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).Template:Ref label
↑ 2.02.1Jensen, Joseph B. (February 2003). "Measuring Distances and Probing the Unresolved Stellar Populations of Galaxies Using Infrared Surface Brightness Fluctuations". Astrophysical Journal583 (2): 712–726. DOI:10.1086/345430.
↑ 3.03.1I. D. Karachentsev (2004). "A Catalog of Neighboring Galaxies". Astronomical Journal127 (4): 2031–2068. DOI:10.1086/382905.
↑Karachentsev, I. D. (2006). "Masses of the local group and of the M81 group estimated from distortions in the local velocity field". Astrophysics49 (1): 3–18. DOI:10.1007/s10511-006-0002-6.
↑ 10.010.1Thilker, David A. (2010). "NGC 404: A Rejuvenated Lenticular Galaxy on a Merger-induced, Blueward Excursion Into the Green Valley". The Astrophysical Journal Letters714 (1): L171–L175. DOI:10.1088/2041-8205/714/1/L171.