2021ApJS..253...39W

We present the fourth catalog of serendipitously discovered compact extragalactic emission-line sources-Hα Dots. A total of 454 newly discovered objects are included in the current survey list. These objects have been detected in searches of moderately deep narrowband images acquired for the ALFALFA Hα project. The catalog of Hα Dots presented in the current paper was derived from searches carried out using ALFALFA Hα images obtained with the KPNO 2.1 m telescope. This results in a substantially deeper sample of Dots compared to our previous lists, which were all discovered in images taken with the WIYN 0.9 m telescope. The median R-band magnitude of the current catalog is 21.59, more than 1.6 mag fainter than the median for the 0.9 m sample (a factor of 4.4x fainter). Likewise, the median emission-line flux of the detected sources is a factor of 4.3x fainter. The line flux completeness limit of the current sample is ∼3 x 10^–16 erg s^–1 cm^–2. We present accurate coordinates, apparent magnitudes, and narrowband line fluxes for each object in the sample. Unlike our previous lists of Hα Dots, the current sample does not include follow-up spectroscopy.