Is NGC 300 a pure exponential disk galaxy?
  NGC 300 is a low-mass disk galaxy in the Sculptor group. In the literature,
it has been identified as a pure exponential disk galaxy, as its luminosity
profile could be well fitted with a single exponential law over many disk scale
lengths (Type I). We investigate the stellar luminosity distribution of NGC 300
using $Hubble$ $Space$ $Telescope$ (HST) archive data, reaching farther and
deeper than any other previous studies. Color magnitude diagrams show a
significant population of old red giant branch (RGB) stars in all fields out to
$R\sim19$ kpc ($32'$), as well as younger populations in the inner regions. We
construct the density profiles of the young, intermediate-aged, and old stellar
populations. We find two clear breaks in the density profiles of the old RGB
and intermediate-aged stars: one down-bending (Type II) at $R\sim5.9$ kpc, and
another up-bending (Type III) at $R\sim8.3$ kpc. Moreover, the old RGB stars
exhibit a negative radial color gradient with an up-bending at $R\sim8$~kpc,
beyond which the stellar populations are uniformly old ($>$7~Gyr) and
metal-poor ($\rm[Fe/H] = -1.6^{+0.2}_{-0.4}$ dex). The outer stellar component
at $R\gtrapprox8$ kpc is, therefore, well separated from the inner disk in
terms of the stellar density and stellar populations. While our results cast
doubt on the currently established wisdom that NGC\,300 is a pure exponential
disk galaxy, a more detailed survey should be carried out to identify the
outskirts as either a disk or a stellar halo.
