### Almost all particles in halo have net negative cooling rate

Justo Antonio Gonzalez Villalba
• 1
• 27 May '22

Hello,

I have seen that TNG300-1 halos have almost all gas particles with net negative cooling rates (GFM_CoolingRate), indicating heating. For example halo 100 has only 52 particles with positive cooling rate and 2769386 with negative, therefore heating.

I was expecting that many gas particles would have a net positive cool rate, consequence of X-Ray emission, or does GFM_CoolingRate not capture X-Ray emission?

Additionally: To compute the total energy loss/gain for the cell, it is necessary to multiply by the cell volume (Mass/Density) or by the inverse of nH number density?

Thanks a lot for your help!

Dylan Nelson
• 27 May '22

Hello,

I imagine that massive halos at z=0 have little gas with net cooling. The situation might be quite different at high redshift, and/or for lower mass halos.

As to what physical processes are actually included in this value, I reference you to Section 2.4 of Vogelsberger+13. X-ray lines, as well as bremsstrahlung, would be included.

For your reference, I attach some old code to compute a "cooling time", which should make more explicit the units and how you would multiply GFM_CoolingRate by density factors:

``````    def coolingTimeGyr(self, code_dens, code_gfmcoolrate, code_u):
""" Calculate a cooling time in Gyr from three code units inputs (i.e. snapshot values) of Density, GFM_CoolingRate, InternalEnergy. """
dens_cgs = self.codeDensToPhys(code_dens, cgs=True) # g/cm^3
ratefact = self.hydrogen_massfrac**2 / self.mass_proton**2 * dens_cgs # 1/(g*cm^3)
coolrate = code_gfmcoolrate * ratefact # erg cm^3/s * (1/g/cm^3) = erg/s/g (i.e. specific rate)
u_cgs_spec = code_u * self.UnitVelocity_in_cm_per_s**2 # i.e. (km/s)^2 to (cm/s)^2, so specific erg/g
t_cool = u_cgs_spec / (-1.0*coolrate) / self.s_in_Gyr

# if lambda_net is positive set t_cool=nan (i.e. actual net heating, perhaps from the background)
w = np.where(code_gfmcoolrate >= 0.0)
t_cool[w] = np.nan

return t_cool.astype('float32')
``````
Justo Antonio Gonzalez Villalba
• 1
• 29 May '22

Thank you for the reference and code. Indeed X-ray emission is included, and the red-shift is z=0 (last snapshot 99)

In reality it seems like the net heating/cooling is actually almost 0, since the average GFM_CoolingRate is of order 1.51e-23 ergcm^3/s and average hydrogen number density is 1.66e-3 1/cm^3, at the end summing up all particles results in 2769386 1.51e-23*1.66e-3 = 6.95e-20 erg/s which is almost 0, compared with the typical X-Ray luminosity of 1e44 erg/s.

So there seems to be a perfect cooling/heating balance globally in the cluster, is this a possible scenario or there is something wrong with these numbers?

For the record, it seems all black holes are in low accretion mode, with BH_MdotBondi/BH_MdotEddington is in between 4.45e-11 and 1.83e-02

Dylan Nelson
• 30 May '22

This is the net cooling rate, and per gas cell. If you separately compute the x-ray emission (e.g. just with the simple bremsstrahlung formula), and integrate over all the gas cells in a cluster, you will see the value is comparable to the erg/s you quote.

Justo Antonio Gonzalez Villalba
• 30 May '22

If I calculate the bolometric cooling rate as [nH**2]x[GFM_CoolingRate ]x[CellVolume] then I obtain -1.87e44 erg/s which is indeed comparable to typical bolometric X-Ray luminosity. However it all looks very smooth, not clearly associated with AGN feedback (see pic). What would be the main heating process responsible of such strong but smooth heating?

Dylan Nelson
• 30 May '22

Do I understand correctly that the total net cooling rate is negative? This indicates net cooling (whereas positive would indicate net heating).

Justo Antonio Gonzalez Villalba
• 30 May '22

Yes the total value of [nH**2]x[GFM_CoolingRate ]x[CellVolume] sum over all gas particles is negative, but I thought that negative cooling rate indicated heating. If negative cooling rate indicates actual cooling then it all makes sense, as this would just be a consequence of the smooth X-Ray luminosity.

Dylan Nelson
• 30 May '22

Yes apologies, if GFM_CoolingRate (the net rate) is positive, this indicates net heating.

If it is negative, this indicates net cooling.

Justo Antonio Gonzalez Villalba
• 30 May '22

Thanks a lot for the support and clarifications!

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