Tracing the evolution of brightest galaxies and diffuse light in galaxy groups

TL;DR

This work uses 100 dissipationless N-body simulations to isolate purely gravitational evolution in pre-virialised, low-mass galaxy groups and to trace the co-evolution of the brightest group galaxy (BGG) and diffuse intragroup light (IGL). By classifying groups into single-BGG, double-BGG, and non-BGG systems and tracking IGL formation and top-ranked galaxy growth from z=1.5 to 0, the paper assesses how magnitude-gap indicators correlate with dynamical age. Key findings show two-phase IGL growth with a median final IGL fraction around 11%, distinct progenitor populations for IGL and m1, and strong, class-wide correlations of gaps such as ΔM4-1 and ΔM5-1 with dynamical age, while gaps involving the brightest satellites better reflect satellite assembly histories. The results provide a controlled framework to interpret IGL and central-galaxy growth in observations and highlight the nuanced role of satellite accretion histories in shaping dynamical state indicators in forming galaxy groups.

Abstract

We present a suite of 100 cosmologically motivated, controlled N-body simulations designed to advance the understanding of the role of purely gravitational dynamics in the early formation of low-mass galaxy groups (~ 1-5 x 10^13 M_sun). In this work, we investigate the temporal evolution of key indicators of dynamical relaxation, with particular emphasis on the secular growth of the diffuse intragroup light (IGL), the four major group galaxies, and the mass distributions of their progenitors. We also assess the diagnostic power of several magnitude gaps between top-ranked galaxies as proxies for dynamical age. As in our previous study, we compare outcomes from three group classes defined by the number of brightest group galaxies (BGGs) present at the end of the simulations. The early assembly of galaxy groups is consistent with a stochastic Poisson process at an approximately constant merger rate. Various dynamical diagnostics - including galaxy pairwise separations, velocity dispersions, and the offset of the first-ranked galaxy from the group barycentre - indicate that single-BGG groups evolve more rapidly towards virialisation than double- and especially non-BGG systems. We further find that first-ranked group members and the IGL, follow distinct growth histories, with the IGL assembled from a more numerous and systematically lower-mass population than the central object. This distinction is particularly pronounced in non-BGG systems, where about one third of the IGL originates from small galaxies, each contributing less than 5% to this component. Among the tested magnitude gaps, the difference between the first- and fourth-ranked galaxies, $ΔM4-1$, proves a more robust indicator of dynamical age for low-mass groups than the conventional $ΔM2-1$ gap. The $ΔM5-1$ and $ΔM6-1$ gaps also perform well and may be preferable in certain contexts.

Figures (12)

• Figure 1: Luminous mass distribution of the four most massive galaxies at the start of the simulations, categorized by group class. Group classes are coded as follows: single-BGG systems (blue solid line), double-BGG systems (orange dotted line), and non-BGG systems (green dashed line).
• Figure 2: Total group mass, $M_{\rm grp}$, as a function of the mass of the first-ranked galaxy, $m_1$, at $z_{\rm ini}=3$. The horizontal dashed line marks the median total mass of all groups, while the vertical dashed line indicates the median mass of the first-ranked galaxies. The absence of any obvious clustering or segregation in the data within the quadrants delineated by these dividing lines suggests no significant bias in the initial conditions. Group classes are coded as follows: single-BGG systems (blue stars), double-BGG systems (orange diamonds), and non-BGG systems (green circles).
• Figure 3: Evolution in the last $\sim 9.3$ Gyr of the runs of various indicators of dynamical state, shown separately for each group class: (a) the fraction of surviving group galaxies relative to $N_{\rm gal}(z_{\rm ref})$; (b) the mass-weighted velocity dispersion of group galaxies; (c) the mass-weighted mean pairwise spatial separation among group members; and (d) the offset between the first-ranked galaxy and the group’s centre of mass, expressed in units of the expected group virial radius (Eq. (\ref{['rvir']})). Data points represent the mean values for each class, with the associated transparent shaded bands surrounding them indicating the standard error. The vertical solid and dashed lines in the panels mark, respectively, the $Q_2$, $Q_1$ and $Q_3$ quartiles of the distribution of turnaround times. Group classes are colour-coded as follows: single-BGG systems (blue), double-BGG systems (orange), and non-BGG systems (green).
• Figure 4: Temporal evolution of various dynamical state indicators based on the IGL, shown separately for each group class: (a) the total IGL mass; (b) the IGL mass fraction; (c) the $m_1$ plus total IGL mass fraction; and (d) the $m_1$ mass fraction. All luminous mass fractions are expressed relative to the invariant total stellar mass of the groups. Data points represent the mean values for each class, with the associated transparent shaded bands surrounding them indicating the standard error. The central vertical solid and dashed left and right lines in the panels mark, respectively, the $Q_2$, $Q_1$ and $Q_3$ quartiles of the distribution of turnaround times for the entire group sample. Group classes are colour-coded as in Fig. \ref{['fig:grp_prop']}.
• Figure 5: Histograms showing the luminous mass distribution of precursor galaxies at $z_{\rm ini}=3$ that contribute at least $5\%$ to the final mass of either the first-ranked group galaxy $m_1$ (solid magenta lines) or the IGL (solid black lines), with median values indicated by vertical dot-dashed and dashed lines, respectively. The distributions are shown separately for each group class: single-BGG (left), double-BGG (middle), and non-BGG (right) panels.