Using data from the Hubble Space Telescope, A. Monachesi et
al. (2013) report the discovery of a faint dwarf galaxy, GHOSTS I. Based on
observations of some of the stars in that galaxy, the estimated distance of
GHOSTS I is ~40 million light years. GHOSTS I appears to be a very isolated
dwarf galaxy as there are no large galaxies anywhere within ~13 million lights
years from it. Also, the process of star-formation seems to be evident within
GHOSTS I, resulting in it being tentatively classified as a dwarf irregular
(dIrr) galaxy. Nevertheless, more observations are probably needed for a more
confident classification.
The Small Magellanic Cloud, a dwarf irregular galaxy located
only 200,000 light years from the Milky Way. Credit: Stéphane Guisard.
In general, most dwarf galaxies that are far from large
galaxies tend to be dIrr galaxies, while those closer to large host galaxies
tend to be dwarf spheroidal (dSph) galaxies. There is a dichotomy between dIrr
and dSph galaxies - dIrr galaxies have ongoing star-formation, while dSph
galaxies do not. It is believed that tidal and ram pressure effects from large
host galaxies probably transformed many star-forming dIrr galaxies into
non-star-forming dSph galaxies. Work by Slater & Bell (2013) show that even
a single close passage by a large galaxy can extinguish star-formation,
transforming a dIrr galaxy to a dSph galaxy.
GHOSTS I is tens of thousands of times fainter than a large
galaxy like the Milky Way or Andromeda. Together with just a few other dwarf
galaxies such as Leo T and Leo P, GHOSTS I is one of the faintest and
least-massive star-forming dwarf galaxies known. Future observations of GHOSTS
I can reveal how such tiny galaxies can retain gas and form stars.
References:
- A. Monachesi et al. (2013), “GHOSTS I: A New Faint very
Isolated Dwarf Galaxy at D = 12 +/- 2 Mpc”, arXiv:1312.0602 [astro-ph.GA]
- Slater & Bell (2013), “Confronting Models of Dwarf
Galaxy Quenching with Observations of the Local Group”, arXiv:1306.1829
[astro-ph.CO]