Intermediate-mass black holes in Green pea galaxies (IMBH-GP) I: A candidate sample from LAMOST and SDSS

The scaling relation of central massive black holes (MBHs) and their host galaxies is well-studied for supermassive BHs (SMBHs, M_BH ≥ 10⁶M_⊙). However, this relation has large uncertainties in the mass range of the intermediate-mass BHs (IMBHs, M_BH ∼ 10³–10⁶M_⊙). Since Green pea (GP) galaxies are luminous compact dwarf galaxies, which may be likely to host less massive SMBHs or even IMBHs, we systematically search for MBHs in a large sample of 2190 GP galaxies at z < 0.4, selected from LAMOST and SDSS spectroscopic surveys. Here, we report a newly discovered sample of 59 MBH candidates with broad Hα lines. This sample has a median stellar mass of 10^8.83±0.11M_⊙ and hosts MBHs with single-epoch virial masses ranging from M_BH ∼ 10^4.7 to 10^8.5M_⊙ (median 10^5.85±0.64M_⊙). Among the 59 MBH candidates, 36 have black hole masses M_BH ≤ 10⁶M_⊙ (IMBH candidates), one of which even has M_BH ≲ 10⁵M_⊙. We find that the M_BH-M_* relation of our MBH sample is consistent with the M_BH-M_bulge relation for SMBHs, while is above the M_BH-M_* relation for MBHs in dwarf galaxies in the same mass range. Furthermore, we show that 25 MBH candidates, including 4 IMBH candidates, have additional evidence of black hole activities, assessed through various methods such as the broad-line width, BPT diagram, mid-infrared color, X-ray luminosity, and radio emission. Our studies show that it is very promising to find IMBHs in GP galaxies, and the BH sample so obtained enables us to probe the connection between the MBHs and compact dwarf galaxies in the low-redshift Universe.