A Catalog Sample of Low-mass Galaxies Observed in X-Rays with Central Candidate Black Holes

Abstract

We present a sample of X-ray-selected candidate black holes in 51 low-mass galaxies with z  ≤ 0.055 and masses up to 10 10 M ⊙ obtained by cross-correlating the NASA-SLOAN Atlas with the 3XMM catalog. We have also searched in the available catalogs for radio counterparts of the black hole candidates and find that 19 of the previously selected sources also have a radio counterpart. Our results show that about 37% of the galaxies of our sample host an X-ray source (associated with a radio counterpart) spatially coincident with the galaxy center, in agreement with other recent works. For these nuclear sources, the X-ray/radio fundamental plane relation allows one to estimate the mass of the (central) candidate black holes, which are in the range of 10 4 –2 × 10 8 M ⊙ (with a median value of ≃3 × 10 7 M ⊙ and eight candidates having masses below 10 7 M ⊙ ). This result, while suggesting that X-ray emitting black holes in low-mass galaxies may have had a key role in the evolution of such systems, makes it even more urgent to explain how such massive objects formed in galaxies. Of course, dedicated follow-up observations both in the X-ray and radio bands, as well as in the optical, are necessary in order to confirm our results.