The photometrics radius

Yunhao Gao
  • 15 Nov '22


Recently I chose data of some galaxies in TNG50-1 and drew a picture to show the relation between the stellar masses bound to subhalos and the stellar masses restricted within the "SubhaloStellarPhotometricsRad". As shown in the figure below, I defined

M_* --- type 4 in the field "SubhaloMassType",
Mp --- the field "SubhaloStellarPhotometricsMassInRad",
{20} --- the field "SubhaloStellarPhotometricsMassInRad".

屏幕截图 2022-11-15 143833.png

And in this figure, I found that some of those galaxies were above the black line (the "y = x" relation), which indicates their stellar masses within the photometrics radius is greater than the total stellar masses bound in their subhalos. And it seems that the photometrics radius is not very correlated with the total stellar masses. I wonder what causes this situation. Does this mean that in TNG there are several galaxies including stars that are not gravitationally bound to subhalos of their galaxies? Is there anything I misunderstand?

Thank you for your time!

Dylan Nelson
  • 15 Nov '22


I would suggest to double-check your code, I cannot find any subhalos with SubhaloStellarPhotometricsMassInRad > SubhaloMassType[4] in TNG50-1 at z=0.

In [2]: sim = simulation("tng50-1", redshift=0.0)

In [3]: x = sim.subhalos('SubhaloMassType')

In [4]: y = sim.subhalos('SubhaloStellarPhotometricsMassInRad')

In [5]: x.shape
Out[5]: (5688113, 6)

In [6]: x = x[:,4]

In [7]: x.shape
Out[7]: (5688113,)

In [8]: y.shape
Out[8]: (5688113,)

In [9]: ww = np.where(y > x)

In [10]: len(ww[0])
Out[10]: 0
Yunhao Gao
  • 1
  • 15 Nov '22

Thanks Dylan! I must apologize that there are indeed some bugs in my code. And now I've got the correct relation between the two quantities.

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