Three Quenched, Faint Dwarf Galaxies in the Direction of NGC 300: New Probes of Reionization and Internal Feedback

Abstract We report the discovery of three faint and ultrafaint dwarf galaxies—Sculptor A, Sculptor B, and Sculptor C—in the direction of NGC 300 ( D = 2.0 Mpc), a Large Magellanic Cloud–mass galaxy. Deep ground-based imaging with Gemini/GMOS resolves all three dwarf galaxies into stars, each displaying a red giant branch indicative of an old, metal-poor stellar population. No young stars or H i gas are apparent, and the lack of a GALEX UV detection suggests that all three systems are quenched. Sculptor C ( D = 2.04 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.13</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.10</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Mpc; M V = −9.1 ± 0.1 mag or L V = (3.7 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.3</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) × 10 5 L ⊙ ) is consistent with being a satellite of NGC 300. Sculptor A ( D = 1.35 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.08</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Mpc; M V = −6.9 ± 0.3 mag or L V = (5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) × 10 4 L ⊙ ) is likely in the foreground of NGC 300 and at the extreme edge of the Local Group, analogous to the recently discovered ultrafaint Tucana B in terms of its physical properties and environment. Sculptor B ( D = 2.48 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.24</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.21</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> Mpc; M V = −8.1 ± 0.3 mag or L V = (1.5 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.5</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> ) × 10 5 L ⊙ ) is likely in the background, but future distance measurements are necessary to solidify this statement. It is also of interest due to its quiescent state and low stellar mass. Both Sculptor A and B are ≳2–4 r vir from NGC 300 itself. The discovery of three dwarf galaxies in isolated or low-density environments offers an opportunity to study the varying effects of ram-pressure stripping, reionization, and internal feedback in influencing the star formation history of the faintest stellar systems.