A What Is the Scale Length in SPARC Data Analysis?

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The discussion focuses on understanding key terms in the SPARC data analysis, particularly the concept of scale length in relation to stellar disks. The scale length (Rd) is defined as the radius where the brightness profile, modeled exponentially, decreases by a factor of e. The term H I refers to neutral hydrogen atoms, which are crucial for mapping the distribution of galactic gas and calculating mass. Additionally, L[3.6] is confirmed to represent solar luminosity, similar to how M[solar] denotes solar mass. Clarifying these terms is essential for interpreting the SPARC data effectively.
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im reading through the sparc data and the accompanying scientific paper

SPARC: MASS MODELS FOR 175 DISK GALAXIES WITH
SPITZER PHOTOMETRY AND ACCURATE ROTATION CURVES

right now defining terms is my biggest problem
the reading through i come across the scale length' references are pg page, c colum, l line

pg2 c2 l10
"Column (9) gives the scale length of the stellar disk (Rd)."
what is the scale length

pg2 c2 l13
"Column (11) gives the total H I mass (M H I )."
i take it that the mass in relation to hydrogen . what is the i referencing

pg2 c2 section2.1. line4
1704844625935.png
i take this to be solar luminosity as a unit
L[3.6]
 
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Scale length refers to the brightness profile when it is modelled as an exponential function (like the paper does). It is defined as the radius at which the central brightness (or whatever else is under consideration) has fallen off by a factor of e.

H I (H-one, not H-i) is a naming convention in astronomy referring to neutral hydrogen atom. Compare II = singly ionised, and III = doubly ionised (edit: obviously not applicable to hydrogen). The 21 cm hydrogen line is produced by H I atoms, letting you map neutral hydrogen distribution and calculate the mass of the (non-ionised) galactic gas.
Wikipedia has an article on this under 'Hydrogen line'.

##L_\odot## is solar luminosity, yes. Same as ##M_\odot## is solar mass.
 
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