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

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.

💡 Research Summary

This paper presents a comprehensive analysis of dwarf galaxies with low neutral hydrogen (HI) mass (M_HI < 10^8 M_⊙), selected from the first data release (DR1) of the FAST All Sky HI Survey (FASHI). Conducted with the highly sensitive Five-hundred-meter Aperture Spherical radio Telescope (FAST), FASHI has detected over 40,000 HI-bright sources, enabling the study of low-mass galaxies that are often too faint for reliable optical photometry and redshift determination.

The study focuses on a sample of 520 low HI-mass galaxies cross-matched with deep optical imaging from the Dark Energy Camera Legacy Survey (DECaLS) Data Release 9. Recognizing that automated photometric pipelines often fail for faint, irregular, or low-surface-brightness galaxies, the authors performed careful, customized photometric measurements on DECaLS g- and r-band images to derive accurate stellar properties.

The first key finding relates to the stellar mass density of these HI-selected dwarfs. They were found to have stellar mass densities similar to those of dwarf elliptical (dE) galaxies. When compared to other dwarf populations, these galaxies exhibit higher stellar mass densities than ultradiffuse galaxies (UDGs) but similar densities to other HI-selected low-surface-brightness (LSB) galaxies, albeit with lower total stellar masses. This suggests a potential evolutionary link among these different classes of low-mass systems.

The most significant insight comes from classifying the galaxies based on the profile of their HI spectral emission lines: single-peaked or double-peaked. The dynamical analysis reveals a clear dichotomy:

• Galaxies with double-peaked HI profiles closely follow the Tully-Fisher Relation (TFR), a tight correlation between rotational velocity and baryonic mass. This indicates they are rotation-supported systems with well-defined disks.
• Galaxies with single-peaked HI profiles tend to follow the Faber-Jackson Relation (FJR), which links stellar velocity dispersion to mass, but with considerable scatter. This suggests these systems are likely dispersion-dominated, meaning their dynamics are primarily governed by the random motions of stars rather than ordered rotation.

The authors interpret these results as evidence that HI-selected dwarf galaxies with single-peaked profiles may share a similar dynamical state (dispersion-dominated) with massive elliptical galaxies, despite orders of magnitude difference in mass. This implies that the relationship between stellar mass and halo mass could maintain a consistent form across both low- and high-mass regimes. The large scatter in the FJR for single-peaked dwarfs may arise from turbulent gas kinematics, irregular morphologies, or ongoing interactions, indicating a diverse population within this category.

In conclusion, the study demonstrates the power of combining high-sensitivity HI blind surveys like FASHI with deep optical imaging to probe the faint end of the galaxy population. The findings offer new perspectives on the structural evolution and formation pathways of low-mass galaxies, highlighting the role of dynamical state (rotation vs. dispersion support) as a key classifier. The FASHI survey, with its unprecedented sensitivity, is poised to place crucial constraints on the lower mass limits of galaxy formation and the connections between different dwarf galaxy types.