Intracluster Stars (ICS)

This page describes the intracluster (or intragroup) stellar component: the diffuse stellar light that fills the host halo outside any single galaxy. SAGE26 tracks ICS as a separate reservoir on every central, with optional assembly-history bookkeeping.

Sources: src/model_infall.c, src/model_mergers.c, src/core_allvars.h (GALAXY struct).

Fields

ICS lives on every galaxy but is only ever non-zero on centrals – at the start of each snapshot any satellite ICS is transferred to the central (see below).

Field	Units	Meaning
`ICS`	10^10 M_sun/h	Current intracluster stellar mass on this central.
`MetalsICS`	10^10 M_sun/h	Metal mass in the ICS reservoir.
`ICS_disrupt`	10^10 M_sun/h	Cumulative stellar mass disrupted into ICS via `disrupt_satellite_to_ICS()`. Only populated when `TrackICSAssembly = 1`.
`ICS_accrete`	10^10 M_sun/h	Cumulative ICS mass inherited from satellites that already carried their own ICS (i.e. former group centrals that fell in). Only populated when `TrackICSAssembly = 1`.
`ICS_sum_mt`	10^10 M_sun/h * code time	Mass-weighted accumulator: sum over all ICS deposition events of `delta_M * t_deposit`. Used to derive the mean assembly lookback as `ICS_sum_mt / (ICS_disrupt + ICS_accrete)`.

The split between ICS_disrupt and ICS_accrete lets you decompose the assembly history into “stars stripped here vs stars that arrived already stripped from a previous host” without needing a full SFH array on the ICS reservoir.

How ICS forms

There are three pathways:

1. Per-snapshot inheritance (always)

infall_recipe() runs once at the start of each snapshot. As it walks the FoF galaxy list, it:

Sums every galaxy’s ICS and MetalsICS into a per-FoF total.
Zeros each satellite’s ICS and MetalsICS.

Assigns the total to the central:

galaxies[centralgal].ICS        = tot_ICS;
galaxies[centralgal].MetalsICS  = tot_ICSMetals;

This makes the FoF central own all of the group’s ICS by construction – which is what you want for plotting (no need to integrate over satellites when measuring a central’s ICS).

If TrackICSAssembly = 1 and an incoming satellite carried ICS, the amount is recorded in the central’s ICS_accrete accumulator and the satellite’s ICS_sum_mt is added to the central’s. This preserves the deposition history of stars that were stripped in an earlier host before this halo absorbed them.

2. Satellite disruption

When disrupt_satellite_to_ICS() fires (see Mergers and disruption), some fraction of the satellite’s stellar mass is added to the central’s ICS:

new_ICS_from_stripping = f_ICS * StellarMass_sat

where f_ICS is set by DynamicDisruptionSplit:

Mode 0: fixed FractionDisruptedToICS.
Mode 1: mass-ratio split f_ICS = 1 - (infallMvir_sat / Mhost)^DisruptionSplitAlpha.
Mode 2: as mode 1, with alpha_eff = DisruptionSplitAlpha * DisruptionSplitCref / c_sat so concentrated satellites resist stripping and deposit more onto the BCG.

The remaining 1 - f_ICS is added to the central’s StellarMass, BulgeMass, and MergerBulgeMass (BCG growth).

When TrackICSAssembly = 1, this event contributes:

ICS_disrupt += new_ICS_from_stripping
ICS_sum_mt += new_ICS_from_stripping * time (with time in code units, the substep midpoint)

3. Satellite merger transfer

When a satellite is merged (rather than disrupted) by deal_with_galaxy_merger() -> add_galaxies_together(), any ICS the satellite already carried is added to the central:

galaxies[t].ICS        += galaxies[p].ICS;
galaxies[t].MetalsICS  += galaxies[p].MetalsICS;

This is bookkeeping rather than new ICS formation – the stars existed already; they just changed hosts.

How to compute the mean ICS assembly time

ICS_sum_mt is a mass-weighted sum: every time delta_M of stars is deposited at code time t, the accumulator gains delta_M * t. The mean deposition time is therefore:

<t_assembly> = ICS_sum_mt / (ICS_disrupt + ICS_accrete)

To convert to lookback time, subtract from the host’s current snapshot age. SAGE26 does not store this derived quantity directly – compute it in post-processing from the three tracked fields.

The denominator is the cumulative deposited mass, not the current ICS. The two can differ if stars merge into the BCG over time (consumed from the ICS reservoir but still counted in the assembly totals).

ICS in the baryon budget

ICS participates in the baryon accounting in infall_recipe():

infalling_mass = reionization_modifier * BaryonFrac * Mvir
               - (StellarMass + ColdGas + HotGas + EjectedMass
                  + BlackHoleMass + ICS + CGMgas)

So ICS stars count toward the halo’s baryon budget for infall purposes – infalling gas is reduced by however much mass is already in the ICS reservoir, as it should be.

ICS does not participate in cooling, star formation, or feedback – the stars are passive once deposited. No SFR is recorded for ICS, no SFH array is maintained, no metal production fires. If you need stellar ages for ICS, derive them in post-processing from the assembly history.

Type-0 vs satellite ICS

Strictly, only Type 0 (centrals) hold ICS. Satellites entering a halo that themselves were former centrals can carry ICS, but at every snapshot the FoF central absorbs all of it (see Pathway 1 above). So in any output snapshot you should typically see:

Type == 0: ICS >= 0 (may be nonzero)
Type == 1 or 2: ICS == 0 (transferred to the central in the same snapshot it entered)

If you find a satellite with nonzero ICS in the output, it’s the result of the satellite having just become a satellite this snapshot and the data being written before the next infall pass – not a bug, but worth knowing when computing population statistics.

Switches and parameters

Parameter	Effect
`TrackICSAssembly`	0 disables `ICS_disrupt` / `ICS_accrete` / `ICS_sum_mt` tracking; the `ICS` field itself is always tracked.
`DynamicDisruptionSplit`	Controls the disruption split between ICS and BCG (see Mergers and disruption).
`FractionDisruptedToICS`	Fixed split fraction (mode 0) or fallback.
`DisruptionSplitAlpha`	Exponent in the mass-ratio split (modes 1 and 2).
`DisruptionSplitCref`	Reference concentration for mode 2.

See parameters.md for full descriptions and defaults.

References

Murante et al. (2007), MNRAS 377, 2 – ICS as a distinct component growing primarily through satellite disruption.
Conroy, Wechsler & Kravtsov (2007), ApJ 668, 826 – BCG-vs-ICS partition during disruption.
Pillepich et al. (2018), MNRAS 475, 648 – mass-ratio dependence of the disruption split.
Contini et al. (2014), MNRAS 437, 3787 – semi-analytic ICS treatment underlying the SAGE26 implementation.