UC Santa Cruz

The stellar halos of nearby galaxies bare the signatures of the mass-assembly processes that have driven galaxy evolution over the last ~10 Gyr. Finding and interpreting these relict clues in galaxies within and beyond the local group offers one of the most promising avenues for understanding how galaxies accumulate their stars over time. To tackle this problem we have performed a systematic study of the wide-field kinematic structure of nearby (D < 30 Mpc) early-type galaxies (ETGs), based on two-dimensional absorption-line stellar spectroscopy out to several effective radii (~3 R_e). The 22 galaxies presented here span a range of environments (field, group, and cluster), intrinsic luminosities (-22.4k<-25.6), and morphologies (S0-E0). The data consist of moderate resolution integrated-stellar-light spectra extracted from the individual slitlets of custom Keck/DEIMOS slitmasks. The complicating effects of strong emission line features are avoided by targeting the spectral region surrounding the near-infrared Calcium II triplet. For each spectrum, we parameterize the line-of-sight velocity distribution (LOSVD) as a truncated Gauss-Hermite series convolved with an optimally weighted combination of stellar templates. These kinematic measurements (V, σ, h₃, and h₄) are combined with literature values to construct spatially resolved maps of large-scale kinematic structure. A variety of kinematic behaviors are observed beyond ~1 R_e, potentially reflecting the stochastic and chaotic assembly of stellar bulges and halos in early-type galaxies.

Next, we describe a global analysis (out to 5 R_e) of kinematics and metallicity in the nearest S0 galaxy, NGC 3115, along with implications for its assembly history. The data include high-quality wide-field imaging and multi-slit spectra of the field stars and globular clusters (GCs). Within two effective radii, the bulge (as traced by the stars and metal-rich GCs) is flattened and rotates rapidly. At larger radii, the rotation declines dramatically, while the characteristic GC metallicities also decrease with radius. We argue that this pattern is not naturally explained by a binary major merger, but instead by a two-phase assembly process where the inner regions have formed in an early violent, dissipative phase, followed by the protracted growth of the outer parts via minor mergers. To test this hypothesis and improve our understanding of the growth of stellar halos we compare our observational results to high-resolution cosmological galaxy simulations. We describe a methodology for visualizing these data and present our initial comparisons between theory and observation, which suggest that the aggregate effects of many minor mergers dictate the large-scale kinematic structure of present day ETGs.