Is NGC 300 a pure exponential disk galaxy?

Abstract

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 $R\sim 19$ kpc ($32^{\prime }$), 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 $R\sim 5.9$ kpc, and another up-bending (Type III) at $R\sim 8.3$ kpc. Moreover, the old RGB stars exhibit a negative radial color gradient with an up-bending at $R\sim 8$~kpc, beyond which the stellar populations are uniformly old ($>$7~Gyr) and metal-poor ($[Fe/H]=-1.6_{-0.4}^{+0.2}$ dex). The outer stellar component at $R⪆8$ 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.

6 pages, 5 figures, 1 appendix, accepted for publication in A&A Letters

References in corpus (9)

• The GALEX Ultraviolet Atlas of Nearby Galaxies
• The ACS Nearby Galaxy Survey Treasury
• Specific SFR profiles in nearby spiral galaxies: quantifying the inside-out formation of disks
• Diverse Stellar Haloes in Nearby Milky Way-Mass Disc Galaxies
• Antitruncated Stellar Disks via Minor Mergers
• The Tip of the Red Giant Branch Distances to Type Ia Supernova Host Galaxies. III. NGC 4038/39 and NGC 5584
• Influence of galaxy stellar mass and observed wavelength on disc breaks in S${}^4$G, NIRS0S, and SDSS data
• Exponential Disks from Stellar Scattering: III. Stochastic Models
• Isolating the young stellar population in the outer disk of NGC 300