As part of our survey of galactic stellar halos , we investigate the structure and stellar populations of the northern outer part of the stellar halo in NGC 55 , a member galaxy of the Sculptor Group , using deep and wide-field V - and I -band images taken with Subaru/Suprime-Cam .
Based on the analysis of the color-magnitude diagrams ( CMDs ) for red-giant-branch ( RGB ) stars , we derive a tip of RGB ( TRGB ) -based distance modulus to the galaxy of ( m - M ) _ { 0 } = 26.58 \pm 0.11 ( d = 2.1 \pm 0.1 { Mpc } ) .
From the stellar density maps , we detect the asymmetrically disturbed , thick disk structure and two metal-poor overdense substructures in the north region of NGC 55 , which may correspond to merger remnants associated with hierarchical formation of NGC 55 ’ s halo .
In addition , we identify a diffuse metal-poor halo extended out to at least z \sim 16 kpc from the galactic plane .
The surface-brightness profiles toward the z -direction perpendicular to the galactic plane suggest that the stellar density distribution in the northern outer part of NGC 55 is described by a locally isothermal disk at z \hskip { 3.0 pt } \raisebox { 1.72 pt } { $ < $ } \hskip { -7.5 pt } \raisebox { -3.01 pt } { $ \sim$ } % \hskip { 3.0 pt } 6 kpc and a likely diffuse metal-poor halo with V -band surface brightness of \mu _ { V } \hskip { 3.0 pt } \raisebox { 1.72 pt } { $ > $ } \hskip { -7.5 pt } \raisebox { -3.01 pt } % { $ \sim$ } \hskip { 3.0 pt } 32 mag arcsec ^ { -2 } , where old RGB stars dominate .
We derive the metallicity distributions ( MDs ) of these structures on the basis of the photometric comparison of RGB stars with the theoretical stellar evolutionary models .
The MDs of the thick disk structures show the peak and mean metallicity of [ Fe/H ] _ { peak } \sim - 1.4 and [ Fe/H ] _ { mean } \sim - 1.7 , respectively , while the outer substructures show more metal-poor features than the thick disk structure .
Combined with the current results with our previous study for M31 ’ s halo , we discuss the possible difference in the formation process of stellar halos among different Hubble types .