I am currently working on selecting high gas fraction galaxies at $z=0$ in the TNG50 simulation, defining the gas fraction as:
FG = M_gas / M_stellar
However, I am facing a problem. My selection returns a surprisingly large number of galaxies with $\mathrm{GF} > 1$ compared to $\mathrm{GF} < 1$. More concerning is that some objects classified as galaxies in the selection have total masses exceeding $10^{12} \, M_\odot$, which clearly correspond to galaxy clusters rather than individual galaxies, for example a subhalo whose id is 0.
I would appreciate your help in understanding whether I am making any mistakes in the parameter selection or in the physical assumptions.
Selection Criteria Applied:
SubhaloFlag Filter
Condition: SubhaloFlag == 1
Purpose: Excludes subhalos not considered gravitationally bound or properly identified.
→ Counter: count_flagged
Minimum Stellar Mass Threshold
Condition: $M{\rm star} \geq 10^7 \, M\odot$
Purpose: Ensures the subhalo contains a significant stellar component to be considered a galaxy.
Purpose: Guarantees the system is dark matter dominated, as expected for galaxies.
→ Counter: count_dm_too_low
Existence of Stars Check
Condition: $M_{\rm star} > 0$
Purpose: Removes purely gaseous subhalos with no stellar content.
→ Counter: count_invalid
Gas Fraction Classification
After applying all the filters above:
If $\mathrm{GF} > 1$: counted as high gas fraction (count_gf_gt1)
If $\mathrm{GF} \leq 1$: counted as low gas fraction (count_gf_lt1)
If $M_{\rm gas} = 0$, classified as $\mathrm{GF} = 0$ (falls under GF < 1)
Results Obtained:
Total subhalos in the snapshot: 5,688,113
Subhalos passing all filters: 16,177
With $\mathrm{GF} > 1$: 2,849
With $\mathrm{GF} < 1$: 13,328
This results in ~17% of the selected galaxies having $\mathrm{GF} > 1$, which seems unexpectedly high. Additionally, the presence of extremely massive objects (> $10^{12} \, M_\odot$) labeled as galaxies suggests a possible problem in the selection criteria.
Question:
Is there a mistake in the way I’m filtering the subhalos, particularly in ensuring that I am not inadvertently including central halos of clusters or intracluster gas? Or could the gas fraction distribution at z=0 in TNG50 naturally include such a high proportion of gas-rich systems?
Code Snippet:
(Note: the lines with missing keys are just illustrative; I can send the full working code if helpful.)
I would sincerely appreciate any guidance on whether the filters I’m applying are correct for isolating galaxies (excluding clusters) and for calculating the gas fraction in a physically meaningful way.
Thank you very much in advance for your help.
Best regards, Matheus Agenor
Dylan Nelson
23 May
I think a more typical definition of galaxy gas fraction would be M_gas / (M_gas + M_stellar).
if you define it just as the ratio of gas to stellar mass, it doesn't surprise me that many galaxies exceed unity.
Till Sawala
1 Jun
In addition to Dylan's comment, I don't see anything in your definition that would prevent you from including galaxy clusters? If you want to be sure that you select what would be considered a galaxy rather than a galaxy cluster, perhaps include an upper limit on the mass.
Dear TNG50 team,
I am currently working on selecting high gas fraction galaxies at $z=0$ in the TNG50 simulation, defining the gas fraction as:
FG = M_gas / M_stellar
However, I am facing a problem. My selection returns a surprisingly large number of galaxies with $\mathrm{GF} > 1$ compared to $\mathrm{GF} < 1$. More concerning is that some objects classified as galaxies in the selection have total masses exceeding $10^{12} \, M_\odot$, which clearly correspond to galaxy clusters rather than individual galaxies, for example a subhalo whose id is 0.
I would appreciate your help in understanding whether I am making any mistakes in the parameter selection or in the physical assumptions.
Selection Criteria Applied:
SubhaloFlag Filter
SubhaloFlag == 1
count_flagged
Minimum Stellar Mass Threshold
count_star_too_low
Dark Matter Dominance Check
count_dm_too_low
Existence of Stars Check
count_invalid
Gas Fraction Classification
count_gf_gt1
)count_gf_lt1
)Results Obtained:
Subhalos passing all filters: 16,177
This results in ~17% of the selected galaxies having $\mathrm{GF} > 1$, which seems unexpectedly high. Additionally, the presence of extremely massive objects (> $10^{12} \, M_\odot$) labeled as galaxies suggests a possible problem in the selection criteria.
Question:
Is there a mistake in the way I’m filtering the subhalos, particularly in ensuring that I am not inadvertently including central halos of clusters or intracluster gas? Or could the gas fraction distribution at z=0 in TNG50 naturally include such a high proportion of gas-rich systems?
Code Snippet:
(Note: the lines with missing keys are just illustrative; I can send the full working code if helpful.)
I would sincerely appreciate any guidance on whether the filters I’m applying are correct for isolating galaxies (excluding clusters) and for calculating the gas fraction in a physically meaningful way.
Thank you very much in advance for your help.
Best regards,
Matheus Agenor
I think a more typical definition of galaxy gas fraction would be
M_gas / (M_gas + M_stellar)
.if you define it just as the ratio of gas to stellar mass, it doesn't surprise me that many galaxies exceed unity.
In addition to Dylan's comment, I don't see anything in your definition that would prevent you from including galaxy clusters? If you want to be sure that you select what would be considered a galaxy rather than a galaxy cluster, perhaps include an upper limit on the mass.