Sunday, April 13, 2014

An Isolated Dwarf Galaxy GHOSTS I

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: St├ęphane 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]