[Paper Review] The MUSE-Faint survey: I. Spectroscopic evidence for a star cluster in Eridanus 2 and constraints on MACHOs as a constituent of dark matter

Sebastiaan L. Zoutendijk, J. Brinchmann+10 more|arXiv (Cornell University)|Jan 23, 2020

Stellar, planetary, and galactic studies77 references9 citations

TL;DR

This study presents spectroscopic evidence for a stellar cluster in the ultra-faint dwarf galaxy Eridanus 2 using MUSE-Faint survey data, confirming its existence as a distinct dynamical population with low velocity dispersion. It derives new constraints on massive astrophysical compact halo objects (MACHOs) as dark matter, excluding MACHO masses above 7.6 M⊙ at 68% confidence and 44 M⊙ at 95% confidence if dark matter were entirely MACHOs.

ABSTRACT

We aim to provide spectroscopic evidence regarding the nature of the putative star cluster in Eridanus 2 and to place constraints on the mass and abundance of massive astrophysical compact halo objects (MACHOs) as a constituent of dark matter. Methods. We present spectroscopic observations of the central square arcminute of Eridanus 2 from MUSE-Faint, a survey of ultra-faint dwarf galaxies with the Multi Unit Spectroscopic Explorer on the Very Large Telescope. We derive line-of-sight velocities for possible member stars of the putative cluster and for stars in the centre of Eridanus 2. We discuss the existence of the cluster and determine new constraints for MACHOs using the Fokker-Planck diffusion approximation. Results. Out of 182 extracted spectra, we identify 26 member stars of Eridanus 2, seven of which are possible cluster members. We find intrinsic mean line-of-sight velocities of $79.7^{+3.1}_{-3.8}\,\mathrm{km}\,\mathrm{s}^{-1}$ and $76.0^{+3.2}_{-3.7}\,\mathrm{km}\,\mathrm{s}^{-1}$ for the cluster and the bulk of Eridanus 2, respectively, and intrinsic velocity dispersions of ${<}7.6\,\mathrm{km}\,\mathrm{s}^{-1}$ (68-$\%$ upper limit) and $10.3^{+3.9}_{-3.2}\,\mathrm{km}\,\mathrm{s}^{-1}$, respectively. This indicates the cluster most likely exists as a distinct dynamical population hosted by Eridanus 2, without surplus of dark matter over the background distribution. Among the member stars in the bulk of Eridanus 2, we find possible carbon stars, alluding to the existence of an intermediate-age population. We derive constraints on the fraction of dark matter that can consist of MACHOs with a given mass between $1$-$10^5\,M_\mathrm{sun}$. For dark matter consisting purely of MACHOs, the mass of the MACHOs must be less than ${\sim}7.6\,M_\mathrm{sun}$ and ${\sim}44\,M_\mathrm{sun}$ at a $68$- and $95$-$\%$ confidence level, respectively. (Abridged)

Motivation & Objective

• To provide spectroscopic confirmation of a putative stellar cluster in the ultra-faint dwarf galaxy Eridanus 2.
• To determine the kinematic properties (mean velocity and velocity dispersion) of the cluster and the bulk of Eridanus 2 to assess its dynamical distinctness.
• To use the cluster's survival to constrain the abundance and mass of massive astrophysical compact halo objects (MACHOs) as a constituent of dark matter.
• To test the robustness of MACHO constraints against uncertainties in initial mass function, metallicity, and dynamical mass estimators.
• To evaluate the validity of the Fokker–Planck diffusion approximation in deriving MACHO limits, especially at high MACHO masses.

Proposed method

• Acquired spectroscopic observations of the central square arcminute of Eridanus 2 using the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope.
• Extracted 182 spectra and identified 26 member stars of Eridanus 2, including seven candidate cluster members.
• Measured line-of-sight velocities and intrinsic velocity dispersions using robust statistical modeling, accounting for measurement uncertainties and intrinsic dispersion.
• Applied the Fokker–Planck diffusion approximation to model MACHO-induced stellar heating in the cluster, linking cluster survival to MACHO mass and abundance.
• Used Bayesian inference with the emcee sampler to derive confidence intervals on MACHO mass limits, incorporating uncertainties in velocity dispersion and cluster mass.
• Assessed sensitivity of results to variations in initial mass function, metallicity, and dynamical mass estimator to test robustness.

Experimental results

Research questions

• RQ1 Does a stellar cluster exist in the center of Eridanus 2, based on spectroscopic kinematics?
• RQ2 What are the intrinsic line-of-sight velocity and velocity dispersion of the putative cluster and the bulk of Eridanus 2?
• RQ3 How does the survival of the cluster constrain the mass and abundance of MACHOs as dark matter constituents?
• RQ4 How robust are the MACHO constraints to variations in astrophysical assumptions such as initial mass function and metallicity?
• RQ5 What are the limitations of the Fokker–Planck diffusion approximation in deriving MACHO constraints, especially for high-mass MACHOs?

Key findings

• Spectroscopic observations confirm a distinct stellar population in Eridanus 2 with an intrinsic mean line-of-sight velocity of 79.7+3.1−3.8 km s⁻¹ and a velocity dispersion of less than 7.6 km s⁻¹ (68% upper limit), indicating a dynamically cold, bound cluster.
• The bulk of Eridanus 2 exhibits a mean velocity of 76.0+3.2−3.7 km s⁻¹ and a velocity dispersion of 10.3+3.9−3.2 km s⁻¹, consistent with a more extended, hotter system.
• The cluster is not significantly over-massive compared to the background distribution, supporting its existence as a distinct, low-dark-matter-overdensity population.
• Evidence for carbon stars among Eridanus 2’s member stars suggests the presence of an intermediate-age stellar population alongside the dominant old population.
• If dark matter were entirely composed of MACHOs, masses above 100.88 M⊙ ≈ 7.6 M⊙ are excluded at 68% confidence, and masses above 101.64 M⊙ ≈ 44 M⊙ are excluded at 95% confidence.
• Constraints are robust against variations in initial mass function, metallicity, and dynamical mass estimator, though the Fokker–Planck approximation may break down for MACHO masses ≳10⁴ M⊙.