Letter to the Editor

Is NGC 300 a pure exponential disk galaxy?

¹ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
e-mail: hanlbomi@gmail.com
² Department of Astronomy, University of Michigan, 311 West Hall, 1085 South University Ave., Ann Arbor, MI, 48109-1107, USA
³ Instituto de Investigación Multidisciplinar en Ciencia y Tecnología, Universidad de La Serena, Raúl Bitrán 1305, La Serena, Chile
⁴ Departamento de Astronomía, Universidad de La Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile
⁵ Department of Physics and Astronomy, 102 Natural Science Building, University of Louisville, Louisville, KY, 40292, USA
⁶ Department of Physics and Astronomy, University of Alabama, Box 870324, Tuscaloosa, AL, 35487-0324, USA
⁷ Vatican Observatory, 00120, Vatican City State

Received: 12 June 2020
Accepted: 20 July 2020

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 can be well fit 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 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 ∼ 19 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, and find two clear breaks in the density profiles of the old RGB and intermediate-aged stars: one down bending (Type II) at R ∼ 5.9 kpc, and another up bending (Type III) at R ∼ 8.3 kpc. Moreover, the old RGB stars exhibit a negative radial color gradient with an upward bend at R ∼ 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 stellar density and stellar population. 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.