X-Ray and Ultraviolet Properties of AGNs in Nearby Dwarf Galaxies

Vivienne F. Baldassare, Amy E. Reines, Elena Gallo, Jenny E. Greene

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50 Scopus citations

Abstract

We present new Chandra X-ray Observatory and Hubble Space Telescope observations of eight optically selected broad-line active galactic nucleus (AGN) candidates in nearby dwarf galaxies (z < 0.055). Including archival Chandra observations of three additional sources, our sample contains all 10 galaxies from Reines et al. (2013) with both broad Hα emission and narrow-line AGN ratios (six AGNs, four composites), as well as one low-metallicity dwarf galaxy with broad Hα and narrow-line ratios characteristic of star formation. All 11 galaxies are detected in X-rays. Nuclear X-ray luminosities range from L 0.5-7keV ≈ 5 × 1039 to 1 × 1042 ergs-1. In all cases except for the star-forming galaxy, the nuclear X-ray luminosities are significantly higher than would be expected from X-ray binaries, providing strong confirmation that AGNs and composite dwarf galaxies do indeed host actively accreting black holes (BHs). Using our estimated BH masses (which range from ∼7 × 104 to 1 × 106 M o), we find inferred Eddington fractions ranging from ∼0.1% to 50%, i.e., comparable to massive broad-line quasars at higher redshift. We use the HST imaging to determine the ratio of UV to X-ray emission for these AGNs, finding that they appear to be less X-ray luminous with respect to their UV emission than more massive quasars (i.e., α OX values an average of 0.36 lower than expected based on the relation between α OX and 2500 luminosity). Finally, we discuss our results in the context of different accretion models onto nuclear BHs.

Original languageEnglish (US)
Article number20
JournalAstrophysical Journal
Volume836
Issue number1
DOIs
StatePublished - Feb 10 2017

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • X-rays: galaxies
  • galaxies: active
  • galaxies: dwarf
  • quasars: supermassive black holes
  • ultraviolet: galaxies

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