2023ApJ...950..163P

Modeling the integrated H I spectra of galaxies has been a difficult task, due to their diverse shapes, but more dynamical information is waiting to be explored in H I line profiles. Based on simple assumptions, we construct a physically motivated model for the integrated H I spectra: Parameterized Asymmetric Neutral hydrogen Disk Integrated Spectrum Characterization (PANDISC). The model shows great flexibility in reproducing the diverse H I profiles. We use Monte Carlo Markov Chain for fitting the model to global H I profiles and produce statistically robust quantitative results. Comparing with several samples of H I data available in the literature, we find the model-fitted widths agree with cataloged velocity widths (e.g., W50) down to S/N ≲ 6. While dynamical information can only be extracted reliably from spectra with S/N > 8. The model is also shown to be useful for applications like the baryonic Tully-Fisher relation (BTFR) and profile-based sample control. By comparing the model parameter v_r to v_flat, we uncover how the H I width is affected by the structure of the rotation curve, following a trend consistent with the difference in the BTFR slope. We also select a sample of spectra with broad wing-like features suggestive of a population of galaxies with unusual gas dynamics. The PANDISC model bears both promise and limitations for potential use beyond H I lines. Further application on the whole ALFALFA sample will enable us to perform large-scale ensemble studies of the H I properties and dynamics in nearby galaxies.