What is meant by "tracer"?

Yun Wang
• 1
• 19 Jun '20

I find that the number of tracers is $2\times 1820^3$ in TNG100-1. Does it mean the number of galaxies here ?
And I also find that the number of gas particles is $1820^3$, the number of DM particles is $1820^3$. So the number of
total particles is $4\times 1820^3$, is that right?

Dylan Nelson
• 1
• 22 Jun '20

Hi Yun,

"Tracers" are PartType3, they are just another type of particle. We usually don't include them in the count, so the total number of particles would be 2x1820^3 or, yes, if you include them, then 4x1820^3.

The number of galaxies is much smaller, ~50,000 for TNG100 and ~400,000 for TNG300, for example.

Yun Wang
• 22 Jun '20

Are "tracers" used to tracing the evolution of the fluid with time ?

Dylan Nelson
• 22 Jun '20

That's right, how they work is described in Genel+2013, and the first time they were used in an example application was Nelson+2013.

Yun Wang
• 1
• 22 Jun '20

Thank you so much, Dylan.

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