The Low-mass and Structured Stellar Halo of M83 Argues Against a Merger Origin for Its Starburst and Extended Neutral Hydrogen Disk*

Abstract A merger origin has been suggested for M83’s massive, metal-rich extended H i disk and nuclear starburst. We observe M83’s stellar halo to test this idea. We train nearest-neighbor star–galaxy separation on wide-area Subaru imaging with Hubble Space Telescope data to map M83’s halo in resolved stars. We find that M83 has an extended, very low-density smooth stellar halo of old and metal-poor [M/H] ∼ −1.15 RGB stars with a mass between 15 and 40 kpc of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>log</mml:mi> <mml:mn>10</mml:mn> </mml:msub> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mo>∗</mml:mo> <mml:mo>,</mml:mo> <mml:mn>15</mml:mn> <mml:mo>−</mml:mo> <mml:mn>40</mml:mn> <mml:mo>,</mml:mo> <mml:mi>maj</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>8.02</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.10</mml:mn> </mml:math> . In addition to M83’s well-known Northern Stream, our ground-based Subaru imaging reveals a new stream to M83’s south, which modeling suggests could be its trailing arm. The combined stream masses are <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>log</mml:mi> <mml:mn>10</mml:mn> </mml:msub> <mml:msub> <mml:mi>M</mml:mi> <mml:mi>stream</mml:mi> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>7.93</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.10</mml:mn> </mml:math> , with metallicity [M/H] = −1.0 ± 0.2. The stream progenitor was only recently accreted, as its stellar populations suggest that it formed stars until 2.1 ± 1.3 Gyr ago. M83 lies on the stellar halo mass–metallicity correlation seen for other Milky Way–mass galaxies, albeit with low stellar halo mass. We infer a total accreted mass of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>log</mml:mi> <mml:mn>10</mml:mn> </mml:msub> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mo>*</mml:mo> <mml:mo>,</mml:mo> <mml:mi>accreted</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>8.7</mml:mn> <mml:msubsup> <mml:mn>8</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.28</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.22</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> , with the most massive past merger having <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mi>log</mml:mi> <mml:mn>10</mml:mn> </mml:msub> <mml:msub> <mml:mi>M</mml:mi> <mml:mrow> <mml:mo>*</mml:mo> <mml:mo>,</mml:mo> <mml:mi>dom</mml:mi> </mml:mrow> </mml:msub> <mml:mo>/</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>8.5</mml:mn> <mml:mo>±</mml:mo> <mml:mn>0.3</mml:mn> </mml:math> . We identify plausible M83 analogs in TNG-50 with similar stellar halos, finding that while a recent accretion can create a prominent stellar stream, such accretions do not trigger starburst activity, nor do they deliver enough gas to form M83’s extended H i disk. We conclude that other nonmerger mechanisms, such as secular evolution or accretion of gas from the intergalactic medium, are likely to be responsible for M83’s remarkable properties.