<p>If you look at the paper associated with that table (Table 2 of <a href="https://ui.adsabs.harvard.edu/abs/2016AJ....152..157L" rel="nofollow noreferrer">Lelli+2016c</a>), it describes the columns in the table that you are asking about as <em>model</em> rotation-curve <em>contributions</em>. 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 <em>observed</em> rotation velocity (<span class="math-container">$V_{\textrm{obs}}$</span>), which comes from combinations of H<span class="math-container">$\alpha$</span> and H I observations. Here is what the paper says about the computed model gas component (<span class="math-container">$V_{\textrm{gas}}$</span>):</p>
<blockquote>
<p>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 <span class="math-container">$M_{\mathrm{H I}}$</span>, where the factor 1.33
considers the contribution of helium. <span class="math-container">$V_{\textrm{gas}}$</span> is either computed using H
I surface density profiles or taken from published mass models.</p>
</blockquote>
<p>The idea is to combine these components to get the total &quot;baryonic&quot; 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.)</p>
<p>In simple terms, the total computed baryonic rotation curve would be <span class="math-container">$V_{\textrm{bar}}^{2} = V_{\textrm{gas}}^2 + V_{\textrm{disk-stars}}^{2} + V_{\textrm{bulge-stars}}^{2}$</span>.</p>
<p>(The <em>observed</em> rotational velocity of the <em>stars</em> is not being considered in this kind of study.)</p>

CC BY-SA 4.0
2023-10-16T13:44:33.647Z
2023-10-16T13:44:33.647Z
Peter Erwin
<p>I was going through the SPARC data on galaxies (<a href="http://astroweb.cwru.edu/SPARC/" rel="nofollow noreferrer">http://astroweb.cwru.edu/SPARC/</a>)
The velocity of gas in the SPARC data is in <a href="http://astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt" rel="nofollow noreferrer">http://astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt</a></p>
<p>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?</p>
<hr />
<p>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</p>

CC BY-SA 4.0
2023-10-14T11:51:01.477Z
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?
ProfRob
CC BY-SA 4.0
2023-10-14T14:28:08.173Z
The flat rotational velocity is here: http://astroweb.cwru.edu/SPARC/BTFR_Lelli2019.mrt
Angela
CC BY-SA 4.0
2023-10-14T14:28:44.967Z
Does $H_I$ data directly measure the mass of gas or velocity of gas?
Angela
CC BY-SA 4.0
2023-10-14T16:54:45.190Z
I still don't understand where you are getting the stellar rotation velocity from. Please be clear exactly what you are comparing with what.
ProfRob
CC BY-SA 4.0
2023-10-15T01:36:31.287Z
In, http://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  http://astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt
Angela
CC BY-SA 4.0
2023-10-16T13:18:50.393Z
Have you tried looking at the paper associated with that table?
Peter Erwin
<observational-astronomy><galaxy><astrophysics><galactic-dynamics>
Gas mass and velocity in galaxy
Angela
# Gas mass and velocity in galaxy

(Asked by: username_0 on 2023-10-14)

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

**Tags:** observational-astronomy, galaxy, astrophysics, galactic-dynamics

> **Comments:**
> **username_1**: 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?  
> **username_0**: The flat rotational velocity is here: http://astroweb.cwru.edu/SPARC/BTFR_Lelli2019.mrt  
> **username_0**: Does $H_I$ data directly measure the mass of gas or velocity of gas?  
> **username_1**: I still don't understand where you are getting the stellar rotation velocity from. Please be clear exactly what you are comparing with what.  
> **username_0**: In, http://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  http://astroweb.cwru.edu/SPARC/MassModels_Lelli2016c.mrt  
> **username_2**: Have you tried looking at the paper associated with that table?  

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## 1 answers

### Answer by username_2 on 2023-10-16. Score: 2 | Accepted answer

If you look at the paper associated with that table (Table 2 of [Lelli+2016c](https://ui.adsabs.harvard.edu/abs/2016AJ....152..157L)), 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.)


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