Hi,
How do I calculate the half-light radius of a galaxy? I see that there is a catalog in Sec-3e in the documentation but it specifies half-light radii in individual photometric bands, which isn't what I need. Is there a way to calculate it from existing data?
Dylan Nelson
4 Jul '23
You could calculate it in 3D from the stellar particles, or you could calculate it in 2D from an image (that you make) of stellar light.
In either case, you would just follow the definition: it is the radius enclosing half of the "total" light.
The supplementary catalog "(e) Stellar Projected Sizes" applies this definition to the 8 bands (that are already available in the snapshot, dust-free) for five different projection directions per subhalo.
Ambica Govind
8 Jul '23
In any case, will the particle data consist of total luminosity and not separate bands(or) are the bands used exhaustive for me to add them up?
Dylan Nelson
8 Jul '23
You can see the docs for the PartType4/GFM_StellarPhotometrics dataset, which is the only pre-computed data of this type in the snapshot.
If you want other bands or other types of luminosities, you could compute them yourself using a code like FSPS or other public tools (based on the star particle mass, age, and metallicity).
Ambica Govind
11 Jul '23
Thanks, Dylan.
Ambica Govind
14 Jul '23
Hello, I have another question related to the same.
I need to calculate the half light radius according to the definition given in Appendix A of this paper. So circularised radii(as in 5e) won't work.
For this I first need to identify the major axis of the galaxy. Is there a way to do this, apart from visually(because I need to run the analysis for some 1000 galaxies). Thank you.
Dylan Nelson
17 Jul '23
If you are following the methodology of that paper, I suggest to identify the major axis in the same way as they do there.
Hi,
How do I calculate the half-light radius of a galaxy? I see that there is a catalog in Sec-3e in the documentation but it specifies half-light radii in individual photometric bands, which isn't what I need. Is there a way to calculate it from existing data?
You could calculate it in 3D from the stellar particles, or you could calculate it in 2D from an image (that you make) of stellar light.
In either case, you would just follow the definition: it is the radius enclosing half of the "total" light.
The supplementary catalog "(e) Stellar Projected Sizes" applies this definition to the 8 bands (that are already available in the snapshot, dust-free) for five different projection directions per subhalo.
In any case, will the particle data consist of total luminosity and not separate bands(or) are the bands used exhaustive for me to add them up?
You can see the docs for the
PartType4/GFM_StellarPhotometricsdataset, which is the only pre-computed data of this type in the snapshot.If you want other bands or other types of luminosities, you could compute them yourself using a code like FSPS or other public tools (based on the star particle mass, age, and metallicity).
Thanks, Dylan.
Hello, I have another question related to the same.
I need to calculate the half light radius according to the definition given in Appendix A of this paper. So circularised radii(as in 5e) won't work.
For this I first need to identify the major axis of the galaxy. Is there a way to do this, apart from visually(because I need to run the analysis for some 1000 galaxies). Thank you.
If you are following the methodology of that paper, I suggest to identify the major axis in the same way as they do there.