The Journey to Dominance: How BCGs Evolve Differently from Other Massive Galaxies

Abstract

We use the L-GALAXIES semi-analytic model to investigate the evolution of Brightest Cluster Galaxies (BCGs) found in clusters at $\rm z \sim 0$. BCGs are typically located in the central region of galaxy clusters, near the bottom of the potential well, exposing them to different environmental conditions compared to galaxies in the cluster outskirts or in the field. As a result, BCGs may follow a distinct evolutionary path and exhibit unique properties. We study the physical properties and merger histories of galaxies in 180 simulated clusters at $z \sim 0$, considering all cluster members with present-day stellar masses above $10^9 \ {\rm M_\odot}$ as the starting points for tracing their merger trees. We compare this sample of galaxies to a control sample of field galaxies and highlight their differences in evolution across cosmic time. We find that BCGs have distinct stellar mass formation histories compared to other massive galaxies from our control sample. Surprisingly, (proto)BCGs consistently become the most massive galaxy of their structure only at z $\sim$ 1.3. Despite this late dominance, (proto)BCGs are found to inhabit regions with higher galaxy and stellar mass density than the most massive galaxy in the structure throughout their entire history, indicating that their evolution is tightly linked to the environment from early times. These conditions shape a distinct evolutionary path for BCGs compared to other massive galaxies in clusters and in the field, underscoring the unique nature of BCGs.

Figures (11)

• Figure 1: Stellar mass assembly histories as a function of redshift (lower axis) and lookback time (upper axis). The figure presents plots of the assembly histories of all structures (top-left) and of different types of structures: Fornax-like (top-right), Virgo-like (bottom-left), and Coma-like (bottom-right), as defined in Section \ref{['sec: defs']}. The solid purple, dashed orange, and dot-dashed green curves in each panel denote the median assembly history of BCGs, the second most massive galaxy in the structure ($\rm 2MM$), and massive field galaxies, respectively. The black inverted triangle with error bars, cyan dot with error bars, and dotted line indicate the median and inter-quartile range of the cluster formation redshift ($z_{14}$), the median and inter-quartile range of the identity redshift ($z_{id}$), and the median half-final stellar mass assembled of BCGs ($z_{50a}$), respectively. The dashed horizontal line marks the threshold where the median BCG reaches 50% of its final stellar mass. The shaded areas, following the same color schemes, encompass the 25th and 75th percentiles over the galaxies in each subsample.
• Figure 2: Stellar mass absolute assembly histories as a function of redshift (lower axis) and lookback time (upper axis). The panels, curves and colors follow the same scheme as in Figure \ref{['fig: sm_hist']}.
• Figure 3: Stellar mass assembly rate histories. The curves and colors follow the same scheme as previous figures. Colored dotted curves are modified Gaussian fit (see Eq. \ref{['eq: mod_gauss']}). The black inverted triangle with error bars and cyan dot with error bars, indicate the median and inter-quartile range of $z_{14}$, and the median and inter-quartile range of $z_{id}$.
• Figure 4: Stellar mass formation histories, i.e., the ratio between the sum of progenitors' stellar masses at a given snapshot and the final stellar mass of the galaxy under consideration. The curves and colors follow the same scheme as in Figure \ref{['fig: sm_hist']}. The dotted vertical line denotes the median redshift when BCGs formed 50% of their final stellar mass. The black inverted triangle with error bars and cyan dot with error bars, indicate the median and inter-quartile range of $z_{14}$, and the median and inter-quartile range of $z_{id}$. The curve for BCGs exceeds unity because the total stellar mass of their progenitor galaxies is greater than the stellar mass of the BCG at $z = 0$. This occurs because satellite galaxies approaching the BCG undergo disruption, with their stellar material being deposited into the BCG’s halo.
• Figure 5: (BCG stellar mass formation histories considering also the halo stellar mass. Differently from Figure \ref{['fig: sm_form_hist']}, the ratio of the progenitors' mass to the final BCG mass does not exceed unity, indicating that these stars were indeed removed from their host galaxies and remain in the BCG's halo.