The stellar content of the nearby SB ( s ) m galaxy NGC 55 is investigated using images obtained with the Gemini South and Canada-France-Hawaii telescopes .
The ( K,H - K ) and ( K,J - K ) color-magnitude diagrams ( CMDs ) of stars near the plane of the disk reveal signatures of large scale star formation during recent and intermediate epochs in the form of red supergiants ( RSGs ) with M _ { K } \sim - 11.5 , and an asymptotic giant branch ( AGB ) that peaks near M _ { K } = -10 .
Comparisons with stellar evolution models suggest that the brightest RSGs have an age near 8 Myr .
A well-defined plume , which stellar evolution models suggest contains stars with masses near the RSG – AGB transition , is detected in CMDs constructed both from infrared and visible wavelength observations .
It is concluded that star formation in the thin disk of NGC 55 has ocurred at a significant rate for at least the past 0.1 – 0.2 Gyr , and this is consistent with other indicators .
The near-infrared spectral-energy distribution ( SED ) of the integrated light near the center of the galaxy is consistent with that in other Magellanic irregular galaxies , indicating that the star-forming history of NGC 55 , when averaged over timescales of 0.1 – 1 Gyr , has likely not been peculiar when compared with other late-type systems .
Evidence is also presented that the disk contains a large population of old ( log ( t _ { yr } ) \sim 10 ) stars , and it is argued that a stable disk has been in place in NGC 55 for a significant fraction of the age of the Universe .

At projected distances in excess of 2 kpc off of the disk plane the brightest AGB stars have ages 10 ^ { +3 } _ { -2 } Gyr .
Thus , despite indications that dust and gas are present in the envelope surrounding the NGC 55 disk , the AGB content suggests that recently formed stars do not occur in large numbers in the extraplanar region .
The ( r ^ { \prime } - i ^ { \prime } ) colors of the RGB in the extraplanar region are consistent with [ Fe/H ] between –2.2 and –0.7 , with the majority of stars having [ Fe/H ] > -1.2 , and the mean metallicity inferred from the RGB color does not change with distance above the disk plane .
Thus , the stellar component in the extraplanar envelope is well mixed , at least in terms of metallicity .
The mean metallicity of RGB stars is in excellent agreement with that measured in the extraplanar HII regions EHR 1 and 2 , suggesting that the age-metallicity relation in this part of NGC 55 has been flat for at least a few Gyr .
Finally , the RGB-tip occurs near i ^ { \prime } = 23.1 in the extraplanar region , and a distance modulus of 26.5 is computed from this feature .