Intermediate-mass black holes (IMBHs), with masses between 100-105 M , fill the gap between stellar mass black holes and the supermassive black holes that reside in galaxy centers. While IMBHs are ...crucial to our understanding of black hole seed formation, black holes of less than 104 M have so far eluded detection by traditional searches. Observations of the infrared coronal lines (CLs) offer us one of the most promising tools to discover IMBHs in galaxies. We have modeled the infrared emission line spectrum that is produced by gas photoionized by an active galactic nucleus (AGN) radiation field and explored, for the first time, the dependence of the infrared CL spectrum on black hole mass over the range of 102-108 M . We show that as the black hole mass decreases, the hardening of the spectral energy distribution of the accretion disk causes infrared coronal lines with the highest ionization potentials to become prominent, revealing a powerful probe of black hole mass in AGNs. We identify key emission line ratios that are most sensitive to black hole mass in the 1-30 m range. With the unprecedented sensitivity of the James Webb Space Telescope, a large number of CLs will be detectable for the first time, providing important insight into the existence and properties of IMBHs in the local universe, and potentially revolutionizing our understanding of this class of object.
Abstract
It is now clear that a significant population of accreting black holes are undetected by commonly employed optical, mid-infrared color, X-ray, and radio selection methods as a result of ...obscuration or contamination of the emission from the nuclear region caused by star formation in the host galaxy. These elusive active galactic nuclei (AGNs) are an important component of the AGN population. They are often found in mergers, where significant black hole growth likely takes place, and in the lowest-mass galaxies or galaxies that lack classical bulges, a demographic that places important constraints on models of supermassive black hole seed formation and merger-free models of AGN fueling. In this work, we demonstrate the power of the James Webb Space Telescope (JWST) in uncovering and characterizing these elusive AGNs. We present an integrated modeling approach in which both the line and emergent continuum are predicted from gas exposed to the ionizing radiation from a young starburst and an AGN, focusing specifically on the spectral diagnostics available through JWST, and provide predictions on the detectability of key diagnostic lines by the near- and mid-infrared spectrometers, assuming typical conditions for the gas. We demonstrate the crucial need for JWST in uncovering low-level accretion activity even in nearby galaxies and out to redshifts of ≈1–3, depending on the ionization parameter, and gas phase metallicity. We present a redshift-dependent selection strategy that can be used to identify promising elusive AGN candidates for future follow-up study. The full suite of simulations is available online, where users can select specific parameters and retrieve the predicted continuum and infrared line luminosities from our models.
Abstract
One of the primary outstanding questions in extragalactic astronomy is the formation and early evolution of the supermassive black holes that are seen in nearly every massive galaxy. ...Low-metallicity dwarf galaxies may offer the most representative local analogs to pristine early galaxies, making them a vital tool in probing black hole seed models through the study of the intermediate-mass black holes (IMBHs) possibly hosted therein; though these dwarf galaxies, and the IMBHs they may host, are typically not as well-studied in this context as their higher-metallicity and higher-mass counterparts. In this paper, we explore the X-ray properties of a sample of 37 low-metallicity dwarf galaxies using archival XMM observations, and we compare the properties of this population against a representative sample of higher-metallicity counterparts. We report the detection of 10 sources with 0.3–10 keV luminosity in excess of 10
40
erg s
−1
within the low-metallicity sample, which we highlight for followup as potential IMBH candidates. Finally, we discuss the differing multiwavelength scaling relations (e.g.,
L
X
–
L
W2
,
L
X
–star formation rate) between the two galaxy populations, as well as the sample’s
L
X
as a function of metallicity.
Abstract
We conduct the first systematic survey of a comprehensive set of the twenty optical coronal lines in the spectra of nearly 1 million galaxies observed by the Sloan Digital Sky Survey (SDSS) ...Data Release 8 catalog. This includes often overlooked high ionization potential lines such as Ar
x
λ
5533, S
xii
λ
7609, Fe
xi
λ
7892, and Fe
xiv
λ
5303. We find that, given the limited sensitivity of SDSS, strong coronal line emission is extremely rare, with only ∼0.03% of the sample showing at least one coronal line, significantly lower than the fraction of galaxies showing optical narrow line ratios (∼17%) or mid-infrared colors (∼2%) indicative of nuclear activity. The coronal line luminosities exhibit a large dynamic range, with values ranging from ∼10
34
to 10
42
erg s
−1
. We find that a vast majority (∼80%) of coronal line emitters in dwarf galaxies (
M
*
< 9.6 × 10
9
M
⊙
) do not display optical narrow line ratios indicative of nuclear activity, in contrast to higher mass galaxies (∼17%). Moreover, we find that the highest ionization potential lines are preferentially found in lower mass galaxies. These findings are consistent with the theory that lower mass black holes found in lower mass galaxies produce a hotter accretion disk, which in turn enhances the higher ionization coronal line spectrum. Future coronal line searches with 30 m class telescopes or JWST may provide a pathway into uncovering the intermediate mass black hole population.
Abstract
Coronal lines are a powerful, yet poorly understood, tool to identify and characterize active galactic nuclei. There have been few large-scale surveys of coronal lines in the general galaxy ...population in the literature so far. Using a novel preselection technique with a flux-to-rms ratio
, followed by Markov Chain Monte Carlo fitting, we searched for the full suite of 20 coronal lines in the optical spectra of almost 1 million galaxies from the Sloan Digital Sky Survey Data Release 8. We present a catalog of the emission-line parameters for the resulting 258 galaxies with detections. The Coronal Line Activity Spectroscopic Survey includes line properties, host-galaxy properties, and selection criteria for all galaxies in which at least one line is detected. This comprehensive study reveals that a significant fraction of coronal-line activity is missed in past surveys based on a more limited set of coronal lines; ∼60% of our sample do not display the more widely surveyed Fe
x
λ
6374. In addition, we discover a strong correlation between coronal-line and Wide-field Infrared Survey Explorer W2 luminosities, suggesting that the mid-infrared flux can be used to predict coronal-line fluxes. For each line we also provide a confidence level that the line is present, generated by a novel neural network, trained on fully simulated data. We find that after training the network to detect individual lines using 100,000 simulated spectra, we achieve an overall true-positive rate of 75.49% and a false-positive rate of only 3.96%.
Abstract
Theoretical studies predict that the most significant growth of supermassive black holes (SMBHs) occurs in late-stage mergers, coinciding with the manifestation of dual active galactic ...nuclei (AGNs), and both major and minor mergers are expected to be important for dual AGN growth. In fact, dual AGNs in minor mergers should be signposts for efficient minor-merger-induced SMBH growth for both the more and less massive progenitor. We identified two candidate dual AGNs residing in apparent minor mergers with mass ratios of ∼1:7 and ∼1:30. Sloan Digital Sky Survey (SDSS) fiber spectra show broad and narrow emission lines in the primary nuclei of each merger while only a narrow O
iii
emission line and a broad and prominent H
α
/N
ii
complex is observed in the secondary nuclei. The FWHMs of the broad H
α
lines in the primary and secondary nuclei are inconsistent in each merger, suggesting that each nucleus in each merger hosts a Type 1 AGN. However, spatially resolved Large Binocular Telescope optical spectroscopy reveals rest-frame stellar absorption features, indicating the secondary sources are foreground stars and that the previously detected broad lines are likely the result of fiber spillover effects induced by the atmospheric seeing at the time of the SDSS observations. This study demonstrates for the first time that optical spectroscopic searches for Type 1/Type 1 pairs similarly suffer from fiber spillover effects as has been observed previously for Seyfert 2 dual AGN candidates. The presence of foreground stars may not have been clear if an instrument with more limited wavelength range or limited sensitivity had been used.
Intermediate-mass black holes (IMBHs) with masses between are crucial to our understanding of black hole seed formation and are the prime targets for the Laser Interferometer Space Antenna, yet black ...holes in this mass range have eluded detection by traditional optical spectroscopic surveys aimed at finding active galactic nuclei (AGNs). In this Letter, we have modeled for the first time the dependence of the optical narrow emission line strengths on the black hole mass of accreting AGN over the range of . We show that as the black hole mass decreases, the hardening of the spectral energy distribution from the accretion disk changes the ionization structure of the nebula. The enhanced high-energy emission from IMBHs results in a more extended partially ionized zone compared with models for higher mass black holes. This effect produces a net decrease in the predicted O iii/Hβ and N ii/H emission line ratios. Based on this model, we demonstrate that the standard optical narrow emission line diagnostics used to identify massive black holes fail when the black hole mass falls below for highly accreting IMBHs and for radiatively inefficient IMBHs with active star formation. Our models call into question the ability of common optical spectroscopic diagnostics to confirm AGN candidates in dwarf galaxies, and indicate that the low-mass black hole occupation fraction inferred from such diagnostics will be severely biased.
Abstract
Ultraluminous X-ray sources (ULXs) are our best laboratories for studying extreme super-Eddington accretion. Most studies of these objects are of relatively persistent sources; however, ...there is growing evidence to suggest a large fraction of these sources are transient. Here we present a sample of five newly reported transient ULXs in the galaxies NGC 4945, NGC 7793, and M81 serendipitously discovered in Swift/XRT observations. Swift monitoring of these sources have provided well-sampled lightcurves, allowing for us to model the lightcurves with the disk-instability model of Hameury & Lasota, which implies durations of 60–400 days and that the mass-accretion rate through the disk is close to or greater than the Eddington rate. Of the three source regions with prior Hubble Space Telescope imaging, color–magnitude diagrams of the potential stellar counterparts show varying ages of the possible stellar counterparts. Our estimation of the rates of these sources in these three galaxies is 0.4–1.3 yr
−1
. We find that, while persistent ULXs dominate the high end of galaxy luminosity functions, the number of systems that produce ULX luminosities are likely dominated by transient sources.
In contrast to massive galaxies with solar or super-solar gas phase metallicities, very few active galactic nuclei (AGNs) are found in low-metallicity dwarf galaxies. Such a population could provide ...insight into the origins of supermassive black holes. Here we report near-IR spectroscopic and X-ray observations of SDSS J105621.45+313822.1, a low-mass, low-metallicity galaxy with optical narrow line ratios consistent with star-forming galaxies but a broad H line and mid-infrared colors consistent with an AGN. We detect the Si vi 1.96 m coronal line and a broad Pa line with an FWHM of 850 25 km s−1. Together with the optical broad lines and coronal lines seen in the Sloan Digital Sky Survey (SDSS) spectrum, we confirm the presence of a highly accreting black hole with mass (2.2 1.3) × 106 M , with a bolometric luminosity of 1044 erg s−1 based on the coronal line luminosity, implying a highly accreting AGN. Chandra observations reveal a weak nuclear point source with erg s−1, ∼2 orders of magnitude lower than that predicted by the mid-infrared luminosity, suggesting that the AGN is highly obscured despite showing broad lines in the optical spectrum. The low X-ray luminosity and optical narrow line ratios of J1056+3138 highlight the limitations of commonly employed diagnostics in the hunt for AGNs in the low-metallicity, low-mass regime.
Abstract
Local low-metallicity dwarf galaxies are relics of the early universe and are thought to hold clues into the origins of supermassive black holes. While recent studies are uncovering a ...growing population of active galactic nuclei (AGNs) in dwarf galaxies, the vast majority reside in galaxies with solar or supersolar metallicities and stellar masses comparable to that of the LMC. Using Multi-Unit Spectroscopic Explorer (MUSE) and Very Large Telescope observations, we report the detection of Fe
x
λ
6374 coronal line emission and a broad H
α
line in the nucleus of SDSS J094401.87−003832.1, a nearby (
z
= 0.0049) metal-poor dwarf galaxy almost 500 times less massive than the LMC. Unlike the emission from the lower-ionization nebular lines, the Fe
x
λ
6374 emission is compact and centered on the brightest nuclear source, with a spatial extent of ≈100 pc, similar to that seen in well-known AGNs. The Fe
x
luminosity is ≈10
37
erg s
−1
, within the range seen in previously identified AGNs in the dwarf-galaxy population. The Fe
x
emission has persisted over the roughly 19 yr time period between the SDSS and MUSE observations, ruling out supernovae as the origin for the emission. The FWHM of the broad component of the H
α
line is 446 ± 17 km s
−1
and its luminosity is ≈1.5 × 10
38
erg s
−1
, corresponding to a black hole mass of ≈ 3150
M
⊙
, in line with its stellar mass if virial mass relations and black hole–galaxy scaling relations apply in this mass regime. These observations, together with previously reported multiwavelength observations, can most plausibly be explained by the presence of an accreting intermediate-mass black hole in a primordial galaxy analog.
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