[Paper Review] A Sample of Massive Black Holes in Dwarf Galaxies Detected via [Fe X] Coronal Line Emission: Active Galactic Nuclei and/or Tidal Disruption Events

Mallory Molina, Amy E. Reines+3 more|arXiv (Cornell University)|Aug 20, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy220 references71 citations

TL;DR

This study identifies 81 dwarf galaxies with [Fe X]λ6374 coronal line emission, indicating accretion onto massive black holes (MBH ≲10⁵ M⊙), most undetected by traditional optical AGN surveys. Using SDSS spectroscopy, it demonstrates that [Fe X] emission—too luminous for stellar sources—probes low-accretion, low-mass BHs in star-forming, blue dwarf galaxies, revealing a hidden population of AGNs and tidal disruption events (TDEs).

ABSTRACT

The massive black hole (BH) population in dwarf galaxies ($M_{ m BH} \lesssim 10^5~M_\odot$) can provide strong constraints on the origin of BH seeds. However, traditional optical searches for active galactic nuclei (AGNs) only reliably detect high-accretion, relatively high-mass BHs in dwarf galaxies with low amounts of star formation, leaving a large portion of the overall BH population in dwarf galaxies relatively unexplored. Here, we present a sample of 81 dwarf galaxies ($M_\star \le 3 imes 10^9~M_\odot$) with detectable [Fe X]$\lambda$6374 coronal line emission indicative of accretion onto massive BHs, only two of which were previously identified as optical AGNs. We analyze optical spectroscopy from the Sloan Digital Sky Survey and find [Fe X]$\lambda$6374 luminosities in the range $L_{ m [Fe\,X]}\approx10^{36}$-$10^{39}$ erg s$^{-1}$, with a median value of $1.6 imes 10^{38}$ erg s$^{-1}$. The [Fe X]$\lambda$6374 luminosities are generally much too high to be produced by stellar sources, including luminous Type IIn supernovae (SNe). Moreover, based on known SNe rates, we expect at most 8 Type IIn SNe in our sample. On the other hand, the [Fe X]$\lambda$6374 luminosities are consistent with accretion onto massive BHs from AGNs or tidal disruption events (TDEs). We find additional indicators of BH accretion in some cases using other emission line diagnostics, optical variability, X-ray and radio emission (or some combination of these). However, many of the galaxies in our sample only have evidence for a massive BH based on their [Fe X]$\lambda$6374 luminosities. This work highlights the power of coronal line emission to find BHs in dwarf galaxies missed by other selection techniques and to probe the BH population in bluer, lower mass dwarf galaxies.

Motivation & Objective

• To identify massive black holes in dwarf galaxies using [Fe X]λ6374 coronal line emission as a probe of low-accretion AGN activity.
• To overcome limitations of traditional optical AGN diagnostics (e.g., BPT diagrams) that miss low-luminosity, low-ionization, and low-accretion-rate sources.
• To assess whether [Fe X] emission in dwarf galaxies is driven by AGN accretion or transient events like tidal disruption events (TDEs) or supernovae.
• To demonstrate that [Fe X] emission is a powerful, complementary method for detecting massive black holes in low-mass, blue, star-forming dwarf galaxies.

Proposed method

• Selected 81 dwarf galaxies from the NASA-Sloan Atlas (NSA v1.0.1) with M⋆ ≤ 3 × 10⁹ M⊙ and detectable [Fe X]λ6374 emission in SDSS single-fiber spectra.
• Measured [Fe X]λ6374 luminosities in the range L[F I] ≈ 10³⁶–10³⁹ erg s⁻¹, with a median of 1.6 × 10³⁸ erg s⁻¹.
• Used optical emission-line diagnostics (e.g., BPT/VO87 diagrams, He II/Hβ, [S II]/Hα vs. [O II]λλ7320,7330/Hα) to assess AGN-like ionization.
• Searched for optical variability in archival SDSS data to identify transient AGN activity.
• Cross-matched with Chandra X-ray data and radio surveys (FIRST, VLASS) to identify X-ray and radio emission consistent with AGN activity.
• Ruled out stellar sources (e.g., Type IIn SNe) by comparing [Fe X] luminosities to known SN rates and luminosities, finding 81 objects exceed SN emission by factors of 6–23.

Experimental results

Research questions

• RQ1 Can [Fe X]λ6374 coronal line emission reliably identify massive black holes in dwarf galaxies missed by optical AGN diagnostics?
• RQ2 Is the observed [Fe X] emission in dwarf galaxies more consistent with AGN accretion or transient events like TDEs or luminous SNe?
• RQ3 What fraction of the low-mass, low-accretion BH population in dwarf galaxies remains undetected by standard optical and IR AGN selection functions?
• RQ4 How do [Fe X] luminosities compare to those expected from known SNe, and can they distinguish between AGN and transient sources?
• RQ5 To what extent do additional AGN indicators (optical variability, X-ray, radio) co-occur with [Fe X] emission in these systems?

Key findings

• The sample of 81 dwarf galaxies with detectable [Fe X]λ6374 emission has a median luminosity of 1.6 × 10³⁸ erg s⁻¹, with luminosities ranging from 10³⁶ to 10³⁹ erg s⁻¹.
• Only two of the 81 galaxies were previously identified as optical AGNs, indicating that [Fe X] emission detects a largely unexplored population of low-accretion BHs.
• [Fe X]λ6374 luminosities are too high to be explained by stellar sources, including luminous Type IIn SNe, with the strongest emission (e.g., in NSA 533731/Mrk 709S) exceeding SN2005ip by a factor of 23.
• The [Fe X] emission in 79 of the 81 galaxies cannot be explained by known SNe rates, with only ~8 expected Type IIn SNe in the sample, far fewer than the number of [Fe X] detections.
• Additional AGN indicators—optical variability, X-ray emission, and radio emission—were found in some systems, but many galaxies are identified solely via [Fe X] luminosity, highlighting its diagnostic power.
• The study confirms that [Fe X]λ6374 emission is a robust probe of massive black holes in low-mass, blue, star-forming dwarf galaxies, particularly those with low accretion rates or transient activity.