Galaxies: IntraCluster Light

A substantial fraction of stars in clusters are not gravitationally bound to any particular galaxy. These stars constitute the so-called intracluster light (ICL). The ICL is distributed around the central galaxy of the cluster and extends to several hundred kpc away from the cluster center (e.g., Murante et al. 2004; Zibetti et al. 2005). This diffuse light is thought to form primarily by the tidal stripping of stars from galaxies that interact and merge during the hierarchical accretion history of the cluster (e.g., Gregg & West. 1998; Rudick et al. 2006; Conroy et al. 2007; Contini et al. 2014). Therefore, the characterization of the ICL provides a direct way of determining the assembly mechanisms occurring inside galaxy clusters. In this sense, the ICL is the signature of how violent the assembly of the cluster has been through its cosmic history. For that reason, it is absolutely key to determine how and when the ICL formed. However, the identification of this light observationally remains difficult and uncertain. Indeed, the typical surface brightness of the ICL is μV gsim26.5 mag arcsec−2 (e.g., Mihos et al. 2005; Zibetti et al. 2005; Rudick et al. 2006) and it is contaminated by foreground and background galaxies. Moreover, it is difficult to dissociate between the ICL and the brightest central galaxy surface brightness profile (e.g., Gonzalez et al. 2005; Krick et al. 2007).

VirgoICL

ICL of Virgo (Mihos et al. 2005)

 

Multiwavelength deep observations are a key tool to understand the origin of the ICL. For this reason, we take advantage of the Hubble Frontier Fields (HFF) survey to investigate the properties of the stellar populations of the ICL of its six massive intermediate redshift (0.3 < z < 0.6) clusters. We carry on this analysis down to a radial distance of 120 kpc from the brightest cluster galaxy. We found that the average metallicity of the ICL is [Fe/H]ICL ∼ − 0.5, compatible with the value of the outskirts of the MilkyWay. The mean stellar ages of the ICL are between 2 and 6 Gyr younger than the most massive galaxies of the clusters. Those results suggest that the ICL of these massive (> 10^15 Msol) clusters is formed by the stripping of MW-like objects that have been accreted at z< 1, in agreement with current simulations.We do not find any significant increase in the fraction of light of the ICL with cosmic time, although the redshift range explored is narrow to derive any strong conclusion. When exploring the slope of the stellar mass density profile, we found that the ICL of the HFF clusters follows the shape of their underlying dark matter haloes, in agreement with the idea that the ICL is the result of the stripping of galaxies at recent times.

image_profm0416

Gradients of age and metallicity as a function of radial distance to the BGC(s) of the clusters. Left-hand panels show the image of the cluster in the F160W filter and overplotted are the different spatial regions in which the SEDs are measured. The central and right-hand panels are the age and metallicity radial gradients derived from the fitting to the SEDs. We also marked in orange the region of the ICL (R > 50 kpc). The colours of the spatial regions in the left-hand panels correspond to the colours of the circles in the age and metallicity gradients.

 

fraction_light_icl_mu

Fraction of light in the V band in the ICL component as a function of redshift. Red filled circles correspond to a slice in surface brightness in the V band from 26 to 27 mag arcsec2. Blue filled circles correspond to all the light fainter than μV = 26 mag arcsec2 while the orange squares correspond to the fraction of ICL light for R < R500 kpc. The green polygon are the fractions derived in Burke et al. (2015), for 13 clusters from CLASH between μB = 25 and 26 mag arcsec^2. The black line is the prediction of Rudick et al. (2011) for the fraction of ICL measured with μV > 26 mag arcsec^2.

density_mass

Slope of the stellar mass density profile for R > 50 kpc versus
the mass of the halo (M200). Blue filled circles are our measured slopes for the HFF clusters. Black and grey squares correspond to the IllustrisTNG simulations taken from Pillepich et al. (2017b), for a volume of 100^and 300^3 Mpc^3. They measured the stellar mass density profiles between 30 kpc to 2Rhm.

 

 

 

 

 

 

 

 

 

 

 

 

 

Artist's (?) impression of the formation of the ICL

Artist’s (?) impression of the formation of the ICL

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