Clarification on the definition of j_z / j_circ in your TNG50 MW/M31-like galaxy catalogue
AMIT MONDAL
24 Oct
In my recent work, I use the TNG50 Milky Way/M31-like galaxy catalogue and aim to isolate the stellar halo component of each galaxy. I found that applying a cut on the circularity parameter can effectively select the spheroidal component of the galaxy, and by further applying a radial cut to exclude the bulge, one can identify the stellar halo stars.
However, I would like to clarify the following points:
What is the appropriate threshold for separating the spheroid — is it ϵ < 0.5, ϵ < 0.65, or |ϵ| < 0.5 (or 0.65)?
In the definition of the circularity parameter, ϵ = j_z / j_circ, does j_z correspond to the z-component of the angular momentum in Cartesian coordinates (i.e., j_z = x v_y − y v_x), or is it computed in cylindrical coordinates?
Is it physically acceptable to perform this analysis using the rotated coordinates and velocities provided in the TNG50 MW/M31-like galaxy catalogue?
Dylan Nelson
1 Nov
I don't think there is any single choice for this threshold, you will find many adopted values in the literature. It would depend on application.
You say "In the definition", i.e. it sounds like you are following some paper or previous work. I would suggest you find any necessary details there.
In my recent work, I use the TNG50 Milky Way/M31-like galaxy catalogue and aim to isolate the stellar halo component of each galaxy. I found that applying a cut on the circularity parameter can effectively select the spheroidal component of the galaxy, and by further applying a radial cut to exclude the bulge, one can identify the stellar halo stars.
However, I would like to clarify the following points:
What is the appropriate threshold for separating the spheroid — is it ϵ < 0.5, ϵ < 0.65, or |ϵ| < 0.5 (or 0.65)?
In the definition of the circularity parameter, ϵ = j_z / j_circ, does j_z correspond to the z-component of the angular momentum in Cartesian coordinates (i.e., j_z = x v_y − y v_x), or is it computed in cylindrical coordinates?
Is it physically acceptable to perform this analysis using the rotated coordinates and velocities provided in the TNG50 MW/M31-like galaxy catalogue?
I don't think there is any single choice for this threshold, you will find many adopted values in the literature. It would depend on application.
You say "In the definition", i.e. it sounds like you are following some paper or previous work. I would suggest you find any necessary details there.