# (Galactic) disc scalelength

• PCB
In summary, disc scalelength refers to the scale length of a disc galaxy, which determines the distribution of stars and can be used to derive its mass distribution. However, the term itself is not well defined and varies between researchers, making it difficult to find a detailed definition.

#### PCB

Can anyone please give m the definition of disc scalelength?

PCB said:
... disc scalelength

I assumed you typo'ed and there should be a gap between scale and length

come back if you have any specific question in what you read

Dave

thanks, I have googled the term and I have found numerous uses of the term, but no definition. If you can help I would very much appreciate it.

from ... http://arxiv.org/pdf/1004.1507.pdf -

Scale length of disc galaxies

comes -----

The scale length determines how the stars are distributed throughout a disk, and can be used to derive its mass distribution,
assuming a specific M/L ratio. Ultimately, this mass distribution is the primary constraint for determining the formation scenariothere are others

Dave

Thanks, that is helpful, but I was looking for a detailed definition so I could understand what, for instance, 2.5 scale lengths meant in a particular situation.

from what I read in a couple of papers whilst doing that searching for you
is that you are not going to get a better definition. It is something that is a little vague and varies between researcherssorry
that's about the best I can do or find

Dave

Thanks very much, your efforts are much appreciated. I cannot help but feel that a term used as much as it is in many formal papers is not well defined, and/or that definition is not easy to find--in this information age.

## 1. What is the (Galactic) disc scalelength?

The (Galactic) disc scalelength refers to the distance at which the surface density of stars in a galaxy's disc decreases by a factor of e (the base of natural logarithms). It is a measure of the size of a galaxy's disc in relation to the distribution of stars within it.

## 2. How is the (Galactic) disc scalelength measured?

The (Galactic) disc scalelength is typically measured by fitting an exponential profile to the surface brightness distribution of stars in a galaxy's disc. This involves plotting the surface brightness as a function of radius and finding the point at which it decreases by a factor of e.

## 3. What does the (Galactic) disc scalelength tell us about a galaxy?

The (Galactic) disc scalelength is a useful parameter for understanding the structure and dynamics of a galaxy. It provides insights into the distribution of stars and the relationship between a galaxy's disc and its other components, such as the bulge and halo.

## 4. How does the (Galactic) disc scalelength vary among different types of galaxies?

The (Galactic) disc scalelength can vary significantly among different types of galaxies. Spiral galaxies, which have well-defined disc structures, tend to have larger scalelengths compared to elliptical galaxies, which have less defined or absent discs. Additionally, the disc scalelength can also vary within a single galaxy, depending on the location in the disc.

## 5. Can the (Galactic) disc scalelength change over time?

Yes, the (Galactic) disc scalelength can change over time due to various factors, such as interactions with other galaxies, mergers, and internal processes like star formation and gas accretion. Changes in the disc scalelength can provide valuable information about a galaxy's evolutionary history and the physical processes at play within it.