JWST’s Constraints on the Substellar IMF in NGC 2024

Abstract A recent study has reported the detection of a turnover in the initial mass function (IMF) below 12 MJup and the possible detection of a minimum mass near 3 MJup in the heavily embedded cluster NGC 2024 (<1 Myr), which was based on a sample of brown dwarf candidates in NIRCam images from the James Webb Space Telescope (JWST). I have used those NIRCam data in conjunction with archival spectra from JWST’s Near-Infrared Spectrograph (NIRSpec) to constrain the substellar IMF in NGC 2024. I present NIRSpec data for 87 sources, 67 of which are within the NIRCam field. Based on those spectra and data from previous studies (e.g., X-ray emission), I have classified 45 of the NIRSpec targets as members of NGC 2024, 17 of which have spectral types indicative of brown dwarfs (M6.5–L; 13 are within NIRCam). The latter have mass estimates as low as ~4 MJup according to theoretical evolutionary models. There remain a few photometric candidates lacking spectra that extend down to the completeness limit of the images (≳2 MJup for AK ≳ 1), so the minimum mass of the IMF has not been detected. It is not possible to measure a reliable substellar IMF from the NIRCam images because of the small number of brown dwarfs encompassed by them, a bias against objects at lower masses due to the high extinctions, and incompleteness at ≳ 10 MJup due to the saturation limit. Thus, there is no evidence in the JWST data for a turnover below 12 MJup.