Modeling the Stellar Kinematics of the Thick Disk and Halo of the Andromeda Galaxy

The SPLASH (Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo) survey has obtained an extensive set of spectra of the resolved stellar population in the disk of the Andromeda galaxy (M31) using the DEIMOS spectrograph on the Keck 10-m telescope. The spectroscopic targets were selected from the Panchromatic Hubble Andromeda Treasury (PHAT) survey of the northeast portion of M31’s disk. In order to better understand the structural subcomponents of M31, we have constructed a toy model of the galaxy’s red giant branch (RGB) star population. We used Python code to construct a model that consists of a thick disk and a halo component. The thick disk is modeled in a (R, phi, z) cylindrical coordinate system with exponential density laws as a function of R and z. The halo is modeled in an (R, theta, phi) spherical polar coordinate system with cored power-law density profile. The kinematics of the thick disk are modeled using the following free parameters: (1) f_rot, the fraction of the neutral hydrogen HI rotation speed v_rot as a function of radius, to model the asymmetric drift of the RGB population; and (2) σ_R, σ_φ, and σ_z, the parameters of the velocity ellipsoid. We have found that projection effects from the thick disk have a negligible effect on the observed line-of-sight velocity dispersion. We also find that the halo is a minority component within the PHAT survey area and has a negligible effect on the observed RGB star kinematics. By forward-modeling these M31 observations, we are able to constrain the structural and kinematical/dynamical properties of M31’s thick disk of RGB stars. We expect that this toy model will be applicable to similar kinematical data of resolved stellar populations in other nearby galaxies – e.g., the Triangulum galaxy (M33). This research was funded in part by the NSF and NASA/STScI. JQ, PS, YA, KM, CV, and TG conducted their research under the auspices of the Science Internship Program (SIP) at UC Santa Cruz. We also wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to work with observations from this mountain.