Triangulum Extended (TREX) Survey: Discovery and Spectroscopic Follow-Up of Broad Emission Lined Luminous Sources (BELLS) in M33’s Star Forming Disk

The Triangulum Galaxy (M33) is the third largest member (by mass) of the Local Group. M33 is a dwarf spiral galaxy well-known for its active star-forming regions that are rich in the ionized gas phase of the interstellar medium (ISM). While inspecting spectra from the Triangulum Extended (TREX) survey, which targets the resolved stellar population in the disk of M33, we discovered six point-like broad emission lined luminous sources (BELLS) in and around the central star-forming region of M33. The BELLS are characterized by Hα (6563 Å) emission line widths as large as ~1000 km/s and the presence of several narrower and relatively uncommon emission lines associated with He I (5876 Å, 6678 Å, and 7065 Å) and other elements. P-Cygni profiles (emission lines with blue shifted absorption) are common among these emission lines, which indicates that they are likely associated with expanding bubbles of ionized material. We analyze spectroscopic observations from the TREX Survey of two such sources from 2018, as well as present follow-up observations from November 2022. The broad emission and other notable spectroscopic features persist for both objects, but a deeper analysis of the differences between the 2018 and 2022 spectra could inform our understanding of the physical processes at play. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to work with observations from this mountain.