Mapping Dark Matter with Extragalactic Stellar Streams: The Case of Centaurus A

Abstract

In the coming decade, thousands of stellar streams will be observed in the halos of external galaxies. What fundamental discoveries will we make about dark matter from these streams? As a first attempt to look at these questions, we model Magellan/Megacam imaging of the Centaurus A (Cen A) disrupting dwarf companion Dwarf 3 (Dw3) and its associated stellar stream, to find out what can be learned about the Cen A dark matter halo. We develop a novel external galaxy stream-fitting technique and generate model stellar streams that reproduce the stream morphology visible in the imaging. We find that there are many viable stream models that fit the data well, with reasonable parameters, provided that Cen A has a halo mass larger than M 200 > 4.70 × 1012 M ⊙. There is a second stream in Cen A’s halo that is also reproduced within the context of this same dynamical model. However, stream morphology in the imaging alone does not uniquely determine the mass or mass distribution for the Cen A halo. In particular, the stream models with high likelihood show covariances between the inferred Cen A mass distribution, the inferred Dw3 progenitor mass, the Dw3 velocity, and the Dw3 line-of-sight position. We show that these degeneracies can be broken with radial-velocity measurements along the stream, and that a single radial velocity measurement puts a substantial lower limit on the halo mass. These results suggest that targeted radial-velocity measurements will be critical if we want to learn about dark matter from extragalactic stellar streams.

New Velocity Measurements of NGC 5128 Globular Clusters Out to 130 kpc: Outer Halo Kinematics, Substructure, and Dynamics*

“…Note, however, that the sign of the velocity gradient would flip if Dw3 was instead located in front of NGC 5128; the plotted set of models assume that Dw3 is at a slightly larger distance from us than NGC 5128. Also, three of the GCs we associated with CenA-MM-Dw3 and its tidal tail are beyond what Pearson et al (2022) considered the extent of the stream.…”

“…Note that the observed blueshifted line-of-sight velocity of Dw3 with respect to NGC 5128 is consistent with the corotating line-of-sight velocity trend found by Müller et al (2021) for 21 out of 28 of NGC 5128ʼs satellites with measured velocities. However, the proper motion predictions from Pearson et al (2022) moves Dw3 in a direction out of the planar satellite structure of NGC 5128 (Müller et al 2018).…”

“…These substructures track the galaxy's active and ongoing merger history. Tidal streams also act as probes of the gravitational potential of the galaxy, and their morphology and kinematics can be used to constrain NGC 5128ʼs dark matter halo mass (Pearson et al 2022). However, determining the velocities for these low-surfacebrightness features is very challenging (Toloba et al 2016).…”

“…Figure 11 plots the velocities of these GCs as a function of distance from the center of CenA-MM-Dw3. Also plotted is a range of stream models from Pearson et al (2022), who explored dynamical models that could reproduce the stream morphology and velocity of the central star cluster associated Positions of all confirmed GCs in NGC 5128 with radial velocity measurements, where the symbol colors correspond to radial velocity. The triangles mark newly identified GCs, and the circles mark previously confirmed GCs.…”

Dynamical Data Mining Captures Disc-Halo Couplings that Structure Galaxies

“…Hunt et al 2022b) in galaxies and occurrence rate in others (e.g. Pearson et al 2022). Simulated realisations of galaxies in disequilibrium are similarly advancing in resolution and scale.…”