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NGC 404 is a field galaxy located about 10 million light years away in the constellationAndromeda. It was discovered by William Herschel in 1784, and is visible through small telescopes. NGC 404 lies just beyond the Local Group and does not appear gravitationally bound to it. It is located within 7 arc-minutes of second magnitude star Mirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".
Physical characteristics
NGC 404 is a very isolated dwarf lenticular galaxy, a bit more luminous and smaller than the Small Magellanic Cloud. Unlike many other early-type galaxies, it is very rich in neutral hydrogen, most of it concentrated in a pair of large rings around it. It also has star formation both in its center and in its outermost regions, albeit at a low level.
Both the outer gas disk and its star formation are assumed to have been triggered by one or several mergers with smaller galaxies roughly 1 billion years ago and it has been proposed NGC 404 is a former spiral galaxy that was transformed into a lenticular one by that event.
At least two techniques have been used to measure distances to NGC 404. The infrared surface brightness fluctuations distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to NGC 404 using this technique in 2003 is 9.9 ± 0.5 Mly (3.03 ± 0.15 Mpc).
However, NGC 404 is close enough that red supergiants can be imaged as individual stars. The light from these stars and knowledge of how they should compare to nearby stars within the Milky Way galaxy allows for direct measurement of the distance to the galaxy. This method is referred to as the tip of the red giant branch (TRGB) method. The estimated distance to NGC 404 using this technique is 10.0 ± 1.2 Mly (3.1 ± 0.4 Mpc). Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly (3.07 ± 0.21 Mpc).
Possible satellite
In 2018 a possible satellite designated Donatiello I was identified. Donatiello I is a dwarf spheroidal galaxy with little recent star formation. Difficulty in establishing the exact distance to the galaxy leaves its status as a satellite unconfirmed.
Materne, J. (April 1979). "The structure of nearby groups of galaxies – Quantitative membership probabilities". Astronomy and Astrophysics. 74 (2): 235–243. Bibcode:1979A&A....74..235M.
^ Mirach's Ghost (NGC 404), The Internet Encyclopedia of Science, David Darling. Accessed on line August 15, 2008.
^ Thilker, David A.; Bianchi, Luciana; Schiminovich, David; Gil de Paz, Armando; et al. (2010). "NGC 404: A Rejuvenated Lenticular Galaxy on a Merger-induced, Blueward Excursion Into the Green Valley". The Astrophysical Journal Letters. 714 (1): L171 – L175. arXiv:1003.4985. Bibcode:2010ApJ...714L.171T. doi:10.1088/2041-8205/714/1/L171. S2CID51511487.
NGC 404 by Hubble Space Telescope; 1.68′ view
Pan-STARRS image of NGC 404
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