A13 Stellar Overdensities null 2018 APJ 854 47 doi:10.3847/1538-4357/aaa4b6 http://dx.doi.org/10.3847/1538-4357/aaa4b6 © 2018. The American Astronomical Society. All rights reserved. [1] Martin, F. N., Ibata, A. R., Rich R. M., et al. 2014. ApJ, 787-19 [2] Crane, J. D., Majewski, S. R., Rocha-Pinto, H. J., et al. 2003, ApJ, 594. L119 [3] Sharma, S., Johnston, K. V., Majewski, S. R., et al. 2010, ApJ, 722, 750 [4] Li T. S., Sheffield A. A., Johnston K. V., et al. 2017, ApJ, 844, 74 [5] Bergemann, M., Sesar, B., Cohen, J. G., et al. 2018, Nature, 555, 334 [6]Widrow, L. M., Gardner, S., Yanny, B., Dodelson, S., & Chen, H.-Y. 2012, ApJLl, 750, L41 [7] Gaia Data Release 2: Mapping the Milky Way disk kinematics. Gaia Collaboration, Katz, D., et al. 2018 [8] Laporte C. F. P., Johnston K. V., Tzanidakis A., 2018, MNRAS Newberg, H. J., Yanny, B., Rockosi, C., et al. 2002, ApJ, 569, 245 Price-Whelan, A. M., Johnston, K. V., et al. 2015, MNRAS, 452, 676 Laporte F. P. C., Johnston, K. V., Gómez A. F., et al. 2018 Laporte, C. F. P., Minchev, I., Johnston, K. V., et al. 2018. MNRAS, 1, 11 Laporte C. F. P., Johnston K. V., Tzanidakis A., 2018, MNRAS Price-Whelan, A. M., Johnston, K. V., Sheffield. A. A., et al. 2015, MNRAS, 452, 676 Slater, C. T., Bell, E. F., Schlafly, E. F., et al. 2014, ApJ, 791, 9 Sheffield. A. A., Price-Whelan. A. M., Tzanidakis. A., Johnston, K. V. et al. 2018, ApJ Contact