Clocking the formation of today’s largest galaxies: wide field integral spectroscopy of brightest cluster galaxies and their surroundings

Abstract

The formation and evolution of local brightest cluster galaxies (BCGs) is investigated by determining the stellar populations and dynamics from the galaxy core, though the outskirts and into the intracluster light (ICL). Integral spectroscopy of 23 BCGs observed out to 4 r e is collected and high signal-to-noise regions are identified. Stellar population synthesis codes are used to determine the age, metallicity, velocity, and velocity dispersion of stars within each region. The intracluster light (ICL) spectra are best modeled with populations that are younger and less metal-rich than those of the BCG cores. The average BCG core age of the sample is 13.3 ± 2.8 Gyr and the average metallicity is [Fe/H] = 0.30 ± 0.09, whereas for the ICL the average age is 9.2 ± 3.5 Gyr and the average metallicity is [Fe/H] = 0.18 ± 0.16. The velocity dispersion profile is seen to be rising or flat in most of the sample (17/23), and those with rising values reach the value of the host cluster's velocity dispersion in several cases. The most extended BCGs are closest to the peak of the cluster's X-ray luminosity. The results are consistent with the idea that the BCG cores and inner regions formed quickly and long ago, with the outer regions and ICL forming more recently, and continuing to assemble through minor merging. Any recent star formation in the BCGs is a minor component, and is associated with the cluster cool core status.

Mapping accreted stars in early-type galaxies across the mass–size plane

“…This is also found to be the case in Oyarzún et al (2019) in which the authors find a flattening in the metallicity profile of z<0.15 early type galaxies beyond a radius of 1.5r e , and conclude the most reasonable explanation of this is stellar accretion to the galaxy outskirts. This result is also seen for samples of brightest cluster galaxies (Edwards et al 2020) who likewise find signatures of the two-phase scenario in profiles of kinematics and metallicity.…”

On the Origin and Evolution of the Intra-Cluster Light: A Brief Review of the Most Recent Developments

“…Similarly to the studies quoted above, they found negative color gradients with increasing distance from the BCG, and an average stellar population age of the ICL of ∼2 Gyr probably from stripping of dwarf/intermediate (log M * < 10) galaxies. Edwards et al [79] studied a sample of 23 local BCG + ICL systems and determined their stellar population age and metallicity. They found that the ICL is best modeled with younger and less metal-rich stars than those in the BCGs and found evident gradients in both age and metallicity from the BCG to the ICL dominated regions.…”

“…By looking at these two properties, it is possible to discern which, among the above quoted mechanisms, is the main responsible mechanism for the formation of the ICL. In the last few years, many authors focused on the age, color and metallicity gradients of BCG + ICL systems [10,16,23,59,62,63,[70][71][72][73][74][75][76][77][78][79][80] and in particular on their shape. It is reasonable to argue that, if mergers are the main responsible mechanisms, we would expect a flat gradient, i.e., no gradient, simply because mergers would tend to mix up the two components (see discussion in [6,77]).…”

Physical properties of brightest cluster galaxies up to redshift 1.80 based on HST data

“…This is an indication of an inside-out growth of BCGs: the inner component forms first and then stops growing while the outer component develops. Edwards et al (2019) gave more evidence to justify this inside-out growth of BCGs by showing that the stars in the ICL are younger and less metal rich than those in the cores of the BCGs. They also showed that the most extended BCGs tend to be close to the X-ray center.…”