{
  "id": "1802.01567",
  "version": "v1",
  "published": "2018-02-05T18:59:59.000Z",
  "updated": "2018-02-05T18:59:59.000Z",
  "title": "Intermediate-mass black holes in dwarf galaxies out to redshift $\\sim$ 2.4 in the Chandra COSMOS Legacy Survey",
  "authors": [
    "M. Mezcua",
    "F. Civano",
    "S. Marchesi",
    "H. Suh",
    "G. Fabbiano",
    "M. Volonteri"
  ],
  "comment": "16 pages, 10 figures, re-submitted to MNRAS after addressing referee's comments",
  "categories": [
    "astro-ph.GA"
  ],
  "abstract": "We present a sample of 40 AGN in dwarf galaxies at redshifts $z \\lesssim$ 2.4. The galaxies are drawn from the \\textit{Chandra} COSMOS-Legacy survey as having stellar masses $10^{7}\\leq M_{*}\\leq3 \\times 10^{9}$ M$_{\\odot}$. Most of the dwarf galaxies are star-forming. After removing the contribution from star formation to the X-ray emission, the AGN luminosities of the 40 dwarf galaxies are in the range $L_\\mathrm{0.5-10 keV} \\sim10^{39} - 10^{44}$ erg s$^{-1}$. With 12 sources at $z > 0.5$, our sample constitutes the highest-redshift discovery of AGN in dwarf galaxies. The record-holder is cid\\_1192, at $z = 2.39$ and with $L_\\mathrm{0.5-10 keV} \\sim 10^{44}$ erg s$^{-1}$. One of the dwarf galaxies has $M_\\mathrm{*} = 6.6 \\times 10^{7}$ M$_{\\odot}$ and is the least massive galaxy found so far to host an AGN. All the AGN are of type 2 and consistent with hosting intermediate-mass black holes (BHs) with masses $\\sim 10^{4} - 10^{5}$ M$_{\\odot}$ and typical Eddington ratios $> 1\\%$. We also study for the first time the evolution, corrected for completeness, of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to $z$ = 0.7. We find that the AGN fraction for $10^{9}< M_{*}\\leq3 \\times 10^{9}$ M$_{\\odot}$ and $L_\\mathrm{X} \\sim 10^{41}-10^{42}$ erg s$^{-1}$ is $\\sim$0.4\\% for $z \\leq$ 0.3 and that it decreases with X-ray luminosity and decreasing stellar mass. Unlike massive galaxies, the AGN fraction is found to decrease with redshift, from $\\sim$0.2\\% for $z \\leq$ 0.3 and $L_\\mathrm{X} \\gtrsim 10^{42}$ erg s$^{-1}$ to 0.04\\% for $0.5 < z \\leq 0.7$, suggesting that AGN in dwarf galaxies evolve differently than those in high-mass galaxies. Mindful of potential caveats, the results seem to favor a direct collapse formation mechanism for the seed BHs in the early Universe.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2018-02-05T18:59:59.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "dwarf galaxies",
      "chandra cosmos legacy survey",
      "intermediate-mass black holes",
      "agn fraction",
      "stellar mass"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 16,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}