Spectroscopic Constraints on the Buildup of Intracluster Light in the Coma Cluster

The stellar content of the intracluster light (ICL) provides unique insight into the hierarchical assembly process of galaxy clusters. We present optical spectra of three ICL regions (μ_g ≈ 25.3-26.2 mag arcsec^-2) in the Coma cluster, located between 100 and 180 kpc from their nearest brightest cluster galaxies (BCGs): NGC 4889 and NGC 4874. Integral-field unit (IFU) spectroscopy with 13.5 hr on-source integration was acquired in an ancillary program within the Sloan Digital Sky Survey-IV MaNGA survey. We stacked the 127 individual fiber spectra in each IFU to achieve a 1σ limiting surface brightness of 27.9 mag arcsec^-2, corresponding to a mean signal-to-noise ratio in the optical of 21.7, 9.0, and 11.7 Å^-1, for each ICL region. We apply stellar population models to the stacked spectra. Our results show that the velocity dispersions of ICL regions are very high (σ ∼ 630 km s^-1), indicating the stars in these regions are tracing the gravitational potential of the cluster, instead of any individual galaxy. The line-of-sight velocities are different from each other by ∼700 km s^-1, while the velocity of each region is similar to the closest BCG. This suggests that the ICL regions are associated with two distinct subclusters centered on NGC 4889 and NGC 4874. The stellar populations of these regions are old and metal-poor, with ages of ${12.7}_{-3.4}^{+1.0}$ , ${6.7}_{-2.0}^{+2.9}$ , and ${9.7}_{-3.5}^{+3.5}$ Gyr, and [Fe/H] of $-{1.0}_{-0.3}^{+0.2}$ , $-{0.6}_{-0.5}^{+0.3}$ , and $-{0.8}_{-0.4}^{+0.3}$ . From the derived age and metallicity, the buildup of ICL in Coma is likely to be through the accretion of low-mass galaxies or the tidal stripping of the outskirts of massive galaxies that have ended their star formation early on, instead of directly from major mergers of massive galaxies.