Title: White Dwarfs in Dwarf Spheroidal Galaxies: A New Class of Compact-Dark-Matter Detectors

Author(s): Juri Smirnov, Ariel Goobar, Tim Linden, and Edvard Mörtsell

First Author’s Institution: Department of Mathematical Sciences, University of Liverpool

Status: Published in Physical Review Letters [open access]

Calcium-rich gap transients

As all-sky surveys like the Zwicky Transient Facility have become increasingly sensitive to dimmer transients, we have found many new types of supernovae (SNe). One such class is called Calcium-rich gap transients, typically found much further away from the centers of their host galaxies compared to the more common Type 1a SNe. Their spectrum lacks Hydrogen Balmer lines, and they are typically found on the outskirts (up to 100 thousand parsecs from the galaxy’s center) of aging elliptical galaxies, far enough to be located in the satellite dwarf galaxies and globular clusters; see Figure 1. They rapidly transition to the nebular emission phase, where high-energy photons from the explosion ionize the ejected gas. The spectra during this phase display strong [Ca II] (λλ7291, 7324 angstroms) lines in the near-infrared, giving rise to the name. The old populations and lack of Hydrogen indicate that white dwarfs (WDs) may be the progenitors of this class of transients. However, the faintness of the light curves and short lifetimes suggest low-mass (0.6 M_⊙ or less) WDs, much lower than typical binary masses (~1.4 M_⊙)  that explain Type 1a SNe. The authors of today’s paper explore the possibility that the progenitor is a single WD ignited by a gravitational encounter with dark matter (DM).

Dwarf spheroidal galaxies are full of fuses

Compact dark matter remnants are theorized as one component contributing to the unseen mass causing many gravitational phenomena we observe in the universe, such as galaxy rotation curves. Compact remnants are typically attributed to primordial black holes (PBHs) since they naturally do not emit light but have oversized gravitational influence. When an object (such as a PBH) has a close encounter with a low-mass WD, dynamical friction can heat the region below the WD’s crust to a sufficient temperature to reignite fusion, causing a runaway reaction and the thermonuclear unbinding of the WD. If PBHs are a significant fraction of dark matter (DM), we should expect to find these transients in the most DM-abundant places. Galaxies like the Milky Way are surrounded by smaller galaxies called Dwarf Spheroidals (dSphs) that are extremely rich in DM. Their stellar populations are older, so more stars have evolved off the main sequence and entered the WD phase. Using a simulated population of Milky Way-like galaxies and their satellites combined with our knowledge of the dark matter halo distribution surrounding galaxies, the authors of today’s paper determined that close encounters between WDs and PBHs that result in thermonuclear explosion are 2-4x more likely to occur in dSph satellite galaxies than in the central galaxy (Figure 2). Therefore, DM-induced SNe may be the mechanism behind the Ca-rich gap transients we observe. 

The nature of dark matter

Using our observations of Ca-rich transients and their measured event rate, we can constrain the interacting PBHs’ properties if they are responsible for low-luminosity thermonuclear SNe. The event rate the authors derive demonstrates that more massive PBHs are less likely to interact with a given WD, but more massive PBHs can also trigger less massive WDs, as small as 0.4 M_⊙. However, such an event would be challenging to detect since lower masses are associated with lower  SN luminosities. Less massive PBHs increase the interaction rate, but the required WD mass is large below 10^21 grams (the mass of a small asteroid compressed to a radius of ~nm). This implies that the SN rate may decrease since more massive WDs are rarer. In Figure 3, the authors compare limits on the number and masses of PBHs derived from the Hyper Suprime-Cam on the Subaru telescope (considered by some to be the most advanced camera in the world). These observations searched for gravitational micro-lensing events caused by PBHs in the Andromeda galaxy, which only detected a single candidate, indicating that PBHs are a non-dominant component of DM under this model. The results of today’s paper suggest that further observations to probe less massive and rarer PBH rates are needed to rule out them as an origin for Ca-rich transients. In particular, JWST observations may be sufficient to resolve individual dSphs at the positions of our current sample of Ca-rich transients.

Featured Image Credit: [ESO/M. Kornmesser; CC BY 4.0]

Edited by Sonja Panjkov

Author’s Note: This article was unintentionally covered a second time. The first bite may be found here.