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

The massive black hole (BH) population in dwarf galaxies (M BH ≲ 105 M ⊙) 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 ⋆ ≤ 3 × 109 M ⊙) with detectable [Fe x]λ6374 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]λ6374 luminosities in the range L [Fe x] ≈ 1036–1039 erg s−1, with a median value of 1.6 × 1038 erg s−1. The [Fe x] λ6374 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 eight Type IIn SNe in our sample. That said, the [Fe x]λ6374 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, and 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]λ6374 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.

The Rapid Formation of Coronal Lines in the Nova V1674 Herculis

The nova V1674 Her (Nova Her 2021) has been reported to have the shortest interval between outburst and coronal line formation of any nova yet observed. We present optical and near-infrared spectroscopy from three epochs ranging from shortly before coronal line formation to shortly after. Taken together with the published results of Woodward et al., they indicate that not only did the coronal lines form very early in the nova’s development, but in going from undetectable to detectable in two days, and undetectable to significant, in six days, they developed very rapidly as well.

Near-infrared Studies of Nova V1674 Herculis: A Shocking Record Breaker

We present near-infrared spectroscopy of Nova Herculis 2021 (V1674 Her), obtained over the first 70 days of its evolution. This fastest nova on record displays a rich emission line spectrum, including strong coronal line emission with complex structures. The hydrogen line fluxes, combined with a distance of 4.7 − 1.0 + 1.3 kpc, give an upper limit to the hydrogen ejected mass of M ej = 1.4 − 1.2 + 0.8 × 10 − 3 M ⊙. The coronal lines appeared at day 11.5, the earliest onset yet observed for any classical nova, before there was an obvious source of ionizing radiation. We argue that the gas cannot be photoionized, at least in the earliest phase, and must be shocked. Its temperature is estimated to be 105.57±0.05 K on day 11.5. Tentative analysis indicates a solar abundance of aluminum and an underabundance of calcium, relative to silicon, with respect to solar values in the ejecta. Further, we show that the vexing problem of whether collisional ionization or photoionization is responsible for coronal emission in classical novae can be resolved by correlating the temporal sequence in which the X-ray supersoft phase and the near-infrared coronal line emission appear.

The central parsec of NGC 3783: a rotating broad emission line region, asymmetric hot dust structure, and compact coronal line region

Using VLTI/GRAVITY and SINFONI data, we investigate the subparsec gas and dust structure around the nearby type 1 active galactic nucleus (AGN) hosted by NGC 3783. The K-band coverage of GRAVITY uniquely allows simultaneous analysis of the size and kinematics of the broad line region (BLR), the size and structure of the near-infrared(near-IR)-continuum-emitting hot dust, and the size of the coronal line region (CLR). We find the BLR, probed through broad Brγ emission, to be well described by a rotating, thick disc with a radial distribution of clouds peaking in the inner region. In our BLR model, the physical mean radius of 16 light-days is nearly twice the ten-day time-lag that would be measured, which closely matches the ten-day time-lag that has been measured by reverberation mapping. We measure a hot dust full-width at half-maximum (FWHM) size of 0.74 mas (0.14 pc) and further reconstruct an image of the hot dust, which reveals a faint (5% of the total flux) offset cloud that we interpret as an accreting or outflowing cloud heated by the central AGN. Finally, we directly measure the FWHM size of the nuclear CLR as traced by the [Ca VIII] and narrow Brγ line. We find a FWHM size of 2.2 mas (0.4 pc), fully in line with the expectation of the CLR located between the BLR and narrow line region. Combining all of these measurements together with larger scale near-IR integral field unit and mid-IR interferometry data, we are able to comprehensively map the structure and dynamics of gas and dust from 0.01 to 100 pc.

Coronal-line forest active galactic nuclei – I. Physical properties of the emission-line regions

ABSTRACT Coronal-line forest (CLiF) active galactic nuclei (AGNs) are characterized by strong high-ionization lines, which contrasts with what is found in most AGNs. Here, we carry out a multiwavelength analysis aimed at understanding the physical processes in the narrow-line region (NLR) of these objects, and at discovering whether they are indeed a special class of AGNs. By comparing coronal emission-line ratios we conclude that there are no differences between CLiF and non-CLiF AGNs. We derive physical conditions of the NLR gas and we find electron densities in the range of 3.6 × 102 to 1.7 × 104 cm−3 and temperatures of 3.7 × 103 to 6.3 × 104 K, suggesting that the ionization mechanism is associated primarily with photoionization by the AGN. We suggest an NLR dominated by matter-bounded clouds to explain the high-ionization line spectrum observed. The mass of the central black hole, derived from the stellar velocity dispersion, shows that most of the objects have values in the interval 107–108 M⊙. Our results imply that CLiF AGNs are not in a separate category of AGNs. In all optical/near-infrared emission-line properties analysed, they represent an extension to the low/high ends of the distribution within the AGN class.