Studies of the kinematic scaling relations, such as the Tully-Fisher relation, Faber-Jackson relations, etc, have often focused on massive galaxies. The lowest masses typically only go down to ~10^9 𝑀⊙, yet galaxies below this threshold are the most common. These galaxies are typically left out because of their defining physical properties: dwarf galaxies are less luminous than regular galaxies, so observations require longer integration times and/or larger collecting areas; they are smaller, so studying their structure requires better spatial resolution; and finally, dwarf galaxies have lower intrinsic velocity dispersions, so an unbiased measurement of their kinematics requires either high spectral resolution or very high signal to noise.
The SH𝛼DE survey was designed to fill this gap, and to deliver a sample of 69 dwarf galaxies with high spectral and spatial resolution H𝛼 observations. The survey was designed with four goals in mind: (i) to test the linearity of galaxy scaling relations over a range in mass and with sufficient spectral resolution to not be observationally limited; (ii) to measure and explain the fraction of dwarf galaxies with asymmetric kinematics; (iii) to study the dynamical effect of star formation feedback in the low-mass regime; and (iv) to study angular momentum accretion.
With SH𝛼DE, we find that there indeed is a bend in the scaling relation! Figure 2 below shows that there exists a lower limit at 𝑆_0.5 ≈ 22.4 km/s, which corresponds to a stellar mass limit of 𝑀_* ≈ 10^8.6 𝑀_⊙. Above this limit, the scaling relation has a slope of 2.58 ± 0.02 and an intercept of 5.16 ± 0.05. This lower limit originates from an apparent lower limit in the observed H𝛼 velocity dispersion at ∼20 km/s. This demonstrates a physical origin for the low-mass bend in the H𝛼 version of the log 𝑀_* - log 𝑆_0.5 scaling relation, rather than a purely observational effect!