Publication Search Results

ID: 159331
Type: article
Authors: Bhattacharya, Souradeep; Arnaboldi, Magda; Gerhard, Ortwin; McConnachie, Alan; Caldwell, Nelson; Hartke, Johanna; Freeman, Kenneth C.
Abstract: Context. The Andromeda (M 31) galaxy displays several substructures in its inner halo. Different simulations associate their origin with either a single relatively massive merger, or with a larger number of distinct, less massive accretions.
Aims: The origin of these substructures as remnants of accreted satellites or perturbations of the pre-existing disc would be encoded in the properties of their stellar populations (SPs). The metallicity and star formation history of these distinct populations leave traces on their deep [O III] 5007 Å planetary nebulae luminosity function (PNLF). By characterizing the morphology of the PNLFs, we constrain their origin.
Methods: From our 54 sq. deg. deep narrow-band [O III] survey of M 31, we identify planetary nebulae in six major inner-halo substructures: the Giant Stream, North East Shelf, G1 Clump, Northern Clump, Western Shelf, and Stream D. We obtain their PNLFs and those in two disc annuli, with galactocentric radii of R_GC = 10-20 kpc and R_GC = 20-30 kpc. We measure PNLF parameters from cumulative fits and statistically compare the PNLFs in each substructure and disc annulus. We link these deep PNLF parameters and those for the Large Magellanic Cloud (LMC) to published metallicities and resolved stellar population-age measurements for their parent SPs.
Results: The absolute magnitudes (M^*) of the PNLF bright cut-off for these sub-populations span a significant magnitude range, despite being located at the same distance and having a similar line-of-sight extinction. The M^* values of the Giant Stream, W Shelf, and Stream D PNLFs are fainter than those predicted by PN evolution models by 0.6, 0.8, and 1.5 mag, respectively, assuming the measured metallicity of the parent stellar populations. The faint-end slope of the PNLF increases linearly with decreasing fraction of stellar mass younger than 5 Gyr across the M 31 regions and the LMC. From their PNLFs, the Giant Stream and NE Shelf are consistent with being stellar debris from an infalling satellite, while the G1 Clump appears to be linked with the pre-merger disc with an additional contribution from younger stars.
Conclusions: The SPs of the substructures are consistent with those predicted by simulations of a single fairly massive merger event that took place 2-3 Gyr ago in M31. Stream D has an unrelated, distinct origin. Furthermore, this study provides independent evidence that the faint-end of the PNLF is preferentially populated by planetary nebulae evolved from older stars.

ID: 159424
Type: article
Authors: Bonaca, Ana; Naidu, Rohan P.; Conroy, Charlie; Caldwell, Nelson; Cargile, Phillip A.; Han, Jiwon Jesse; Johnson, Benjamin D.; Kruijssen, J. M. Diederik; Myeong, G. C.; Speagle, Joshua S.; Ting, Yuan-Sen; Zaritsky, Dennis
Abstract: The origins of most stellar streams in the Milky Way are unknown. With improved proper motions provided by Gaia EDR3, we show that the orbits of 23 Galactic stellar streams are highly clustered in orbital phase space. Based on their energies and angular momenta, most streams in our sample can plausibly be associated with a specific (disrupted) dwarf galaxy host that brought them into the Milky Way. For eight streams we also identify likely globular cluster progenitors (four of these associations are reported here for the first time). Some of these stream progenitors are surprisingly far apart, displaced from their tidal debris by a few to tens of degrees. We identify stellar streams that appear spatially distinct, but whose similar orbits indicate they likely originate from the same progenitor. If confirmed as physical discontinuities, they will provide strong constraints on the mass loss from the progenitor. The nearly universal ex situ origin of existing stellar streams makes them valuable tracers of galaxy mergers and dynamical friction within the Galactic halo. Their phase-space clustering can be leveraged to construct a precise global map of dark matter in the Milky Way, while their internal structure may hold clues to the small-scale structure of dark matter in their original host galaxies.

ID: 159615
Type: article
Authors: Carter, Courtney; Conroy, Charlie; Zaritsky, Dennis; Ting, Yuan-Sen; Bonaca, Ana; Naidu, Rohan P.; Johnson, Benjamin D.; Cargile, Phillip A.; Caldwell, Nelson; Speagle, Joshua; Han, Jiwon Jesse
Abstract: Ancient, very metal-poor (VMP) stars offer a window into the earliest epochs of galaxy formation and assembly. We combine data from the H3 Spectroscopic Survey and Gaia to measure metallicities, abundances of α elements, stellar ages, and orbital properties of a sample of 482 VMP ([Fe/H] 70% of VMP stars near the disk are on prograde orbits and this fraction increases toward lower metallicities. This result is unexpected if metal-poor stars are predominantly accreted from many small systems with no preferred orientation, as such a scenario would imply a mostly isotropic distribution. Furthermore, we find there is some evidence for higher fractions of prograde orbits among stars with lower [α/Fe]. Isochrone-based ages for main-sequence turn-off stars reveal that these VMP stars are uniformly old (≍12 Gyr) irrespective of the α abundance and metallicity, suggesting that the metal-poor population was not born from the same well-mixed gas disk. We speculate that the VMP population has a heterogeneous origin, including both in situ formation in the ancient disk and accretion from a satellite with the same direction of rotation as the ancient disk at early times. Our precisely measured ages for these VMP stars on prograde orbits show that the Galaxy has had a relatively quiescent merging history over most of cosmic time, and implies the angular momentum alignment of the Galaxy has been in place for at least 12 Gyr.

ID: 157675
Type: article
Authors: Bonaca, Ana; Conroy, Charlie; Cargile, Phillip A.; Naidu, Rohan P.; Johnson, Benjamin D.; Zaritsky, Dennis; Ting, Yuan-Sen; Caldwell, Nelson; Han, Jiwon Jesse; van Dokkum, Pieter
Abstract: The archeological record of stars in the Milky Way opens a uniquely detailed window into the early formation and assembly of galaxies. Here we use 11,000 main-sequence turn-off stars with well-measured ages, $[\mathrm{Fe}/{\rm{H}}]$ , $[\alpha /\mathrm{Fe}]$ , and orbits from the H3 Survey and Gaia to time the major events in the early Galaxy. Located beyond the Galactic plane, $1\lesssim | Z| /\mathrm{kpc}\lesssim 4$ , this sample contains three chemically distinct groups: a low-metallicity population, and low-α and high-α groups at higher metallicity. The age and orbit distributions of these populations show that (1) the high-α group, which includes both disk stars and the in situ halo, has a star formation history independent of eccentricity that abruptly truncated 8.3 ± 0.1 Gyr ago (z ≃ 1); (2) the low-metallicity population, which we identify as the accreted stellar halo, is on eccentric orbits and its star formation truncated $10.2{.}_{-0.1}^{+0.2}$ Gyr ago (z ≃ 2); (3) the low-α population is primarily on low-eccentricity orbits and the bulk of its stars formed less than 8 Gyr ago. These results suggest a scenario in which the Milky Way accreted a satellite galaxy at z ≍ 2 that merged with the early disk by z ≍ 1. This merger truncated star formation in the early high-α disk and perturbed a fraction of that disk onto halo-like orbits. The merger enabled the formation of a chemically distinct, low-α disk at z ≲ 1. The lack of any stars on halo-like orbits at younger ages indicates that this event was the last significant disturbance to the Milky Way disk.

ID: 157303
Type: article
Authors: Bonaca, Ana; Conroy, Charlie; Hogg, David W.; Cargile, Phillip A.; Caldwell, Nelson; Naidu, Rohan P.; Price-Whelan, Adrian M.; Speagle, Joshua S.; Johnson, Benjamin D.
Abstract: The 100° long thin stellar stream in the Milky Way halo, GD-1, has an ensemble of features that may be due to dynamical interactions. Using high-resolution MMT/Hectochelle spectroscopy we show that a spur of GD-1-like stars outside of the main stream are kinematically and chemically consistent with the main stream. In the spur, as in the main stream, GD-1 has a low intrinsic radial velocity dispersion, σ_{V_r} ≲ 1 km s^-1, is metal-poor, [Fe/H] ≈ -2.3, and has little intrinsic spread in the [Fe/H] and [α/Fe] abundances, which point to a common globular cluster progenitor. At a fixed location along the stream, the median radial velocity offset between the spur and the main stream is smaller than 0.5 km s^-1, comparable to the measurement uncertainty. A flyby of a massive, compact object can change orbits of stars in a stellar stream and produce features like the spur observed in GD-1. In this scenario, the radial velocity of the GD-1 spur relative to the stream constrains the orbit of the perturber and its current on-sky position to ≈5000 deg². The family of acceptable perturber orbits overlaps the stellar and dark-matter debris of the Sagittarius dwarf galaxy in present-day position and velocity. This suggests that GD-1 may have been perturbed by a globular cluster or an extremely compact dark-matter subhalo formerly associated with Sagittarius.

ID: 156872
Type: article
Authors: Caldwell, Nelson; Bonaca, Ana; Price-Whelan, Adrian M.; Sesar, Branimir; Walker, Matthew G.
Abstract: We present new kinematic data for the Ophiuchus stellar stream. Spectra have been taken of member candidates at the MMT telescope using Hectospec, Hectochelle, and Binospec, which provide more than 1800 new velocities. Combined with proper-motion measurements of stars in the field by the Gaia-DR2 catalog, we have derived stream membership probabilities, resulting in the detection of more than 200 likely members. These data show the stream extends to more than three times the length shown in the discovery data. A spur to the main stream is also detected. The high-resolution spectra allow us to resolve the stellar velocity dispersion, found to be 1.6 ± 0.3 km s^-1.

ID: 157699
Type: article
Authors: Girardi, Léo; Boyer, Martha L.; Johnson, L. Clifton; Dalcanton, Julianne J.; Rosenfield, Philip; Seth, Anil C.; Skillman, Evan D.; Weisz, Daniel R.; Williams, Benjamin F.; Bhattacharya, Antara Raaghavi; Bressan, Alessandro; Caldwell, Nelson; Chen, Yang; Dolphin, Andrew E.; Fouesneau, Morgan; Goldman, Steven; Guhathakurta, Puragra; Marigo, Paola; Mukherjee, Sagnick; Pastorelli, Giada; Quirk, Amanda; Soraisam, Monika; Trabucchi, Michele
Abstract: The presence of asymptotic giant branch (AGB) stars in clusters provides key constraints for stellar models, as has been demonstrated with historical data from the Magellanic Clouds. In this work, we look for candidate AGB stars in M31 star clusters from the Panchromatic Hubble Andromeda Treasury survey. Our photometric criteria selects stars brighter than the tip of the red giant branch, which includes the bulk of the thermally pulsing AGB stars as well as early-AGB stars and other luminous cool giants expected in young stellar populations (e.g., massive red supergiants, and intermediate-mass red helium-burning stars). The AGB stars can be differentiated, a posteriori, using the ages already estimated for our cluster sample. 937 candidates are found within the cluster aperture radii, half (∼450) of which are very likely cluster members. Cross-matching with additional databases reveals two carbon stars and 10 secure variables among them. The field-corrected age distribution reveals the presence of young supergiants peaking at ages smaller than 10⁸ yr, followed by a long tail of AGB stars extending up to the oldest possible ages. This long tail reveals the general decrease in the numbers of AGB stars from initial values of $\sim 50\times {10}^{-6}{M}_{\odot }^{-1}$ at 10⁸ yr down to $\sim 5\times {10}^{-6}{M}_{\odot }^{-1}$ at 10¹⁰ yr. Theoretical models of near-solar metallicity reproduce this general trend, although with localized discrepancies over some age intervals, whose origin is not yet identified. The entire catalog is released together with finding charts to facilitate follow-up studies.

ID: 157674
Type: article
Authors: Naidu, Rohan P.; Conroy, Charlie; Bonaca, Ana; Johnson, Benjamin D.; Ting, Yuan-Sen; Caldwell, Nelson; Zaritsky, Dennis; Cargile, Phillip A.
Abstract: In the ΛCDM paradigm, the Galactic stellar halo is predicted to harbor the accreted debris of smaller systems. To identify these systems, the H3 Spectroscopic Survey, combined with Gaia, is gathering 6D phase-space and chemical information in the distant Galaxy. Here we present a comprehensive inventory of structure within 50 kpc from the Galactic center using a sample of 5684 giants at $| b| \gt 40^\circ $ and $| Z| \gt 2\,\mathrm{kpc}$ . We identify known structures including the high-α disk, the in situ halo (disk stars heated to eccentric orbits), Sagittarius (Sgr), Gaia-Sausage-Enceladus (GSE), the Helmi Streams, Sequoia, and Thamnos. Additionally, we identify the following new structures: (i) Aleph ([Fe/H] = -0.5), a low-eccentricity structure that rises a surprising 10 kpc off the plane, (ii) and (iii) Arjuna ([Fe/H] = -1.2) and I'itoi ([Fe/H] 80% of the halo is built by two massive (M_⋆ ∼ 10⁸-10⁹M_☉) accreted dwarfs: GSE ([Fe/H] = -1.2) within 25 kpc and Sgr ([Fe/H] = -1.0) beyond 25 kpc. This explains the relatively high overall metallicity of the halo ([Fe/H] ≍ -1.2). We attribute ≳95% of the sample to one of the listed structures, pointing to a halo built entirely from accreted dwarfs and heating of the disk.

ID: 156232
Type: article
Authors: Raymond, John C.; Caldwell, Nelson; Fesen, R. A.; Weil, K. E.; Boumis, P.; di Cicco, D.; Mittelman, D.; Walker, S.
Abstract: The large, faint supernova remnant (SNR) G70.0─21.5 is believed to be the result of a Type Ia supernova some 90,000 yr ago at a distance of ∼1 kpc based on the Gaia proper motion and parallax of an unusual white dwarf. We have obtained narrow passband optical images and high- resolution spectroscopy to determine shock speeds of 70 to 110 km s⁻¹. The shock itself is unusual in that the sharp Hα filaments arise from a very thin postshock zone where preshock neutral atoms are rapidly excited and ionized. Combining the shock speed with the remnant's estimated age, distance, and diameter, we investigate the SNR evolution in the post-Sedov phase. One-dimensional models that ignore such factors as magnetic fields, cosmic rays, and thermal conduction are marginally consistent with the observations.

ID: 157700
Type: article
Authors: Voggel, Karina T.; Seth, Anil C.; Sand, David J.; Hughes, Allison; Strader, Jay; Crnojevic, Denija; Caldwell, Nelson
Abstract: Tidally stripped galaxy nuclei and luminous globular clusters (GCs) are important tracers of the halos and assembly histories of nearby galaxies, but are difficult to reliably identify with typical ground-based imaging data. In this paper we present a new method to find these massive star clusters using Gaia DR2, focusing on the massive elliptical galaxy Centaurus A (Cen A). We show that stripped nuclei and GCs are partially resolved by Gaia at the distance of Cen A, showing characteristic astrometric and photometric signatures. We use this selection method to produce a list of 632 new candidate luminous clusters in the halo of Cen A out to a projected radius of 150 kpc. Adding in broadband photometry and visual examination improves the accuracy of our classification. In a spectroscopic pilot program we have confirmed five new luminous clusters, which includes the 7th and 10th most luminous GC in Cen A. Three of the newly discovered GCs are further away from Cen A than all previously known GCs. Several of these are compelling candidates for stripped nuclei. We show that our novel Gaia selection method retains at least partial utility out to distances of ∼25 Mpc and hence is a powerful tool for finding and studying star clusters in the sparse outskirts of galaxies in the local universe.

ID: 158763
Type: article
Authors: Zaritsky, Dennis; Conroy, Charlie; Naidu, Rohan P.; Cargile, Phillip A.; Putman, Mary; Besla, Gurtina; Bonaca, Ana; Caldwell, Nelson; Han, Jiwon Jesse; Johnson, Benjamin D.; Speagle, Joshua S.; Ting, Yuan-Sen
Abstract: We report the discovery of 15 stars in the H3 survey that lie, in projection, near the tip of the trailing gaseous Magellanic Stream (MS). The stars have Galactocentric velocities ^-1, Galactocentric distances of ?40 to 80 kpc (increasing along the MS), and [Fe/H] consistent with that of stars in the Small Magellanic Cloud. These 15 stars comprise 94% (15 of 16) of the H3 observed stars to date that have R_GAL > 37.5 kpc, -350 km s^-1 _GSR ^-1, and are not associated with the Sagittarius Stream. They represent a unique portion of the Milky Way's outer halo phase space distribution function and confirm that unrelaxed structure is detectable even at radii where H3 includes only a few hundred stars. Due to their statistical excess, their close association with the MS and HI compact clouds in the same region, both in position and velocity space, and their plausible correspondence with tidal debris in a published simulation, we identify these stars as debris of past Magellanic Cloud encounters. These stars are evidence for a stellar component of the tidal debris field far from the Clouds themselves and provide unique constraints on the interaction.

ID: 156233
Type: article
Authors: Zaritsky, Dennis; Conroy, Charlie; Zhang, Huanian; Naidu, Rohan P.; Bonaca, Ana; Caldwell, Nelson; Cargile, Phillip A.; Johnson, Benjamin D.
Abstract: The timing argument provides a lower limit on the mass of the Milky Way. Using a sample of 32 stars at R > 60 kpc drawn from the H3 Spectroscopic Survey and mock catalogs created from published numerical simulations, we find that M₂₀₀ > 0.91 × 10¹² M_☉ with 90% confidence. We recommend using this limit to refine the allowed prior mass range in more complex and sophisticated statistical treatments of Milky Way dynamics. The use of such a prior would have significantly reduced many previously published uncertainty ranges. Our analysis suggests that the most likely value of M₂₀₀ is ≈1.5 × 10¹² M_☉, but establishing this as the Milky Way mass requires a larger sample of outer halo stars and a more complete analysis of the inner halo stars in H3. The imminent growth in the sample of outer halo stars due to ongoing and planned surveys will make this possible.

ID: 154576
Type: article
Authors: Bhattacharya, Souradeep; Arnaboldi, Magda; Caldwell, Nelson; Gerhard, Ortwin; Blaña, Matías; McConnachie, Alan; Hartke, Johanna; Guhathakurta, Puragra; Pulsoni, Claudia; Freeman, Kenneth C.
Abstract: Context. The age-velocity dispersion relation is an important tool to understand the evolution of the disc of the Andromeda galaxy (M 31) in comparison with the Milky Way.
Aims: We use planetary nebulae (PNe) to obtain the age-velocity dispersion relation in different radial bins of the M 31 disc.
Methods: We separate the observed PNe sample based on their extinction values into two distinct age populations in the M 31 disc. The observed velocities of our high- and low-extinction PNe, which correspond to higher- and lower-mass progenitors, respectively, are fitted in de-projected elliptical bins to obtain their rotational velocities, V_ϕ, and corresponding dispersions, σ_ϕ. We assign ages to the two PN populations by comparing central-star properties of an archival sub-sample of PNe, that have models fitted to their observed spectral features, to stellar evolution tracks.
Results: For the high- and low-extinction PNe, we find ages of ̃2.5 and ̃4.5 Gyr, respectively, with distinct kinematics beyond a deprojected radius R_GC = 14 kpc. At R_GC = 17-20 kpc, which is the equivalent distance in disc scale lengths of the Sun in the Milky Way disc, we obtain σ_{ϕ, 2.5 Gyr} = 61 ± 14 km s^-1 and σ_{ϕ, 4.5 Gyr} = 101 ± 13 km s^-1. The age-velocity dispersion relation for the M 31 disc is obtained in two radial bins, R_GC = 14-17 and 17-20 kpc.
Conclusions: The high- and low-extinction PNe are associated with the young thin and old thicker disc of M 31, respectively, whose velocity dispersion values increase with age. These values are almost twice and three times that of the Milky Way disc stellar population of corresponding ages, respectively. From comparison with simulations of merging galaxies, we find that the age-velocity dispersion relation in the M 31 disc measured using PNe is indicative of a single major merger that occurred 2.5-4.5 Gyr ago with an estimated merger mass ratio ≈1:5.

ID: 155308
Type: article
Authors: Bhattacharya, Souradeep; Arnaboldi, Magda; Hartke, Johanna; Gerhard, Ortwin; Comte, Valentin; McConnachie, Alan; Caldwell, Nelson
Abstract: Context. The Andromeda (M 31) galaxy subtends nearly 100 square degrees on the sky. Any study of its halo must therefore account for the severe contamination from the Milky Way halo stars whose surface density displays a steep gradient across the entire M 31 field of view. Aims: Our goal is to identify a population of stars firmly associated with the M 31 galaxy. Planetary nebulae (PNe) are one such population that are excellent tracers of light, chemistry, and motion in galaxies. We present a 16 square degree survey of the disc and inner halo of M 31 with the MegaCam wide-field imager at the CFHT to identify PNe, and characterise the luminosity-specific PN number and PN luminosity function (PNLF) in M 31. Methods: PNe were identified via automated detection techniques based on their bright [O III] 5007 Å emission and absence of a continuum. Subsamples of the faint PNe were independently confirmed by matching with resolved Hubble Space Telescope sources from the Panchromatic Hubble Andromeda Treasury and spectroscopic follow-up observations with HectoSpec at the MMT. Results: The current survey reaches two magnitudes fainter than the previous most sensitive survey. We thus identify 4289 PNe, of which only 1099 were previously known. By comparing the PN number density with the surface brightness profile of M 31 out to ∼30 kpc along the minor axis, we find that the stellar population in the inner halo has a luminosity- specific PN number value that is seven times higher than that of the disc. We measure the luminosity function of the PN population and find a bright cut-off and a slope consistent with previous determinations. Interestingly, it shows a significant rise at the faint end, present in all radial bins covered by the survey. This rise in the M 31 PNLF is much steeper than that observed for the Magellanic clouds and Milky Way bulge. Conclusions: The significant radial variation of the PN specific frequency value indicates that the stellar population at deprojected minor-axis radii larger than ∼10 kpc is different from that in the disc of M 31. The rise at the faint end of the PNLF is a property of the late phases of the stellar population. M 31 shows two major episodes of star formation and the rise at the faint end of the PNLF is possibly associated with the older stellar population. It may also be a result of varying opacity of the PNe. Based on observations obtained with MegaCam, a joint project of the CFHT and CEA/DAPNIA, at the Canada-France-Hawaii Telescope (CFHT), which is operated by the National Research Council (NRC) of Canada, the Institut National des Sciences de l'Univers of the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii. The observations at the CFHT were performed with care and respect from the summit of Maunakea which is a significant cultural and historic site.

ID: 154407
Type: article
Authors: Conroy, Charlie; Bonaca, Ana; Cargile, Phillip; Johnson, Benjamin D.; Caldwell, Nelson; Naidu, Rohan P.; Zaritsky, Dennis; Fabricant, Daniel; Moran, Sean; Rhee, Jaehyon; Szentgyorgyi, Andrew; Berlind, Perry; Calkins, Michael L.; Kattner, ShiAnne; Ly, Chun
Abstract: Modern theories of galaxy formation predict that the Galactic stellar halo was hierarchically assembled from the accretion and disruption of smaller systems. This hierarchical assembly is expected to produce a high degree of structure in the combined phase and chemistry space; this structure should provide a relatively direct probe of the accretion history of our Galaxy. Revealing this structure requires precise 3D positions (including distances), 3D velocities, and chemistry for large samples of stars. The Gaia satellite is delivering proper motions and parallaxes for >1 billion stars to G ≈ 20. However, radial velocities and metallicities will only be available to G ≈ 15, which is insufficient to probe the outer stellar halo (≳10 kpc). Moreover, parallaxes will not be precise enough to deliver high-quality distances for stars beyond ̃10 kpc. Identifying accreted systems throughout the stellar halo therefore requires a large ground-based spectroscopic survey to complement Gaia. Here we provide an overview of the H3 Stellar Spectroscopic Survey, which will deliver precise stellar parameters and spectrophotometric distances for ≈200,000 stars to r = 18. Spectra are obtained with the Hectochelle instrument at the MMT, which is configured for the H3 Survey to deliver resolution R ≈ 23,000 spectra covering the wavelength range 5150-5300 Å. The survey is optimized for stellar halo science and therefore focuses on high Galactic latitude fields (| b| > 30^\circ ), sparsely sampling 15,000 sq. degrees. Targets are selected on the basis of Gaia parallaxes, enabling very efficient selection of bona fide halo stars. The survey began in the fall of 2017 and has collected 88,000 spectra to-date. All of the data, including the derived stellar parameters, will eventually be made publicly available via the survey website: h3s urvey.rc.fas.harvard.edu.

ID: 154515
Type: article
Authors: Conroy, Charlie; Naidu, Rohan P.; Zaritsky, Dennis; Bonaca, Ana; Cargile, Phillip; Johnson, Benjamin D.; Caldwell, Nelson
Abstract: The Galactic stellar halo is predicted to have formed at least partially from the tidal disruption of accreted dwarf galaxies. This assembly history should be detectable in the orbital and chemical properties of stars. The H3 Survey is obtaining spectra for 200,000 stars and, when combined with Gaia data, is providing detailed orbital and chemical properties of Galactic halo stars. Unlike previous surveys of the halo, the H3 target selection is based solely on magnitude and Gaia parallax the survey therefore provides a nearly unbiased view of the entire stellar halo at high latitudes. In this paper we present the distribution of stellar metallicities as a function of Galactocentric distance and orbital properties for a sample of 4232 kinematically selected halo giants to 100 kpc. The stellar halo is relatively metal- rich, =-1.2, and there is no discernible metallicity gradient over the range 6 =-1.2, and there is no discernible metallicity gradient over the range 6 _{gal gal gal} > 30 kpc, respectively. The Sagittarius stream dominates the metallicity distribution at 20─40 kpc for stars on prograde orbits. The Gaia─Enceladus merger remnant dominates the metallicity distribution for radial orbits to ≈30 kpc. Metal-poor stars with [Fe/H] < −2 are a small population of the halo at all distances and orbital categories. We associate the "in situ" stellar halo with stars displaying thick disk chemistry on halo-like orbits; such stars are confined to | z| < 10 {kpc}. The majority of the stellar halo is resolved into discrete features in chemical─orbital space, suggesting that the bulk of the stellar halo formed from the accretion and tidal disruption of dwarf galaxies. The relatively high metallicity of the halo derived in this work is a consequence of the unbiased selection function of halo stars and, in combination with the recent upward revision of the total stellar halo mass, implies a Galactic halo metallicity that is typical for its mass.

ID: 150492
Type: article
Authors: Crnojević, D.; Sand, D. J.; Bennet, P.; Pasetto, S.; Spekkens, K.; Caldwell, Nelson; Guhathakurta, P.; McLeod, Brian A.; Seth, A.; Simon, J. D.; Strader, J.; Toloba, E.
Abstract: The Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS) is constructing a wide-field map of the resolved stellar populations in the extended halos of these two nearby, prominent galaxies. We present new Magellan/Megacam imaging of a ∼3 deg² area around Centaurus A (Cen A), which filled in much of our coverage to its south, leaving a nearly complete halo map out to a projected radius of ∼150 kpc and allowing us to identify two new resolved dwarf galaxies. We have additionally obtained deep Hubble Space Telescope (HST) optical imaging of 11 out of the 13 candidate dwarf galaxies identified around Cen A and presented in Crnojević et al. 2016a: seven are confirmed to be satellites of Cen A, while four are found to be background galaxies. We derive accurate distances, structural parameters, luminosities, and photometric metallicities for the seven candidates confirmed by our HST/ACS imaging. We further study the stellar population along the ∼60 kpc long (in projection) stream associated with Dw3, which likely had an initial brightness of M _V ∼ ‑15 and shows evidence for a metallicity gradient along its length. Using the total sample of 11 dwarf satellites discovered by the PISCeS survey, as well as 13 brighter previously known satellites of Cen A, we present a revised galaxy luminosity function for the Cen A group down to a limiting magnitude of M _V ∼ ‑8, which has a slope of ‑1.14 ± 0.17, comparable to that seen in the Local Group and in other nearby groups of galaxies.

ID: 151819
Type: article
Authors: Johnson, Christian I.; Caldwell, Nelson; Michael Rich, R.; Mateo, Mario; Bailey, John I.
Abstract: NGC 6402 is among the most massive globular clusters in the Galaxy, but little is known about its detailed chemical composition. Therefore, we obtained radial velocities and/or chemical abundances of 11 elements for 41 red giant branch stars using high resolution spectra obtained with the Magellan-M2FS instrument. We find NGC 6402 to be only moderately metal-poor with = -1.13 dex (σ = 0.05 dex) and to have a mean heliocentric radial velocity of -61.1 km s^-1 (σ = 8.5 km s^-1). In general, NGC 6402 exhibits mean composition properties that are similar to other inner Galaxy clusters, such as [α/Fe] ˜+0.3 dex, [Cr,Ni/Fe] ˜ 0.0 dex, and = -0.08 dex. Similarly, we find large star-to-star abundance variations for O, Na, Mg, Al, and Si that are indicative of gas that experienced high temperature proton-capture burning. Interestingly, we not only detect three distinct populations but also find large gaps in the [O/Fe], [Na/Fe], and [Al/Fe] distributions that may provide the first direct evidence of delayed formation for intermediate composition stars. A qualitative enrichment model is discussed where clusters form stars through an early (≲5-10 Myr) phase, which results in first generation and `extreme' composition stars, and a delayed phase (≳40 Myr), which results in the dilution of processed and pristine gas and the formation of intermediate composition stars. For NGC 6402, the missing intermediate composition stars suggest the delayed phase terminated prematurely, and as a result the cluster may uniquely preserve details of the chemical enrichment process.

ID: 154610
Type: article
Authors: Mutlu-Pakdil, Burçin; Sand, David J.; Walker, Matthew G.; Caldwell, Nelson; Carlin, Jeffrey L.; Collins, Michelle L.; Crnojević, Denija; Mateo, Mario; Olszewski, Edward W.; Seth, Anil C.; Strader, Jay; Willman, Beth; Zaritsky, Dennis
Abstract: The ultra-faint dwarf galaxy Leo V has shown both photometric overdensities and kinematic members at large radii, along with a tentative kinematic gradient, suggesting that it may have undergone a close encounter with the Milky Way. We investigate these signs of disruption through a combination of (i) high precision photometry obtained with the Hubble Space Telescope (HST), (ii) two epochs of stellar spectra obtained with the Hectochelle Spectrograph on the MMT, and (iii) measurements from the Gaia mission. Using the HST data, we examine one of the reported stream-like overdensities at large radii, and conclude that it is not a true stellar stream, but instead a clump of foreground stars and background galaxies. Our spectroscopic analysis shows that one known member star is likely a binary, and challenges the membership status of three others, including two distant candidates that had formerly provided evidence for overall stellar mass loss. We also find evidence that the proposed kinematic gradient across Leo V might be due to small number statistics. We update the systemic proper motion of Leo V, finding ({μ }_α \cos δ ,{μ }_δ )=(0.009+/- 0.560, -0.777 ± 0.314) mas yr^-1, which is consistent with its reported orbit that did not put Leo V at risk of being disturbed by the Milky Way. These findings remove most of the observational clues that suggested Leo V was disrupting; however, we also find new plausible member stars, two of which are located >5 half-light radii from the main body. These stars require further investigation. Therefore, the nature of Leo V still remains an open question.

ID: 145816
Type: article
Authors: Johnson, Christian I.; Rich, R. Michael; Caldwell, Nelson; Mateo, Mario; Bailey, John I., III; Olszewski, Edward W.; Walker, Matthew G.
Abstract: Photometric and spectroscopic analyses have shown that the Galactic bulge cluster Terzan 5 hosts several populations with different metallicities and ages that manifest as a double red horizontal branch (HB). A recent investigation of the massive bulge cluster NGC 6569 revealed a similar, though less extended, HB luminosity split, but little is known about the cluster's detailed chemical composition. Therefore, we have used high-resolution spectra from the Magellan–M2FS and VLT–FLAMES spectrographs to investigate the chemical compositions and radial velocity distributions of red giant branch and HB stars in NGC 6569. We found the cluster to have a mean heliocentric radial velocity of ‑48.8 km s^‑1 (σ = 5.3 km s^‑1 148 stars) and =-0.87 dex (19 stars), but the cluster's 0.05 dex [Fe/H] dispersion precludes a significant metallicity spread. NGC 6569 exhibits light- and heavy-element distributions that are common among old bulge/inner Galaxy globular clusters, including clear (anti)correlations between [O/Fe], [Na/Fe], and [Al/Fe]. The light-element data suggest that NGC 6569 may be composed of at least two distinct populations, and the cluster's low =-0.11 dex indicates significant pollution with r-process material. We confirm that both HBs contain cluster members, but metallicity and light-element variations are largely ruled out as sources for the luminosity difference. However, He mass fraction differences as small as ΔY ∼ 0.02 cannot be ruled out and may be sufficient to reproduce the double HB.