Mirach’s Goblin: Discovery of a dwarf spheroidal galaxy behind the Andromeda galaxy

¹ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
e-mail: delgado@ari.uni-heidelberg.de
² School of Astronomy, Institute for Research in Fundamental Sciences (IPM), Tehran, 19395-5531 Iran
³ Fundación G. Galilei – INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
⁴ Instituto de Astrofísica de Canarias (IAC), Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Tenerife, Spain
⁶ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁷ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, 782-0436 Macul, Santiago, Chile
⁸ Special Astrophysical Observatory, Nizhniy Arkhyz, Karachai-Cherkessia, 369167 Russia
⁹ Nuovo Orione, 72024 Oria, Italy
¹⁰ Cornell Center for Astrophysics and Planetary Science, Space Sciences Building, Cornell University, Ithaca, NY, 14853 USA
¹¹ Centre for Astrophysics & Supercomputing, Swinburne University of Technology, Hawthorn, VIC, 3122 Australia
¹² Department of Physics & Astronomy, San José State University, One Washington Square, San Jose, CA, 95192 USA
¹³ University of California Observatories, 1156 High Street, Santa Cruz, CA, 95064 USA

Received: 25 April 2018
Accepted: 9 October 2018

Abstract

Context. It is of broad interest for galaxy formation theory to carry out a full inventory of the numbers and properties of dwarf galaxies, both satellite and isolated, in the Local Volume.

Aims. 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 (β And) in a deep image taken with an amateur telescope.

Methods. The color-magnitude diagram (CMD) 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 is 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 CMD. At this distance, the absolute magnitude (M_V = −8.3), surface brightness (μ_V = 26.5 mag arcsec⁻²), and stellar content of Donatiello I are similar to the “classical” Milky Way companions Draco or Ursa Minor.

Results. The projected position and distance of Donatiello I are consistent with this object 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.