Comment on the paper "On the correlation between the local dark matter and stellar velocities"

TL;DR

This paper critiques a previous study on the correlation between local dark matter and stellar velocities, clarifying inaccuracies and reaffirming earlier claims about their kinematic relationship.

Contribution

It provides a detailed clarification of earlier findings and addresses misrepresentations in a recent critique regarding dark matter and stellar velocity correlations.

Findings

Previous claims about dark matter-stellar velocity correlation are reaffirmed.

The critique's conclusions are based on misrepresented data.

Clarification supports the original theoretical models.

Abstract

We briefly comment on the paper "On the correlation between the local dark matter and stellar velocities" (Bozorgnia et al. 2018), which purports to test our previous claims regarding the correlation of stellar and dark matter kinematics (Herzog-Arbeitman et al. 2017; Necib et al. 2018). The conclusions of these papers are not accurately presented in Bozorgnia et al. (2018), and we thus clarify them here.

Full text

Comment on the paper

“On the correlation between the local dark matter and stellar velocities”

Mariangela Lisanti

Department of Physics, Princeton University, Princeton, NJ 08544, USA

Lina Necib

Walter Burke Institute for Theoretical Physics, California Institute of Technology, Pasadena, CA 91125, USA

We briefly comment on the paper “On the correlation between the local dark matter and stellar velocities” [1], which purports to test our previous claims regarding the correlation of stellar and dark matter kinematics [3, 4]. The conclusions of these papers are not accurately presented in [1], and we thus clarify them here.

In the first study of the Eris simulation, which is the key paper [1] compares to, [3] stated in the abstract: “Using the high resolution Eris simulation […], we demonstrate that metal-poor stars are indeed effective tracers for the local, virialized dark matter velocity distribution.” The choice of qualifier “virialized” throughout this early work was deliberate. While we compared the metal-poor stars to the total dark matter distribution (and not the virialized component specifically), we expected the Eris dark matter halo to be largely virialized. This is because Eris had an early accretion history, with no major mergers after $z\sim 3$ [2].

The work of [1] did not observe a correlation between metal-poor stars and the total local dark matter distribution in the six Auriga halos, a fact which we do not dispute and largely expect for galaxies with more active merger histories than Eris. Indeed, we observed precisely the same behavior in the Fire simulations [4]. The discrepancy in [1] can potentially arise if a relevant fraction of the local dark matter is not virialized, or originated from non-luminous subhalos/smooth accretion. This would need to be directly tested before drawing conclusions regarding our findings in [3, 4].

Because we observed the same discrepancy as [1] when we first began studying the Fire halos, we performed a more extensive analysis that divided the local dark matter into separate populations based on origin. As detailed in [4], this allowed us to conclude the following:

1. The most metal-poor stars ($\text{[Fe/H]}\lesssim-2$) trace the dark matter accreted from the oldest mergers ($z\gtrsim 3$). This dark matter population appears to be fully relaxed, or virialized.

2. More intermediate metallicity stars ($-2\lesssim\text{[Fe/H]}\lesssim-1$) trace the dark matter accreted from younger mergers ($z\lesssim 3$). This correspondence holds for dark matter that is in debris flow, but is less robust for streams.

3. Dark matter originating from non-luminous satellites or smooth accretion is not necessarily traced by stars.

Again, neither [3] nor [4] claimed that metal-poor stars should trace the total local dark matter distribution, as suggested by [1].

Characterizing the dark matter distribution near the Solar position is a challenging open problem. In tackling this question, [4] took the approach of finding tracers for separate dark matter populations, as defined by origin. This approach proved fruitful, and it would be interesting to see a similar complete analysis of the Auriga simulations in order to discuss the validity of our conclusions.