|
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
|
