NGC 404 is afield galaxy[5] located about 10 millionlight years away in theconstellationAndromeda. It was discovered byWilliam Herschel in 1784, and is visible through small telescopes.[6] NGC 404 lies just beyond theLocal Group and does not appear gravitationally bound to it. It is located within 7arc-minutes of second magnitude starMirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".[6][7]
NGC 404 is a very isolated dwarflenticular galaxy, a bit more luminous and smaller than theSmall Magellanic Cloud.[8] Unlike many other early-type galaxies, it is very rich inneutral hydrogen, most of it concentrated in a pair of large rings around it.[9] It also hasstar formation both in its center[10] and in its outermost regions, albeit at a low level.[11]
Both the outer gas disk and its star formation are assumed to have been triggered by one or severalmergers with smaller galaxies roughly 1 billion years ago[11] and it has been proposed NGC 404 is a formerspiral galaxy that was transformed into a lenticular one by that event.[10]
At least two techniques have been used to measure distances to NGC 404. The infraredsurface 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 ± 0.5Mly (3.03 ± 0.15Mpc).[2]
However, NGC 404 is close enough thatred supergiants can be imaged as individual stars. The light from these stars and knowledge of how they should compare to nearby stars within theMilky Way galaxy allows for direct measurement of the distance to the galaxy. This method is referred to as thetip 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).[3] Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly (3.07 ± 0.21 Mpc).[a]
In 2018 a possible satellite designatedDonatiello I was identified.[14] Donatiello I is adwarf spheroidal galaxy with little recent star formation.[14] Difficulty in establishing the exact distance to the galaxy leaves its status as a satellite unconfirmed.[14]
^abJensen, Joseph B.; Tonry, John L.; Barris, Brian J.; Thompson, Rodger I.; et al. (February 2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations".Astrophysical Journal.583 (2):712–726.arXiv:astro-ph/0210129.Bibcode:2003ApJ...583..712J.doi:10.1086/345430.S2CID551714.
^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.
^abMirach's Ghost (NGC 404),The Internet Encyclopedia of Science, David Darling. Accessed on line August 15, 2008.
^abThilker, 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.
