The Darkest Galaxy

In Job 38:19–20, God challenges Job and his friends with these queries, “Where does darkness reside? Do you know the paths to their dwellings?” Thousands of years later, we know that the book of Job got it right. Darkness is not merely the absence of light. It is a variety of substances with specific geographical locations in the universe.

In the 1930s, Caltech astronomer Fritz Zwicky recognized that huge amounts of hidden dark matter were necessary to explain the dynamics of galaxies and galaxy clusters.¹ Today, we understand that every galaxy is dominated by dark matter, both ordinary dark matter (made up of particles like protons, neutrons, and electrons that efficiently interact with photons) and the much more abundant exotic dark matter (made up of particles like neutrinos and axions that interact weakly with photons or not at all). Yet not all galaxies contain the same proportions of dark stuff.

Hot big bang inflationary models that posit that most dark matter is cold (moving at velocities much less than the speed of light) predict that the smaller the galaxy, the greater the proportion of its total mass comprised of dark matter. In particular, subdwarf galaxies will contain proportionately greater amounts of dark matter than galaxies the size of the Milky Way (MWG) and Andromeda galaxies or even larger dwarf galaxies like the Magellanic Clouds.

The easiest and most direct way astronomers determine the proportionate amount of dark matter in a galaxy is by measuring the galaxy’s mass-to-light ratio, the quotient between a galaxy’s total mass and its luminosity. Medium- and large-sized galaxies (those ranging from a little more than a tenth to ten times the mass of the MWG) exhibit mass-to-light ratios ranging from two to ten times greater than the Sun’s. Dwarf galaxies (ranging from a hundredth to a tenth the mass of the MWG) manifest mass-to-light ratios up to a few hundred times the Sun’s. In a recent issue of the Astrophysical Journal, a team of 12 astronomers led by Joshua Simon determined that the subdwarf galaxy Segue 1 reveals the highest known mass-to-light ratio of any galaxy.²

Segue 1 is a dwarf spheroidal galaxy discovered in 2006 by the Sloan Digital Sky Survey. It is located about 75,000 light-years away in the constellation of Leo. It ranks as one of the faintest known galaxies. Its integrated luminosity is only about 300 times that of the Sun.

Simon’s team first developed and applied new techniques for identifying stars that are members of Segue 1. These methods included using measurements of the velocities, metallicities, colors, magnitudes (apparent brightnesses), and spectra of stars in the vicinity of Segue 1 and a Bayesian statistical analysis that they described in a companion paper.³ The measured colors and magnitudes of the identified stars showed that the luminosity of Segue 1 has a value of 340 times the Sun’s luminosity.

Through an exhaustive spectroscopic survey of Segue 1’s stars, Simon’s team determined, for the first time, an accurate picture of Segue 1’s dynamics. This determination removed any doubt that Segue 1 is a true dwarf galaxy. Previous teams of astronomers had speculated that Segue 1 was merely a globular cluster stripped away from the Sagittarius Dwarf Elliptical Galaxy by the Milky Way’s tidal forces. Simon and his team write in their paper, “We showed that there is no observational evidence supporting the possibility of tidal disruption.”⁴ They add, “We also determined that contamination by Sgr stream stars is significantly lower than previously estimated.”⁵

With an accurate picture of Segue 1’s dynamics, the team established that Segue 1’s mass, contained within the half-light radius of Segue 1, is 580,000 times the mass of the Sun. This value gives Segue 1 a mass-to-light ratio at its half-light radius of 3,400 times that manifested by the Sun.

There is no evidence or any astronomical expectation that Segue 1’s dark matter halo is truncated at such a small radius as its half-light radius. Therefore, the mass-to-light ratio value of 3,400 times that of the Sun is a lower limit. The total mass-to-light ratio for Segue 1 easily could be ten to a hundred times greater.

Even with a mass-to-light ratio of just 3,400 times that of the Sun, Segue 1 ranks as the darkest known galaxy. The density of dark matter within the inner 38 parsecs (1 parsec=3.258 light-years or 19.18 trillion miles) of Segue 1 equals 2.5 solar masses per cubic parsec.
This is, by far, the highest dark matter density yet determined by astronomers.

The measured dark matter density for Segue 1 is good news for two reasons. First, Segue 1’s dark matter density value is entirely consistent with predictions arising from hot big bang inflationary models where most of the universe’s exotic matter is in a cold state. These cosmic models are consistent with the big bang creation models the Bible uniquely described more than 2,000 years ago.⁶ Thus, the new measurement of Segue 1’s dark matter density yields yet another confirmation of the biblical cosmic creation model.

Second, with such a high dark matter density, Segue 1 is an excellent candidate for detecting and determining the properties of exotic dark matter particles. The masses and abundances of such particles would provide yet another tool for testing the biblical creation model and possibly ruling out competing models.