The heaviest neutron stars and lightest black holes expected to be produced by stellar evolution leave the mass-range 2.2 M _ { \odot } \lesssim m \lesssim 5 M _ { \odot } largely unpopulated .
Objects found in this so-called lower mass gap likely originate from a distinct astrophysical process .
Such an object , with mass 2.6 M _ { \odot } was recently detected in the binary merger GW190814 through gravitational waves by LIGO/Virgo .
Here we show that black holes in the mass gap are naturally assembled through mergers and accretion in AGN disks , and can subsequently participate in additional mergers .
We compute the properties of AGN-assisted mergers involving neutron stars and black holes , accounting for accretion .
We find that mergers in which one of the objects is in the lower mass gap represent up to 4 % of AGN-assisted mergers detectable by LIGO/Virgo .
The lighter object of GW190814 , with mass 2.6 M _ { \odot } , could have grown in an AGN disk through accretion .
We find that the unexpectedly high total mass of 3.4 M _ { \odot } observed in the neutron star merger GW190425 may also be due to accretion in an AGN disk .