Exploring the Invisible Milky Way

Title: Sizing from the Smallest Scales: The Mass of the Milky Way

Authors: M. K. Rodriguez Wimberly, M. C. Cooper, D. C. Baxter, M. Boylan-Kolchin, J. S. Bullock, S. P. Fillingham, A. P. Ji, L. V. Sales, J. D. Simon

First Author’s Institution: Center for Cosmology, Department of Physics & Astronomy, 4129 Reines Hall, University of California, Irvine, CA 92697, USA

Status: Submitted to MNRAS.

Dark Matter Halos and Satellite Galaxies

Dark matter, the mysterious matter in the Universe that doesn’t interact with light, seems to be a significant component of most galaxies (with some notable exceptions). A galaxy like the Milky Way consists of a disk of gas and stars surrounded by a halo of dark matter. Within galaxy halos, there are smaller galaxies known as “satellites” (some famous ones include the Magellanic Clouds) that are gravitationally bound to the more massive “host” galaxy.

Satellite galaxies have orbital parameters that depend on the mass of the host, so they can be useful for determining the mass of dark matter halo. The mass of the Milky Way’s dark matter halo has been estimated through various methods, with the most recent estimates placing it in the range 0.70 – 1.55 x 10¹² M_☉. The authors of today’s paper use a combination of galaxy  simulations and observations of satellite galaxies to further constrain the mass of the Milky Way’s dark matter halo. 

The Dark Matter Mass of the Milky Way

The Phat Exploring the Local Volume In Simulations (phELVIS) are high resolution simulations that allow us to get a close up look at galaxy halos. For the analysis, the authors focus on 12 simulations of Milky Way-like galaxies which include a disk (in the form of a gravitational potential), in addition to a dark matter halo. On the observation side, the authors use a sample of 44 Milky Way satellites with precise stellar motion measurements from Gaia.

The goal here is to take the very detailed simulations that represent the Milky Way, and compare them to the actual kinematics of Milky Way satellites. The simulated halos are divided into 3 bins based on their mass: low mass (<10¹² M_☉), intermediate mass (~10¹² M_☉), high mass (~2 x 10¹² M_☉). Whichever bin of simulations best matches the observations indicates which mass range the Milky Way’s halo falls in.

From the Gaia observations, the authors can calculate the Galactocentric velocities of the satellites, that is, the velocities of the satellites with respect to the Milky Way center. The authors look at the distribution of three velocities: the radial velocity (the most commonly available), tangential velocity, and total velocity (the combination of radial and tangential velocities). For the sample of 44 Milky Way satellites, the distribution of radial velocities match the simulated halos of intermediate mass; however, that’s not the case for the tangential and total velocities. 

Figure 1. Distribution of 3 Galactocentric velocities: radial (panel a), tangential (panel b), and total velocity (panel c). The horizontal axis shows the velocities with respect to the galaxy center and the vertical axis shows the fraction of objects with a particular velocity. The solid black line represents the 34 satellites observed with Gaia with better measurements. The dashed and dotted lines each represent one of the mass bins of simulated halos: low mass (burgundy), intermediate mass (aqua), high mass (sienna). The intermediate mass curve best matches the observed curve for all 3 velocity distributions. (Figure 4 in the paper).

Some of these galaxies might have high uncertainties in their velocities, so the authors select a subsample of 34 galaxies with better constrained measurements (uncertainties smaller than 30% of the velocity) for further analysis. Figure 1 above shows the distribution of the three velocity measurements for this subsample of 34 satellite galaxies. In this case, unlike the larger sample, the observed velocities match the intermediate mass simulated halos for all 3 velocity distributions. To narrow down the mass estimate, the authors re-run the analysis using different limits of the intermediate mass bin.

The Conclusion

The author’s of today’s paper use high-resolution simulations of Milky Way-like galaxy halos and observations of Milky Way satellites to constrain the mass of the Milky Way’s dark matter halo; the best results place it between 1 and 1.2 x 10¹² M_☉. The authors note that in the future, it might be possible to further refine the mass of the Milky Way’s dark matter halo, whether through future Gaia releases or with upcoming deep and wide surveys like the LSST and Nancy Grace Roman Telescope.

Astrobite edited by Luna Zagorac

Featured image credit: ESA/Gaia/DPAC

• About the Author

About Gloria Fonseca Alvarez

I'm a fifth year graduate student at the University of Connecticut. My research focuses on the inner environments of supermassive black holes. I am currently working on measuring black hole properties from the spectral energy distributions of quasars in the Sloan Digital Sky Survey. As a Nicaraguan astronomer, I am also involved in efforts to increase the participation of Central American students in astronomy research.

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