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Paper   IPM / Astronomy / 15469
School of Astronomy
  Title:   Mirach's Goblin: Discovery of a dwarf spheroidal galaxy behind the Andromeda galaxy
1.  D. Martinez-Delgado
2.  E. K. Grebel
3.  B. Javanmardi
4.  W. Boschin
5.  N. Longeard
6.  J. A. Carballo-Bello
7.  D. Makarov
8.  M. A. Beasley
9.  G. Donatiello
10.  M. P. Haynes
11.  D. A. Forbes
12.  A. J. Romanowsky
  Status:   Published
  Journal: Astronomy & Astrophysics
  Vol.:  620
  Year:  2018
  Supported by:            ipm IPM
It is of broad interest for galaxy formation theory to carry out a full inventory of the numbers and properties of dwarf galaxies in the Local Volume, both satellites and isolated ones. Ultra-deep imaging in wide areas of the sky with small amateur telescopes can help to complete the census of these hitherto unknown low surface brightness galaxies, which cannot be detected by the current resolved stellar population and HI surveys. We report the discovery of Donatiello I, a dwarf spheroidal galaxy located one degree from the star Mirach (Beta And) in a deep image taken with an amateur telescope. The color--magnitude diagram obtained from follow-up observations obtained with the Gran Telescopio Canarias (La Palma, Spain) reveals that this system is beyond the Local Group and is mainly composed of old stars. The absence of young stars and HI emission in the ALFALFA survey are typical of quenched dwarf galaxies. Our photometry suggests a distance modulus for this galaxy of (m-M)=27.6 +/- 0.2 (3.3 Mpc), although this distance cannot yet be established securely owing to the crowding effects in our color--magnitude diagram. At this distance, Donatiello I's absolute magnitude (M_v =-8.3), surface brightness (mu_v=26.5 mag arcsec^-2) and stellar content are similar to the "classical" Milky Way companions Draco or Ursa Minor. The projected position and distance of Donatiello I are consistent with being a dwarf satellite of the closest S0-type galaxy NGC 404 ("Mirach's Ghost"). Alternatively, it could be one of the most isolated quenched dwarf galaxies reported so far behind the Andromeda galaxy.

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