Dark Energy Survey finds new dwarf galaxies

Up to nine new dwarf galaxies close to the Galaxy and the Magellanic Clouds have recently been found. Three of the new objects are definitely dwarf galaxies whilst the others may be fainter dwarfs or even globular clusters. Imaging data taken from the Dark Energy Survey (DES),  a five-year effort to photograph a large portion of the southern sky, has been analysed and these new objects were found. Two separate teams of astronomers (eight objects reported in Bechtol et al. 2015 and nine reported in Koposov et al. 2015) announced these findings after looking at the first year of DES data.

The Magellanic Clouds and the Auxiliary Telescopes at the Paranal Observatory in Chile. Six of the nine newly discovered satellites are shown (insets and enlarged). The other three possible dwarfs are outside the field of view. The insets show images of the three most visible objects (Eridanus 2, Horologium 1 and Pictoris 1) and each span 13×13 arcminutes on the sky. (Credit: V. Belokurov and S. Koposov, IoA, Cambridge; Y. Beletsky, Carnegie Observatories)

The closest of these new objects is 97,000 light-years away, about halfway to the Magellanic Clouds, in the constellation Reticulum. The most distant and most luminous of these objects is 1.2 million light-years away in the constellation Eridanus. The newly discovered objects are a billion times dimmer than the Milky Way, and a million times less massive. Their dimness and small size makes them incredibly difficult to find. Automatic object finding software is used to detect slight stellar overdensities. Colour information can then be used to filter out background stars to produce a candidate dwarf galaxy.

The projected positions of the objects are close to the Magellanic Clouds, suggesting some may be physically associated with the Clouds. They may even be linked with the extensive Magellanic Stream of cold, atomic hydrogen gas that spans a large part of the southern sky near the two Clouds. Spectroscopic confirmation of stars in each new object is still required to accurately determine their distances, chemical composition and structure

This animation shows how difficult it is to spot dwarf galaxy candidates in the Dark Energy Camera’s images. The first image is a snapshot of DES J0335.6-5403. This object sits roughly 100,000 light-years from Earth, and contains very few stars — only about 300 could be detected with DES data. The second image shows the detectable stars that likely belong to this object. (Credit: Fermilab/Dark Energy Survey)

Why are these discoveries significant? These ultra-faint galaxies are the smallest, least luminous and most dark matter dominated galaxies known. Most simulations of structure formation, using cold dark matter, predict hundreds of dwarf galaxies should be in orbit around the Galaxy. Until recently, only about 30 dwarfs were known. The current DES results increase this number by about 30%. The complete 5 year DES survey area is predicted to contain between 19-37 dwarf galaxies. It is unclear what the total number of dwarfs linked to the Galaxy is, due to incompleteness in depth (faintness) and sky coverage of various surveys. Thus it is still unclear the discrepancy between the number of dwarfs detected and that predicted by simulations.

Strangely, the majority of dwarf galaxies in orbit around the Galaxy appear to lie on a “great circle” on the sky. That is they do not seem to be isotropically distributed around the Galaxy. Recently, similar (planar) distributions of dwarf galaxies have been detected around our Local Group neighbour M31 (Andromeda), and many dwarfs appear to have the same sense of rotation about their host.  It is not yet clear whether these peculiar dwarf galaxy distributions are caused by ‘normal’ structure formation or other processes. Koposov et al. and Bechtol et al. both suggest that some of the new dwarfs found in DES may have been associated with the Magellanic Clouds in the past (harking back to previous suggestions of this nature by Donald Lynden-Bell in the mid-1970s). Perhaps they are Magellanic dwarfs not Galaxy dwarfs.

All-sky image showing the locations of known dwarf galaxies associated with the Galaxy. The new DES dwarfs are close to the positions of the LMC and SMC (bottom, left). (Credit: A. Frebel, MIT)

Finally, the dominance of dark matter in dwarf galaxies make these objects excellent targets for indirect dark matter searches. Dark matter annihilation produces gamma rays. Geringer-Sameth et al. (2015) have looked at Fermi-LAT data for the newly discovered DES dwarf, Reticulum 2. They detect a signal between 2 GeV and 10 GeV that exceeds the background. The detection is between 2.3 and 3.7 sigma, depending on the background model used. Despite this uncertainty, Reticulum 2 has the most significant gamma-ray signal of any known dwarf galaxy. Other sources of gamma rays (e.g. a background extragalactic source, or internal millisecond pulsars or young massive stars) still need to be ruled out.

For more information, see

[Glen Mackie]

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