Stellar fluxes as dark matter maps

Title: Mapping dark matter with extragalactic stellar fluxes: the case of Centaurus A

Authors: Sarah Pearson, Adrian M. Price-Whelan, David W. Hogg, Anil C. Seth, David J. Sand, Jason AS Hunt, Denija Crnojevic

Institution of the first author: Center for Cosmology and Particle Physics, Department of Physics, New York University, 726 Broadway, New York, NY 10003, USA

Status: Submitted to AAS reviews [open access]

Have you watched leaves falling into a river and swirling in the current? It’s hard to see where the water is flowing, but the leaves are good tracers of this movement. If you are experienced, you may be able to infer the underwater terrain from the water current. A similar phenomenon occurs when a dwarf galaxy falls into a massive galaxy. The stars are scattered from the dwarf galaxy and stretch out in a stream. Stars are easy to observe and they are good tracers of the invisible dark matter in a galaxy, which governs their motion. As explained in this Astrobite, astronomers can use stellar fluxes to learn more about dark matter (DM) in galaxies.

In the Milky Way, stellar fluxes have been used to infer DM distribution and even DM substructure. We can model these stars in detail because we can observe their full 3D positions and velocities. This is much harder to do for outer galaxies, where distances are hard to come by and speeds even harder. What can we learn from the stellar fluxes of outer galaxies? The authors of today’s article turn their attention to Cen A (NGC5128), a nearby massive galaxy.

Figure 1: The outer stellar halo of the Centaurus A galaxy, colored by the density of Red Giant Branch stars. The Dw3 stream is traced by 13 black control points and the second stream is traced by three white dots. Figure 1 in the article.

Data and method

Cen A hosts a known stellar stream associated with its dwarf satellite galaxy Dw3. Figure 1 shows the stellar density in the outer halo of Cen A. Two flows are visible in this map, but we will focus on the Dw3 flow. Since these flows are about 4 Mpc apart, the authors only use the locations of the flow in the sky, as indicated by the black dots in Figure 1. The goal is to use this information to find the orbit of the progenitor dwarf galaxy (Dw3), which may then constrain the DM distribution of the massive central galaxy (Cen A).

In order to model this stellar flux, the authors develop a new flux adjustment technique. Checkpoints are placed with the naked eye to indicate the known location of the Dw3 stream. The inputs to this model are the control points and the radial velocity of the progenitor (Dw3). To calculate the orbit of Dw3, we must also enter the gravitational potentials of Cen A and Dw3. But isn’t gravitational potential something we want to figure out with modeling? That’s why the authors start with a reasonable fiduciary potential, then explore what happens if you vary it.

Given the known inputs, the authors simulate how the stars disperse from the ancestor and form a stream. They use a publicly available code gala to embed the orbit of Dw3 into the potential of Cen A. The code releases stars from Dw3 at each time step and tracks where they go under the combined gravitational forces of Dw3 and Cen A The resulting stellar flux is compared to the control data points, and the best-fitting model should match all of them. Next, the authors also run a more accurate N-body simulation with the same initial setup as the best-fitting model to ensure that the gala results are reliable.

Results

Same image as Fig 1, but with an overhanging green stream.  The stream flows from the upper right corner to the middle left corner and then to the lower left corner.  The color gradient goes from white to green, which corresponds to 0 km/s to -300 km/s in radial speed.
Figure 2: The best fitting model for the Dw3 flow superimposed on the Cen A image, where the colors show the radial velocity relative to Cen A. The white arrow indicates the movement of Dw3. Modified from figure 4 in the article.

Fixed Halo Cen A: from the fiducial model, the authors find a range of models capable of reproducing the shape of the Dw3 flow. The best fitting model is shown in Fig. 2, as projected onto the sky. The stream looks directly at the sky, and the best-fit orbits have high velocity in the direction of the stream and low velocity in the perpendicular direction. Logic. What happens when you vary the Cen A halo?

Degeneration: We only match the shape of the stream on the sky, which is basically not enough information to uniquely constrain its story. The reason is illustrated in Figure 3. If you halve the mass of the host and the satellite dwarf, the orbital speeds will be lower and the stars will scatter more slowly. But if you wait longer, the satellite will show exactly the same disturbance and the resulting stream (shown in the top two panels of Fig. 3) will look the same. Not having the ability to observe this current 6 billion years ago, we can only observe the current at t=0 in the lower plots. If we only have the spatial position of the flow and we don’t know how long it has been perturbing, we will not be able to distinguish between high-mass and low-mass potentials. This is a problem since that was our goal at the beginning!

Top row: two panels with stellar fluxes of similar shape but the color is deeper in the left panel, corresponding to a higher radial velocity.  Bottom row: The left panel shows a curve with three ripples from -6 Gyr to 0 Gyr.  The right panel shows a curve with the same ripples but stretched on the x-axis, from -12 Gyr to 0 Gyr.
Figure 3: The comparison between two streams evolved into a halo of high mass (left) and low mass (right). Top row: Dots indicate stars in the stream, colored by radial velocity in the Cen A image. Lines trace the orbit of the ancestor. Bottom line: the distance between the model flow and Cen A, as a function of time. The high mass flux is modeled over 6 billion years while the low mass flux is modeled over 12 billion years. Fig 6 in the paper.

How to break this degeneration: The difference between these two scenarios lies in the radial speed and the distance. Even though the shape of the flows looks identical in Figure 3, the color bar shows that they have different radial velocities. This is a direct result of the orbital velocity of the progenitor dwarf galaxy, which depends on the mass of the host halo. Even though the radial velocity of the stream has not been measured, even observing part of the stream can help determine the mass of the host galaxy. The authors of this article identify readily available instruments that can take this measurement, and it’s up to observers to point telescopes at Dw3!

Outlook

Future telescopes like the Vera Rubin Observatory and the Roman Space Telescope will be able to detect thousands of extragalactic stellar streams. The tools built by today’s authors will pave the way for exploring the dark matter halos of all these galaxies!

Astrobite edited by Benjamin Cassese

Featured image credit: Sarah Pearson

About Zili Shen

Hi! I hold a doctorate. astronomy student at Yale University. My research focuses on ultra-diffuse galaxies and their populations of globular clusters. Since my arrival at Yale, I have worked on two galaxies “without dark matter” NGC1052-DF2 and DF4. I coped with the pandemic and worked from home baking sourdough bread and baking various cookies and cakes, reading books ranging from philosophy to virology, doing daily hikes or errands and watching too of TV shows.

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