INTERMEDIATE-MASS BLACK HOLES
Intermediate-mass black holes (IMBHs) are the missing link between stellar-mass and supermassive black holes (SMBHs), with BH masses in between 100-10^6 solar masses, and constitute the long-sought seed black holes from which SMBHs form.
IMBHs should be present in globular clusters, at the nucleus of low-mass galaxies, and in the outskirts of large galaxies (e.g., in the form of ultraluminous X-ray sources) after tidal stripping of merging satellite low-mass galaxies. Given the key role IMBHs could play in SMBH and galaxy growth, I have searched for IMBHs in ultraluminous X-ray sources and in low-mass dwarf galaxies using a combination of X-ray, radio, and optical observations. For a broad review on observational evidence for intermediate-mass black holes, see:
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Formation scenarios for IMBHs. Seed BHs in the early Universe could form from Population III stars, from mergers in dense stellar clusters formed out either from the second generation of stars or from inflows in protogalaxies, or from direct collapse of dense gas in protogalaxies, and grow via accretion and merging to 10^9 solar masses by z~7. SMBHs could also directly form by mergers of protogalaxies at z~6. Those seed BHs that did not grow into SMBHs can be found in the local Universe as leftover IMBHs. Credit: Mezcua (2017), International Journal of Modern Physics D, vol. 26, No 11.
IN DWARF GALAXIES
The presence of IMBHs is expected in star-forming dwarf galaxies that have not significantly grown through mergers/accretion: these should resemble those galaxies formed in the early Universe, with low mass and low metallicity, that hosted the seed BHs from which SMBHs and larger galaxies grew. In Mezcua et al. (2016, 2018) we discovered that a population of IMBHs does exist in dwarf galaxies and constructed the first complete sample of 40 AGN located in dwarf galaxies up to redshift z~2.4. This constitutes the largest and deepest sample of its kind and allowed us to study the evolution of AGN fraction with stellar mass, X-ray luminosity, and redshift in dwarf galaxies out to z = 0.7.
For this we used the Chandra X-ray data of the COSMOS survey, which is one of the largest (2.2 deg2) surveys with a complete, deep, multiwavelength dataset. The results were subject of a NASA press release. See:
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AGN fraction as function of redshift for two complete X-ray luminosity bins, two complete stellar mass bins, and three complete redshift bins. Values derived using the 40 AGN dwarf galaxies from COSMOS-Legacy. The low value of AGN fraction and its decrease with stellar mass suggest a direct collapse formation scenario for the seed BHs. The possible decrease of AGN fraction with redshift suggests that dwarf galaxies evolve differently than massive galaxies (whose AGN fraction increases with redshift). Credit: Mezcua et al. (2018b)
Stacked X-ray detections of dwarf galaxies in the 0.5–2 keV band. Images have been smoothed with a Gaussian of radiusï‚ =ï‚ 2. Color scales are in counts. Credit: Mezcua et al. (2016).
Using the VLA radio data of the COSMOS survey, we found an even deeper (out to z~3.4) sample of AGN in dwarf galaxies. Their high radio luminosities and the finding of jet efficiencies >10% in more than 50% of the sample indicate that dwarf galaxies can host radio jets as powerful as those of massive radio galaxies whose jet mechanical feedback can strongly affect the formation of stars in the host galaxy:
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Using MaNGA integral field unit (IFU) spectroscopy we have found a new sample of 23 dwarf galaxies that show AGN ionization signatures in spatially-resolved emission line diagnostic diagrams. The AGN signatures are largely missed by integrated emission line diagnostics. The bolometric luminosity of these 23 new AGN candidates is <~10^40 erg/s, fainter than that of other AGN in dwarf galaxies. The AGN emission is in most cases offset from the optical center of the dwarf galaxy and shows a symmetric morphology, which indicates that either the AGN are of-nuclear, that the central emission of the galaxy is dominated by star formation, or that the AGN are turned-off and we are observing a past ionization burst. The finding of this sample of hidden and faint AGN has important implications for population studies of AGN in dwarf galaxies and for seed black hole formation models.
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MaNGA analysis for one of the dwarf galaxy hosting a type 1 AGN candidate . Left : Location of each MaNGA spaxel on the [NII]-BPT used to distinguish between ionization by AGN/LINER (red spaxels), star-formation (blue spaxels), or composite (green spaxels). The black square marks the median BPT location of those spaxels classied as AGN/LINER, the grey square the SDSS (single-fiber) BPT location; Center : Spatial distribution of the BPT-classified spaxels (color-coded as in the left panel). Empty squares mark the IFU coverage, grey squares those spaxels with continuum SNR > 1. The N shows the number of AGN/LINER spaxels used in the analysis and stacking; Right : SDSS composite image. The pink hexagon shows the IFU coverage. Credit: Mezcua & DomÃnguez Sánchez (2020).
IN ULTRALUMINOUS X-RAY SOURCES
Ultraluminous X-ray sources (ULXs) are extragalactic, off-nuclear X-ray sources brighter than Galactic X-ray binaries but dimmer than X-ray emitting galaxies. The brightest ULXs are suggested to be IMBHs accreting at sub-Eddington rates, while those with X-ray luminosities below 2×10^40 erg/s can be explained as stellar-mass BHs with super-Eddington accretion. In order to clarify their nature, I investigated the radio counterparts of ULXs using the VLA, VLBA, and the European VLBI Network in combination with observations with the Chandra and Swift X-ray satellites. This yielded the detection of compact radio emission in three ULXs, for which a black hole mass in the IMBH regime is estimated, and the first detection of extended jet emission from an IMBH. See:
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TOP: HST image of the extreme ULX NGC 2276-3c. The VLA 5 GHz radio contours are plotted as (5, 6, 7, 8, 9, 10) times the off-source rms noise of 0.04 mJy/beam. The green circle denotes the Chandra position of the ULX with a diameter of 0.6 arcsec (Mezcua et al. 2013c). BOTTOM: EVN 1.6 GHz image of NGC 2276-3c. The synthesized beam size is 16.4 mas x 13.1 mas.The off-source rms noise is 8 microJy/beam. Credit: Mezcua et al. (2015a).
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Radio observations of extreme ULXs: revealing the most powerful ULX radio nebula ever or the jet of an intermediate-mass black hole? Mezcua, M.; Roberts, T. P.; Sutton, A. D.;Lobanov, A. P. 2013, MNRAS, 436, 3128
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Revealing the nature of the ULX and X-ray population of the spiral galaxy NGC 4088. Mezcua, M.; Fabbiano, G.; Gladstone, J. C.; Farrell, S. A.; Soria, R. 2014, ApJ, 785, 121
Combining X-ray detections from the Chandra Source Catalog (Version 2) with galaxies from the Sloan Digital Sky Survey we have recently constructed a sample of 144 candidates to Hyperluminous X-ray sources, which have X-ray luminosities above 10^41 erg/s. Their X-ray emission is significantly in excess of the expected contribution from star formation, suggesting that they are produced by accretion onto black holes more massive than stars. We estimate that at least ∼20 of the HLX candidates are consistent with IMBHs:
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VLBA image of the ULX CXO J133815.6+043255 at 4.4 GHz. The synthesized beam size is 4.94 mas x 1.95 mas. The 7.6 GHz contours are plotted as (-3, 4, 4.5, 5) the off-source r.m.s. noise of 23.5 microJy/beamô€€€. Credit: Mezcua et al. (2018c)
THE BEST IMBHs
The BH mass range of IMBHs is usually taken as 100 < BH mass < 10^6 solar masses. However, some authors consider the upper limit to be in 10^5 solar masses instead of 10^6, and name those BHs with 10^5 – 10^6 solar masses as ‘low-mass’ BHs instead of IMBHs. The are a few hundreds of low-mass BHs; however, IMBHs for which the BH mass is below 10^5 solar masses and has been derived using different methods, there are only a few. Here some of the best cases:
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NGC 4395. See here a paper on this bulgeless galaxy: http://adsabs.harvard.edu/abs/2015ApJ…809..101D
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RGG118. Here the paper on the discovery of this dwarf galaxy: http://adsabs.harvard.edu/abs/2015ApJ…809L..14B
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HLX-1. This is one of those cases in which the IMBH is off-nuclear, a ULX. Here one of the papers where they estimate the BH mass: http://adsabs.harvard.edu/abs/2012Sci…337..554W