Deep g ^ { \prime } r ^ { \prime } i ^ { \prime } z ^ { \prime } images obtained with the Gemini Multi-Object Spectrograph ( GMOS ) on Gemini North are used to investigate the stellar content in the outer regions of the nearby Sc galaxies NGC 2403 and M33 .
The field observed in NGC 2403 covers galactocentric distances between 5 and 11 kpc perpendicular to the line of sight ( R _ { GC } ^ { LOS } ) , and 7 and 19 kpc along the plane of the disk ( R _ { GC } ^ { disk } ) .
The red giant branch ( RGB ) -tip occurs at i ^ { \prime } = 23.6 \pm 0.1 , and the Cepheid and RGB-tip distance scales for NGC 2403 are in good agreement .
The number density of bright main sequence stars in this field experiences a steep cut-off at R _ { GC } ^ { disk } \sim 10 kpc , which is consistent with the expected truncation radius of the disk predicted from studies of edge-on spiral galaxies .
While very young stars are restricted to R _ { GC } ^ { disk } < 10 kpc , a population of bright asymptotic giant branch ( AGB ) stars is present throughout the entire GMOS field , indicating that star formation occured outside of the present-day star-forming disk of NGC 2403 during intermediate epochs .
The AGB stars are not in a tidal stream ; in fact , the ratio of AGB stars above the RGB-tip to those below the RGB-tip does not change with radius , indicating that the bright AGB stars are uniformly mixed with the fainter stellar content throughout the field .
The AGB luminosity function ( LF ) scales with r - band surface brightness over a wide range of radii throughout the main body of NGC 2403 , indicating that the age distribution of stars in the outer regions of the present-day star-forming disk is not skewed to younger values than in the inner disk .
Based on the color of stars on the upper portions of the RGB it is concluded that metallicity changes across the field , with [ Fe/H ] = -0.8 \pm 0.1 ( random ) \pm 0.3 ( systematic ) at R _ { GC } ^ { LOS } 
= 5 kpc , and [ Fe/H ] = -2.2 \pm 0.2 ( random ) \pm 0.8 
( systematic ) at R _ { GC } ^ { LOS } = 11 kpc .

The M33 field samples R _ { GC } ^ { LOS } between 8 and 10 kpc and R _ { GC } ^ { disk } between 14 and 17 kpc .
Bright AGB stars are detected in this field , and the ratio of bright AGB stars to stars on the upper RGB is at least as large as that measured in the outer regions of NGC 2403 ; thus , an intermediate-age population occurs well outside of the young star-forming disk of M33 .
The color of stars on the upper RGB suggests that [ Fe/H ] = -1.0 \pm 0.3 ( random ) \pm 0.3 ( systematic ) in this field .

The globular cluster systems of both NGC 2403 and M33 contain objects that formed during intermediate epochs , and it is suggested that the luminous AGB stars detected in the current study are the field counterparts of these clusters .
The detection of intermediate-age stars in the outer regions of these galaxies is consistent with models in which late-type spiral galaxies formed more slowly than earlier-type systems .