HI-detected Dwarf Galaxies in the FASHI Survey: Insights from Single- and Double-Peaked Emission-Line Samples

TL;DR

This study analyzes HI-rich, low-mass dwarf galaxies from the FASHI DR1 catalog by leveraging DECaLS photometry to produce a robust optical counterpart sample of 351 galaxies with $M_{ m HI} < 10^8\,M_\odot$. The authors demonstrate that these HI-dwarf systems predominantly exhibit low stellar mass surface densities similar to dwarf ellipticals, and they split the sample into single- and double-peaked HI profiles to probe their dynamical states. Double-peaked galaxies populate the baryonic Tully–Fisher relation, consistent with rotation-dominated halos, while single-peaked systems lie on the Faber–Jackson relation and the stellar-mass fundamental plane, indicating velocity-dispersion–dominated kinematics and potentially lower halo masses or dark-matter deficiencies. The work suggests an evolutionary link between HI-rich dwarfs and dEs and highlights decoupling between gas and stellar kinematics in dwarfs, underscoring the need for spatially resolved HI and stellar kinematic studies to constrain dark matter halo properties. These findings advance our understanding of formation pathways for low-mass galaxies and provide a framework for interpreting HI profile shapes as proxies for dynamical state.

Abstract

We present a sample of low HI mass dwarf galaxies ($M_{\rm HI} < 10^8 M_\odot$) detected by The FAST All Sky HI Survey (FASHI) project. Due to the faint and irregular morphology of these galaxies, the default photometry is often inaccurate. Therefore, we utilized The Dark Energy Camera Legacy Survey (DECaLS) data to perform careful photometric measurements, and find that the low HI mass galaxies have similar stellar mass densities to dwarf elliptical (dE) galaxies. Compared to other dwarf galaxy populations, the HI-selected dwarfs exhibit higher stellar mass densities than ultradiffuse galaxies, and similar densities to HI-selected low-surface-brightness galaxies, albeit with lower stellar masses, suggesting a possible evolutionary connection among these populations. By classifying the galaxies according to their HI spectral-line profiles, we show that the double-peaked sources conform closely to the Tully-Fisher relation, whereas the single-peaked sources follow the Faber-Jackson relation but with large scatter. This indicates that the single-peaked systems are likely dispersion dominated and that the relationship between stellar mass and halo mass in such systems may remain consistent across both low- and high-mass regimes. These findings suggest that HI-selected dwarf galaxies with single-peaked HI profiles may share a similar dynamical state with massive ellipticals, offering new insights into their structural evolution and the diversity of formation pathways for low-mass galaxies.

Figures (12)

• Figure 1: The H i-to-stellar-mass ratio as a function of $g - r$ color for the low H i mass sample presented in this work (open circles), overplotted with FASHI targets that have reliable optical counterparts (filled circles) as a reference. All points are color coded by their $r$-band magnitude. The similar trend between the two samples indicates the statistical reliability of the optical counterpart identification for our H i-selected galaxies.
• Figure 2: Stellar mass versus H i mass for the FASHI sample, with optical counterparts matched from the SGA catalog with spectroscopic redshifts. The H i-selected dwarf studied in this work is highlighted in red.
• Figure 3: Stellar mass and mass surface density of various stellar systems, including star forming galaxies 2006ApJS..164..334F2020MNRAS.493...87T, elliptical galaxies and early-type galaxies 2006ApJS..164..334F2006ApJS..165...57C. To compare with the dwarf galaxy population, we also show the Local Group dwarf galaxies 2011MNRAS.414.3699M, ultracompact dwarfs 2008AA...487..921M, dwarf elliptical galaxies 2008AA...486..697M2009AA...496..683M2009AJ....138.1037C, low-surface-brightness galaxies Du2015, and UDGs from coma 2015ApJ...813...23V. The low H i mass galaxies are shown with red cross signs, which are at the similar region as dwarf elliptical galaxies. The surface density is roughly 1-2 orders of magnitude lower than the mass density of elliptical or UCDs, implying that no bright bulge formed when the dwarf galaxies were rich in H i gas. We also show the star cluster systems in gray and black dots 2005ApJS..161..304M2009ApJS..180...54J. The solid lines indicate the galaxy size of 100 pc, 1 kpc, and 10 kpc.
• Figure 4: Examples of the growth curve for three targets (ID from left: 40703, 35982, 40772). The left and right peaks are indicated by dashed lines in the lower-right panel of each subplot. The apparent dip between the two peaks is likely caused by noise fluctuations, or not significant enough for the S/N. To account for this, we add random noise at the rms level to each channel 500 times and remeasure the K parameter. The resulting distribution of K values is shown as a histogram in the upper-left panel of each subplot. In the left example, the condition K + Kerr $<$ 0 is satisfied, and the spectrum is classified as double peaked. The right panel example also exhibits a double-peaked profile by eye, but the significance is insufficient for a robust classification in this work.
• Figure 5: Inclination-corrected $0.5\times$W50 velocity vs the baryonic and stellar mass for double peaked (top panels) and single peaked (bottom panels). The open circles are the sample from the Spitzer Photometry and Accurate Rotation Curves 2016AJ....152..157L, which defines baryonic Tully-Fisher relation 2016ApJ...816L..14L. Our H i low-mass with double-peaked H i profiles follows the trend of the scaling relation, while the single-peaked targets are on or above baryonic Tully-Fisher relation. Our H i sample is colored by the stellar mass surface density.