Parameterized Asymmetric Neutral Hydrogen Disk Integrated Spectrum Characterization (PANDISC). I. Introduction to a Physically Motivated H i Model

Abstract

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.