# Gas mass and velocity in galaxy

I was going through the SPARC data on galaxies (http://astroweb.cwru.edu/SPARC/) The velocity of gas in the SPARC data is in http://astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt

The gas velocity seems to be different from the observed rotational velocity of the stars. Does it mean that the gas and the stars are rotating at different velocities around the galaxy's center? What does HI data actually measure? The velocity of atomic hydrogen?

In, astroweb.cwru.edu/SPARC/BTFR_Lelli2019.mrt, column 6 gives Vf and column 12 gives Vmax. This I am comparing with column 6 (Vgas) in astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt

• Are you talking about differences in the gas velocity, disk velocity and bulge velocity in that catalogue? There is no mention of the rotation velocities of stars? Oct 14, 2023 at 11:51
• The flat rotational velocity is here: astroweb.cwru.edu/SPARC/BTFR_Lelli2019.mrt Oct 14, 2023 at 14:28
• I still don't understand where you are getting the stellar rotation velocity from. Please be clear exactly what you are comparing with what. Oct 14, 2023 at 16:54
• In, astroweb.cwru.edu/SPARC/BTFR_Lelli2019.mrt, column 6 gives Vf and column 12 gives Vmax. This I am comparing with column 6 (Vgas) in astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt Oct 15, 2023 at 1:36
• Have you tried looking at the paper associated with that table? Oct 16, 2023 at 13:18

If you look at the paper associated with that table (Table 2 of Lelli+2016c), it describes the columns in the table that you are asking about as model rotation-curve contributions. In other words, for each possible mass component in the galaxy -- neutral gas, stars in the disk, stars in the bulge -- they compute the expected rotation curve if that component was the only mass present. These are to be compared with the observed rotation velocity ($$V_{\textrm{obs}}$$), which comes from combinations of H$$\alpha$$ and H I observations. Here is what the paper says about the computed model gas component ($$V_{\textrm{gas}}$$):

The gas contribution is calculated using the formula from Casertano (1983), which solves the Poisson equation for a disk with finite thickness and arbitrary density distribution. We assume a thin disk with a total mass of 1.33 $$M_{\mathrm{H I}}$$, where the factor 1.33 considers the contribution of helium. $$V_{\textrm{gas}}$$ is either computed using H I surface density profiles or taken from published mass models.

The idea is to combine these components to get the total "baryonic" model rotation curve (how fast the observed gas should be rotating if all the mass consisted of just the neutral gas and the stars in the disk and bulge), to see how much of a discrepancy there is at different radii. (Said discrepancy is presumably due to the presence of dark matter, or else due to gravity behaving differently than we normally assume.)

In simple terms, the total computed baryonic rotation curve would be $$V_{\textrm{bar}}^{2} = V_{\textrm{gas}}^2 + V_{\textrm{disk-stars}}^{2} + V_{\textrm{bulge-stars}}^{2}$$.

(The observed rotational velocity of the stars is not being considered in this kind of study.)