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Discovery of an isolated dwarf spheroidal galaxy
neighboring the Local Group

Russian version

    Our recent observations with the Hubble Space Telescope (August 29, 2014) led to revealing a highly isolated dwarf spheroidal galaxy KKs 3 situated at a distance of 2.12±0.07 Mpc from the Milky Way. The dSph galaxy locates on the southern sky (RA=02h24m44.4s, Dec.=-73°30'51", J2000.0) and has a stellar mass about 1/10000 of the Milky Way mass.
    The gas-poor dSph systems with old stellar population are found almost exclusively in the virial domain of groups and clusters. It is generally considered that the concentration of dSph galaxies to the halos of massive galaxies is due to processes of gas stripping and strangulation of irregular dwarfs that suppresses further star formation. If these mechanisms that are operative in dense environments are paramount then dSph objects should be absent in the general field of low densities. However, energetic events associated with active star formation in dwarf systems at an early epoch might deplete gas resources. In such cases, relic quenched dSph galaxies may occur among isolated objects.
    The search and discovery of isolated spheroidal dwarfs constitutes an important interest for cosmology, given the small observed number of dwarfs with respect to their expected number in the standard cosmological model. Over the last decade, about 30 dSph galaxies have been discovered inside the Local Group through systematic search in the vicinity of M31 and more that a dozen dSphs were discovered in the nearby group around M81. These discoveries resulted from targeted searches within small parts of the sky. The hunt for isolated spheroidal dwarfs is very difficult because it requires a survey of large sky area and considerable sensitivity. Objects devoid of neutral hydrogen and high contrast HII-regions are usually invisible in optical and HI-surveys. Only very nearby dSph galaxies, inevitably of low surface brightness, may be revealed if they resolve into individual stars. So far, only a single isolated spheroidal dwarf, KKR 25, was found in the Local Group neighborhood at a distance of 1.93 Mpc. It has been also discovered by our team in 1999.
Karachentsev I.D., Makarova L.N., Makarov D.I., Tully R.B. Rizzi L. A new isolated dSph galaxy near the Local Group, MNRAS Letters

Contact - Makarov D.I.
Fig.3. Landscape of neighbouring galaxies around KKs 3. The colour and size of the circles represent the morphological type and luminosity of a galaxy according to the given scales. Three circles outline spheres of zero-velocity surfaces around the three massive groups: the Local Group and groups around NGC5128 (CenA) and NGC253. Red stars locate five dSph galaxies that in varying degrees are isolated systems
Fig.1. HST/ACS image of KKs 3 through the F606W filter. The image size is 3.4x3.4 arcmin. The 7.5 arcsec region highlighted by the white square near the centre of the galaxy is shown in the lower right corner to contain a globular cluster
Fig.2. Colour-magnitude diagram of resolved stars in KKs 3. Photometric errors are indicated by the bars at the right side of the CMD. The tip of the red giant branch is indicated by the dotted line