Eridanus II
Eridanus II is significant, in a general sense, because the widely accepted Lambda CDM cosmology predicts the existence of many more dwarf galaxies than have yet been observed.
One of the most notable current efforts is the Dark Energy Survey (DES), which makes extensive use of one of the new generation of Chilean telescopes, the 4 m Blanco instrument at the Cerro Telolo Inter-American Observatory (Bechtol et al., 2015: 1).
These observations included more detailed spectral data and also focused on Eridanus II's central globular cluster (Crnojević et al., 2016; Zaritsky et al., 2016; Li et al., 2016).
Additional data have been obtained from a re-examination of older radio telescope surveys which included the region of the sky occupied by Eridanus II (Westmeier et al., 2015).
Standing on the galactic plane at the position of the Sun, facing the center of the galaxy, Eridanus II would be on the right and below, about half-way down the sky from the horizontal.
[4] For reasons to be discussed in the concluding section, most researchers now believe that Eridanus II is an extremely long-period (i.e., several billion years per orbit) satellite of the Milky Way, probably beginning only its second approach to our galaxy.
The Hubble Constant is also believed to change over time, so that orbital dynamics on the scale of megaparsecs and billions of years cannot simply be computed using Newton's law of gravitation.
The velocity measurements of Li et al. (2016) made use of light emitted by Eridanus II approximately one million years ago.
The material making up Eridanus II must orbit about the galactic center, but there is no evidence of a well-defined plane or concerted direction of rotation.
A number of workers have speculated about an association between the Magellanic Clouds and various dwarf galaxies in the Local Group, including Eridanus II.
As they note, the evidence for such pre-existing association is not compelling, but it does explain an otherwise "alarming" number of small galaxies found along a relatively narrow celestial corridor.
[7] The stars in Eridanus II are largely consistent with a very old (~10 billion years) and low-metal ([Fe/H] < −1) population, similar to other small dwarf galaxies as well as many globular clusters.
The Red Giant Branch (RGB) is relatively vertical, ruling out any large proportion of young (250 million years or less), metal-rich stars (Crnojević et al., 2016: 2–3).
Li et al. (2016) calculated the mean metallicity of Eridanus II by measuring the size of the calcium triplet absorption peaks in spectra from 16 individual stars on the RGB.
These are (1) the partial confirmation of the predictions of ΛCDM cosmology concerning the number of small, faint dwarf galaxies in the Local Group; (2) the questions Eridanus II raises about the history of the Milky Way and the Magellanic Clouds; and (3) the constraints placed on the nature of dark matter by the unanticipated finding of an apparently stable globular cluster at the heart of this strange little galaxy.
As noted in the introductory section, one of the principle aims of the Dark Energy Survey was to determine whether the numbers of faint dwarf galaxies predicted by ΛCDM cosmology actually existed.
The second, and perhaps related, point is that the Sloan Survey "revealed that there appears to be a gap in the distribution of effective radii between globular clusters (GCs) and dwarfs which extends across a large range of luminosities."
That is, absent finding a new population intermediate between globular clusters and the current crop of rather robust galactic dwarves, we may be forced to conclude that there is something special about certain scales of dark matter organization.
In an important recent paper, Brandt (2016) has argued that the presence of a stable globular cluster near the center of Eridanus II places severe constraints on certain possible forms of dark matter.
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