Signatures of a Tidally Induced Spiral Arm at the Anticenter of the Milky Way and a Kinematically Extended Anticenter Stream Using DESI DR2

Abstract

Using the Dark Energy Spectroscopic Instrument Milky Way Survey (DESI MWS), we examine the 6D space of the anticenter region of the stellar disk (150$^\circ$ $<$ Galactic longitude $<$ 220$^\circ$) using 61,883 main sequence turn-off stars. We focus on two well-known stellar overdensities in the anticenter, the Monoceros Ring (MRi) and Anticenter Stream (ACS). We find that the MRi overdensity has kinematics consistent with a tidally induced spiral arm, a type of dynamic spiral arm created by an interaction with a satellite galaxy, most likely the Sagittarius Dwarf Spheroidal galaxy (Sgr). We use the kinematics of the MRi to calculate the two most recent passage times of Sgr are 0.25 $\pm$ 0.09 Gyrs and 1.10 $\pm$ 0.23 Gyrs from the present day. We validate that the ACS is kinematically decoupled from the MRi because they are moving in opposite radial and vertical directions. We find that the kinematics associated with the ACS are not confined to our defined overdensity. The features we see in the ACS region are likely part of a broader distribution of stars with the same kinematic signature as detected in other places, like the vertical wave in the outer disk and phase spiral.

Figures (10)

• Figure 1: Two-dimensional histogram of the color--absolute magnitude diagram of the MWS MAIN-BLUE sample using 70 bins in each direction. The magenta box shows our MSTO star selection, which is defined as $3.3<M_g<5.7$ and $0.2< g-r <0.4$.
• Figure 2: Density distribution of our MWS MAIN-BLUE sample in $l$--$b$ with 180 bins along $l$ and 90 bins along $b$. The pink box represents our anticenter sample coordinates between $150^\circ<l<220^\circ$ and $20^\circ<b<40^\circ$. We focus on the lowest latitude of the anticenter region, where we can observe the MRi in the DESI footprint and extending to the end of the ACS region that is above the DESI Galactic latitude limit. The gaps in data correspond to regions where the DESI MWS does not target due to high dust extinction.
• Figure 3: Left: Two-dimensional histogram of the $R$--$Z$ projection of the MSTO anticenter sample with five bins per kpc in both directions. The top left panel shows the density distribution, and the bottom left panel is color mapped to the median $V_\phi$ distribution. We see stars with comparable disk velocities extend to large $R$ and $Z$. Right: $X$--$Y$ projection using the same sample of stars as the left panels, with five bins per kpc in both directions. The top right panel shows the density distribution of our sample, and the bottom right panel is color mapped to the median $V_\phi$ distribution. We find that, after excluding stars with halo-like velocities, the median $V_\phi$ of this sample is 214 km s$^{-1}$, consistent with thin disk $V_\phi$ at the solar neighborhood.
• Figure 4: Top: Projection in $l$--$b$ space of the uncorrected density distribution of the MSTO sample with one bin per degree in $l$ and two bins per degree in $b$. Middle: Same as above, except showing the density distribution corrected for incompleteness. Bottom: Same as above, mapped to the completeness ratio. We see that the middle panel is more homogeneous than the top panel due to the completeness correction.
• Figure 5: Top: $l$--$b$ projection of the MSTO anticenter sample with $R>14$ kpc where color is mapped to the median value of $V_R$ (left), $V_Z$ (middle), and [Fe/H] (right) with one bin per degree along both axes. Bottom: $R$--$Z$ of the MSTO anticenter sample with the same colormapping as above. We see distinct kinematic groups in $V_R$ and $V_Z$ where the blue/yellow regions, illustrating $-V_R$ and $-V_Z$, are located in the region associated with the MRi overdensity. The red/purple regions, representing $+V_R$ and $+V_Z$, correspond to approximately the ACS region. The horizontal black dashed line is $Z=5$ kpc and indicates our defined separation between the MRi region and other higher $Z$ stars.