[Paper Review] Surface photometry of BCGs and intracluster stars in Lambda-CDM

Andrew P. Cooper, Liang Gao+5 more|arXiv (Cornell University)|Jul 21, 2014

Astronomy and Astrophysical Research3 citations

TL;DR

This study uses a semi-analytic particle tagging method applied to high-resolution ΛCDM N-body simulations to model the surface photometry of brightest cluster galaxies (BCGs) and intracluster light (ICL). It finds that over 90% of BCG stellar mass originates from accreted galaxies, with a diffuse outer component formed by tidal stripping, and that 80–95% of stellar mass below μV ≈ 26.5 mag arcsec⁻² in the cluster center is associated with accreted stars in the BCG envelope.

ABSTRACT

We simulate the phase-space distribution of stellar mass in 9 massive Lambda-CDM galaxy clusters by applying the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N-body simulations (M200 = 7.5 to 33 x 10^14 Msol). The resulting surface brightness (SB) profiles of brightest cluster galaxies (BCGs) match well to observations. On average, stars formed in galaxies accreted by the BCG account for ~90 per cent of its total mass (the remainder is formed in situ). In circular BCG-centred apertures, the superposition of multiple debris clouds (each ~10 per cent of the total BCG mass) from different progenitors can result in an extensive outer diffuse component, qualitatively similar to a 'cD envelope'. These clouds typically originate from tidal stripping at z < 1 and comprise both streams and the extended envelopes of other massive galaxies in the cluster. Stars at very low SB contribute a significant fraction of the total cluster stellar mass budget: in the central 1 Mpc^2 of a z ~ 0.15 cluster imaged at SDSS-like resolution, our fiducial model predicts 80-95 per cent of stellar mass below a SB of mu_V = 26.5 mag arcsec^2 is associated with accreted stars in the envelope of the BCG. The ratio of BCG stellar mass (including this diffuse component) to total cluster stellar mass is ~30 per cent.

Motivation & Objective

• To model the surface brightness profiles of BCGs and intracluster light (ICL) in ΛCDM galaxy clusters using high-resolution simulations.
• To determine the origin and spatial distribution of stellar mass in BCGs, particularly the contribution from accreted galaxies versus in situ formation.
• To assess the role of tidal stripping and merger histories in forming the diffuse outer components of BCGs, resembling cD envelopes.
• To evaluate the consistency of semi-analytic models with observed low-surface-brightness stellar components in massive clusters.
• To quantify the fraction of total cluster stellar mass in the BCG and ICL, and test model convergence under varying resolution and feedback assumptions.

Proposed method

• Applied the semi-analytic particle tagging method of Cooper et al. to the Phoenix suite of high-resolution N-body simulations (M200 ≈ 7.5–33 × 10¹⁴ M☉).
• Tracked stellar mass from accreted galaxies by tagging particles formed in progenitor galaxies before merger.
• Simulated surface brightness profiles in circular apertures centered on BCGs to compare with observational data.
• Used a surface brightness threshold of μV ≈ 26.5 mag arcsec⁻² to isolate the accreted component of the BCG.
• Assessed numerical convergence by testing resolution effects on subhalo structure and star formation histories.
• Compared simulated surface photometry with observational data from Gonzalez et al. (2007) and Donzelli et al. (2011), and with idealized merger simulations.

Experimental results

Research questions

• RQ1 What fraction of a BCG’s stellar mass originates from accreted galaxies versus in situ star formation in the cluster environment?
• RQ2 How do tidal stripping and merger histories contribute to the formation of diffuse, low-surface-brightness components in BCGs?
• RQ3 To what extent does the observed intracluster light (ICL) in massive clusters arise from stars stripped from cluster galaxies in the ΛCDM framework?
• RQ4 How well do semi-analytic models with particle tagging reproduce observed surface brightness profiles of BCGs and ICL?
• RQ5 What is the contribution of accreted stars to the total stellar mass budget in the central Mpc² of a cluster, particularly at low surface brightness levels?

Key findings

• Over 90% of the total stellar mass in BCGs originates from stars formed in galaxies that were accreted, with the remainder formed in situ.
• The diffuse outer component of BCGs, resembling a cD envelope, is primarily formed by the superposition of multiple debris clouds from different progenitor galaxies, each contributing ≥10% of the BCG mass.
• In the central 1 Mpc² of a z ≈ 0.15 cluster, 80–95% of the stellar mass below a surface brightness of μV ≈ 26.5 mag arcsec⁻² is associated with accreted stars in the BCG envelope.
• The ratio of BCG stellar mass (including the diffuse component) to total cluster stellar mass is approximately 30%.
• A surface brightness cut at μV ≈ 26.5 mag arcsec⁻² recovers about 30% of the BCG stellar mass, with nearly all of it being accreted, and requires only a ~20% correction for light from other cluster members.
• The model shows good agreement with observations of Gonzalez et al. (2007) and Donzelli et al. (2011), but slightly overestimates the effective radius R₅₀ by 0.2 dex compared to the SerExp relation of Bernardi et al. (2012), suggesting potential overprediction of progenitor luminosities.