Which Profile Is More Appropriate To Represent Dark Matter Haloes & Estimating Galaxy Total Mass? (Cosmology)

Rotation curves are major tools for determining the dynamical mass distribution in the Milky Way and spiral galaxies. Discoveries of extended rotation curves have suggested the presence of dark matter in spiral galaxy haloes. It has led to many studies that estimated the galaxy total mass, mostly by using the Navarro Frenk and White (NFW) density profile.

Now, Jiao and colleagues aimed at verifying how the choice of the dark-matter profile may affect the predicted values of extrapolated total masses.

They have considered the recent Milky Way rotation curve (MW RC) for two reasons, firstly because of its unprecedented accuracy, and secondly because the Galactic disk is amongst the least affected by past major mergers having fully reshaped the initial disk.

They found that, for calculating the dark-matter contribution to the Milky Way rotation curve, the use of NFW profile (or its generalized form, gNFW) generates apparently inconsistent results, e.g., an increase of the baryonic mass leads to increase of the dark matter mass.

In addition, it has been found that NFW and gNFW profile narrow the total mass range, leading to a possible methodological bias particularly against small milky way (MW) masses.

Fig. 1. Top: Contribution to the rotation curve of different baryonic models and model components. Red points indicate the rotation curve of the Milky Way. The error-bars are estimated via bootstrapping and include the systematic uncertainties from the neglected term (see text). Bottom: Fit of the rotation curve by the best-fit model (solid blue curve, total mass of 2.6 × 10¹¹ M), and with the most massive MW model for which the χ² probability reaches P=0.05 (orange dash-dotted line, total mass of 18 × 10¹¹ M), both associated to the baryonic distribution from model I of Pouliasis et al. © Jiao et al.

Finally, they suggested, the use of Einasto profile is more appropriate to represent cold dark matter haloes and found that the Milky Way slightly decreasing rotation curve favors total mass that can be as small as 2.6 ×10¹¹ M, disregarding any other dynamical tracers further out in the Milky Way. It is inconsistent with values larger than 18 ×10¹¹ M for any kind of cold dark matter (CDM) halo profiles, under the assumption that stars and gas do not influence the predicted dark matter distribution in the Milky Way.

“Our study encourages the use of the Einasto profile for characterizing rotation curves with the aim of evaluating their total masses.”

— concluded authors of the study

Reference: Yongjun JIAO, Francois HAMMER, Jianling WANG, Yanbin YANG, “Which Milky Way masses are consistent with the slightly declining 5-25 kpc rotation curve?”, Arxiv, pp. 1-10, 2021. https://arxiv.org/abs/2107.00014

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