{
  "id": "2006.00013",
  "version": "v1",
  "published": "2020-05-29T18:00:05.000Z",
  "updated": "2020-05-29T18:00:05.000Z",
  "title": "UV Spectral-Slopes at $z=6-9$ in the Hubble Frontier Fields: Lack of Evidence for Unusual or Pop III Stellar Populations",
  "authors": [
    "Rachana Bhatawdekar",
    "Christopher J. Conselice"
  ],
  "comment": "13 pages, 9 figures, Submitted to MNRAS",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We present new measurements of the UV spectral slope $\\beta$ for galaxies at $z=6-9$ in the Frontier Field cluster MACSJ0416.1-2403 and its parallel field, to an unprecedented level of low stellar mass. To calculate $\\beta$, we fit synthetic stellar population models to the observed spectral energy distribution and calculate its value by fitting a power law to the best-fit spectrum. This is the first derivation of rest-frame UV colours of galaxies extending out to $z=9$ for the Frontier Fields program that probes magnitudes as faint as $M\\mathrm{_{UV}=-13.5}$. We find no correlation between $\\beta$ and rest-frame UV magnitude $M_{1500}$ at all redshifts but we find a strong correlation between $\\beta$ and stellar mass, with lower mass galaxies exhibiting bluer UV slopes. At $z=7$ we find that the bluest value of our sample is $\\beta=-2.32\\pm0.31$, which is redder than previously reported values at similar redshifts in the literature, whereas at $z\\sim9$ we find that our bluest data point has a value of $\\beta=-2.63\\pm0.12$. Thus, we find no evidence for extreme stellar populations or evidence for Pop III stars in low-luminosity galaxies at $z>6$. Additionally, we find a strong correlation between $\\beta$ and SFR such that galaxies with low SFRs exhibit bluer slopes, which appear to get bluer with increasing redshift at a given SFR. We also find a star formation main sequence up to $z = 9$ with a rising SFRs with increasing stellar mass. All of these relations show that $\\beta$ values correlate with a process that drives both the overall star formation rate and stellar mass assembly. Furthermore, as we also observe no trend between $\\beta$ and specific star formation rate (sSFR), this suggests that whatever is setting $\\beta$ is not a local process but a global one driven by the scale of the galaxy.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-05-29T18:00:05.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "hubble frontier fields",
      "synthetic stellar population models",
      "stellar mass",
      "uv spectral-slopes",
      "galaxies exhibiting bluer uv"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 13,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}