The ghost of Mirach and the elf of Mirach: a new galaxy discovered near the local group

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

Authors: David Martinez-Delgado, Eva K. Grebel, Behnam Javanmardi, Walter Boschin, Nicolas Longeard, Julio A. Carballo-Bello, Dmitry Makarov, Michael A. Beasley, Giuseppe Donatiello, Martha P. Haynes, Duncan A. Forbes, Aaron J. Romanowsky

Institution of the first author: Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Heidelberg, Germany

State: Accepted in Astronomy & Astrophysics

Astrobite original: Mirach's Ghost and Mirach's Goblin: A New Galaxy Found Near the Local Group by Daniel Berke

Statement by Daniel Berke: one of the authors of this article (Duncan Forbes) is a faculty member of my university, but I realized after I had chosen this article, I have no participation in the article itself.

Despite the title, this is not an accidental Halloween publication scheduled for Thanksgiving, but a full explanation will take a little time, so be patient: the brightest star in the Andromeda constellation is known as Mirach, a red giant star of magnitude 2. Located only seven arch minutes in the sky (the full Moon has about 13 minutes of arc as reference) is NGC 404, the lenticular galaxy Closer. NGC 604 is 10 million light years away, just a little beyond the Local Group of galaxies that contains our own Milky Way, the Andromeda galaxy (Messier 31), the Great and the Small Magellanic cloud, and several dozen dwarf galaxies satellites. Due to the proximity of NGC 404 to a very bright star, historically it was very difficult to see or photograph it, especially for observers with the first telescopes. Its diffuse and nebulous appearance could easily be mistaken for a fuzzy internal reflex of Mirach in the first telescopes, which led to her calling it "The ghost of Mirach" (This fact, combined with its number in NGC, led me to make a Extremely obvious joke, see Figure 1.)

Figure 1: Image of NGC 604, "Mirach Fantastic" taken by the Hubble Space Telescope. Public domain image.

Today's article covers the discovery of another galaxy a little further away from Mirach in the sky (close to a degree), but so small and diffuse that it was only discovered a few years ago. In September 2016 an amateur astronomer named Giuseppe Donatiello (who is listed as one of the authors of the article) was taking long exposures of the area around the Andromeda galaxy, when he noticed a faint spot on one of his images that did not fit with no known characteristic, as seen in Figure 2.

Figure 2: The image of the discovery of Donatiello 1, is the result of combining approximately ten hours of observation with a refractor telescope of 127 mm during four nights. Figure 1 in the original article.

The first author of today's article, David Martinez-delgado, discovered the image after Donatiello published it on Facebook, and contacted him to ensure timely follow-up on professional telescopes and to be sure that he would receive his own credit for his discovery. They were able to take observations with the telescope Italian Nazionale Galileo (TNG) of 3.58 meters and the Gran Telescopio Canarias (GTC), both part of Roque de los Muchachos Observatory on the Spanish island of La Palma. (The images of these two observations are shown below in Figure 3.) As a recognition of their discovery, the galaxy was named "Donatiello 1".

Figure 3: Images of Donatiello 1 taken with the TNG (left) and GTC (right). The observation of TNG represents a total observation time of 24,300 seconds, while the GTC observed a total of 3,300 seconds. The extremely bright and overexposed object in the upper right part of the image on the left (which has a field of view of just over 2 °) is Mirach, with NGC 404, the small black object at the 2 o'clock position. in point to the side. Figure 2 in the original article.

Using the observations they had taken, the authors were able to make estimates of some of the properties of Donatiello 1. It appears to be a spheroidal dwarf galaxy that is not forming stars, similar to many small galaxies orbiting the Milky Way and the Andromeda galaxy within of the Local Group. Deriving its distance proved difficult, but the authors estimated that it is located between 8.1 and 11.4 million light years. This comfortably places it beyond the Local Group, which means that it is not gravitationally bound to it. If it is 10 million light years from the Milky Way it should probably be only 211,000 light years away from NGC 404, and possibly associated with it. Interestingly, there is some evidence that NGC 604 has recently undergone an interaction or collision; it has been discovered that it has an active star formation ring, unusual for a lenticular galaxy. Donatiello 1 is also remarkably elongated and non-spheroidal, consistent with a recent interaction with NGC 404, but the authors point out that more research is needed to explore this interesting possibility.

Figure 4: Two graphs in super-galactic coordinates They show the location of Donatiello 1 (marked as Do I) compared to the Local Group and some relatively close galaxies. Donatiello 1 is marked with a red star; the black line that goes from it represents the range of distances in which it is probably located. NGC 404 is the orange dimmer point almost below the symbol for Donatiello 1. The colors indicate the type of galaxy according to the morphological type standard, with red which usually means "elliptical" and blue which means "spiral". The large black circle around the Milky Way and Andromeda indicates the gravitational limits of the Local Group. The red stars indicate the position of particularly isolated dwarf galaxies; the use of one to mark the position of Donatiello 1 represents the uncertainty in its position and association with NGC 404. Figure 8 of the original article.

In these days when professional telescopes are getting bigger and faster all the time, it is inspiring to know that amateur astronomers can make important discoveries like this one. Amateur and professional astronomers have worked together to discover new aspects of the Universe, and who knows what new wonders are waiting to be discovered tomorrow!