The Hobby-Eberly Telescope Chemical Abundances of Stars in the Halo (CASH) project aims to characterize the nature of the early universe through the study of metal-poor stars in the stellar halo of the galaxy. Once completed, this will be the largest set of abundances determined for metal-poor stars from high resolution spectra. In this paper, we present chemical abundances and trends of eleven elements for the first ~80 stars of the ~500 star study. These 80 stars serve as a pilot sample to test the automated stellar parameter and abundance determination pipeline newly developed for the CASH project called CASHCODE. Among the pilot sample, two stars with [Fe/H]<-3.5 were discovered and their abundance analysis is discussed. / text

The stellar halo of the Milky Way is populated by mostly old and metal-poor stars. As dynamical timescales are of order ~Gyrs at these large distances, accreted stellar substructures, such as dwarf galaxies or globular clusters, survive here as coherent entities longer than anywhere else in the Galaxy. These substructures represent our “fossil record” which can be used to reconstruct the Galaxy’s complex past. In this work, we seek to identify the structures found in the far reaches of the stellar halo as a step towards a correct interpretation of this fossil record. The advent of all-sky surveys in the Gaia era has ignited a prosperous period for this field of Galactic archaeology, but exploring the distant Milky Way (>10 kpc) with Gaia is difficult. Parallax measurements are much less accurate beyond the Solar neighborhood, though Gaia’s proper motions remain useful out to large radii. To push Gaia into the distant Galaxy, we combined these astrometric data with u-band photometry from the Canada-France Imaging Survey (CFIS). We exploited CFIS’ excellent photometric quality and depth (which extends 3 magnitudes deeper than that of the Sloan Digital Sky Survey) to use blue horizontal branch stars (BHBs) as a tracer population with well-measured distances. We first examined the distribution of BHBs using the OPTICS (Ordering Points To Identify the Clustering Structure) clustering algorithm to visualize the hierarchical nature of outer halo substructure. We then identified several well-known satellites, including a group of stars in the vicinity of a distant globular cluster (NGC 5466). Analysis of their kinematics suggested a few of these BHBs outside the cluster’s tidal radius were co-moving with NGC 5466, implying they may be tidal debris from this system. Interestingly, a stream had previously been detected extending from this globular cluster. However, its properties had not been studied in the decade since its discovery, and previous dynamical models were unable to reproduce many of the reported features. As one of the (allegedly) longest globular cluster streams on the sky - and given its distance and utility to constrain the Milky Way’s mass at large Galactic radius - we sought to explore this structure further. We subsequently used red giant branch stars (RGBs) identified in CFIS to try to better quantify the characteristics of the putative stream. We were able to filter these data and obtain a sample of stars that are fully consistent with stream membership and which span approximately 31 degrees of sky. Combined with the BHBs, we used these populations to trace the path of the stream, its distance and distance gradient across the stream’s longitude, and additionally estimated a lower limit to the stream’s luminosity. Our measurements suggest that the stream is at least 11% of the luminosity of the cluster. We then compared our observational data to dynamical models, which showed generally good agreement with the observed stream. This success reflects the updated properties of data measured in this work, and the inclusion of new data (especially proper motions). Our model suggests that the pericenter and apocenter of NGC 5466's orbit are 6.4 and 43 kpc, respectively, resulting in a very eccentric orbit (ε = 0.74). We also find evidence that the cluster experienced a recent interaction (within the past ~100 Myrs) with the Galactic disk, suggesting that the primary source of mass loss in this system may be caused by disk-shocking. The NGC 5466 stellar stream also exhibits an interesting heliocentric gradient in the leading arm, which our simplistic spherical halo model does not fully reproduce. Dynamical experiments with various halo shapes fit to this stream will prove interesting for future work. For local cosmology in particular, long, thin, dynamically cold stellar streams are ideal systems for constraining properties of the Milky Way’s dark matter halo, and streams at large radius are especially useful for measuring the Galaxy's mass interior to the stream. In this respect, we anticipate that NGC 5466 will be exceptionally useful as a probe of the shape, mass, and dark substructure of the Milky Way's distant dark matter halo. / Graduate

We motivate the importance of understanding the kinematics and dynamics of the Milky Way stellar halo both in unravelling the formation history and evolution of our host Galaxy and in the more general context of galaxy dynamics. We present a cleaned picture of the kinematics of the smooth component of the stellar halo: we develop a method to quantify the average distance error on a sample of stars based on the idea of Schoenrich et al. (2012), but adapted so that it uses velocity information only on average. We use this scheme to construct an analytic distance calibration for Blue Horizontal Branch (BHB) field halo stars in Sloan colours and demonstrate that our calibration is a) more accurate than the ones available and b) unbiased w.r.t. metallicity and colour. We measure the rotation of the smooth component of the stellar halo with a tool-set of four estimators that use either only the l.o.s. velocities or the full 3D motion. From two samples of BHB stars from the Sloan Digital Sky Survey, we favour a non-rotating single halo. We critique conflicting results in the literature based on similar samples and trace back the disagreement (either in the sign of rotation or in the morphology of the halo) to sample contaminations and/or neglect account of the halo geometry. We propose a scheme that generalizes any isotropic spherical model to a model where the potential is axisymmetric and the distribution function is a function of the three actions. The idea is to approximate the Hamiltonian as a function of the actions with a library of quadratic fits to surfaces of constant energy in action space and then make explicit the dependence of the energy on the three actions in the ergodic distribution function. The transparency of the physics implied by the model we achieve, should make it possible to combine our spheroidal models to the f(J)-models of Binney (2010) for the disks and of Pontzen & Governato (2013) for the dark-matter halo, and obtain a complete actions-defined dynamical model of the Milky Way Galaxy.

According to the paradigm of lambda-CDM cosmology, the stellar halo ofour Galaxy has been built-up over time through the accretion of other galaxiesand star clusters. The remnants of some of these are still observable today asstellar streams, but are typically very faint and difficult to resolve amidst the farmore numerous foreground Milky Way stars. The VelHel-4 stream, discoveredby Helmi et al. [2017], consists of seven members selected based on their energiesand angular momenta. Further studies of these stars has shown evidence ofglobular cluster (GC) abundance patterns, suggesting that the stream originatedfrom a GC progenitor, but a larger sample is needed to verify this signature. Theobjective of this thesis is to find new candidate members of the VelHel-4 stellarstream in order to better characterize its properties and to confirm a possibleGC origin.The preliminary selection of stars was done kinematically, by computing theorbital actions and energies using astrometric data and radial velocities for abright subset of the Gaia DR3 database, and then analyzing the clustering ofstream members in different combinations of action space. The selected samplewas then cleaned by analyzing the positions of these stars in a colour-magnitudediagram. In total, 34 stars were included in the final selection. Follow-up high-resolution spectroscopy of these candidates is needed to study their stellar abun-dances and confirm the possible GC origin of this stream.