Eridanus II
The Eridanus II Dwarf is a low-surface brightness dwarf galaxy in the constellation Eridanus. Eridanus II was independently discovered by two groups in 2015, using data from the Dark Energy Survey. This galaxy is probably a distant satellite of the Milky Way. Eridanus II contains a centrally located globular cluster; and is the smallest, least luminous galaxy known to contain a globular cluster. Crnojević et al., 2016. 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. The search for just such bodies was one of the motivations for the ongoing Dark Energy Survey observations. Eridanus II has special significance because of its apparently stable globular cluster. The stability of this cluster, near the center of such a small, diffuse, galaxy places constraints on the nature of dark matter.
Discovery and history of observations
Since the end of the Twentieth century, the most widely accepted cosmologies have been built on the foundations of the ΛCDM model which, in turn, is founded on the bedrock of the Big Bang cosmologies of the 1960s and 1970s. In the simplest terms, ΛCDM adds dark energy and cold dark matter to the Big Bang in order to explain the major features of the universe we observe today. ΛCDM describes a universe whose mass is dominated by dark matter. In such a universe, galaxies might be thought of as accretions of normal matter onto the largest concentrations of dark matter. However, ΛCDM does not predict any particular scale of CDM concentrations. In fact, it suggests that there ought to be tens or hundreds of smaller dark matter bodies for each observable galaxy the size of our own Milky Way galaxy. These should contain much less baryonic matter than a “normal” galaxy. Thus, we should observe many, very faint, satellite galaxies around the Milky Way.Until about 1990, however, only about 11 Milky Way satellites were known. The difference between the number of satellites known and the number expected in ΛCDM is referred to as the "missing dwarf" or "substructure" problem. Simon & Geha also discuss various cosmological and astrophysical "fixes" which might reconcile theory and observation without requiring a great many new dwarf galaxies. Efforts have been underway to determine whether the predicted population of faint satellite galaxies could be observed, and many new dwarf satellites are now being reported. One of the most notable current efforts is the Dark Energy Survey, 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. As of early 2016, the results have been promising, with over a dozen new satellite galaxies observed and reported.
Eridanus II is one these newly discovered satellites. The discovery was made independently by two groups working from the DES data, and their results were published simultaneously in 2015. The DES group and a third group of researchers conducted more detailed follow-up observations in late 2015, using both of the Magellan instruments at Las Campanas, Chile. These observations included more detailed spectral data and also focused on Eridanus II's central globular cluster. Finally, Crnojević et al. also conducted observations in early 2016 using the Byrd Green Bank radio telescope at Green Bank, West Virginia, USA. 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.
Properties
Location
Eridanus II is located deep in the southern sky. Since Eridanus II is a faint, diffuse object, spread over several arc-minutes of the sky, its position cannot be stated with great precision. The most detailed observations are probably those of Crnojević et al., who report celestial coordinates of RA 3h 44m 20.1s and Dec −43° 32' 0.1". These correspond to galactic coordinates of l = 249.7835°, b = −51.6492°. 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.The distance to Eridanus II has been estimated using a variety of methods. All rely on fitting the observed stars to a curve on a color-magnitude diagram, then comparing the luminosity of stars from the target galaxy with the luminosity of stars from equivalent positions on the CMD in galaxies of known distance, after various corrections for the estimated age and metallicity. See, e.g., Sand et al.. The results have been fairly consistent: 330 kpc , 380 kpc , and 366 ± 17 kpc . Whatever the exact distance value, Eridanus II is the most distant of currently known bodies which are likely satellites of the Milky Way.
Velocity
Determining whether or not Eridanus II is, in fact, a satellite galaxy depends in part on an understanding of its velocity. Li et al. have recently taken up that challenging series of measurements. Most of the difficulty relates to the fact that, while Eridanus II is distant in astronomical terms, it is too close in cosmological terms. Not only are spectral redshifts quite small at this distance, but the galaxy cannot be treated as a point object. Li et al. were forced to look at the spectra of individual stars, all of which were moving with respect to each other at speeds not much less than that of Eridanus II with respect to the observers, who were also moving at appreciable speeds around the center of the Earth, the Sun, and the center of the Milky Way galaxy. In spite of these difficulties, Li et al. were able to obtain a very tight distribution of velocities centering on 75.6 km/sec in a direction away from us. However, since the Sun's rotation about the center of the Milky Way is presently carrying us almost directly away from Eridanus II, Eridanus II's motion is actually carrying it toward the center of the galaxy at about 67 km/sec.While these observations solve the problem of radial velocity, the movement of Eridanus II towards the center of the Milky Way galaxy, they cannot solve the problem of transverse velocity, motion at right angles to the line between Eridanus II and the Milky Way. That is, we cannot determine whether Eridanus II is orbiting the Milky Way, or simply moving in its direction from outside the system. Li et al. report that Eridanus II does not exhibit a "tail" or gradient of lower velocity stars in a particular direction, which might give a clue to that galaxy's transverse velocity. However, they point out that an object similar to Eridanus II would need a total velocity of about 200 km/sec to escape capture by the Milky Way. Given its radial velocity of 75 km/sec, Eridanus II would need a transverse velocity of some 185 km/sec to avoid capture—certainly possible, but not likely. In addition, they point to the results of detailed simulation studies of the Local Group. All objects situated similarly to Eridanus II in these simulations were determined to be satellites of the Milky Way. For reasons to be discussed in the concluding section, most researchers now believe that Eridanus II is an extremely long-period satellite of the Milky Way, probably beginning only its second approach to our galaxy.
Eridanus II is moving toward the center of the Milky Way at 67 km/sec. However, applying the current value of the Hubble Constant, the space between the two galaxies is also increasing at about 26 km/sec. 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. In addition, the speed of light delay must be considered. The velocity measurements of Li et al. made use of light emitted by Eridanus II approximately one million years ago. At the present moment, Eridanus II is probably only around 300 kpc away and has accelerated significantly beyond the observed 67 km/sec toward the Milky Way.
Size, shape, and rotation
Eridanus II does not have a spherical shape, and its ellipticity has been estimated at 0.45. Its size depends on assumptions about mass distribution and three-dimensional structure. Crnojević et al. find that their data are consistent with a simple exponential distribution of mass and a half-light radius of 277 ±14 pc, with an apparent half-light diameter of 4.6 arcmin to observers on Earth.A galactic structure of this small size is not expected to show signs of coherent rotation. In their studies of Eridanus II's velocity, Li et al. found no velocity gradient or anisotropy which would suggest coherent rotation. 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.
Relationship to other objects
A number of workers have speculated about an association between the Magellanic Clouds and various dwarf galaxies in the Local Group, including Eridanus II. The Magellanic Clouds are two satellite galaxies of the Milky Way, which are both presently about 60 kpc distant, and separated by 24 kpc from each other. This work is reviewed—briefly, but cogently—by Koposov et al.. Koposov and co-workers note that the Clouds show significant signs of distortion characteristic of tidal stress. This stress may have been induced by proximity to the Milky Way, but simulations suggest that it is more likely a result of interactions between the Clouds themselves ; Diaz & Bekki ).[|Koposov's group] suggest that the Magellanic Clouds are of the right size and age to have been part of a loosely-bound association of small galaxies which has been captured by the Milky Way, resulting in a scatter of small galaxies, including Eridanus II, roughly aligned along the trajectory of the Clouds. 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. In addition, similar clusters of dwarf galaxies are known to inhabit specific corridors around other major galaxies in the Local Group.
Pawlowski et al. also note Eridanus II's alignment with the Magellanic Clouds, but doubt that Eridanus II is properly part of a Magellanic cluster of dwarf galaxies because of its considerable distance from the other suspected members of the group. On the other hand, they argue for the existence of a well-defined plane running from the Andromeda Galaxy to the Milky Way. This plane, only 50 kpc thick, but up to 2 Mpc wide, includes 10 presently-known dwarves, all more than 300 kpc from any of the major galaxies of the Local Group. These workers observe that Eridanus II is not as well confined to the plane as are other members, and suggest that this may have something to do with its distant alignment to the Magellanic Clouds.