DOLoRes@TNG confirms the extragalactic nature of Donatiello I, a new dwarf galaxy in the outskirts of the Local Group discovered by an Italian amateur astronomer

Its name is Donatiello I and it is a new entry in the list of galaxies located in the neighbourhood of the Local Galaxy Group. This object was discovered in 2016 by the amateur astronomer G. Donatiello (Oria, Italy). It appeared as a diffuse unresolved source in images taken by Donatiello himself in 2010-2013 from the Pollino National Park (Southern Italy). Donatiello's equipment was a small 127mm refractor equipped with a CCD camera.

In order to unveil the true nature of this source, Donatiello contacted Dr. W. Boschin (TNG staff) and proposed deep imaging observations with the instrument DOLoRes. In October and November 2016 the object was pointed for a couple of hours during Discretionary Director Time and the reduced images clearly revealed that it is a partially resolved spheroidal galaxy (See Fig. 1).

Indeed, a first attempt to measure the distance of the object was performed by D. Martinez Delgado (University of Heidelberg) and his science team by analyzing the TNG data. They could build the colour- magnitude diagram of the galaxy stars and infer that Donatiello I is a nearby dwarf galaxy, possibly located at the border of the Local Galaxy Group. Further very deep observations performed with the Gran Telescopio Canarias in January 2017 allowed Martinez Delgado et al. to conclude that its distance is probably 3.3 +/- 0.1 Mpc, thus locating the galaxy in the outskirts of the Local Group.

Donatiello I is similar in size, brightness and stellar content to some classical Milky Way dwarf companions like Draco or Ursa Mayor. Even more interestingly, its projected position (see Fig. 2) and distance are consistent with being a dwarf satellite of the S0-type galaxy NGC 404 (popularly known as the "Mirach's Ghost").

The discovery of Donatiello I is important for at least two reasons. First, the search for dwarf galaxies is a hot topic in modern cosmology. In fact, the Lambda-CDM paradigm predicts a large number of these small objects in the Local Volume, thus it is of broad interest for galaxy formation theories to carry out a full inventory of the number and properties of dwarf galaxies. The characterization of the newly discovered galaxy should be framed within this context. Second, Donatiello I is another proof of the significant contributions that well prepared amateur astronomers can provide to the astrophysical research, especially when they work in sinergy with professional astronomers.

Fig. 1: r'-band image of the dwarf galaxy Donatiello I obtained with the instrument DOLoRes@TNG (Credits: W. Boschin/TNG and G. Donatiello).

Fig. 2: DSS2 image of the field of Donatiello I and NGC 404 (Credits: DSS2/STScI).

More information in the accepted paper: Martinez Delgado, D.; Grebel, E. K.; Javanmardi, D.; Boschin, W.; et al., 2018, Astronomy & Astrophysics, in press (eprint arXiv:1810.04741).